3.1 GENEs
Where are genes located? Activity
A gene occupies a specifc position on one type Etimating the number of
o chromosome. human gene
Experiments in which dierent varieties o plant or animals are crossed In October 1970 Scientifc
show that genes are linked in groups and each group corresponds to one American published an estimate
o the types o chromosome in a species. For example, there are our that the human genome might
groups o linked genes in ruit fies and our types o chromosome. Maize consist o as many as 10 million
has ten groups o linked genes and ten types o chromosome and in genes. How many times greater
humans the number o both is 23. than the current predicted
number is this? What reasons
Each gene occupies a specic position on the type o chromosome where can you give or such a huge
it is located. This position is called the locus o the gene. Maps showing the overestimate in 1970?
sequence o genes along chromosomes in ruit fies and other organisms
were produced by crossing experiments, but much more detailed maps
can now be produced when the genome o a species is sequenced.
7q36.2
7q35
7q33
7q32.2
7q31.33
7q31.31
7q31.1
7q22.2
7 q 21 . 3
7 q 21 .13
7 q 21 .11
7q11.22
7 q 12 .1
7 q 12 . 3
7 q 14.1
7 q 14. 3
7 q 15 . 2
7 q 21 .1
7 q 21 . 3
7q22.2
Figure 1 Chromosome 7: an example o a human chromosome. It consists o a single DNA
molecule with approximately 170 million base pairs about 5% o the human genome. The
pattern o banding, obtained by staining the chromosome, is diferent rom other human
chromosomes. Several thousand genes are located on chromosome 7, mostly in the light
bands, each o which has a unique identiying code. The locus o a ew o the genes on
chromosome 7 is shown
What are alleles? Figure 2 Diferent coat colours in mice
The various specifc orms o a gene are alleles.
Gregor Mendel is usually regarded as the ather o genetics. He crossed
varieties o pea plants, or example tall pea plants with dwar peas and
white-fowered pea plants with purple-fowered. Mendel deduced that
the dierences between the varieties that he crossed together were due
to dierent heritable actors. We now know that these pairs o heritable
actors are alternative orms o the same gene. For example there are
two orms o the gene that infuences height, one making pea plants tall
and the other making the plants dwar.
These dierent orms are called alleles. There can be more than two
alleles o a gene. One o the rst examples o multiple alleles to be
discovered is in mice. A gene that infuences coat colour has three
alleles, making the mice yellow, grey and black. There are three alleles
o the gene in humans that determines ABO blood groups. In some cases
there are large numbers o dierent alleles o a gene, or example the
gene that infuences eye colour in ruit fies.
As alleles are alternative orms o the same gene, they occupy the same
position on one type o chromosome they have the same locus. Only
one allele can occupy the locus o the gene on a chromosome. Most
animal and plant cells have two copies o each type o chromosome, so
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3 Genetics
we can expect two copies o a gene to be present. These could be two o
the same allele o the gene or two dierent alleles.
Diferences between alleles
Alleles difer rom each other by one or a ew bases only.
A gene consists o a length o DNA, with a base sequence that can be
hundreds or thousands o bases long. The dierent alleles o a gene have
slight variations in the base sequence. Usually only one or a very small
number o bases are dierent, or example adenine might be present at
a particular position in the sequence in one allele and cytosine at that
position in another allele.
Positions in a gene where more than one base may be present are called
single nucleotide polymorphisms, abbreviated to SNPs and pronounced
snips. Several snips can be present in a gene, but even then the alleles o
the gene dier by only a ew bases.
Comparing genes
Use o a database to determine diferences in the base sequence o a gene
in two species
One outcome o the Human Genome Project is C hoose Fast A and the sequence should
that the techniques that were developed have appear. C opy the sequence and paste it into
enabled the sequencing o other genomes. This a .txt fle or notepad fle.
allows gene sequences to be compared. The results Repeat with a number o dierent species that
o this comparison can be used to determine you want to compare and save the fles.
evolutionary relationships. Also, the identifcation
o conserved sequences allows species to be chosen To have the computer align the sequence or
or exploring the unction o that sequence. you, download the sotware called ClustalX
Go to the website called GenBank and run it.
( http://www.ncbi. nlm.nih.gov/pubmed/) In the File menu, choose Load Sequences.
Choose gene rom the search menu. S elect your fle. Your sequences should show
up in the C lustalX window.
Enter the name o a gene plus the organism,
such as cytochrome oxidase 1 (COX1 ) or pan Under the Alignment menu choose Do
(chimpanzee) . Complete Alignment. The example below
shows the sequence alignment o 9 dierent
Move your mouse over the section Genomic organisms.
regions, transcripts, and products until
Nucleotide Links appears. Figure 3
144
3.1 GENEs
Data-baed quetion: COX-2, smoking and stomach cancer
COX-2 is a gene that codes or the enzyme 2 a) Calculate the total percentage o the patients
cyclooxygenase. The gene consists o over
6,000 nucleotides. Three single nucleotide that were smokers and the total percentage
polymorphisms have been discovered that
are associated with gastric adenocarcinoma, o controls that were smokers. [2]
a cancer o the stomach. One o these SNPs
occurs at nucleotide 1 1 95. The base at this b) Explain the conclusion that can be drawn
nucleotide can be either adenine or guanine. A rom the dierence in the percentages. [2]
large survey in China involved sequencing both
copies o the COX-2 gene in 357 patients who 3 Deduce, with a reason, whether G or A
had developed gastric adenocarcinoma and in at nucleotide 1 1 95 is associated with an
985 people who did not have the disease. All o increased risk o gastric adenocarcinoma. [2]
these people were asked whether they had ever
smoked cigarettes. 4 Discuss, using the data, whether the risk o
gastric adenocarcinoma is increased equally
in all smokers. [2]
GG AG or AA
Table 1 shows the 3 5 7 patients with gastric Smokers 9.8% 43.7%
adenocarcinoma categorized according to 40.0%
whether they were smokers or non-smokers and Non-smokers 9.5%
whether they had two copies o COX-2 with G AG or AA
at nucleotide 1 1 95 (GG) or at least one copy o Table 1 Patients with cancer 35.6%
the gene with A at this position (AG or AA) . The
results are shown as percentages. Table 2 shows Smokers GG
the same categorization or the 985 people who 9.4%
did not have this cancer.
1 Predict, using the data, which o bases G or 12.6% 42.4%
A is more common at nucleotide 1 1 95 in Non-smokers
the controls. [2] Table 2 Patients without cancer
Mutation Activity
New alleles are ormed by mutation. New allele
New alleles are ormed rom other alleles by gene mutation. Mutations are Recent research into mutation
random changes there is no mechanism or a particular mutation being involved nding the base
carried out. The most signifcant type o mutation is a base substitution. sequence o all genes in parents
One base in the sequence o a gene is replaced by a dierent base. For and their ofspring. It showed that
example, i adenine was present at a particular point in the base sequence there was one base mutation per
it could be substituted by cytosine, guanine or thymine. 1.2 108 bases. Calculate how
many new alleles a child is likely
A random change to an allele that has developed by evolution over to have as a result o mutations
perhaps millions o years is unlikely to be benefcial. Almost all in their parents. Assume that
mutations are thereore either neutral or harmul. Some mutations there are 25,000 human genes
are lethal they cause the death o the cell in which the mutation and these genes are 2,000 bases
occurs. Mutations in body cells are eliminated when the individual dies, long on average.
but mutations in cells that develop into gametes can be passed on to
ospring and cause genetic disease. Source: Campbell, CD, et al. (2012)
Estimating the human mutation
rate using autozygosity in a founder
population. Nature Genetics, 44:
1277-1281. doi: 10.1038/ng.2418
145
3 Genetics
TOK sickle cell anemia
What criteria can be used to The causes o sickle cell anemia, including a base
distinguish between correlation and substitution mutation, a change to the base sequence
cause and efect? o mRNA transcribed rom it and a change to the
sequence o a polypeptide in hemoglobin.
There is a correlation between high
requencies o the sickle-cell allele Sickle-cell anemia is the commonest genetic disease in the world.
in human populations and high rates It is due to a mutation o the gene that codes or the alpha-globin
o inection with Falciparum malaria. polypeptide in hemoglobin. The symbol or this gene is Hb. Most
Where a correlation exists, it may humans have the allele HbA. I a base substitution mutation converts
or may not be due to a causal link. the sixth codon o the gene rom GAG to GTG, a new allele is ormed,
Consider the inormation in fgure 4 called HbS. The mutation is only inherited by ospring i it occurs in a
to decide whether sickle-cell anemia cell o the ovary or testis that develops into an egg or sperm.
causes inection with malaria.
When the HbS allele is transcribed, the mRNA produced has GUG as its
a) b) sixth codon instead o GAG, and when this mRNA is transcribed, the
sixth amino acid in the polypeptide is valine instead o glutamic acid. This
Key 05 change causes hemoglobin molecules to stick together in tissues with low
Frequency of Hbs allele (%) oxygen concentrations. The bundles o hemoglobin molecules that are
ormed are rigid enough to distort the red blood cells into a sickle shape.
1520 1015 510
These sickle cells cause damage to tissues by becoming trapped in blood
Figure 4 Map ( a) shows the requency o capillaries, blocking them and reducing blood fow. When sickle cells
the sickle cell allele and map return to high oxygen conditions in the lung, the hemoglobin bundles
(b) shows malaria afected areas in break up and the cells return to their normal shape. These changes occur
Arica and Western Asia time ater time, as the red blood cells circulate. Both the hemoglobin and
the plasma membrane are damaged and the lie o a red blood cell can be
shortened to as little as 4 days. The body cannot replace red blood cells at
a rapid enough rate and anemia thereore develops.
So, a small change to a gene can have very harmul consequences
or individuals that inherit the gene. It is not known how oten this
mutation has occurred but in some parts o the world the HbS allele is
remarkably common. In parts o East Arica up to 5% o newborn babies
have two copies o the allele and develop severe anemia. Another 35%
have one copy so make both normal hemoglobin and the mutant orm.
These individuals only suer mild anemia.
Figure 5 Micrographs o sickle cells and normal red blood cells
146
3.1 GENEs
Wha is a genome? Activity
The genome is the whole of the genetic information of Ethic of genome reearch
an organism.
Ethical questions about
Among biologists today the word genome means the whole o the genome research are worth
genetic inormation o an organism. Genetic inormation is contained in discussing.
DNA, so a living organisms genome is the entire base sequence o each
o its DNA molecules. Is it ethical to take a DNA
sample from ethnic groups
In humans the genome consists o the 46 molecules that orm around the world and
the chromosomes in the nucleus plus the DNA molecule in the sequence it without their
mitochondrion. This is the pattern in other animals, though the permission?
number o chromosomes is usually dierent.
Is it ethical for a biotech
In plant species the genome is the DNA molecules o chromosomes company to patent the
in the nucleus plus the DNA molecules in the mitochondrion and base sequence of a gene to
the chloroplast. prevent other companies
from using it to conduct
The genome o prokaryotes is much smaller and consists o the DNA research freely?
in the circular chromosome, plus any plasmids that are present.
Who should have access to
the Human Genome Projec this genetic information?
Should employers,
The entire base sequence of human genes was insurance companies and
sequenced in the Human Genome Project. law enforcement agencies
know our genetic makeup?
The Human Genome Project began in 1 990. Its aim was to fnd the
base sequence o the entire human genome. This project drove rapid
improvements in base sequencing techniques, which allowed a drat
sequence to be published much sooner than expected in 2000 and a
complete sequence in 2003.
Although knowledge o the entire base sequence has not given us an
immediate and total understanding o human genetics, it has given us
what can be regarded as a rich mine o data, which will be worked by
researchers or many years to come. For example, it is possible to predict
which base sequences are protein-coding genes. There are approximately
2 3 , 000 o these in the human genome. O riginally, estimates or the
number o genes were much higher.
Another discovery was that most o the genome is not transcribed.
Originally called junk DNA, it is being increasingly recognized
that within these junk regions, there are elements that aect gene
expression as well as highly repetitive sequences, called satellite DNA.
The genome that was sequenced consists o one set o chromosomes it
is a human genome rather than the human genome. Work continues
to fnd variations in sequence between dierent individuals. The vast
majority o base sequences are shared by all humans giving us genetic
unity, but there are also many single nucleotide polymorphisms which
contribute to human diversity.
Since the publication o the human genome, the base sequence o many
other species has been determined. Comparisons between these genomes
reveal aspects o the evolutionary history o living organisms that were
previously unknown. Research into genomes will be a developing theme
o biology in the 2 1 st century.
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3 Genetics
techniques used for genome sequencing
Developments in scientifc research ollow improvements in technology: gene
sequencers, essentially lasers and optical detectors, are used or the sequencing
o genes.
The idea o sequencing the entire human genome fuorescent marker is used or the copies
seemed impossibly dicult at one time but ending in each o the our bases.
improvements in technology towards the end o The samples are mixed together and all the
the 20th century made it possible, though still very DNA copies are separated in one lane o a gel
ambitious. These improvements continued once the according to the number o nucleotides.
project was underway and drat sequences were
thereore completed much sooner than expected. A laser scans along the lane to make the
Further advances are allowing the genomes o other fuorescent markers fuoresce.
species to be sequenced at an ever increasing rate. An optical detector is used to detect the
To sequence a genome, it is rst broken up into colours o fuorescence along the lane.
small lengths o DNA. Each o these is sequenced There is a series o peaks o fuorescence,
separately. To nd the base sequence o a ragment corresponding to each number o
o DNA, single-stranded copies o it are made nucleotides
using DNA polymerase, but the process is stopped A computer deduces the base sequence rom
beore the whole base sequence has been copied the sequence o colours o fuorescence
by putting small quantities o a non-standard detected.
nucleotide into the reaction mixture. This is done
separately with non-standard nucleotides carrying
each o the our possible DNA bases. Four samples
o DNA copy o varying length are produced, each
with one o our DNA bases at the end o each
copy. These our samples are separated according
to length by gel electrophoresis. For each number
o nucleotides in the copy there is a band in just
one o the our tracks in the gel, rom which the
sequence o bases in the DNA can be deduced.
