92 Biology Term 3 STPM Chapter 16 Selection and Speciation 16 4. Variation, gathered as a result of mutation and selection over a long period of time, results in speciation. One group may be split into two and may become two species if a chromosome of one group becomes non-homologous. The resulting two species will be reproductively isolated. 5. For example, in the history of evolution of horses and donkeys, after the two species were formed even if they are crossed with each other, the offspring i.e. the mule produced is sterile. Continuous and Discontinuous Variations Variation can be divided into continuous and discontinuous variations. Continuous variation 1. Continuous variation is the difference of a character which is of degree that is quantitative and can be measured such as mass, height, length, volume and intelligence. 2. The character is controlled by polygenes. Each gene with a dominant allele contributes very little to the phenotype. 3. The character is easily influenced by environmental factors. An example is too much eating will increase ones weight. 4. The character exhibits a normal distribution. The distribution curve that is frequency plotted against the parameter is symmetrical, as shown in Figure 16.1. 5. Analysis of data is done statistically by finding the means and standard deviation. Discontinuous variation 1. Discontinuous variation is the difference of a character that is qualitative and can be observed, felt or tested serologically, such as colour of flower, texture of hair and blood group. 2. The character is controlled by ordinary Mendelian gene. The genes segregate and assort independently. 3. The character is not easily influenced by environmental factors. An example is flower colour of most plants. 4. The character does not exhibit normal distribution. The distribution curve is not symmetrical. 5. Analysis of data is by simple counting. The usual Mendelian ratios of 3 : 1 or 9 : 3 : 3 : 1 are obtained in the analysis of data using bar chart, as shown in Figure 16.2. Summary Continuous variation: 1. Difference in degree (quantitative) 2. Controlled by polygenes 3. Easily influenced by the environment 4. Normally distributed 5. Statistically analysed Discontinuous variation: 1. Difference in character (qualitative) 2. Controlled by Mendelian genes 3. Not so easily influenced by the environment 4. Non-symmetrically distributed 5. Analysed by simple counting Figure 16.1 Normal distribution of continuous variation Parameter Frequency Figure 16.2 Bar chart of discontinuous variation Parameter Frequency
93 Biology Term 3 STPM Chapter 16 Selection and Speciation 16 Environmental variation: 1. Caused by food, water, soil and climate. 2. Non-inheritable 3. Affects quantitatives Genetic variation: 1. Caused by mutation, crossing-over, independent assortment, different sexual partners and random fusion of gametes. 2. Inheritable Environmental and Genetic Variations Variation can also be divided into environmental and genetic variations. Environmental variation 1. Environmental variation is the difference that is caused by environmental factors and influences. Examples of such factors are food, water, soil factors and climate, as shown in Figure 16.3. 2. Food can help in the development of full genetic potential for animals. Lack of food for pregnant mothers affect the birth weight of their young. Food directly influences the weight of growing young animals. 3. Water affects growth of all organisms. Inadequate water supply affects seedling growth despite its genetic superiority. 4. Soil factors, such as fertility or mineral ion contents, positively influence plant growth. The pH of soil affects colour of flower as in Hydrangea. The flower is pink when the soil is acidic and blue when it is alkaline. This may affect the attraction of insects for its pollination. 5. Climatic factors, such as temperature affect the expression of certain genes. Classic examples are found in Himalayan rabbit and Siamese cat. High temperature prevents the expression of temperature-sensitive genes and only in the extremities of the body do the black pigment form. 6. Environmental variation cannot be inherited by the next generation. It does affect the evolution of organisms but to a lesser degree. 7. In conclusion, environmental factors seldom affect qualitative characters but easily affect quantitative characters. Genetic variation 1. Genetic variation is the difference of a character that is caused by genes. It is therefore inherited by the future generations. 2. They are ultimately caused by mutations. Mutations, especially point mutations, introduce new alleles. 3. Chromosomal mutations introduce more genetic materials in terms of addition of chromosomal part or numbers. 4. These mutations produce new genotypes that bring about evolution over many generations, as shown in Figure 16.4. Summary Water Soil Climate Food Figure 16.3 Different environmental variations
94 Biology Term 3 STPM Chapter 16 Selection and Speciation 16 A B C D A B B C A B C E C C A C Species A Species C Species B Different gene mutations Different chromosomal mutations B A B Figure 16.4 How mutations brought about speciation 5. More often, new alleles brought in are reshuffled. There are several ways of reshuffling, as shown in Figure 16.5. A D B B Crossing-over Independent assortment a d b A A D D B BA Parents Gametes a b D bA D Ba D ba D BA D bA d ba D ba d 16 combinations Offspring A B A BD D A B A BD d A B a bD D A B a bD d A B A bD d A B A bD d A b a bD D A b a bD d A B a BD D A B a BD d a B a bD D a B a bD d A B a bD D A B a bD d a b a b D D a b a b D d Figure 16.5 The reshuffling of alleles
