LECTURE NOTE WEEK 8

Chapter 5

Population Genetics

Learning Outcomes

5.1 Gene pool concept
Explain population genetics, gene pool
a llele freq uencies , g enotyp e freq uencies a nd
genetic equilibrium

Population

A localized group of individuals
that:

A group of individuals belongs to
same species that occupying a
particular habitat that are
capable of interbreeding and
producing fertile offspring.

Population
Genetics

Involves the study of genes in a
population including distributions
and changes in genotype and
phenotype frequency

Studies of the genes/ allele frequencies in population.

in response to the processes of
natural selection, genetic drift,
mutation and gene flow.

Gene Pool

Gene pool is the total number of alleles of all the
individuals in a population at any one time.

A population whose allele and genotypes
frequencies do not change from
generation to generation.

Genetic equilibrium

A population whose allele and genotypes
frequencies do not change from
generation to generation.

•In population genetics, a gene pool is the complete set
of unique alleles in a species or population.

•The gene pool consists of all alleles at all gene loci in all
individuals of the population.

A A A A Aa aa

Learning Outcomes

5.1 Hardy-Weinberg Law

a) State the Hardy-Weinberg Law.
b)Exp la in the five a ssump tions of H a rd y-W einb erg La w
for genetic equilibrium:
i.large population size;
ii.random mating;
iii.no mutation;
iv.no migration; and
v. no natural selection.

The history of Hardy-
Weinberg equilibrium

IIn 1908, G. H. Hardy (an English
mathematician) and
W. Weinberg (a German physician)
independently identified a
mathematical relationship between
alleles and genotypes in populations.

Hardy Weinburg Equilibrium
definition

In genetic equilibrium, the frequency of allele and genotype in a
population will remain constant from generation
to generation

If a p op ula tion is not evolving , it is in g enetic
equilibrium and the

allele frequency does not change.

When a population evolves, the
allele frequency in the population will change.

Five i. Large population size
a ssumption
s of Hardy- ii. Random fertilization / random mating
Weinberg
Law for iii. No net mutations
genetic
equilibrium: iv. No migration

v.No natural selection

i. Large
population size

Allele frequencies in a small
population are more likely to change
by random fluctuations (that is by
genetic drift) than are allele
frequencies in a large population.

ii. Random
fertilization /
random mating

In random fertilization, each
individual in a population has an
equal chance of mating with any
individual of the opposite sex.

iii. No net
mutations

When there is no mutation, the gene
pool remains the same.
By changing one allele into another
(by mutation) the gene pool is
altered.

iv. No migration

N o migration of individuals into or out
of a population.
So, no exchange of alleles with other
populations that might have different
allele frequencies.

v. No natural
selection

If natural selection does not occur,
allele frequencies remain the same.
If natural selection is occurring, the
allele frequencies will change, and
the population will evolve.

Hardy-Weinberg Equation

Equation for allele Equation for allele
frequency frequency

p+ q= 1 p2 + 2pq + q2 = 1

Key:

p = dominant allele frequency
q = recessive allele frequency
p2 = homozygous dominant genotype
2pq = heterozygous genotype
q2 = homozygous recessive genotype