The major advance in technology that speeded up
base sequencing by automating it is this:
Coloured fuorescent markers are used to Figure 6 Sequencing read from the DNA of Pinor Noir variety
mark the DNA copies. A dierent colour o of grape
148
3.2 ChrOmOsOmEs
3.2 Coooe
Udertadig Applicatio
Prokaryotes have one chromosome consisting Cairnss technique or measuring the length
o a circular DNA molecule. o DNA molecules by autoradiography.
Some prokaryotes also have plasmids but Comparison o genome size in T2
eukaryotes do not. phage, Escherichia coli, Drosophila
melanogaster, Homo sapiens and
Eukaryote chromosomes are linear Paris japonica.
DNA molecules associated with histone
proteins. Comparison o diploid chromosome numbers
o Homo sapiens, Pan troglodytes, Canis
In a eukaryote species there are familiaris, Oryza sativa, Parascaris equorum.
diferent chromosomes that carry diferent
genes. Use o karyotypes to deduce sex and diagnose
Down syndrome in humans.
Homologous chromosomes carry the same
sequence o genes but not necessarily the skill
same alleles o those genes.
Use o online databases to identiy the locus o
Diploid nuclei have pairs o homologous a human gene and its protein product.
chromosomes.
nature of ciece
Haploid nuclei have one chromosome o
each pair. Developments in scientic research ollow
improvements in techniques: autoradiography
The number o chromosomes is a characteristic was used to establish the length o DNA
eature o members o a species. molecules in chromosomes.
A karyogram shows the chromosomes o
an organism in homologous pairs o
decreasing length.
Sex is determined by sex chromosomes and
autosomes are chromosomes that do not
determine sex.
Bacterial chromoome
Prokaryotes have one chromosome consisting
o a circular DNA molecule.
The structure of prokaryotic cells was described in sub-topic 1 .2. In
most prokaryotes there is one chromosome, consisting of a circular DNA
molecule containing all the genes needed for the basic life processes
of the cell. The DNA in bacteria is not associated with proteins, so is
sometimes described as naked.
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3 Genetics
Because only one chromosome is present in a prokaryotic cell, there
is usually only a single copy o each gene. Two identical copies are
present briefy ater the chromosome has been replicated, but this is a
preparation or cell division. The two genetically identical chromosomes
are moved to opposite poles and the cell then splits in two.
Plasmids
Some prokaryotes also have plasmids but eukaryotes
do not.
Plasmids are small extra DNA molecules that are commonly ound in
prokaryotes but are very unusual in eukaryotes. They are usually small,
circular and naked, containing a ew genes that may be useul to the cell
but not those needed or its basic lie processes. For example, genes or
antibiotic resistance are oten located in plasmids. These genes are benecial
when an antibiotic is present in the environment but are not at other times.
Plasmids are not always replicated at the same time as the chromosome
o a prokaryotic cell or at the same rate. Hence there may be multiple
copies o plasmids in a cell and a plasmid may not be passed to both cells
ormed by cell division.
C opies o plasmids can be transerred rom one cell to another, allowing
spread through a population. It is even possible or plasmids to cross
the species barrier. This happens i a plasmid that is released when a
prokaryotic cell dies is absorbed by a cell o a dierent species. It is a
natural method o gene transer between species. Plasmids are also used
by biologists to transer genes between species articially.
Figure 1 (a) Circular DNA molecule from trimethoprim genes to help the
a bacterium (b) Bacterium preparing resistance plasmid spread
to divide
disinfectant resistance
penicillin family
resistance streptomycin family
resistance
vancomycin
resistance
Figure 2 The pLW1043 plasmid
Usig autoradiography to measure DnA molecules
Developments in scientifc research ollow improvements in techniques:
autoradiography was used to establish the length o DNA molecules in chromosomes.
Quantitative data is usually considered to be Developments in microscopy have allowed images
the strongest type o evidence or or against a to be produced o structures that were previously
hypothesis, but in biology it is sometimes images invisible. These sometimes conrm existing ideas
that provide the most convincing evidence. but sometimes also change our understanding.
150
3.2 ChrOmOsOmEs
Autoradiography was used by biologists rom chromosome was a single DNA molecule or
the 1 940s onwards to discover where specic more than one, but the images produced by
substances were located in cells or tissues. Cairns answered this question. They also
John Cairns used the technique in a dierent revealed replication orks in DNA or the rst
way in the 1 960s. He obtained images o whole time. Cairnss technique was used by others
D NA molecules rom E. coli bacteria. At the to investigate the structure o eukaryote
time it was not clear whether the bacterial chromosomes.
Measurig the legth of DnA molecules
Cairnss technique for measuring the length of DNA molecules by autoradiography.
John Cairns produced images o DNA molecules The images produced by Cairns showed that the
rom E.coli using this technique: chromosome in E. coli is a single circular D NA
molecule with a length o 1 ,1 00 m. This is
Cells were grown or two generations in remarkably long given that the length o the E coli
a culture medium containing tritiated cells is only 2 m.
thymidine. Thymidine consists o the base
thymine linked to deoxyribose and is used Autoradiography was then used by other
by E. coli to make nucleotides that it uses in researchers to produce images o eukaryotic
D NA replication. Tritiated thymidine contains chromosomes. An image o a chromosome rom
tritium, a radioactive isotope o hydrogen, so the ruit fy Drosophila melanogaster was produced
radioactively labelled DNA was produced by that was 1 2,000 m long. This corresponded
replication in the E. coli cells. with the total amount o DNA known to be in a
D. melanogaster chromosome, so or this species
The cells were then placed onto a dialysis at least a chromosome contains one very long
membrane and their cell walls were digested DNA molecule. In contrast to prokaryotes, the
using the enzyme lysozyme. The cells were molecule was linear rather than circular.
gently burst to release their DNA onto the
surace o the dialysis membrane.
A thin lm o photographic emulsion was
applied to the surace o the membrane and
let in darkness or two months. During that
time some o the atoms o tritium in the DNA
decayed and emitted high energy electrons,
which react with the lm.
At the end o the two-month period the Figure 3
lm was developed and examined with a
microscope. At each point where a tritium
atom decayed there is a dark grain. These
indicate the position o the DNA.
Eukaryote chromosomes
Eukaryote chromosomes are linear DNA molecules
associated with histone proteins.
Chromosomes in eukaryotes are composed o DNA and protein. The
DNA is a single immensely long linear DNA molecule. It is associated
with histone proteins. Histones are globular in shape and are wider
151
3 Genetics
than the DNA. There are many histone molecules in a chromosome,
with the DNA molecule wound around them. Adjacent histones in the
chromosome are separated by short stretches o the DNA molecule that
are not in contact with histones. This gives a eukaryotic chromosome the
appearance o a string o beads during interphase.
Diferences between chromosomes
Figure 4 In an electron micrograph the In a eukaryote species there are diferent chromosomes
histones give a eukaryotic chromosome that carry diferent genes.
the appearance of a string of beads during
interphase Eukaryote chromosomes are too narrow to be visible with a light
microscope during interphase. During mitosis and meiosis the
OH PH chromosomes become much shorter and atter by supercoiling, so are
visible i stains that bind either DNA or proteins are used. In the frst stage
7S DNA phe 16S o mitosis the chromosomes can be seen to be double. There are two
thr val chromatids, with identical DNA molecules produced by replication.
23S
When the chromosomes are examined during mitosis, dierent types
cyt b pro PL leu can be seen. They dier both in length and in the position o the
N1 centromere where the two chromatids are held together. The centromere
can be positioned anywhere rom close to an end to the centre o the
glu ile chromosome.
N6 gln f-met
There are at least two dierent types in every eukaryote but in most
control loop ala N2 species there are more than that. In humans or example there are
23 types o chromosome.
N5 ribosomal RNA asn trp
leu OL Every gene in eukaryotes occupies a specifc position on one type o
transfer RNAs cys chromosome, called the locus o the gene. Each chromosome type
protein coding gene tyr thereore carries a specifc sequence o genes arranged along the linear
DNA molecule. In many chromosomes this sequence contains over a
ser thousand genes.
his ser OX1 Crossing experiments were done in the past to discover the sequence o
genes on chromosome types in Drosophila melanogaster and other species.
N4 asp The base sequence o whole chromosomes can now be ound, allowing
more accurate and complete gene sequences to be deduced.
arg N3gly OX3 OX2
lys Having the genes arranged in a standard sequence along a type o
ATPase chromosome allows parts o chromosomes to be swapped during meiosis.
Figure 5 Gene map of the human mitochondrial Homologous chromosomes
chromosome. There are genes on both of the
two DNA strands. The chromosomes in the Homologous chromosomes carry the same sequence o
nucleus are much longer, carry far more genes genes but not necessarily the same alleles o those genes.
and are linear rather than circular
I two chromosomes have the same sequence o genes they are
homologous. Homologous chromosomes are not usually identical to
each other because, or at least some o the genes on them, the alleles
are dierent.
I two eukaryotes are members o the same species, we can expect each
o the chromosomes in one o them to be homologous with at least one
chromosome in the other. This allows members o a species to interbreed.
152
3.2 ChrOmOsOmEs
Data-baed quetion: Comparing the chromosomes of mice Activity
and humans
micocope invetigation of galic
Figure 6 shows all of the types of chromosome in mice and in coooe
humans. Numbers and colours are used to indicate sections of mouse
chromosomes that are homologous to sections of human chromosomes. 1 Garlic has large chromosomes so is an
ideal choice for looking at chromosomes.
Mouse and human genetic similarities Cells in mitosis are needed. Garlic bulbs
grow roots ifthey are kept for 3 or 4 days
Mouse chromosomes Human chromosomes with their bases in water, at about 25C.
Root tips with cells in mitosis are yellow
1234567 8 9 1234567 8 9 in colour, not white.
10 8 8 7 19 11
93
6 19 19
8
7
22 49 4 11
11 19
11 2 15 4 15
18 15 3 3 19 6
1 10 11
16
1 20 1 7 16
10 3
12 garlic bulb polystyrene
4 13 11 1 water at 25C disc with
hole cut
10 11 12 13 14 15 16 17 18 through
10 11 12 13 14 15 16 17 18 beaker
2 3 5 16 10
6 22 7 7 22 6 18
7 6 10 8 3 16 5
10 2 14 5 14 22 21
22 16 8 12 21 6 18
21 5 19
19 13 18
12 17 2
19 20 21 22 X Y 2 Root tips are put in a mixture ofa stain
19 X Y that binds to the chromosomes and
11 X Y acid, which loosens the connections
9 between the cell walls. A length ofabout
5 mm is suitable. Ten parts ofaceto-
10 orcein to one part of 1.0 mol dm-3
hydrochloric acid gives good results.
Figure 6 Chromosomes
1 Deduce the number of types of chromosomes in mice and [2] 5 mm long garlic stainacid mixture
in humans. root tip
2 Identify the two human chromosome types that are most [2] watch glass
similar to mouse chromosomes.
3 Identify mouse chromosomes which contain sections that are
not homologous to human chromosomes. [2] 3 The roots are heated in the stainacid
mixture on a hot plate, to 80C for
4 Suggest reasons for the many similarities between the mouse 5 minutes. One of the root tips is put
on a microscope slide, cut in half and
and human genomes. [2] the 2.5 mm length furthest from the
end of the root is discarded.
5 Deduce how chromosomes have mutated during the evolution
of animals such as mice and humans. [2]
root tip
watch glass
Comparing the genome sizes 8 hot plate
71 set at
Comparison of genome size in T2 phage, Escherichia 80 C
coli, Drosophila melanogaster, Homo sapiens and 62
Paris japonica.
53
The genomes of living organisms vary by a huge amount. The smallest 4
genomes are those of viruses, though they are not usually regarded as
living organisms. The table on the next page gives the genome size of 4 A drop ofstain and a cover slip is added
one virus and four living organisms. and the root tip is squashed to spread
out the cells to form a layer one cell
One of the four living organisms is a prokaryote. It has much the thick. The chromosomes can then be
smallest genome. The genome size of eukaryotes depends on the size examined and counted and the various
and number of chromosomes. It is correlated with the complexity phases ofmitosis should also be visible.
of the organism, but is not directly proportional. There are several
reasons for this. The proportion of the DNA that acts as functional thumb pressing down to
genes is very variable and also the amount of gene duplication varies. squash root tip
cover microscope folded
slip slide lter paper
153
3 Genetics
Organism Genome size Description
(million base pairs)
T2 phage Virus that attacks
0.18 Escherichia coli
Escherichia coli Gut bacterium
Drosophila melanogaster 5 Fruit fy
Homo sapiens 140 Humans
Paris japonica 3,000 Woodland plant
150,000
Finding the loci of human genes
Use o online databases to identiy the locus o a human gene and its
protein product.
The locus o a gene is its particular position on together with the total number o gene loci on
homologous chromosomes. Online databases can that chromosome.
be used to fnd the locus o human genes. There
is an example o such a database in the Online Gene name Description of gene
Mendelian Inheritance in Man website, maintained
by Johns Hopkins University. DRD4 A gene that codes or a dopamine
receptor that is implicated in a variety o
Search or the abbreviation OMIM to open the neurological and psychiatric conditions.
home page.
CFTR A gene that codes or a chloride channel
Choose Search Gene Map. protein. An allele o this gene causes
cystic brosis.
Enter the name o a gene into the Search
Gene Map box. This should bring up a table HBB The gene that codes or the beta-globin
with inormation about the gene, including its subunit o hemoglobin. An allele o this
locus, starting with the chromosome on which gene causes sickle cell anemia.
the gene is located. Suggestions o human
genes are shown on the right. F8 The gene that codes or Factor VIII, one
o the proteins needed or the clotting o
An alternative to entering the name o a gene blood. The classic orm o hemophilia is
is to select a chromosome rom 1 22 or one caused by an allele o this gene.
o the sex chromosomes X or Y. A complete
sequence o gene loci will be displayed, TDF Testis determining actor the gene that
causes a etus to develop as a male.
Haploid nuclei
Haploid nuclei have one chromosome o each pair.
A haploid nucleus has one chromosome o each type. It has one ull
set o the chromosomes that are ound in its species. Haploid nuclei in
humans contain 23 chromosomes or example.
Gametes are the sex cells that use together during sexual reproduction.
Gametes have haploid nuclei, so in humans both egg and sperm cells
contain 23 chromosomes.
154
3.2 ChrOmOsOmEs
Diploid nuclei Figure 7 Mosses coat the trunks of the laurel
trees in this forest in the Canary Islands.