95 Biology Term 3 STPM Chapter 16 Selection and Speciation 16 6. One way is through crossing-over, which is the exchange between parts of sister chromatids of homologous chromosomes. It causes variation by rearranging the sequence of alleles of linked genes. For example, grey-bodied and normal-winged Drosophila when crossed with that of ebony-bodied and vestigial-winged will only produce grey-bodied normal-winged and ebony-bodied vestigialwinged if no crossing over occurs. They will produce normalbodied vestigial-winged and ebony-bodies normal-winged when there is a crossing over. Crossing-over can produce many combinations because a chromosome may contain up to 5000 genes. Therefore, the number of combinations is 2n, where n is the number of genes in a chromosome. 7. Another way is independent assortment of homologous chromosomes. This applies to new alleles introduced to different chromosomes. It occurs in metaphase I where the random orientation of paired maternal and paternal chromosomes produces new combinations depending on the haploid number of a species. Variation is equal to 2n where n is the haploid number. In humans with haploid number of 23, the variation is more than 8 million! 8. Another shuffling is the random fusion of gametes. Though there is one ovum produced each month for humans, the number of spermatozoa produced is millions. Therefore, the combination of alleles formed is also very high. 9. Finally, sexual reproduction also brings about variation through the shuffling of genes from two individuals. Each gamete is formed from meiosis where two sets of chromosomes are reduced to form one. There are tremendous numbers of combinations from two unrelated parents. Quick Check 1 1. Tabulate the differences between continuous and discontinuous variation. 2. Explain how alleles, hybridisation and polygenes can cause variation. Natural Selection 1. Natural selection is the differential survival of some members of a species due to certain advantages possessed in a certain environment. The advantage may be their ability to comouflage so they would not be easily targetted by predators and would have better chance in catching prey compared to others. 2. The advantage is due to the genotypes of the members that confer certain morphological, behavioural or physiological characters. Such characters are controlled by alternative alleles. Exam Tips Remember the differences between continuous and discontinuous variations with examples. Remember also the source of genetic variation and the causes of environment variation, and the importance of variation in a population. 2013
96 Biology Term 3 STPM Chapter 16 Selection and Speciation 16 3. All members of a species are born with genetic variation caused by mutation, meiosis or sexual reproduction. The alleles are inheritable and expressed in a combination to produce certain advantage. 4. Some members are better adapted; they have more suitable characters than others and higher chances of survival. Therefore, when there is a change in the environment, species with the ability to adapt to these changes will survive and have the chance to reproduce. 5. As an example, the black peppered moth, Biston betularia, which were originally more susceptible to predators, flourished when the changes in the environment started during the industrial revolution. Black soot from coal-burning factories covered the trees protecting the moth from predation as its black-coloured body comouflaged well with the soot. in light background in dark background Figure 16.6 Peppered moths in different background and ease of predation 6. Similarly, finches with big beaks survive better in droughts compared to finches with smaller beaks. This is due to their ability to crack large and hard seeds, an ability absent in smaller finches. As seeds are scarce during droughts, these bigger-beaked finches survive well. VIDEO Natural Selection and the Owl Butterfly
97 Biology Term 3 STPM Chapter 16 Selection and Speciation 16 7. As the variation are determined by genes, those that survive will pass the advantage to the next generation. Conversely, those that cannot adapt will die without leaving any young. 8. Therefore, natural selection is actually differential reproduction. There are differences in reproduction between different individual. Those that have certain advantageous genes or alleles can reproduce as they live longer. 9. It is also a change in gene frequency in a population. For example, the black recessive allele in the peppered moth is in low proportion but later as the environment changes, its frequency increases. 10. The significance of natural selection is that it perpetuates the species. It enables a certain percentage of the species to survive, though the majority would die. 11. In the long term, it enables evolution to occur. This will cause the formation of new species after prolong isolation of a breakaway popution. 12. There are three forms of natural selection as represented in Figure 16.7. Frequency Then Later Beginning Measurement of character Stabilising Measurement of character Directional Measurement of character Disruptive Figure 16.7 The three types of natural selections (a) Stabilising selection 1. Stabilising selection is a form of natural selection in which the moderate type is increased while the extreme types are decreased. 2. It involves, more often, little or no changes in the morphology of the organism. 3. It does not lead to evolution, even if the allele frequency of the moderate increase or is maintained and the allele frequencies of the extreme dissapear. 2014
98 Biology Term 3 STPM Chapter 16 Selection and Speciation 16 4. It occurs in most populations when there is no change in the environment. The moderate pehenotype is the most common. 5. The species is well adapted to the environment. The favourable characters are maintained and the members survive successfully. 6. For example: (a) Under moderate temperature conditions, the fur of mammals is of moderate length from generation to generation. The moderate hair length is best suited for moderate temperature. (b) Human birth weight is around 3.2 kg. Any big variation to this weight is not healthy, as bigger babies endanger their lives as well as their mothers' lives whereas underweight babies are also unhealthy as they are likely to have some defects. (c) Horseshoe crabs or king crabs are of the same morphology and size since the time they evolved until now. This indicates that they are successfully adapted to their habitat of the coastal area. (b) Directional selection 1. Directional selection is a form of natural selection in which the extreme type is favoured. 2. Either the lower or higher extreme is selected. It cause the phenotype to shift to the opposite extreme. 3. It occurs when there is a change in the environment usually in gradual pace producing evolutionary forces and increasing selection pressures. 4. Directional selection produces a change in the morphological or structural character of the species which would likely lead to species evolution. 5. It usually occurs when there are mutations followed by changes in allele frequencies. 6. Polygenes are likely to be involved when quantitative characters are increasing or decreasing. 7. The existing type is not well-adapted to environmental change thus, a different form is selected. 8. For example: (a) When the temperature becomes colder, the mammals in the region will have longer fur and vice versa as the generation goes by. (b) The black peppered moth will increase in number and pigment intensity as the area becomes more industrialised and dirty. (c) Insects will become more resistant to insecticides. The pests will become more resistant to pesticides, and bacteria more resistant to antibiotics as more such chemicals are used. Summary Stabilising selection: 1. Moderates selected 2. No morphological changes 3. Well adapted 4. Maintain stabilising forces until environment changes 5. E.g. moderate for length in moderate temperatures Directional selection: 1. Extreme favoured 2. In gradual environmental change 3. Cause structural changes 4. Quantitative characters 5. E.g. increase in fur length as temperature increase Disruptive selection: 1. Moderate not selected 2. Two extremes produced 3. Two populations isolated 4. E.g. in gradual rise of mountain range producing hot and cold regions.
99 Biology Term 3 STPM Chapter 16 Selection and Speciation 16 (c) Disruptive selection 1. Disruptive selection is a form of natural selection in which the two extreme types are selected for but the moderates are selected against. This is also shown in Figure 16.7. 2. This causes the presence of two distinct groups or the existence of polymorphism. 3. The continuality of a normal distribution of a wider range of measurements is disrupted. It forms two smaller groups of normal distributions, each with different means. 4. For example: (a) In an area with two distinct sub-regions like on the top of a mountain with cold temperature and a desert low land, mammals living in the mountain top will have extremely long fur and those of the lowland will have short fur. There is no mammal of moderately long fur. (b) In the Galapagos Islands the finches either have big or small beaks. There are few or none with moderate beaks, as they are unable to compete with those with bigger beaks to get bigger seeds or with smaller beaks to get smaller seeds. (c) In Africa, the Papillo swallow-tailed butterflies are either darker orange or whitish colour for the females but there is a range with moderately yellow colour type for the males. This is because the moderate yellow colour ones are favoured by birds as food. The brightly orange and the whitish-coloured ones are avoided as they mimic poisonous types of other species. Males have very short lifespan, so they are newly emerged. Sexual selection 1. Sexual selection is a form of natural selection in which one of the sexes selects the other as a partner for sexual reproduction based on certain characteristics. The genes of such partner will be transmitted to the next generation. 2. Therefore, the allele frequencies that determine such characteristics will be increased over the generations. This will result in the differences in external features of the sexes. 3. Usually, it is the females that select males. Rarely, it is the males that select females. 4. This causes the males to evolve into different forms. They can become larger, have longer tails, brighter colours, have antlers, and behave differently, including being able to sing and dance in specific manners. 5. This leads to sexual dimorphism (Figure 16.8).