Diploid nuclei have pairs of homologous chromosomes. Mosses are unusual because their cells are
haploid. In most eukaryotes the gametes are
A diploid nucleus has two chromosomes of each type. It has two full haploid but not the parent that produces them
sets of the chromosomes that are found in its species. Diploid nuclei in
humans contain 46 chromosomes for example. Figure 8 Trillium luteum cell with a diploid
number of 12 chromosomes. Two of each
When haploid gametes fuse together during sexual reproduction, a type of chromosome are present
zygote with a diploid nucleus is produced. When this divides by mitosis,
more cells with diploid nuclei are produced. Many animals and plants
consist entirely of diploid cells, apart from the cells that they are using to
produce gametes for sexual reproduction.
Diploid nuclei have two copies of every gene, apart from genes on the
sex chromosomes. An advantage of this is that the effects of harmful
recessive mutations can be avoided if a dominant allele is also present.
Also, organisms are often more vigorous if they have two different alleles
of genes instead of just one. This is known as hybrid vigour and is the
reason for strong growth of F1 hybrid crop plants.
Chromosome numbers
The number of chromosomes is a characteristic feature
of members of a species.
One of the most fundamental characteristics of a species is the number
of chromosomes. Organisms with a different number of chromosomes
are unlikely to be able to interbreed so all the interbreeding members of
a species need to have the same number of chromosomes.
The number of chromosomes can change during the evolution of a
species. It can decrease if chromosomes become fused together or increase
if splits occur. There are also mechanisms that can cause the chromosome
number to double. However, these are rare events and chromosome
numbers tend to remain unchanged over millions of years of evolution.
Comparing chromosome numbers
Comparison of diploid chromosome numbers of Homo sapiens, Pan troglodytes,
Canis familiaris, Oryza sativa, Parascaris equorum.
The Oxford English Dictionary consists of twenty eukaryotes. Some have a few large chromosomes
large volumes, each containing a large amount and others have many small ones.
of information about the origins and meanings of
words. This information could have been published All eukaryotes have at least two different types of
in a smaller number of larger volumes or in a larger chromosome, so the diploid chromosome number
number of smaller volumes. There is a parallel is at least four. In some cases it is over a hundred.
with the numbers and sizes of chromosomes in The table on the next page shows the diploid
chromosome number of selected species.
155
3 Genetics
scientifc name Englih Diploid chromoome
o pecie name number
4
Parascaris horse
equorum threadworm 24
46
Oryza sativa rice 48
78
Homo sapiens humans
Pan troglodytes chimpanzee
Figure 9 Who has more chromosomes a dog or its owner? Canis amiliaris dog
Data-baed quetion: Diferences in chromosome number
Plant Chromoome number Animal
Haplopappus gracilis 4 Parascaris equorum (horse threadworm)
Luzula purpurea (woodrush) 6 Aedes aegypti (yellow ever mosquito)
Crepis capillaris 8 Drosophila melanogaster (ruity)
Vicia aba (eld bean) 12 Musca domestica (house y)
Brassica oleracea (cabbage) 18 Chorthippus parallelus (grasshopper)
Citrullus vulgaris (water melon) 22 Cricetulus griseus (Chinese hamster)
Lilium regale (royal lily) 24 Schistocerca gregaria (desert locust)
Bromus texensis 28 Desmodus rotundus (vampire bat)
Camellia sinesis (Chinese tea) 30 Mustela vison (mink)
Magnolia virginiana (sweet bay) 38 Felis catus (domestic cat)
Arachis hypogaea (peanut) 40 Mus musculus (mouse)
Cofea arabica (cofee) 44 Mesocricetus auratus (golden hamster)
Stipa spartea (porcupine grass) 46 Homo sapiens (modern humans)
Chrysoplenum alterniolium (saxirage) 48 Pan troglodytes (chimpanzee)
Aster laevis (Michaelmas daisy) 54 Ovis aries (domestic sheep)
Glyceria canadensis (manna grass) 60 Capra hircus (goat)
Carya tomentosa (hickory) 64 Dasypus novemcinctus (armadillo)
Magnolia cordata 76 Ursus americanus (American black bear)
Rhododendron keysii 78 Canis amiliaris (dog)
Table 1
1 There are many different chromosome numbers 3 Explain why the size of the genome of a
in the table, but some numbers are missing, species cannot be deduced from the number
for example, 5, 7, 1 1 , 1 3. Explain why none of chromosomes. [1 ]
of the species has 1 3 chromosomes. [3] 4 Suggest, using the data in table 1 , a change
2 Discuss, using the data in the table, the in chromosome structure that may have
hypothesis that the more complex an occurred during human evolution. [2]
organism is, the more chromosomes it has. [4]
156
3.2 ChrOmOsOmEs
sex determination female male
XX XY
Sex is determined by sex chromosomes and autosomes
are chromosomes that do not determine sex. X Y
X X
There are two chromosomes in humans that determine sex: XX
XX XY
the X chromosome is relatively large and has its centromere near
the middle. XY
the Y chromosome is much smaller and has its centromere near 1 female : 1 male
the end.
Figure 10 Determination of gender
Because the X and Y chromosomes determine sex they are called the sex
chromosomes. All the other chromosomes are autosomes and do not
affect whether a fetus develops as a male or female.
The X chromosome has many genes that are essential in both males and
females. All humans must therefore have at least one X chromosome.
The Y chromosome only has a small number of genes. A small part of the
Y chromosome has the same sequence of genes as a small part of the X
chromosome, but the genes on the remainder of the Y chromosome
are not found on the X chromosome and are not needed for female
development.
One Y chromosome gene in particular causes a fetus to develop as a
male. This is called either S RY or TD F. It initiates the development of
male features, including testes and testosterone production. Because of
this gene a fetus with one X and one Y chromosome develops as a male.
A fetus that has two X chromosomes and no Y chromosome does not
have the TDF gene so ovaries develop instead of testes and female sex
hormones are produced, not testosterone.
Females have two X chromosomes. Females pass on one of their two
X chromosomes in each egg cell, so all offspring inherit an X chromosome
from their mother. The gender of a human is determined at the moment
of fertilization by one chromosome carried in the sperm. This can either
be an X or a Y chromosome. When sperm are formed, half contain the X
chromosome and half the Y chromosome. Daughters inherit their fathers
X chromosome and sons inherit his Y chromosome.
Karyogram
A karyogram shows the chromosomes of an organism
in homologous pairs of decreasing length.
The chromosomes of an organism are visible in cells that are in mitosis,
with cells in metaphase giving the clearest view. S tains have to be used
to make the chromosomes show up. Some stains give each chromosome
type a distinctive banding pattern.
If dividing cells are stained and placed on a microscope slide and are
then burst by pressing on the cover slip, the chromosomes become
spread. O ften they overlap each other, but with careful searching a cell
can usually be found with no overlapping chromosomes. A micrograph
can be taken of the stained chromosomes.
157
3 Genetics Originally analysis involved cutting out all the chromosomes and
arranging them manually but this process can now be done digitally. The
TOK chromosomes are arranged according to their size and structure. The
position of the centromere and the pattern of banding allow chromosomes
To what extent is determining gender that are of a different type but similar size to be distinguished.
or sporting competition a scientifc As most cells are diploid, the chromosomes are usually in homologous
question? pairs. They are arranged by size, starting with the longest pair and
ending with the smallest.
Gender testing was introduced at
the 1968 Olympic games to address Figure 11 Karyogram o a human emale, with fuorescent staining
concerns that women with ambiguous
physiological genders would have
an unair advantage. This has proven
to be problematic or a number o
reasons. The chromosomal standard
is problematic as non-disjunction can
lead to situations where an individual
might technically be male, but might
not defne hersel in that way. People
with two X chromosomes can develop
hormonally as a male and people with
an X and a Y can develop hormonally
as a emale.
The practice o gender testing was
discontinued in 1996 in part because
o human rights issues including the
right to sel-expression and the right to
identiy one's own gender. Rather than
being a scientifc question, it is more
airly a social question.
Figure 12 Child with trisomy 21 or Karyotypes and Down syndrome
Down syndrome
Use o karyotypes to deduce sex and diagnose Down
158 syndrome in humans.
A karyogram is an image of the chromosomes of an organism,
arranged in homologous pairs of decreasing length. A karyotype is a
property of an organism it is the number and type of chromosomes
that the organism has in its nuclei. Karyotypes are studied by looking
at karyograms. They can be used in two ways:
1 To deduce whether an individual is male or female. If two XX
chromosomes are present the individual is female whereas one X
and one Y indicate a male.
2 To diagnose Down syndrome and other chromosome abnormalities.
This is usually done using fetal cells taken from the uterus during
pregnancy. If there are three copies of chromosome 2 1 in the
karyotype instead of two, the child has Down syndrome. This is
sometimes called trisomy 21 . While individuals vary, some of the
component features of the syndrome are hearing loss, heart and
vision disorders. Mental and growth retardation are also common.
3.3 mEiOsis
Data-based questions: A human karyotype [2]
The karyogram shows the karyotype of a fetus. [4]
1 State which chromosome type is [2]
[2]
a) longest
b) shortest.
2 Distinguish between the structure of
a) human chromosome 2 and chromosome 1 2
b) the human X and Y chromosome.
3 Deduce with a reason the sex of the fetus.
4 Explain whether the karyotype shows any abnormalities.
Figure 13
3.3 meo
Udertadig Applicatio
One diploid nucleus divides by meiosis to Non-disjunction can cause Down syndrome
produce our haploid nuclei. and other chromosome abnormalities. Studies
showing age o parents inuences chances o
The halving o the chromosome number allows non-disjunction.
a sexual lie cycle with usion o gametes.
Methods used to obtain cells or karyotype
DNA is replicated beore meiosis so that all analysis e.g. chorionic villus sampling and
chromosomes consist o two sister chromatids. amniocentesis and the associated risks.
The early stages o meiosis involve pairing o skill
homologous chromosomes and crossing over
ollowed by condensation. Drawing diagrams to show the stages o
meiosis resulting in the ormation o our
Orientation o pairs o homologous haploid cells.
chromosomes prior to separation is random.
nature of ciece
Separation o pairs o homologous
chromosomes in the rst division o meiosis Making careul observations: meiosis was
halves the chromosome number. discovered by microscope examination o
dividing germ-line cells.
Crossing over and random orientation promotes
genetic variation.
Fusion o gametes rom diferent parents
promotes genetic variation.
159
3 Genetics
the discovery of meiosis
Making careful observations: meiosis was discovered by microscope examination
of dividing germ-line cells.
When improved microscopes had been developed chromosome number is doubled by ertilization. The
in the 1 9th century that gave detailed images o observation led to the hypothesis that there must be
cell structures, it was discovered that some dyes a special nuclear division in every generation that
specifcally stained the nucleus o the cell. These dyes halves the chromosome number.
revealed thread-like structures in dividing nuclei that
were named chromosomes. From the 1 880s onwards Nuclear divisions unlike mitosis had already
a group o German biologists carried out careul and been observed during gamete development in
detailed observations o dividing nuclei that gradually both animals and plants. These divisions were
revealed how mitosis and meiosis occur. identifed as the method used to halve the
chromosome number and they were named
We can appreciate the considerable achievements o meiosis. The sequence o events in meiosis was
these biologists i we try to repeat the observations eventually worked out by careul observation o
that they made. The preparation o microscope cells taken rom the ovaries o rabbits ( Oryctolagus
slides showing meiosis is challenging. Suitable tissue cuniculus) between 0 and 2 8 days old. The
can be obtained rom the developing anthers inside advantage o this species is that in emales meiosis
a lily bud or rom the testis o a dissected locust. begins at birth and occurs slowly over many days.
The tissue must be fxed, stained and then squashed
on a microscope slide. Oten no cells in meiosis are
visible or the images are not clear enough to show
details o the process. Even with prepared slides
made by experts it is difcult to understand the
images as chromosomes orm a variety o bizarre
shapes during the stages o meiosis.
A key observation was that in the horse threadworm Figure 1
(Parascaris equorum) there are two chromosomes
in the nuclei o egg and sperm cells, whereas the
ertilized egg contains our. This indicated that the
one diploid cell 2n Meiosis in ouline
meiosis I
One diploid nucleus divides by meiosis to produce four
two haploid cells n n haploid nuclei.
meiosis II nn
Meiosis is one o the two ways in which the nucleus o a eukaryotic
four haploid cells n n cell can divide. The other method is mitosis, which was described in
sub-topic 1 .6. In meiosis the nucleus divides twice. The frst division
Figure 2 Overview of meiosis produces two nuclei, each o which divides again to give a total o our
nuclei. The two divisions are known as meiosis I and meiosis II.
The nucleus that undergoes the frst division o meiosis is diploid it
has two chromosomes o each type. Chromosomes o the same type are
known as homologous chromosomes. Each o the our nuclei produced
by meiosis has just one chromosome o each type they are haploid.
Meiosis involves a halving o the chromosome number. It is thereore
known as a reduction division.
The cells produced by meiosis I have one chromosome o each type, so
the halving o the chromosome number happens in the frst division,
160
3.3 mEiOsis
not the second division. The two nuclei produced by meiosis I have the Figure 4 Fledgling owls (bottom) produced by
haploid number o chromosomes, but each chromosome still consists o a sexual life cycle have diploid body cells but
two chromatids. These chromatids separate during meiosis II, producing mosses (top) have haploid cells
our nuclei that have the haploid number o chromosomes, with each
chromosome consisting o a single chromatid.
Meiosis and sexual life cycles
The halving of the chromosome number allows a sexual
life cycle with fusion of gametes.
The lie cycles o living organisms can be sexual or asexual. In an asexual
lie cycle the ospring have the same chromosomes as the parent so are
genetically identical. In a sexual lie cycle there are dierences between the
chromosomes o the ospring and the parents, so there is genetic diversity.
In eukaryotic organisms, sexual reproduction involves the process o
ertilization. Fertilization is the union o sex cells, or gametes, usually
rom two dierent parents. Fertilization doubles the number o
chromosomes each time it occurs. It would thereore cause a doubling
o chromosome number every generation, i the number was not also
halved at some stage in the lie cycle. This halving o chromosome
number happens during meiosis.
Meiosis can happen at any stage during a sexual lie cycle, but in animals
it happens during the process o creating the gametes. Body cells are
thereore diploid and have two copies o most genes.