100 Biology Term 3 STPM Chapter 16 Selection and Speciation 16 Female Male Fish (guppy) Male Female Female Male Amphibian (frog) Female Male Bird (peacock) Female Male Mammal (seal) Figure 16.8 Sexual selection in fish, amphibian, reptile, bird and mammal 5. For example, sexual selection in the elephant seal has reached its extreme. Males are more than twice the size of females. A giant and ferocious male will choose the best territory by the seaside. Other males will be driven away. A few hundred females will be fertilised by the most successful male that can guard a big territory. This is also known as intrasexual selection as the males compete among themselves for females. 6. In fishes, there are males and females which differ in size and colour. An example is one species of African Tilapia. The males are more colourful and bigger in size. They are territorial and furiously guard their territories of cleaned stone slabs. The females will choose the clean slabs for laying eggs, which are immediately fertilised by the males. This is the usual form also known as intersexual selection where females choose the males. Summary 1. Female selects male 2. Based on: (a) colour (b) size (c) voice (d) behaviour 3. Produces sexual dimorphism 4. E.g. in vertebrates 5. Produces unfavourable characters for survival 6. Increases selection pressure and more variation
101 Biology Term 3 STPM Chapter 16 Selection and Speciation 16 7. The birds of paradise of Papua New Guinea have long wings and tails. Males dance back and forth to attract females. Males that are not so colourful or cannot dance in a manner to the females’ liking will end up without partners. This results in the evolution of the present colour and behaviour of these types of birds. This vast difference in form will be perpetuated between the two sexes. 8. Sexual selection has certain disadvantages: (a) The bigger size of males makes them more visible to predators making them easy targets. It also makes them less mobile to catch food. (b) Similarly, their bright colours make them more noticeable to preys and predators. (c) Noisy males are also easily noticed by predators. (d) Males that are too engrossed in mating and dancing rituals fall easy prey to predators as well. 9. Therefore, sexual selection can only cause the evolution of males to a certain extent. Nature will balance the increase in size, colour and behaviour up to a certain point. 10. Sexual selection has the advantage of strong and healthy characteristics being passed to the future generation. Males are selected based on their strength, ability to look for food and their healthy plumage or their facial colour as in certain primates such as the proboscis monkey. 11. Sexual selection does play a role in the evolution of new species. It can cause isolation of populations into smaller groups as the females are attracted to newly mutated phenotypes. This might have caused the many species of birds of paradise in Papua New Guinea. Polymorphism or genetic polymorphism 1. Polymorphism is a condition within a population where two or more contrasting phenotypes are clearly exhibited. 2. As an example, in western and central Africa, the inborn disease of sickle-cell anaemia is high. The phenotypes as determined by the genotypes are as follows: (a) Seriously anaemic, homozygous dominant (SS) = 2.5% (b) Moderately anaemic, heterozygous dominant (Ss) = 30% (c) Normal, homozygous recessive (ss) = 67.5% 3. These phenotypes are controlled by a single locus with two alleles, as shown in Figure 16.9. 4. Polymorphism is divided into two types as follows: (a) Transient polymorphism (i) It is an intermediate stage that exists in the course of evolution when there is a change in the environment. (ii) A certain phenotype is favoured by the environment and is being selected for causing the phenotype increase. Normal red blood cell ss Red blood cells half sickle-shaped Ss All sickleshaped SS Figure 16.9 The three genotypes and phenotypes of sickle-cell anaemia 2014
102 Biology Term 3 STPM Chapter 16 Selection and Speciation 16 (iii) The environmental changes may only last for a short period, in evolutionary sense, a few hundred years, and the condition is reversed. This results in the existence of contrasting phenotypes. (iv) The phenomenon may exist for a short period or it can be for long periods, depending on the selection pressure. Selection pressure is how fast a certain phenotype increases or decreases. (v) Formerly, when malaria was incurable, normal people fell sick and died. However, heterozygous people were resistant to the parasite attack and survived. This resulted in an increase in the sickle-celled allele. Now, people with the allele are selected against as they are anaemic and weak. (b) Balanced polymorphism (i) It is a state that exists due to random mating of the members of a population. (ii) None of the phenotype is favoured or selected against by nature. (iii) There is no change in the environment. (iv) The phenomenon may have existed a very long time ago. The frequencies of each phenotype may vary slightly from generation to generation due to the population size and random nature of the mating. (v) An example is the ABO blood group that exists in human population. The proportions of people with A, B, AB and O groups vary from place to place. Artificial Selection Artificial selection is selection made by men since the dawn of civilisation that results in the evolution of crops and domesticated animals. Breeding of domesticated animals and crop plants 1. Domesticated animals and crop plants are bred by men more than ten thousand years ago. 2. They originated from wild species. 3. Plants and animals with the best traits were selected and their genes kept for future use. 4. Later, men crossed the best plants and animals with each other. The best among the offsprings were selected and bred for the next generation. 5. This led to the wide varieties and species of domesticated animals and crop plants as there are today. Exam Tips You should be able to explain, with the help of graphs, stabilising, directional, and disruptive selections together with the meaning and causes of genetic drift in a population. 2000
103 Biology Term 3 STPM Chapter 16 Selection and Speciation 16 6. Modern breeders make use of induced mutation to create more varieties and species. 7. Genetic engineering further created more transgenic varieties. 8. Examples of artificial selection or breeding are as follows: (a) Cattle. There are different varieties of cows for different purposes and there are variations in different countries. Jersey cow is bred for milk and Hereford is bred for meat. (b) Dogs. There are hundreds of varieties of dogs of different sizes, colour, form and hair length. Among them are Alsatian, Doberman, Dalmatian, Rotweiler, Bulldog, Chihuahua, Pom-pom, Pekinese and Poodle. (c) Crop. All the cereals originate from grass-like varieties with tiny seeds. Controlled or selective breeding (Please refer to 17.1) Inbreeding 1. Inbreeding is a form of controlled breeding in which related animals or plants are crossed. 2. The closest form of inbreeding is selfing. This can only occur in plants. 3. Another form of inbreeding is mating among siblings, between offspring and parents, and among cousins. 4. The purpose of such mating is to produce homozygosity among the alleles. This is to produce a pure line in which the quality of a certain character is fixed. 5. Examples of pure line exist in both domesticated animals and crops such as certain breeds of cattle, sheep, wheat, rice and oil palm. 6. Inbreeding produces a number of disadvantages. (a) It decreases hybrid vigour. (b) It decreases variability. (c) The organism grows slowly. (d) Inbred offsprings are more susceptible to diseases. (e) The yield decreases. (f) The size of inbred offspring gets smaller. (g) Inbred offspring becomes less fertile. (h) Inbred offspring have shorter lifespan. Outbreeding 1. Outbreeding is a form of breeding in which unrelated animals or plants are crossed. 2. It includes the crossings of different pure lines, breeds varieties, subspecies or species.
104 Biology Term 3 STPM Chapter 16 Selection and Speciation 16 3. The purpose of such mating is to produce heterozygosity or hybrid vigour. 4. Hybrids have the advantage of growing faster, being more resistant to diseases, increase yield and size of the organism, more fertile and longer lifespan. 5. Hybrids have a disadvantage of losing its pure character or uniformity. 6. The second or subsequent generations produce a wild range of characters, from one extreme to the opposite. The size can be from smallest to the biggest. Sperm bank 1. Animal sperm bank (a) Animal sperm bank is the storage of sperm from animals of proven genetic stock and used for controlled breeding. (b) The sperm is collected from studs and deep frozen in liquid nitrogen. (c) It offers many advantages for breeding purposes over the traditional method. (i) It enables easy management of artificial insemination. Many female animals can be fertilised in a short period of time. (ii) Sperm from a single male animal can be used to fertilise thousands of females. This is because the sperm can be diluted and little is required each time. (iii) It enables the sperm to be used vastly and all over the world, as it is easily transported. (iv) The sperm can be stored for a long time. It can still be used even after the death of the male animal. (v) The farmer does not need to care for the male breeding stock. The job can be left to experts in the research station. (vi) As there is no direct contact between male and female animals, sexually transmitted diseases can be avoided. Both sperm and instruments used for insemination are sterilised. (vii) With good record keeping of the stock, any gene defect can be traced and controlled. (viii) Inbreeding can be cut down to the minimum, as different sperm bank maintains different pure line of stock. 2. Human sperm bank (a) A human sperm bank is the storage of human sperm, usually operated by hospitals to inseminate wives whose husbands are sterile. (b) The hospital obtains the sperm from healthy fathers, free of genetic and contagious diseases.
105 Biology Term 3 STPM Chapter 16 Selection and Speciation 16 (c) Private hospitals or commercial sperm banks may accept individuals who wish to have children in the future. (d) Therefore, the human sperm bank create a lot of controversies. (i) The question of ethics. Artificial insemination is viewed as morally wrong in many religions to manipulate with human sperm and ovum. Human reproduction is the gift from God. (ii) The question of legality. In law, the biological father must take care of his children. The biological son can claim for maintenance and inheritance of wealth from the father. Can the biological father claim back the son who is artificially fathered? (iii) The question of eugenics. Eugenics is the creation of a “super human race”. The sperm from a distinguished man with outstanding characteristics in a certain field may be used to fertilise an ovum from a similarly calibre woman to produce smart or gifted children of the next generation. If sperm bank can be established, what is stopping the misuse of it? Gene (germplasm) bank 1. Gene or germplasm bank is a storage in low temperature which preserves genetic material. In plants, the storage can be seed and tissue banks while in animals, the storage can be sperm and egg banks. 2. In plants, it is possible to unfreeze the material and propagate it and a new plant is produced. However, in animals, a living female is required for artificial insemination but for bird eggs, a chick can be hatched from the preserved egg by incubating the egg. In corals, fragments are taken and stored in water tanks under controlled conditions. 3. In general, it is a form of conservation in which part of the DNA molecules with a gene as in gene library to the big tree in a forest can be considered as gene banks. Thus, it can be part of ex situ conservation or even in situ conservation especially for endangered species. 4. In an effort to conserve agricultural biodiversity, gene banks are used to store and conserve the plant genetic resources of major crop plants and their wild crop relatives. The Svalbard Global Seed Vault is probably the most well-known out of the many gene banks in the world. 5. There are many types of gene banks: (a) Seed bank The seed bank preserves dried seeds by storing them at a very low temperature. Spores of ferns are conserved in seed banks. The largest rice seed bank in the world is the International Rice Research Institute in Manila. Exam Tips Remember the meaning and description of artificial selection, controlled breeding, including inbreeding and outbreeding. You should know the uses of animal sperm bank and the controversies on the use of human sperm bank.