Meiosis is a complex process and it is not at the moment clear how it
developed. What is clear is that its evolution was a critical step in the
origin o eukaryotes. Without meiosis there cannot be usion o gametes
and the sexual lie cycle o eukaryotes could not occur.
Data-baed queton: Life cycles
Figure 3 shows the lie cycle o humans and 1 Outline fve similarities between the lie
mosses, with n being used to represent the haploid cycle o a moss and o a human. [5]
[5]
number o chromosomes and 2n to represent the 2 Distinguish between the lie cycles o
diploid number. Sporophytes o mosses grow on a moss and a human by giving fve
the main moss plant and consist o a stalk and a dierences.
capsule in which spores are produced.
egg
n
sperm egg sperm
n n n
human male zygote moss zygote
2n 2n 2n
human female plant
2n n
Key spore sporophyte
mitosis n 2n
meiosis
Figure 3 fer t i l i za t i o n
161
3 Genetics
Replicatio of DnA before meiosis
2n interphase DNA is replicated before meiosis so that all chromosomes
consist of two sister chromatids.
During the early stages o meiosis the chromosomes gradually shorten
by supercoiling. As soon as they become visible it is clear that each
chromosome consists o two chromatids. This is because all DNA in
2n homologous the nucleus is replicated during the interphase beore meiosis, so each
chromosomes chromosome consists o two sister chromatids.
2n meiosis I Initially the two chromatids that make up each chromosome are
genetically identical. This is because DNA replication is very accurate and
the number o mistakes in the copying o the DNA is extremely small.
n n meiosis II We might expect the D NA to be replicated again between the frst and
the second division o meiosis, but it does not happen. This explains how
nn nn the chromosome number is halved during meiosis. One diploid nucleus,
in which each chromosome consists o two chromatids, divides twice
Figure 5 Outline of meiosis to produce our haploid nuclei in which each chromosome consists o
one chromatid.
Bivalets formatio ad crossig over
The early stages of meiosis involve pairing of homologous
chromosomes and crossing over followed by condensation.
Some o the most important events o meiosis happen at the start o meiosis
I while the chromosomes are still very elongated and cannot be seen with
a microscope. Firstly homologous chromosomes pair up with each other.
Because DNA replication has already occurred, each chromosome consists
o two chromatids and so there are our DNA molecules associated in each
pair o homologous chromosomes. A pair o homologous chromosomes is
bivalent and the pairing process is sometimes called synapsis.
Soon ater synapsis, a process called crossing over takes place. The molecular
details o this need not concern us here, but the outcome is very important.
A junction is created where one chromatid in each o the homologous
chromosomes breaks and rejoins with the other chromatid. Crossing over
occurs at random positions anywhere along the chromosomes. At least one
crossover occurs in each bivalent and there can be several.
Figure 6 A pair of homologous Because a crossover occurs at precisely the same position on the two
chromosomes contains four chromatids involved, there is a mutual exchange o genes between the
chromatids and is sometimes called chromatids. As the chromatids are homologous but not identical, some
a tetrad. Five chiasmata are visible alleles o the exchanged genes are likely to be dierent. Chromatids with
in this tetrad, showing that crossing new combinations o alleles are thereore produced.
over can occur more than once
Radom orietatio of bivalets
Orientation of pairs of homologous chromosomes prior to
separation is random.
While pairs o homologous chromosomes are condensing inside the
nucleus o a cell in the early stages o meiosis, spindle microtubules are
growing rom the poles o the cell. Ater the nuclear membrane has
162
3.3 mEiOsis
broken down, these spindle microtubules attach to the centromeres o MITOSIS
the chromosomes.
either or
The attachment o the spindle microtubules is not the same as in mitosis.
The principles are these: MEIOSIS
E ach chromosome is attached to one pole only, not to both. Figure 7 Comparison of attachment
of chromosomes to spindle
The two homologous chromosomes in a bivalent are attached to microtubules in mitosis and meiosis
dierent poles.
The pole to which each chromosome is attached depends on which
way the pair o chromosomes is acing. This is called the orientation.
The orientation o bivalents is random, so each chromosome has an equal
chance o attaching to each pole, and eventually o being pulled to it.
The orientation o one bivalent does not aect other bivalents. The
consequences o the random orientation o bivalents are discussed in
the section on genetic diversity later in this topic.
Halving the chromosome number
Separation o pairs o homologous chromosomes in the
frst division o meiosis halves the chromosome number.
The movement o chromosomes is not the same in the frst division o
meiosis as in mitosis. Whereas in mitosis the centromere divides and the two
chromatids that make up a chromosome move to opposite poles, in meiosis
the centromere does not divide and whole chromosomes move to the poles.
Initially the two chromosomes in each bivalent are held together
by chiasmata, but these slide to the end o the chromosomes and
then the chromosomes can separate. This separation o homologous
chromosomes is called disjunction. One chromosome rom each bivalent
moves to one o the poles and the other chromosome to the other pole.
The separation o pairs o homologous chromosomes to opposite poles
o the cell halves the chromosome number o the cell. It is thereore
the frst division o meiosis that is the reduction division. Because one
chromosome o each type moves to each pole, both o the two nuclei
ormed in the frst division o meiosis contain one o each type o
chromosome, so they are both haploid.
Obtaining cells from a fetus
Methods used to obtain cells or karyotype analysis e.g. chorionic villus sampling
and amniocentesis and the associated risks.
Two procedures are used or obtaining cells The second procedure is chorionic villus sampling.
containing the etal chromosomes needed or A sampling tool that enters through the vagina is
producing a karyotype. Amniocentesis involves used to obtain cells rom the chorion, one o the
passing a needle through the mother's abdomen membranes rom which the placenta develops.
wall, using ultrasound to guide the needle. This can be done earlier in the pregnancy than
The needle is used to withdraw a sample o amniocentesis, but whereas the risk o miscarriage
amniotic luid containing etal cells rom the with amniocentesis is 1 %, with chorionic villus
amniotic sac. sampling it is 2%.
163
3 Genetics
Diagrams of the stages of meiosis
Drawing diagrams to show the stages of meiosis resulting in the formation of four
haploid cells.
In mitosis our stages are usually recognized: Usually we draw biological structures rom
prophase, metaphase, anaphase and telophase. actual specimens, oten looking at them down
Meiosis can also be divided into these stages, but a microscope. Preparation o microscope slides
each stage happens twice: in meiosis I and then a showing meiosis is worth attempting but it is
second time in meiosis II. The main events o each challenging. Permanent slides usually have more
stage in mitosis also happen in meiosis: cells visible in meiosis than temporary mounts,
but even then it is difcult to interpret the
prophase: condensation o chromosomes; structure o bivalents rom their appearance. This
is why we usually construct diagrams o meiosis
metaphase: attachment o spindle microtubules; rather than draw stages rom specimens on
microscope slides!
anaphase: movement o chromosomes to
the poles;
telophase: decondensation o chromosomes.
The frst division o meiosis
Prophase i nuclear membrane
Cell has 2n chromosomes (double spindle microtubules
chromatid) : n is haploid number of and centriole
chromosomes.
Prophase I
Homologous chromosomes pair (synapsis) .
bivalents aligned
Crossing over occurs. on the equator
metaphase i Metaphase I
Spindle microtubules move homologous pairs homologous
to equator ofcell. chromosomes
being pulled to
Orientation of paternal and maternal opposite poles
chromosomes on either side of equator
is random and independent of other
homologous pairs.
Anaphase i
Homologous pairs are separated. One
chromosome of each pair moves to each
pole.
Telophase i Anaphase I
Chromosomes uncoil. During interphase cell has divided
that follows, no replication occurs. across the equator
Telophase I
Reduction of chromosome number from
diploid to haploid completed.
Cytokinesis occurs.
164
3.3 mEiOsis
The second division of meiosis
Prophae ii
Chromosomes, which still consist of two
chromatids, condense and become visible.
Prophase II
metaphae ii
Metaphase II
Anaphae ii
Centromeres separate and chromatids are
moved to opposite poles.
Anaphase II
Telophae ii
Chromatids reach opposite poles.
Nuclear envelope forms.
Cytokinesis occurs.
Telophase II
Meiosis and genetic variation
Crossing over and random orientation promotes genetic
variation.
When two parents have a child, they know that it will inherit an
unpredictable mixture of characteristics from each of them. Much of
the unpredictability is due to meiosis. Every gamete produced by a
parent has a new combination of alleles meiosis is a source of endless
genetic variation.
Apart from the genes on the X and Y chromosomes, humans have two
copies of each gene. In some cases the two copies are the same allele and
there will be one copy of that allele in every gamete produced by the
parent. There are likely to be thousands of genes in the parents genome
165
3 Genetics
Activity where the two alleles are dierent. Each o the two alleles has an equal
chance o being passed on in a gamete. Let us suppose that there is
I g is the number o genes a gene with the alleles A and a. Hal o the gametes produced by the
in a genome with diferent parent will contain A and hal will contain a.
alleles, 2g is the number
o combinations othese Let us now suppose that there is another gene with the alleles B and b.
alleles that can be generated Again hal o the gametes will contain B and hal b. However, meiosis
by meiosis. I there were can result in gametes with dierent combinations o these genes: AB, Ab,
just 69 genes with diferent aB and ab. There are two processes in meiosis that generate this diversity.
alleles (3 in each othe
23 chromosome types in 50% a B B
humans) there would be probability A b a
590,295,810,358,705,
700,000 combinations. Bb b b
Assuming that all humans Aa B A
are genetically diferent, and
that there are 7,000,000 prophase I 50% a telophase I
humans, calculate the probability A
percentage o all possible b
genomes that currently exist. a
Figure 9 B
A
166
metaphase I
Figure 8 Random orientation in metaphase I
1. Random orientation o bivalents
In metaphase I the orientation o bivalents is random and the orientation
o one bivalent does not infuence the orientation o any o the others.
Random orientation o bivalents is the process that generates genetic
variation among genes that are on dierent chromosome types.
For every additional bivalent, the number o possible chromosome
combinations in a cell produced by meiosis doubles. For a haploid number
o n, the number o possible combinations is 2n. For humans with a
haploid number o 23 this amounts to 223 or over 8 million combinations.
2. Crossing over
Without crossing over in prophase I, combinations o alleles on
chromosomes would be orever linked together. For example, i one
chromosome carried the combination CD and another carried cd, only
these combinations could occur in gametes. Crossing over allows linked
genes to be reshufed, to produce new combinations such as Cd and cD.
It increases the number o allele combinations that can be generated by
meiosis so much that it is eectively innite.
Fertilization and genetic variation
Fusion o gametes rom diferent parents promotes
genetic variation.
The usion o gametes to produce a zygote is a highly signicant event
both or individuals and or species.
It is the start o the lie o a new individual.
It allows alleles rom two dierent individuals to be combined in one
new individual.
3.3 mEiOsis
The combination o alleles is unlikely ever to have existed beore.
Fusion o gametes thereore promotes genetic variation in a species.
Genetic variation is essential or evolution.
no-disjuctio ad Dow sydrome
Non-disjunction can cause Down syndrome and other chromosome abnormalities.
Meiosis is sometimes subject to errors. Most other trisomies in humans are so serious
One example o this is when homologous that the ospring do not survive. Babies are
chromosomes ail to separate at anaphase. This sometimes born with trisomy 1 8 and trisomy
is termed non-disjunction. This can happen with 1 3. Non-disjunction can also result in the
any o the pairs o homologous chromosomes. birth o babies with abnormal numbers o sex
Both o the chromosomes move to one pole and chromosomes. Klineelters syndrome is caused
neither to the other pole. The result will be a by having the sex chromosomes XXY. Turners
gamete that either has an extra chromosome or syndrome is caused by having only one sex
is defcient in a chromosome. I the gamete is chromosome, an X.
involved in human ertilization, the result will
be an individual with either 45 or diploid parent cell with
47 chromosomes. two chromosome 21
An abnormal number o chromosomes non-disjunction gamete with no
will oten lead to a person possessing a during meiosis chromosome 21
syndrome, i.e. a collection o physical
signs or symptoms. For example gamete with two cell dies
trisomy 21 , also known as Down chromosome 21
syndrome, is due to a non-disjunction
event that leaves the individual with fusion of normal haploid
three o chromosome number 21 gametes gamete
instead o two. While individuals vary,
some o the component eatures o trisomy: zygote with
the syndrome include hearing loss, three chromosome 21
heart and vision disorders. Mental and
growth retardation are also common. Figure 10 How non-disjunction can give rise to Down syndrome
Paretal age ad o-disjuctio trisomy 21
all chromosomal
Studies showing age o parents infuences chances o 14 abnormalitiesincidence (% of all live births)
non-disjunction
12
The data presented in fgure 1 1 shows the relationship between
maternal age and the incidence o trisomy 21 and o other 10
chromosomal abnormalities.
8
1 Outline the relationship between maternal age and the incidence
6
o chromosomal abnormalities in live births. [2]
4
2 a) For mothers 40 years o age, determine the probability that
2
they will give birth to a child with trisomy 21 . [1 ]
0
b) Using the data in fgure 1 1 , calculate the probability that a 20 40 60
maternal age (years)
mother o 40 years o age will give birth to a child with a
Figure 11 The incidence of trisomy 21
chromosomal abnormality other than trisomy 21 . [2] and other chromosomal abnormalities
as a function of maternal age
167
3 Genetics
3 Only a small number of possible chromosomal abnormalities
are ever found among live births, and trisomy 21 is much the
commonest. Suggest reasons for these trends. [3]
4 Discuss the risks parents face when choosing to postpone [2]
having children.
3.4 inhertance
Udertadig Applicatio
Mendel discovered the principles o inheritance Inheritance o ABO blood groups.
with experiments in which large numbers o Red-green colour-blindness and hemophilia as
pea plants were crossed.
examples o sex-linked inheritance.
Gametes are haploid so contain one allele o Inheritance o cystic brosis and Huntingtons
each gene.
disease.
The two alleles o each gene separate into Consequences o radiation ater nuclear
diferent haploid daughter nuclei during meiosis.
bombing o Hiroshima and Nagasaki and the
Fusion o gametes results in diploid zygotes nuclear accidents at Chernobyl.
with two alleles o each gene that may be the
same allele or diferent alleles. skill
Dominant alleles mask the efects o recessive Construction o Punnett grids or predicting the
alleles but co-dominant alleles have joint efects. outcomes o monohybrid genetic crosses.
Many genetic diseases in humans are due to Comparison o predicted and actual outcomes
recessive alleles o autosomal genes. o genetic crosses using real data.
Some genetic diseases are sex-linked and some Analysis o pedigree charts to deduce the
are due to dominant or co-dominant alleles. pattern o inheritance o genetic diseases.
The pattern o inheritance is diferent with nature of ciece
sex-linked genes due to their location on sex
chromosomes. Making quantitative measurements with
replicates to ensure reliability: Mendels genetic
Many genetic diseases have been identied in crosses with pea plants generated numerical data.
humans but most are very rare.
Radiation and mutagenic chemicals increase
the mutation rate and can cause genetic
disease and cancer.
168
3.4 iNhEriTANCE
Mendel and the principles of inheritance Figure 1 Hair styles are acquired
characteristics and are ortunately not
Mendel discovered the principles of inheritance inherited by ofspring
with experiments in which large numbers of pea
plants were crossed.
When living organisms reproduce, they pass on characteristics to their
ospring. For example, when blue whales reproduce, the young are
also blue whales they are members o the same species. More than
this, variations, such as the markings on the skin o a blue whale, can be
passed on. We say that the ospring inherit the parents characteristics.
However, some characteristics cannot be inherited. S cars seen on the
tails o some blue whales caused by killer whale attacks and cosmetic
surgery in humans are examples o this. According to current theories,
acquired characteristics such as these cannot be inherited.
Inheritance has been discussed since the time o Hippocrates and earlier.
For example, Aristotle observed that children sometimes resemble
their grandparents more than their parents. Many o the early theories
involved blending inheritance, in which ospring inherit characters
rom both parents and so have characters intermediate between those o
their parents. Some o the observations that biologists made in the rst
hal o the 1 9th century could not be explained by blending inheritance,
but it was not until Mendel published his paper Experiments in Plant
Hybridization that an alternative theory was available.
Mendels experiments were done using varieties o pea plant, each o
which reliably had the same characters when grown on its own. Mendel
careully crossed varieties o pea together by transerring the male pollen
rom one variety to the emale parts in fowers o another variety. He
collected the pea seeds that were ormed as a result and grew them to
nd out what their characters were. Mendel repeated each cross with
many pea plants. He also did this experiment with seven dierent pairs
o characters and so his results reliably demonstrated the principles o
inheritance in peas, not just an isolated eect.
In 1 866 Mendel published his research. For over thirty years his ndings
were largely ignored. Various reasons have been suggested or this. One
actor was that his experiments used pea plants and there was not great
interest in the pattern o inheritance in that species. In 1 900 several
biologists rediscovered Mendels work. They quickly did cross-breeding
experiments with other plants and with animals. These conrmed that
Mendels theory explained the basis o inheritance in all plants and animals.
Replicates and reliability in Mendels experiments
Making quantitative measurements with replicates to ensure reliability: Mendel's
genetic crosses with pea plants generated numerical data.
Gregor Mendel is regarded by most biologists as characteristics such as red or white fower colour
the ather o genetics. His success is sometimes that can easily be ollowed rom one generation
attributed to being the rst to use pea plants to the next. They can also be crossed to produce
or research into inheritance. Peas have clear hybrids or they can be allowed to sel-pollinate.
169
3 Genetics
In act Mendel was not the rst to use pea cross pollinating peas:
plants. Thomas Andrew Knight, an English pollen from another plant is dusted on
horticulturalist, had conducted research at to the stigma here
Downton Castle in Hereordshire in the late
1 8th century and published his results in the pollen is collected
Philosophical Transactions o the Royal S ociety. from the anthers
Knight made some important discoveries:
lower petal
male and emale parents contribute equally to called the keel
the ospring; self pollinating peas:
if the ower is left untouched, the anthers
characters such as white fower colour
that apparently disappear in ospring can inside the keel pollinate the stigma
reappear in the next generation, showing that
inheritance is discrete rather than blending; Figure 2 Cross and sel pollination
one character such as red fower colour (a) Prediction based on
can show a stronger tendency than the blending inheritance
alternative character.
tall plants 3 dwarf plants
Although Mendel was not as pioneering in his
experiments as sometimes thought, he deserves pea plants with an
credit or another aspect o his research. Mendel intermediate height
was a pioneer in obtaining quantitative results and (b) Actual results
in having large numbers o replicates. He also did tall plants 3 dwarf plants
seven dierent cross experiments, not just one.
Table 1 shows the results o his monohybrid crosses. pea plants as tall
as the tall parent
It is now standard practice in science to include
repeats in experiments to demonstrate the Figure 3 Example o a monohybrid cross experiment. All the
reliability o results. Repeats can be compared to hybrid plants produced by crossing two varieties together
see how close they are. Anomalous results can be had the same character as one o the parents and the
identied and excluded rom analysis. Statistical character o the other parent was not seen. This is a clear
tests can be done to assess the signicance o alsifcation o the theory o blending inheritance
dierences between treatments. It is also standard
practice to repeat whole experiments, using a
dierent organism or dierent treatments, to test
a hypothesis in dierent ways. Mendel should
thereore be regarded as one o the athers o
genetics, but even more we should think o him
as a pioneer o research methods in biology.
Paental plants hybid plants Ofsping om sel-pollinating te ybids ratio
Tall stem dwar stem 2.84 : 1
Round seed wrinkled seed All tall 787 tall : 277 dwar 2.96 : 1
Yellow cotyledons green cotyledons 3.01 : 1
Purple fowers white fowers All round 5474 round : 1850 wrinkled 3.15 : 1
Full pods constricted pods 2.95 : 1
Green unripe pods yellow unripe pods All yellow 6022 yellow : 2001 green 2.82 : 1
Flowers along stem fowers at stem tip 3.14 : 1
All purple 705 purple : 224 white
All ull 882 ull : 299 constricted
All green 428 green : 152 yellow
All along stem 651 along stem : 207 at tip
Table 1
170
3.4 iNhEriTANCE
Gamete Figure 4 Pollen on the anthers o a fower
contains the male gamete o the plant. The
Gametes are haploid so contain one allele o each gene. male gametes contain one allele o each o
the plants
Gametes are cells that fuse together to produce the single cell that is the
start of a new life. They are sometimes called sex cells, and the single cell Figure 5 Most crop plants are pure-bred strains
produced when male and female gametes fuse is a zygote. Male and female with two o the same allele o each gene
gametes are different in size and motility. The male gamete is generally
smaller than the female one. It is usually able to move whereas the female 171
gamete moves less or not at all. In humans, for example, the sperm has a
much smaller volume than the egg cell and uses its tail to swim to the egg.
Parents pass genes on to their offspring in gametes. Gametes contain
one chromosome of each type so are haploid. The nucleus of a gamete
therefore only has one allele of each gene. This is true of both male
and female gametes, so male and female parents make an equal genetic
contribution to their offspring, despite being very different in overall size.
Zygote
Fusion o gametes results in diploid zygotes with two
alleles o each gene that may be the same allele or
diferent alleles.
When male and female gametes fuse, their nuclei j oin together, doubling
the chromosome number. The nucleus of the zygote contains two
chromosomes of each type so is diploid. It contains also two alleles of
each gene.
If there were two alleles of a gene, A and a, the zygote could contain two
copies of either allele or one of each. The three possible combinations are
AA, Aa and aa.
Some genes have more than two alleles. For example, the gene for
AB O blood groups in humans has three alleles: IA, IB and i. This gives six
possible combinations of alleles:
three with two of the same allele, IAIA, IBIB and ii
three with two different alleles, IAIB, IAi and IBi.
segregation of allele
The two alleles o each gene separate into diferent
haploid daughter nuclei during meiosis.
During meiosis a diploid nucleus divides twice to produce four haploid
nuclei. The diploid nucleus contains two copies of each gene, but the
haploid nuclei contain only one.
If two copies of one allele of a gene were present, each of the haploid
nuclei will receive one copy of this allele. For example, if the two
alleles were PP, every gamete will receive one copy of P.
If two different alleles were present, each haploid nucleus will
receive either one of the alleles or the other allele, not both. For
example, if the two alleles were Pp, 50% of the haploid nuclei would
receive P and 50% would receive p.
3 Genetics
TOK The separation o alleles into dierent nuclei is called segregation. It
breaks up existing combinations o alleles in a parent and allows new
Did mendel alter his results for combinations to orm in the ospring.
publication?
Dominant, recessive and co-dominant alleles
In 1936, the English statistician
R.A. Fisher published an analysis Dominant alleles mask the efects o recessive alleles but
o Mendels data. His conclusion co-dominant alleles have joint efects.
was that the data o most, i not
all, o the experiments have been In each o Mendels seven crosses between dierent varieties o pea
alsied so as to agree closely with plant, all o the ospring showed the character o one o the parents, not
Mendels expectations. Doubts still the other. For example, in a cross between a tall pea plant and a dwar
persist about Mendel's data a pea plant, all the ospring were tall. The dierence in height between
recent estimate put the chance o the parents is due to one gene with two alleles:
getting seven ratios as close to 3:1 as
Mendels at 1 in 33,000. the tall parents have two copies o an allele that makes them tall, TT
1 To get ratios as close to 3:1 as the dwar parents have two copies o an allele that makes them dwar, tt
Mendel's would have required a
miracle o chance. What are the they each pass on one allele to the ospring, which thereore has one
possible explanations apart rom a o each allele, Tt
miracle o chance?
when the two alleles are combined in one individual, it is the allele
2 Many distinguished scientists, or tallness that determines the height because the allele or tallness
including Louis Pasteur, are is dominant
known to have discarded results
when they did not t a theory. Is it the other allele, that does not have an eect i the dominant allele is
acceptable to do this? How can we present, is recessive.
distinguish between results that
are due to an error and results that In each o Mendels crosses one o the alleles was dominant and the other
alsiy a theory? What standard do was recessive. However, some genes have pairs o alleles where both have an
you use as a student in rejecting eect when they are present together. They are called co-dominant alleles. A
anomalous data? well-known example is the fower colour o Mirabilis jalapa. I a red-fowered
plant is crossed with a white-fowered plant, the ospring have pink fowers.
there is an allele or red fowers, CR
there is an allele or white fowers, CW
these alleles are co-dominant so C RC W gives pink fowers.
The usual reason or dominance o one allele is that this allele codes or
a protein that is active and carries out a unction, whereas the recessive
allele codes or a non-unctional protein.
Figure 6 There are co-dominant alleles of the gene for coat
colour in Icelandic horses.
172
3.4 iNhEriTANCE
Punnett grids parents: TT tt
tall stem dwarf stem
Construction of Punnett grids for predicting the genotype
outcomes of monohybrid genetic crosses. phenotype
Monohybrid crosses only involve one character, or example the eggs or pollen T t
height o a pea plant, so they involve only one gene. Most crosses start
with two pure-breeding parents. This means that the parents have F1 hybrids genotype Tt
two o the same allele, not two dierent alleles. Each parent thereore
produces just one type o gamete, containing one copy o the allele. phenotype tall stem
Their ospring are also identical, although they have two dierent
alleles. The ospring obtained by crossing the parents are called F1 t pollen eggs t
hybrids or the F1 generation. T T
TT
The F1 hybrids have two dierent alleles o the gene, so they can each ta l l
produce two types o gamete. I two F1 hybrids are crossed together, tT Tt
or i an F1 plant is allowed to sel-pollinate, there are our possible tall tall
outcomes. This can be shown using a 2 2 table, called a Punnett grid tt
ater the geneticist who rst used this type o table. The ospring o a dwarf
cross between two F1 plants are called the F2 generation.
Figure 7 Explanation of Mendels 3:1 ratio
To make a Punnett grid as clear as possible the gametes should be
labeled and both the alleles and the character o the our possible parents: CRCR CWCW
outcomes should be shown on the grid. It is also useul to give an red owers white owers
overall ratio below the Punnett grid. gen o ty pe
phenotype
Figure 7 shows Mendels cross between tall and dwar plants. It
explains the F2 ratio o three tall to one dwar plant. CR CW
Figure 8 shows the results o a cross between red and white fowered
plants o Mirabilis jalapa. It explains the F2 ratio o one red to two pink
to one white fowered plant.
Data-based questons: Coat colour in the house mouse F1 hybrids genotype CRCW
In the early years o the 2 0th century, many crossing experiments phenotype pink owers
were done in a similar way to those o Mendel. The French geneticist
Lucien C unot used the house mouse, Mus musculus, to see whether C W pollen CRCR eggs CW
the principles that Mendel had discovered also operated in animals. CR CR
He crossed normal grey-coloured mice with albino mice. The hybrid red
mice that were produced were all grey. These grey hybrids were
crossed together and produced 1 98 grey and 72 albino ospring. CWCR CWCW
pink pink
CWCW
white
1 Calculate the ratio between grey and albino ospring, showing
your working. [2] Figure 8 A cross involving co-dominance
2 Deduce the colour o coat that is due to a recessive allele, with
two reasons or your answer. [3]
3 Choose suitable symbols or the alleles or grey and albino coat
and list the possible combinations o alleles o mice using your
symbols, together with the coat colours associated with each
combination o alleles. [3]
173
3 Genetics
4 Using a Punnett grid, explain how the observed ratio o grey
and albino mice was produced. [5]
t ica annulata 5 The albino mice had red eyes in addition to white coats. Suggest
Figure 9 how one gene can determine whether the mice had grey ur
and black eyes or white ur and red eyes. [2]
Data-based questions: The two-spot ladybird
Adalia bipunctata is a species o ladybird. In North America ladybirds are
called ladybugs. The commonest orm o this species is known as typica.
There is a rarer orm called annulata. B oth orms are shown in fgure 9.
Figure 10 F hybrid ofspring 1 C ompare the typica and annulata orms o Adalia bipunctata. [2 ]
1
2 The dierences between the two orms are due to a single [2]
gene. I male and emale typica are mated together, all the
ospring are typica. S imilarly, the ospring produced when
annulata orms are mated are all annulata. Explain the
conclusions that can be drawn.