106 Biology Term 3 STPM Chapter 16 Selection and Speciation 16 (b) Tissue bank In this technique, buds, protocorm, tissues, meristematic cells and stem cells of animals are conserved in a nutrient medium in liquid nitrogen. This is another technique used to preserve both plants and animals in a much smaller space than keeping them in a botanical garden or zoo. (c) Cryobank In this technique, a seed or embryo is preserved at very low temperatures. It is usually preserved in liquid nitrogen at –196°C. This is helpful for the conservation of species facing extinction. (d) Pollen bank This is a method in which pollen grains are stored. Plants in the endangered list can be propagated with one set of chromosome from the pollen. (e) Field gene bank This can be natural forest in a national park, marine park or wild animal sanctuary where the plants or animals are conserved. This also can be a man-made botanical garden, research institute, zoo or aquarium to collect and propagate endangered plants or breed captive animals. Importance of Artificial Selection 1. Artificial selection has been used by ancient people more than ten thousand years to domesticate animals and plant crops. The earliest domesticated animals include goats, sheeps and cows. Wheat seem to be the earliest cereal cultivated. 2. The evolution of plants can be studied through artificially bred crops. One of them is from wild cabbage, Brassica oleracea. From this species, distinct vegetables have been developed i.e. the modern cabbage, broccoli, brussel sprouts, cauliflower, collard greens, kale and kohlrabi have been developed. This is a classic case of one ansector evolving into so many types though not naturally. 3. It was used by Darwin and many scientists to show that artificial selection could be used as evidence for evolution. Artificial selection by breeders developing so many breeds of dog in just a few generations has influenced Darwin to fomulate his theory of natural selection in evolution. Natural selection has since become a branch of biological science. 4. Artificial selection is used in selective breeding and development of crops and domesticated animals. (a) Many varieties of outstanding cereals have been developed that revolutionalised food production. Special breeds of wheat for bread making, pastry, cakes and brewing are Summary Artificial selection is important in: 1. domestication of plants and animals 2. studying evolution 3. acting as evidence for evolution 4. selective breeding 5. genetically manipulation of plants and animals
107 Biology Term 3 STPM Chapter 16 Selection and Speciation 16 developed through selective breeding. Similarly, corn breeds have been specially developed for specific use. High yielding rice breeds have been developed in Philippines. (b) Besides cereals, vegetables, horticultural plants, fruit trees, palms and rubber trees are selectively bred. Each breed has distinctive economic value. (c) Farm animals are also selectively bred for specific value. Many domesticated animal has been bred especially for human needs or appreciation as pets. (d) Artificial selection is now used in conservation of endangered species. As the endangered species becomes so few, human intervention using knowledge gained in artificial selection has to be carried out before species extinction. 5. From artificial selection, scientists have used biotechnology and recombinant technology to engineer specific plants and animals. These crops are genetically manipulated to increase yield, be resistant to diseases and become special with a specific character such as uncoventional colour. Similarly, animals are genetically manipulated to produce therapeutic proteins in milk. Quick Check 2 1. What are intrasexual selection, intersexual selection and sexual dimorphism? 2. What is polymorphism? What does it signify in evolution? 3. State five advantages of using animal sperm banks. 4. State three controversies about the use of human sperm bank. 16.2 Speciation 1. Speciation is the formation of new species from one or more existing species. 2. It can be divided into two types, such as intraspecific speciation and interspecific speciation (Figure 16.10). Speciation Intraspecific speciation Allopatric speciation Geographical barrier Sympatric speciation Reproductive barrier 1 species Interspecific speciation 2 species Hybrid Chromosome doubling New species Figure 16.10 Types of speciation Learning Outcomes Students should be able to: (a) explain the processes of isolation, genetic drift, hybridisation and adaptive radiation; (b) explain the importances of speciation in relation to evolution. VIDEO Speciation