3 When typica is mated with annulata, the F1 hybrid ospring are
Figure 11 F ofspring not identical to either parent. Examples o these F1 hybrid
2
ospring are shown in fgure 1 0. Distinguish between the F1
Activity
hybrid ospring and the typica and annulata parents. [3]
ABO blood groups
It is possible for two parents to have 4 I F1 hybrid ospring are mated with each other, the ospring
an equal chance of having a child with include both typica and annulata orms, and also ospring with
blood group A, B, AB or O. What would the same wing case markings as the F1 hybrid ospring.
be the genotypes of the parents?
a) Use a genetic diagram to explain this pattern o inheritance. [6]
b) Predict the expected ratio o phenotypes. [2]
ABO blood groups recessive to both IA and IB. The reasons or two
alleles being co-dominant and the other allele
Inheritance of ABO blood groups. being recessive are as ollows:
The ABO blood group system in humans is an All o the three alleles cause the production o
example o co-dominance. It is o great medical a glycoprotein in the membrane o red blood
importance: beore blood is transused, it is vital cells.
to fnd out the blood group o a patient and
ensure that it is matched. Unless this is done, IA alters the glycoprotein by addition o acetyl-
there may be complications due to coagulation galactosamine. This altered glycoprotein is
o red blood cells. One gene determines the ABO absent rom people who do not have the allele IA
blood group o a person. The genotype IAIA gives so i exposed to it they make anti-A antibodies.
blood group A and the genotype IBIB gives group
B . Neither IA nor IB is dominant over the other IB alters the glycoprotein by addition o
allele and a person with the genotype IAIB has galactose. This altered glycoprotein is not
a dierent blood group, called AB. There is a present in people who do not have the allele IB
third allele o the ABO blood group gene, usually so i exposed to it they make anti-A antibodies.
called i. A person with the genotype ii is in blood
group O . The genotypes IAi and IBi give blood
groups A and B respectively, showing that i is
174
3.4 iNhEriTANCE
The genotype IAIB causes the glycoprotein to either o the IA or IB alleles is also present
be altered by addition o acetyl-galactosamine the glycoprotein is altered by addition o
and galactose. As a consequence neither acetyl-galactosamine or galactose. IAIA and IAi
anti-A nor anti-B antibodies are produced. thereore give the same phenotype, as do IBIB
This genotype thereore gives a dierent and IBi.
phenotype to IAIA and IBIB so the alleles IA and
IB are co- dominant. The allele i is recessive because it does not
cause the production o a glycoprotein. IAIA
The allele i is recessive because it causes and IAi thereore give the same phenotype and
production o the basic glycoprotein: i so do IBIB and IBi.
Group A Group O
anti-A anti-B anti-A anti-B
Group B Group AB
anti-A anti-B anti-A anti-B
Figure 12 Blood group can easily be determined using test cards
tesing predicions in cross-breeding experimens
Comparison of predicted and actual outcomes of genetic crosses using real data.
It is in the nature o science to try to fnd general one ace showing and 500 times with the other
principles that explain natural phenomena and ace showing.
not just to describe individual examples o a
phenomenon. Mendel discovered principles o An important skill in biology is deciding
inheritance that have great predictive power. whether the results o an experiment are close
We can still use them to predict the outcomes o enough to the predictions or us to accept that
genetic crosses. Table 2 lists possible predictions in they ft, or whether the dierences are too great
monohybrid crosses. and either the results or the predictions must
be alse. An obvious trend is that the greater
The actual outcomes o genetic crosses do not the dierence between observed and expected
usually correspond exactly with the predicted results, the less likely that the dierence is
outcomes. This is because there is an element o due to chance and the more likely that the
chance involved in the inheritance o genes. The predictions do not ft the results.
tossing o a coin is a simple analogy. We expect
the coin to land 50% o times with each o its two To assess obj ectively whether results ft
aces uppermost, but i we toss it 1 ,000 times we predictions, statistical tests are used. For genetic
do not expect it to land precisely 500 times with crosses the chi-squared test can be used. This test
is described later in the book in sub-topic 4.1 .
175
3 Genetics
Cross Predicted outcome Example
Pure-breeding parents one with All o the ofspring will have the same All ofspring o a cross between pure-
dominant alleles and one with character as the parent with dominant breeding tall and dwar pea plants
recessive alleles are crossed. alleles. will be tall.
Pure-breeding parents that have All o the ofspring will have the same All ofspring o a cross between red
and white owered Mirabilis jalapa
diferent co-dominant alleles character and the character will be plants will have pink owers.
are crossed. diferent rom either parent.
Two parents each with one Three times as many ofspring have 3:1 ratio o tall to dwar pea plants
dominant and one recessive the character o the parent with rom a cross between two parents
allele are crossed. dominant alleles as have the character that each have one allele or tall
o the parent with the recessive height and one allele or dwar
alleles. height.
A parent with one dominant and Equal proportions o ofspring with 1:1 ratio rom a cross between a
one recessive allele is crossed the character o an individual with a dwar pea plant and a tall plant with
with a parent with two recessive dominant allele and the character o one allele or tall height and one or
alleles. an individual with recessive alleles. dwar height .
Table 2
Figure 13 Antirrhinum fowers Data-based questions: Analysing genetic crosses
(a) wild type, (b) peloric
1 C harles D arwin crossed pure breeding wild- type Antirrhinum
majus plants, which have bilaterally symmetric fowers, with
pure breeding plants with peloric fowers that are radially
symmetric. All the F1 ospring produced bilaterally symmetric
fowers. D arwin then crossed the F plants together. In the F
12
generation there were 88 plants with bilaterally symmetric
fowers and 37 with peloric fowers.
a) Construct a Punnett grid to predict the outcome o the cross
between the F1 plants. [3]
b) Discuss whether the actual results o the cross are close
enough to support the predicted outcome. [2]
c) Peloric Antirrhinum majus plants are extremely rare in wild
populations o this species. Suggest reasons or this. [1 ]
2 There are three varieties o pheasant with eather coloration
called light, ring and bu. When light pheasants were bred
together, only light ospring were produced. S imilarly, when
ring were crossed with ring, all the ospring were ring. When
bu pheasants were crossed with bu there were 75 light
ospring, 68 ring and 1 41 bu.
a) Construct a Punnett grid to predict the outcome o [3]
breeding together bu pheasants.
b) Discuss whether the actual results o the cross are close [2]
enough to support the predicted outcome.
176
3.4 iNhEriTANCE
3 Mary and Herschel Mitchell investigated the inheritance o a
character called poky in the ungus Neurospora crassa. Poky strains
o the ungus grow more slowly than the wild-type. The results
are shown in table 3.
male paent Feale paent Nube o wld Nube o poky
type ofspng ofspng
Wild type Wild type 90
Poky Poky 9,691 10,591
Wild type Poky 0 7,905
Poky Wild type 0 43
4,816
Table 3
a) Discuss whether the data fts any o the Mendelian ratios in
table 1 (page 1 70) . [2]
b) Suggest a reason or all the ospring being poky in a cross
between wild type and poky strains when a wild type is the
male parent. [2]
c) Suggest a reason or a small number o poky ospring in a
cross between wild type and poky strains when a wild type
is the emale parent. [1 ] Figure 14 Feather coloration rom a buf pheasant
Genetic diseases due to recessive alleles Aa Aa
Aa Aa
Many genetic diseases in humans are due to recessive
alleles of autosomal genes.
A genetic disease is an illness that is caused by a gene. Most genetic
diseases are caused by a recessive allele o a gene. The disease thereore
only develops in individuals that do not have the dominant allele o the
gene, usually because they have two copies o the recessive allele. I a
person has one allele or the genetic disease and one dominant allele,
they will not show symptoms o the disease, but they can pass on the
recessive allele to their ospring. These individuals are called carriers.
Genetic diseases caused by a recessive allele usually appear
unexpectedly. B oth parents o a child with the disease must be carriers,
but as they do not show symptoms o the disease, they are unaware o
this. The probability o these parents having a child with the disease is 25
per cent (see fgure 1 5) . Cystic fbrosis is an example o a genetic disease
caused by a recessive allele. It is described later in this sub-topic.
Other causes of genetic diseases AA Aa aA aa
not carrier
Some genetic diseases are sex-linked and some are due carrier
to dominant or co-dominant alleles.
do not develop the disease
A small proportion o genetic diseases are caused by a dominant allele. develops the genetic disease
It is not possible to be a carrier o these diseases. I a person has one
dominant allele then they themselves will develop the disease. I one Figure 15 Genetic diseases caused
by a recessive allele
177
3 Genetics
Bb bb parent has the allele or the disease, the chance o a child inheriting it
Bb bb is 50 per cent (see fgure 1 6) . Huntingtons disease is an example o a
genetic disease caused by a dominant allele. It is described later in this
Bb bb sub-topic.
develops the does not develop
A very small proportion o genetic diseases are caused by co-dominant
disease the disease alleles. An example is sickle-cell anemia. The molecular basis o this
disease was described in sub-topic 3.1 . The normal allele or hemoglobin
Figure 16 Genetic diseases caused is HbA and the sickle cell allele is HbS. Figure 1 7 shows the three
by a dominant allele possible combinations o alleles and the characteristics that result.
Individuals that have one HbA and one HbS allele do not have the same
characteristics as those who have two copies o either allele, so the
alleles are co-dominant.
Most genetic diseases aect males and emales in the same way but
some show a dierent pattern o inheritance in males and emales.
This is called sex linkage. The causes o sex linkage and two examples,
red-green colour- blindness and hemophilia, are described later in this
sub-topic.
alleles : HbA HbA alleles : HbA Hbs alleles : HbS HbS
characteristics :
characteristics : characteristics : - susceptible to malaria
- susceptible to - increased resistance - severe anemia
malaria
- not anemic to malaria sickle-cell shape
- mild anemia
normal red blood
cell shape
Figure 17 Efects o HbA and HbS alleles
Cystic fbrosis and Huntingtons disease
Inheritance o cystic fbrosis and Huntingtons disease.
Cystic fbrosis is the commonest genetic disease secretions, making them very viscous. Sticky
in parts o Europe. It is due to a recessive allele mucus builds up in the lungs causing inections
o the CFTR gene. This gene is located on and the pancreatic duct is usually blocked so
chromosome 7 and the gene product is a chloride digestive enzymes secreted by the pancreas do
ion channel that is involved in secretion o sweat, not reach the small intestine.
mucus and digestive juices.
In some parts o Europe one in twenty people
The recessive alleles o this gene result in
chloride channels being produced that do not have an allele or cystic fbrosis. As the allele
unction properly. S weat containing excessive
amounts o sodium chloride is produced, but is recessive, a single copy o the allele does not
digestive juices and mucus are secreted with
insufcient sodium chloride. As a result not have any eects. The chance o two parents
enough water moves by osmosis into the
bot hb e ing a carri er o th ea llele i s _1_ _1_ ,
wh ich i The chance o such p nt
_1__ 20 20
s 400 . are s ha v i ng
a child with cystic fbrosis can be ound using a
Punnett grid.
178
3.4 iNhEriTANCE
father Because of the late onset, many people diagnosed
Cc with Huntingtons disease have already had
children. A genetic test can show before
Cc symptoms would develop whether a young
person has the dominant allele, but most people
C CC Cc at risk choose not to have the test.
normal normal
( carrier) About one in 1 0,000 people have a copy of
the Huntingtons allele, so it is very unlikely
mother Cc for two parents both to have a copy. A person
can nonetheless develop the disease if only
cC cc one of their parents has the allele because it is
dominant.
c normal cystic
father
(carrier) brosis Hh
ratio 3 normal : 1 cystic brosis
Huntingtons disease is due to a dominant Hh
allele of the HTT gene. This gene is located on
chromosome 4 and the gene product is a protein h Hh hh
named huntingtin. The function of huntingtin is H u n t i n gt o n s normal
still being researched.
disease
The dominant allele of HTT causes degenerative
changes in the brain. Symptoms usually start mother hh
when a person is between 30 and 50 years old.
C hanges to behaviour, thinking and emotions h Hh hh
become increasingly severe. Life expectancy after H u n t i n gt o n s normal
the start of symptoms is about 20 years. A person
with the disease eventually needs full nursing care disease
and usually succumbs to heart failure, pneumonia
or some other infectious disease. ratio 1 normal : 1 Huntingtons disease
sex-linked gene
The pattern o inheritance is diferent with
sex-linked genes due to their location on sex
chromosomes.
Plants such as peas are hermaphrodite they can produce both male and
female gametes. When Thomas Andrew Knight did crossing experiments
between pea plants in the late 1 8th century, he discovered that the
results were the same whichever character was in the male gamete and
which in the female gamete. For example, these two crosses gave the
same results:
pollen from a plant with green stems placed onto on the stigma of a
plant with purple stems;
pollen from a plant with purple stems placed onto on the stigma of a
plant with green stems.
179
3 Genetics
white eye red eye Plants always give the same results when reciprocal crosses such as these
XrXr XRY are carried out, but in animals the results are sometimes dierent. An
inheritance pattern where the ratios are dierent in males and emales is
Xr XrXR Y called sex linkage.
Xr red XR
One o the rst examples o sex linkage was discovered by Thomas
XrXR XrY Morgan in the ruit fy, Drosophila. This small insect is about 4 mm long
red white and completes its lie cycle in two weeks, allowing crossing experiments
to be done quickly with large numbers o fies. Most crosses in Drosophila
XrY do not show sex linkage. For example, these reciprocal crosses give the
white same results:
red eye white eye normal-winged males vestigial-winged emales;
XRXR XrY
vestigial-winged males normal-winged emales.
XR XRXR Y
XR red Xr These crosses gave dierent results:
XRXr
red XRY red-eyed males white-eyed emales gave only red-eyed ospring;
XRY red
red white-eyed males red-eyed emales gave red-eyed emales and
white-eyed males.
Key
XR X chromosome with allele Geneticists had observed that the inheritance o genes and o
chromosomes showed clear parallels and so genes were likely to be
for red eye (dominant) located on chromosomes. It was also known that emale Drosophila
Xr X chromosome with allele have two copies o a chromosome called X and males only have one
copy. Morgan deduced that sex linkage o eye colour could thereore
for white eye (recessive) be due to the eye colour gene being located on the X chromosome.
Y Y chromosome Male Drosophila also have a Y chromosome, but this does not carry
the eye-colour gene.
Figure 18 Reciprocal sex-linkage
Figure 1 8 explains the inheritance o eye colour in Drosophila. In crosses
crosses involving sex linkage, the alleles should always be shown as a superscript
letter on a letter X to represent the X chromosome. The Y chromosome
should also be shown though it does not carry an allele o the gene.