108 Biology Term 3 STPM Chapter 16 Selection and Speciation 16 Intraspecific Speciation 1. Intraspecific speciation is the formation of a new species from an existing species. 2. In the beginning, a small group is reproductively isolated thus interrupting the gene flow in the population. Such small group of members tend to stay together and interbreed among themselves. 3. This isolated group will be exposed to mutation and recombination of the alleles among themselves through sexual reproduction. This results in a change in the allele frequency and genotype frequency. 4. This isolated group is called deme. Later, it becomes variety or subspecies. 5. After the accumulation of mutations and recombination for around a million years, the chromosomes of this isolated group and the main population become so different that they are no longer homologous. Chromosomal mutations in the structures can also result in the chromosomes becoming non-homologous. 6. Finally, the isolated group is considered as a new species. If the new species is mated to the original group, they produce sterile hybrid. 7. Intraspecific speciation can be divided into allopatric and sympatric speciation as describe later. Interspecific Speciation or Hybridisation 1. Interspecific speciation is the formation of one species from the hybridisation of two species as shown in figure 16.10. 2. This occurs naturally in plants, e.g in Spartina grass, a type of grass that live in marshy areas in North America and other continents. 3. In the beginning, two related species crossed and form a hybrid. Such related species are similar to each other in form and structure. 4. The hybrid is sterile because meiosis cannot occur normally, as the chromosomes are not homologous. This results in imbalance gametes and no gametes either egg cells or male nuclei are produced. 5. However, chromosome doubling can occur in plants. This is the result of the presence of natural inhibitor for formation of spindle fibre such as colchicine. 6. Chromosome doubling can occur in mitosis or meiosis. In mitosis, it is the nondisjunction of chromatids. 7. In meiosis, it is the nondisjunction of chromosomes in meiosis I and chromatids in meiosis II. Summary Intraspecific speciation: 1. Produced from one existing species 2. Separated reproductively 3. Has its own mutations 4. Overtime 5. Chromosomes become non-homologous 6. Reproductively isolated 7. Interbreeding permanently cannot occurs Interspecific speciation: 1. Produced from hybridisation of two species 2. Usually in plants 3. Hybrid is sterile 4. Chromosome doubling 5. New species is formed 2012
109 Biology Term 3 STPM Chapter 16 Selection and Speciation 16 8. In mitosis, it can happen in the formation of a bud by the meristem. This results in a branch, a runner or a separate plant that is polyploid and fertile. 9. Similarly, this can happen in meiosis during the formation of pollen and embryo sac by the hybrid sterile plant. Non-disjunction produces unreduced pollen or ovum that may fertilise one another. A polyploid seed and later, a fertile plant is produced. 10. A new species of plant can form this way in a short time. It is fertile and cannot interbreed with either of the original ancestral species. This new species is called allopolyploid, described again under chromosomal mutation in Chapter 17. (Figure 17.49) Allopatric speciation or isolation 1. Allopatric speciation is the intraspecific formation of a new species that requires the original group to be physically isolated. 2. The population is separated in the form of a geographical barrier. 3. Examples of the barrier are mountain ranges, oceans, river or habitat, as shown in Figure 16.11. Forest Grassland River Grassland Arid grassland Forest Grassland Mountain Grassland Ocean Island Land Ocean Mountain range Habitat River Figure 16.11 Different geographical barriers that cause allopatric speciation 4. Mountain ranges are formed when one land plate moves against another, such as the Himalayan range. Animals on either side of the range cannot come in contact and interbreed freely. 2012 2016
110 Biology Term 3 STPM Chapter 16 Selection and Speciation 16 5. Sinking of lands are classical examples of how oceans or straits isolate population. This is especially so in ancient time when Australia is linked to Asia by land bridges. The formation of islands by under ocean volcanic eruption such as in Galapagos Islands also isolates the islands by oceans. 6. The formation of river can isolate animals, such as certain waterfearing rodents like rats. Bigger rivers or Grand Canyon can isolate larger animals. 7. A forest fire can create a grassland habitat across dense forest. This can isolate ground-fearing primates moving from one side to another. A modern highway that cuts across a tropical rainforest has the same effect. 8. Isthmus or land can separate aquatic animals into two oceans or lakes. Originally, all of the fish in one big lake can freely interbreed. The sinking of water later created two lakes. Thus, the fish are separated into two groups. Another example is the isthmus of Panama, which separated the Pacific Ocean from Gulf of Mexico in Central America. Sympatric speciation or isolation 1. Sympatric speciation is the formation of a new species by intraspecific way where there is no physical isolation. 2. Even though the organisms still live physically close to each other, they are isolated in different ways by mechanisms as discussed below. Isolation 1. Isolation is a prerequisite in the course of intraspecific speciation during evolution. 