Red-green colour-blindness and hemophilia
Red-green colour-blindness and hemophilia as examples of sex-linked inheritance.
Many examples o sex linkage have been
discovered in humans. They are almost all due
to genes located on the X chromosome, as there
are very ew genes on the Y chromosome. Two
examples o sex-linked conditions due to genes on
the X chromosomes are described here: red-green
colour-blindness and hemophilia.
Red-green colour-blindness is caused by Figure 19 A person with red-green colour-blindness cannot clearly
a recessive allele o a gene or one o the distinguish between the colours o the fowers and the leaves
photoreceptor proteins. These proteins are made
by cone cells in the retina o the eye and detect
specic wavelength ranges o visible light.
180
3.4 iNhEriTANCE
proteins involved in the clotting o blood. Lie
expectancy is only about ten years i hemophilia
is untreated. Treatment is by inusing Factor VIII,
purifed rom the blood o donors.
The gene or Factor VIII is located on the X
chromosome. The allele that causes hemophilia
is recessive. The requency o the hemophilia
allele is about 1 in 1 0,000. This is thereore the
requency o the disease in boys. Females can
be carriers o the recessive hemophilia allele but
they only develop the disease i both o their X
chromosomes carry the allele. The requency in
___1 __
1 0,000
( )girls theoretically is 2 = 1 in 1 00,000,000.
In practice, there have been even ewer cases o
Figure 20 Blood should stop quickly owing rom a pricked girls with hemophilia due to lack o Factor VIII
fnger but in hemophiliacs bleeding continues or much longer
as blood does not clot properly than this. One reason is that the ather would
Males have only one X chromosome, which they have to be hemophiliac and decide to risk passing
inherit rom their mother. I that X chromosome
carries the red-green colour-blindness allele then on the condition to his children.
the son will be red-green colour-blind. In parts
o northern Europe the percentage o males with XH Xh XH Y KE Y
this disability is as high as 8%. Girls are red-green Xh XH XH Y XH X chromosome carrying
colour-blind i their ather is red-green colour-
blind and they also inherit an X chromosome the allele for normal
carrying the recessive gene rom their mother.
We can predict that the percentage o girls with blood clotting
colour-blindness in the same parts o Europe to be Xh X chromosome carrying
8% 8% = 0.64%. The actual percentage is about
0.5%, ftting this prediction well. the allele for hemophilia.
Whereas red-green colour-blindness is a mild Xh XH XH Y
disability, hemophilia is a lie- threatening genetic XH normal XH
disease. Although there are some rarer orms o
the disease, most cases o hemophilia are due XH Xh XH Y
to an inability to make Factor VIII, one o the carrier normal
Xh Y
hemophiliac
Pedigree charts
Analysis ofpedigree charts to deduce the pattern ofinheritance ofgenetic diseases.
It isnt possible to investigate the inheritance o squares and circles are shaded or cross-
genetic diseases in humans by carrying out cross hatched to indicate whether an individual is
experiments. Pedigree charts can be used instead aected by the disease;
to deduce the pattern o inheritance. These are the
usual conventions or constructing pedigree charts: parents and children are linked using a T, with
the top bar o the T between the parents;
males are shown as squares;
Roman numerals indicate generations;
emales are shown as circles;
181
3 Genetics
Arabic numbers are used or individuals in their children will be albino, we could only
each generation. expect to see that ratio i the parents had very
large numbers o children. The actual ratio o
Example 1 Albinism in humans 1 in 2 is not unexpected and does not show
that our deductions about the inheritance o
generation I albinism are incorrect.
12 Example 2 Vitamin D-resistant rickets
generation II D e du ctio ns:
1234 Two unaected parents only have unaected
children but two aected parents have
Key: children with vitamin D-resistant rickets,
normal pigmentation suggesting that this disease is caused by a
albino dominant allele.
D e ductio ns: The ospring o the parents in generation I
are all aected daughters and unaected sons.
Two o the children are albino and yet the This suggests sex linkage although the number
parents both have normal pigmentation. This o ospring is too small to be sure o the
suggests that albinism is caused by a recessive inheritance pattern.
allele (m) and normal pigmentation by a
dominant allele (M) . I vitamin D-resistant rickets is caused by a
dominant X-linked allele, daughters o the
There are both daughters and sons with ather in generation I would inherit his X
albinism suggesting that the condition is not chromosome carrying the dominant allele, so
sex-linked. Both males and emales are albino all o his daughters would have the disease.
only i they have two copies o the recessive This data in the pedigree shows that this and
albinism allele (mm) . so supports the theory.
The albino children must have inherited an Similarly i vitamin D-resistant rickets is
allele or albinism rom both parents. caused by a dominant X-linked allele, the
mother with the disease in generation II
Both parents must also have one allele or would have one X chromosome carrying
normal pigmentation as they are not albino. the dominant allele or the disease and one
The parents thereore have the alleles Mm. with the recessive allele. All o her ospring
would have a 50% chance o inheriting this X
The chance o a child o these parents having chromosome and o having the disease. The
data in the pedigree fts this and so supports
albinism is 1 . Although on average 1 in 4 o the theory.
4
Key:
vitamin D-resistant rickets
not aected
Figure 21 Pedigree of a family with cases of vitamin D-resistant rickets
182
3.4 iNhEriTANCE
Data-based questons: Deducing genotypes from pedigree charts
The pedigree chart in I 34
fgure 22 shows fve 12
generations o a amily
aected by a genetic disease. II 8 9 10 11 12 13 14 15
1 2 3 4 56 7 34
1 Explain, using evidence III
12
rom the pedigree,
whether the IV
condition is due to a 12 34 56 78
recessive or a dominant V ?? unaected male
unaected female
allele. [3] ?? 34 aected male
aected female
12
2 Explain what the Figure 22 Example of a pedigree chart
probability is o the
individuals in
generation V having:
a) two copies o a 3 Deduce, with reasons, the possible alleles o:
recessive allele;
b) one recessive and one dominant a) 1 in generation III;
allele;
b) 1 3 in generation II. [2]
c) two copies o the dominant
allele. [3] 4 Suggest two examples o genetic diseases that
would ft this inheritance pattern. [2]
Genetic diseases in humans Figure 23 Alleles from two parents come
together when they have a child. There is a
Many genetic diseases have been identifed in humans small chance that two recessive alleles will
but most are very rare. come together and cause a genetic disease
Several genetic diseases have already been described in this sub-topic, 183
including sickle-cell anemia, cystic fbrosis, hemophilia and Huntingtons
disease. There are other well-known examples, such as phenylketonuria
( PKU) , Tay-S achs disease and Marans syndrome.
Medical research has already identifed more than 4,000 genetic diseases
and more no doubt remain to be ound. Given this large number o
genetic diseases, it might seem surprising that most o us do not suer
rom any o them. The reason or this is that most genetic diseases are
caused by very rare recessive alleles which ollow Mendelian patterns o
inheritance. The chance o inheriting one allele or any specifc disease
is small but to develop the disease two alleles must be inherited and the
chance o this is extremely small.
It is now possible to sequence the genome o an individual human
relatively cheaply and quickly and large numbers o humans are being
sequenced to allow comparisons. This research is revealing the number
o rare recessive alleles that a typical individual is carrying that could
cause a genetic disease. Current estimates are that the number is
between 75 and 200 alleles among the 25,000 or so genes in the human
genome. An individual can only produce a child with a genetic disease
due to one o these recessive alleles i the other parent o the child has
the same rare allele.
3 Genetics
Figure 24 Abraham Lincolns eatures Causes of mutation
resemble Marans syndrome but a more
recent theory is that he sufered rom MEN2B, Radiation and mutagenic chemicals increase the mutation
another genetic disease rate and can cause genetic disease and cancer.
Figure 25 The risk o mutations due to A gene consists o a length o DNA, with a base sequence that can be
radiation rom nuclear waste is minimized hundreds or thousands o bases long. The dierent alleles o a gene have
by careul storage slight variations in the base sequence. Usually only one or a very small
number o bases are dierent. New alleles are ormed rom other alleles
by gene mutation.
A mutation is a random change to the base sequence o a gene. Two
types o actor can increase the mutation rate.
Radiation increases the mutation rate i it has enough energy to
cause chemical changes in DNA. Gamma rays and alpha particles
rom radioactive isotopes, short-wave ultraviolet radiation and
X-rays are all mutagenic.
Some chemical substances cause chemical changes in DNA and so are
mutagenic. Examples are benzo[a] pyrene and nitrosamines ound
in tobacco smoke and mustard gas used as a chemical weapon in the
First World War.
Mutations are random changes there is no mechanism or a particular
mutation being carried out. A random change to an allele that has
developed by evolution over perhaps millions o years is unlikely to
be benefcial. Almost all mutations are thereore either neutral or
harmul. Mutations o the genes that control cell division can cause
a cell to divide endlessly and develop into a tumour. Mutations are
thereore a cause o cancer.
Mutations in body cells, including those that cause cancer, are
eliminated when the individual dies, but mutations in cells that develop
into gametes can be passed on to ospring. This is the origin o genetic
diseases. It is thereore particularly important to minimize the number
o mutations in gamete-producing cells in the ovaries and testes. Current
estimates are that one or two new mutations occur each generation in
humans, adding to the risk o genetic diseases in children.
Consequences of nuclear bombing and accidents at nuclear
power stations
Consequences of radiation after nuclear bombing of Hiroshima and Nagasaki and
the nuclear accidents at Chernobyl.
The common eature o the nuclear bombing people either died directly or within a ew
o Hiroshima and Nagasaki and the nuclear months. The health o nearly 1 00,000 survivors
accidents at Three Mile Island and Chernobyl is has been ollowed since then by the Radiation
that radioactive isotopes were released into the Eects Research Foundation in Japan. Another
environment and as a result people were exposed 26,000 people who were not exposed to
to potentially dangerous levels o radiation. radiation have been used as a control group.
By 201 1 the survivors had developed 1 7,448
When the atomic bombs were detonated over tumours, but only 853 o these could be
Hiroshima and Nagasaki 1 50,000250,000
184
3.4 iNhEriTANCE
attributed to the eects o radiation rom the into the atmosphere in total. The eects were
atomic bombs. widespread and severe:
Apart rom cancer the other main eect o the 4 km2 o pine orest downwind o the reactor
radiation that was predicted was mutations, turned ginger brown and died.
leading to stillbirths, malormation or death. The
health o 1 0,000 children that were etuses when Horses and cattle near the plant died rom
the atomic bombs were detonated and 77,000 damage to their thyroid glands.
children that were born later in Hiroshima and
Nagasaki has been monitored. No evidence has Lynx, eagle owl, wild boar and other wildlie
been ound o mutations caused by the radiation. subsequently started to thrive in a zone
There are likely to have been some mutations, around Chernobyl rom which humans were
but the number is too small or it to be statistically excluded.
signifcant even with the large numbers o
children in the study. Bioaccumulation caused high levels o
radioactive caesium in fsh as ar away as
Despite the lack o evidence o mutations due Scandinavia and Germany and consumption
to the atomic bombs, survivors have sometimes o lamb contaminated with radioactive
elt that they were stigmatized. Some ound that caesium was banned or some time as ar away
potential wives or husbands were reluctant to as Wales.
marry them or ear that their children might
have genetic diseases. Concentrations o radioactive iodine in the
environment rose and resulted in drinking
The accident at Chernobyl, Ukraine, in 1 986 water and milk with unacceptably high levels.
involved explosions and a fre in the core o a
nuclear reactor. Workers at the plant quickly More than 6,000 cases o thyroid cancer
received atal doses o radiation. Radioactive have been reported that can be attributed
isotopes o xenon, krypton, iodine, caesium and to radioactive iodine released during the
tellurium were released and spread over large accident.
parts o Europe. About six tonnes o uranium
and other radioactive metals in uel rom the According to the report Chernobyls Legacy
reactor was broken up into small particles by Health, Environmental and Socio-Economic
the explosions and escaped. An estimated 5,200 Impacts, produced by The Chernobyl Forum,
million GBq o radioactive material was released there is no clearly demonstrated increase in
solid cancers or leukemia due to radiation in
the most aected populations.
12 Incidence per 100,000 in BelarusCases per 100,000 Actvty
adults (1934)
Cangng ates of tyod cance
10 adolescents (1518) When would you expect the cases
children (014) o thyroid cancer in young adults to
start to drop, based on the data in
8 fgure 26?
6 185
4
2
0
1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004v
Figure 26 Incidence of thyroid cancer in Belarus after the Chernobyl accident
3 Genetics
Data-baed quetion: The aftermath of Chernobyl
Mutations can cause a cell to become a tumour cell. The release of
6.7 tonnes of radioactive material from the nuclear power station
at Chernobyl in 1 986 was therefore the cause of large numbers of
deaths due to cancer. The UN C hernobyl Forum stated that up
to 4, 000 people may ultimately die as a result of the disaster, but
Green Party members of the European Parliament commissioned
a report from a radiation scientist, which gave an estimate of
30,000 to 60,000 extra deaths. One way of obtaining an estimate
is to use data from previous radiation exposures, such as the
detonation of nuclear warheads at Hiroshima and Nagasaki in
1 945. The data below is an analysis of deaths due to leukemia and
cancer between 1 950 and 1 990 among those exposed to radiation
from these warheads. It was published by the Radiation Effects
Research Foundation.
radiation Numbe of death Etimate of exce Pecentage of death
doe ange in people expoed death ove contol attibutable to
( sv) to adiation goup adiation expoue
Leukemia
0.0050.2 70 10
0.20.5 27 13 48
0.51 23 17 74
Figure 27 Humans have been excluded from >1 56 47
a large zone near the Chernobyl reactor. Some Cancer
plants and animals have shown deformities
that may be due to mutations 0.0050.2 3391 63 2
0.20.5 646 76 12
0.51 342 79 23
>1 308 121 39
1 Calculate the percentage of excess deaths over control groups
due to leukemia in people exposed to (a) 0.005-0.02 Sv
(sieverts) of radiation (b) >1 Sv of radiation. [4]
2 Construct a suitable type of graph or chart to represent the data in
the right-hand column of the table, including the two percentages
that you have calculated. There should be two y-axes, for the
leukemia deaths and the cancer deaths. [4]
3 Compare the effect of radiation on deaths due to leukemia [3]
and deaths due to cancer.
4 Discuss, with reasons, what level of radiation might be [4]
acceptable in the environment.
186
3 .5 GENETiC mODiFiCATiON AND BiOTEChNOlOGy
3.5 Genetc odfcaton and botecnoog
Udertadig Applicatio
Gel electrophoresis is used to separate proteins Use o DNA proling in paternity and orensic
or ragments o DNA according to size. investigations.
PCR can be used to ampliy small amounts o DNA. Gene transer to bacteria with plasmids using
restriction endonucleases and DNA ligase.
DNA proling involves comparison o DNA.
Assessment o the potential risks and benets
Genetic modication is carried out by gene associated with genetic modication o crops.
transer between species.
Production o cloned embryos by somatic-cell
Clones are groups o genetically identical nuclear transer.
organisms, derived rom a single original
parent cell. skill
Many plant species and some animal species Design o an experiment to assess one actor
have natural methods o cloning. afecting the rooting o stem-cuttings.
Animals can be cloned at the embryo stage by Analysis o examples o DNA proles.
breaking up the embryo into more than one Analysis o data on risks to monarch butteries
group o cells.
o Bt crops.
Methods have been developed or cloning adult
animals using diferentiated cells.
nature of ciece
Assessing risks associated with scientic research: scientists attempt to assess the risks associated
with genetically modied crops or livestock.
Gel electrophorei DNA samples
negative electrode 2
Gel electrophoresis is used to separate proteins or
ragments o DNA according to size. sample well
gel
Gel electrophoresis involves separating charged molecules in an electric
eld, according to their size and charge. Samples are placed in wells cast 1
in a gel. The gel is immersed in a conducting fuid and an electric eld positive electrode
is applied. Molecules in the sample that are charged will move through
the gel. Molecules with negative and positive charges move in opposite large fragments 2
directions. Proteins may be positively or negatively charged so can be direction of
separated according to their charge. migration
The gel used in gel electrophoresis consists o a mesh o laments that small fragments
resists the movement o molecules in a sample. DNA molecules rom 1
eukaryotes are too long to move through the gel, so they must be
broken up into smaller ragments. All DNA molecules carry negative Figure 1 Procedure for gel electrophoresis
charges so move in the same direction during gel electrophoresis, but not
187
3 Genetics
at the same rate. Small ragments move aster than large ones so they
move urther in a given time. Gel electrophoresis can thereore be used
to separate ragments o DNA according to size.
Figure 2 Small samples o DNA being DnA amplifcatio by PCR
extracted rom ossil bones o a Neanderthal
or amplifcation by PCR PCR can be used to amplify small amounts of DNA.
The polymerase chain reaction is used to make large numbers o
copies o DNA. It is almost always simply called PCR. The details
o this technique are described in sub-topic 2.7. Only a very small
amount o DNA is needed at the start o the process in theory just
a single molecule. Within an hour or two, millions o copies can
be made. This makes it possible to study the DNA urther without
the risk o using up a limited sample. For example, DNA extracted
rom ossils can be amplifed using PC R. Very small amounts o D NA
rom blood, semen or hairs can also be amplifed or use in orensic
investigations.
PCR is not used to copy the entire set o DNA molecules in a sample
such as blood or semen. White blood cells contain all chromosomes o
the person rom whom the blood came, or example, and together the
sperm cells in a sample o semen contain a mans entire genome. Instead
PCR is used to copy specifc DNA sequences. A sequence is selected or
copying by using a primer that binds to the start o the desired sequence.
The primer binds by complementary base pairing.
The selectivity o PCR allows particular desired sequences to be copied
rom a whole genome or even greater mixture o DNA. One test or the
presence o genetically modifed ingredients in oods involves the use o a
primer that binds to the genetically modifed DNA. Any such DNA present
is amplifed by the PCR, but i there is none present the PCR has no eect.
Data-based questions: PCR and Neanderthals Neanderthal and between the humans and thefrequency of
chimpanzees.number of dierences /%
The evolution o groups o living organisms can
be studied by comparing the base sequences o 25
their DNA. I a species separates into two groups,
dierences in base sequence between the two humanNeanderthal
species accumulate gradually over long periods o 20
time. The number o dierences can be used as an
evolutionary clock. 15 humanhuman humanchimp
Samples o DNA were recently obtained 10
rom ossil bones o a Neanderthal ( Homo
neanderthalensis) . They were amplifed using PC R. 5
A section o the Neanderthal mitochondrial DNA
was sequenced and compared with sequences 0
rom 994 humans and 1 6 chimpanzees. 0 5 10 15 20 25 30 35 40 45 50 55 60 65
number of dierences in base sequence
The bar chart in fgure 3 shows how many base-
sequence dierences were ound within the Figure3 Number o dierences in base sequences
sample o humans, between the humans and the between humans, chimps and Neanderthals
188
3 .5 GENETiC mODiFiCATiON AND BiOTEChNOlOGy
1 State the most common number o dierences this classifcation is supported by the data in
in base sequence between pairs o humans. [1 ]
the bar chart. [3]
2 Humans and Neanderthals are both classifed 3 Suggest a limitation to drawing any
in the genus Homo and chimpanzees are
classifed in the genus Pan. Discuss whether conclusion rom the humanNeanderthal
comparison. [1 ]
DnA proflig Figure 4 DNA profles are oten reerred to as
DNA fngerprints as they are used in a similar
DNA profling involves comparison o DNA. way to real fngerprints to distinguish one
individual rom all others
DNA profling involves these stages:
A sample o DNA is obtained, either rom a known individual or
rom another source such as a ossil or a crime scene.
Sequences in the DNA that vary considerably between individuals
are selected and are copied by PCR.
The copied DNA is split into ragments using restriction endonucleases.
The ragments are separated using gel electrophoresis.
This produces a pattern o bands that is always the same with DNA
taken rom one individual. This is the individual's DNA profle.
The profles o dierent individuals can be compared to see which
bands are the same and which are dierent.
Paterity ad oresic ivestigatios
Use o DNA profling in paternity and orensic investigations.
DNA profling is used in orensic investigations. DNA profling is also used in paternity
investigations. These are done to fnd out
Blood stains on a suspects clothing could be whether a man is the ather o a child. There are
shown to come rom the victim. various reasons or paternity investigations being
requested.
Blood stains at the crime scene that are not
rom the victim could be shown to come rom Men sometimes claim that they are not the
the suspect. ather o a child to avoid having to pay the
mother to raise the child.
A single hair at the crime scene could be
shown to come rom the suspect. Women who have had multiple partners
may wish to identiy the biological ather o
Semen rom a sexual crime could be shown to a child.
come rom the suspect.
In each example the DNA profle o material rom A child may wish to prove that a deceased
the crime scene is compared with the DNA profle man was their ather in order to show that
o a sample o DNA taken rom the suspect or the they are their heir.
victim. I the pattern o bands matches exactly it
is highly likely that the two samples o DNA are D NA profles o the mother, the child and the
rom the same person. This can provide very strong man are needed. DNA profles o each o the
evidence o who committed the crime. Some samples are prepared and the patterns o bands
countries now have databases o DNA profles, which are compared. I any bands in the childs profle
have allowed many criminal cases to be solved. do not occur in the profle o the mother or
man, another person must be the ather.
189
3 Genetics
Figure 6 Genes have been transerred rom Aalysis o DnA profles
daodil plants to rice, to make the rice
produce a yellow pigment in its seeds Analysis o examples o DNA profles.
190 Analysis o DNA profles in orensic investigations is straightorward:
two DNA samples are very likely to have come rom the same person
i the pattern o bands on the profle is the same.
victim
specimen
1
2 suspects
3
Figure 5 Which o the three suspects DNA fngerprints matches the
specimen recovered rom the crime scene?
Analysis o DNA profles in paternity investigations is more complicated.
Each o the bands in the childs DNA profle must be the same as a band
in the biological mother or athers profle. Every band in the childs
profle must be checked to make sure that it occurs either in the mothers
profle or in the profle o the man presumed to be the ather. I one or
more bands do not, another man must have been the biological ather.
Geetic modifcatio
Genetic modifcation is carried out by gene transer
between species.
Molecular biologists have developed techniques that allow genes to
be transerred between species. The transer o genes rom one species
to another is known as genetic modifcation. It is possible because the
genetic code is universal, so when genes are transerred between species,
the amino acid sequence translated rom them is unchanged the same
polypeptide is produced.
Genes have been transerred rom eukaryotes to bacteria. One o the
early examples was the transer o the gene or making human insulin to
a bacterium. This was done so that large quantities o this hormone can
be produced or treating diabetics.
Genetic modifcation has been used to introduce new characteristics
to animal species. For example, goats have been produced that secrete
milk containing spider silk protein. Spider silk is immensely strong, but
spiders could not be used to produce it commercially.
Genetic modifcation has also been used to produce many new varieties
o crop plant. These are known as genetically modifed or GM crops. For
example genes rom snapdragons have been transerred to tomatoes to
produce ruits that are purple rather than red. The production o golden
rice involved the transer o three genes, two rom daodil plants and
3 .5 GENETiC mODiFiCATiON AND BiOTEChNOlOGy
one rom a bacterium, so that the yellow pigment -carotene is produced
in the rice grains.
Actvt
Scientists have an obligation to consider the ethical implications o their
research. Discuss the ethics o the development o golden rice. -carotene is
a precursor to vitamin A. The development o golden rice was intended as a
solution to the problem o vitamin A defciency, which is a signifcant cause o
blindness among children globally.
techniques for gene ransfer o baceria Bacterial cell
Plasmid
Gene transer to bacteria with plasmids using restriction
endonucleases and DNA ligase. mRNA extracted from
human pancreatic cells
Genes can be transerred rom one species to another by a variety
o techniques. Together these techniques are known as genetic mRNA Plasmid obtained
engineering. Gene transer to bacteria usually involves plasmids, from bacteria
restriction enzymes and DNA ligase. cDNA
mRNA treated Plasmid
A plasmid is a small extra circle o DNA. The smallest plasmids with reverse cut with
have about 1 ,000 base pairs (1 kbp) , but they can have over transcriptase restriction
1 ,000 kbp. They occur commonly in bacteria. The most abundant to make enzyme
plasmids are those with genes that encourage their replication in complementary
the cytoplasm and transer rom one bacterium to another. There DNA (cDNA) Plasmid and
are thereore some parallels with viruses but plasmids are not cDNA fused
pathogenic and natural selection avours plasmids that coner an using DNA ligase
advantage on a bacterium rather than a disadvantage. Bacteria
use plasmids to exchange genes, so naturally absorb them and Recombinant
incorporate them into their main circular DNA molecule. Plasmids plasmid
are very useul in genetic engineering. introduced into
host cells
Restriction enzymes, also known as endonucleases, are enzymes
that cut DNA molecules at specifc base sequences. They can be Bacteria
used to cut open plasmids and also to cut out desired genes rom multiply in
larger DNA molecules. Some restriction enzymes have the useul a fermenter
property o cutting the two strands o a DNA molecule at dierent and produce
points. This leaves single-stranded sections called sticky ends. The insulin
sticky ends created by any one particular restriction enzyme have
complementary base sequences so can be used to link together Separation and
pieces o DNA, by hydrogen bonding between the bases. purication of
human insulin
DNA ligase is an enzyme that joins DNA molecules together frmly
by making sugarphosphate bonds between nucleotides. When Human insulin
the desired gene has been inserted into a plasmid using sticky ends can be used
there are still nicks in each sugarphosphate backbone o the DNA by diabetic
but DNA ligase can be used to seal these nicks. patients
An obvious requirement or gene transer is a copy o the gene being Figure 7 shows the steps involved in one
transerred. It is usually easier to obtain messenger RNA transcripts o example o gene transer. It has been used
genes than the genes themselves. Reverse transcriptase is an enzyme to create genetically modifed E. coli bacteria
that makes DNA copies o RNA molecules called cDNA. It can be used that are able to manuacture human insulin,
to make the DNA needed or gene transer rom messenger RNA. or use in treating diabetes
191
3 Genetics
Figure 8 The biohazard symbol indicates any Assessing the risks o genetic modifcation
organism or material that poses a threat to the
health of living organisms especially humans Assessing risks associated with scientifc research:
scientists attempt to assess the risks associated with
Figure 9 GM corn (maize) is widely grown in genetically modifed crops or livestock.
North America
There have been many ears expressed about the possible dangers
o genetic modifcation. These ears can be traced back to the 1 970s
when the frst experiments in gene transer were being conducted.
Paul Berg planned an experiment in which DNA rom the monkey
virus S V40 was going to be inserted into the bacterium E. coli. O ther
biologists expressed serious concerns because SV40 was known to
cause cancer in mice and E. coli lives naturally in the intestines o
humans. There was thereore a risk o the genetically engineered
bacterium causing cancer in humans.
Since then many other risks associated with genetic modifcation
have been identifed. There has been ferce debate both among
scientists and between scientists and non-scientists about the
saety o the research and the saety o using genetically modifed
organisms. This has led to bans being imposed in some countries,
with potentially useul applications o GM crops or livestock let
undeveloped.
Almost everything that we do carries risks and it is not possible to
eliminate risk entirely, either in science or in other aspects o our
lives. It is natural or humans to assess the risk o an action and decide
whether or not go ahead with it. This is what scientists must do
assess the risks associated with their research beore carrying it out.
The risks can be assessed in two ways:
What is the chance o an accident or other harmul consequence?
How harmul would the consequence be?
I there is a high chance o harmul consequences or a signifcant
chance o very harmul consequences then research should not
be done.
Risks and benefts o GM crops is disagreement, because gene transer to crop
plants is a relatively recent procedure, the issues
Assessment o the potential risks involved are very complex and in science it oten
and benefts associated with genetic takes decades or disputes to be resolved.
modifcation o crops.
Potential benefts can be grouped into
GM crops have many potential benefts. These environmental benefts, health benefts and
have been publicized widely by the corporations agricultural benefts. Economic benefts o GM
that produce GM seed, but they are questioned crops are not included here, because they cannot
by opponents o the technology. Even basic be assessed on a scientifc basis using experimental
issues such as whether GM crops increase yields evidence. It would be impossible in the time
and reduce pesticide and herbicide use have available or IB students to assess all claimed
been contested. It is not surprising that there
192
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