2. Other than physical isolation, there are many ways organisms can be isolated, especially after mutation but before fertilisation. This is also known as prezygotic mechanisms that prevent the formation of viable zygotes. (a) Seasonal (temporal) isolation. The organisms are physically close together but the time of reproduction divides them into two or more groups. (i) An example is a species of plants such as durian that may have members flowering at different time of the year. Those that flower at the same time can interbreed while the rest is isolated. (ii) Another example is frogs that do call or respond for mating after rain at certain time of the year due to temperature difference. Others that do not call or respond are isolated. (b) Ecological isolation. The organisms are isolated by occupying different ecological niches. This may be in the form of food for animals or changes in the soil acidity. (i) An example is the finches in Galapagos Islands that feed on insects, suddenly feed on the blood of the chicks of pelicans and injured adults. Summary Allopatric speciation: 1. Separated by geographical barrier such as: (a) Mountain ranges (b) Oceans (c) Rivers (d) Stripping of land Sympatric speciation: 1. Separated by reproductively in terms of: (a) Season (b) Ecology (c) Behaviour (d) Mechanic (e) Physiology 2012 2015 Exam Tips Sympatric speciation and prezygotic isolating mechanisms. (STPM 2015 essay question)
111 Biology Term 3 STPM Chapter 16 Selection and Speciation 16 (ii) Seeds of the same species of plants may be dispersed on acidic and alkaline soil nearby. The flowers produced are of different colours. Now, the colour difference forces the two groups to occupy different niches as they attract different insects for pollination. (c) Behavioural isolation. The animals are isolated because new sounds, dancing or rituals of males fail to attract some of the females. (i) Butterflies, fishes and birds dance or perform certain rituals, such as the Paradise birds in Papua New Guinea. If that behaviour is changed, it may attract a different group of females. (ii) Similarly, the call or song of the males may fail to attract one group but is likely to attract another. (d) Mechanical isolation. The sexual organs or genitalia fail to match during mating of animals or there are changes in the floral structure in plants. (i) An example is in dog where the Alsatian will fail to mate with a Chihuahua, as the genitalia are no longer compatible. (ii) In flowers, a mutation occurs that result in smaller tubular flower excluding larger insects from visiting them, thus bringing about isolation. (e) Physiological (gametic) isolation. There are chemicals that prevent the gametes from fertilisation. So, the gametes are chemically incompatible and will not fuse to form zygotes. (i) The reproductive tract of the female produce chemicals that can kill the spermatozoa. The fertilisation process may fail to occur, even after the entry of a spermatozoon. (ii) Similarly, in plants the stigma fails to produce chemical for the germination of the pollen. It may be that the style may produce toxic chemicals that inhibit the growth of the pollen tube. 3. After fertilisation, certain groups are still isolated. This is known as postzygotic isolation. 2016 (a) Hybrid lethality (inviability). The young produced are too weak to survive or compete. In Northern America, there are two types of frogs: one from the north and the other from the south. When they mate, the hybrid cannot grow to become adults. (b) Hybrid sterility. The hybrid formed do not produce gametes as meiosis fails due to non-homologous chromosomes. There are many examples in animals but more often, they are manmade. An example is mule, formed by a cross between the stallion and female donkey. (c) Hybrid emasculation. The amphidiploid hybrid becomes fertile as in plants. It cannot be mated back to the original ancestral stock. An example is the American grass Spartina. Exam Tips Remember the meaning, types, importances and examples of speciation. Also remember the mechanisms that prevent hybrids from passing on their genes and various isolating mechanisms during evolution.
112 Biology Term 3 STPM Chapter 16 Selection and Speciation 16 Genetic Drift 1. Genetic drift also known as Sewell Wright effect is the change in allelic frequency of a population due to chance and not due to natural selection, as a result of sampling error from generation to generation. 2. This also occurs when a population is isolated into a smaller group known as bottle-neck effect or migration of a small group to an island known as founder effect. 3. Genetic drift (Figure 16.2) is common in population that undergo regular cycles of extinction and recolonisation. This occurs in ecosystem that have patchy distribution where each patch is a small population. 4. There are many causes of genetic drift. (a) There is non-random crossing of the various genotypes. This results in non-random recombination of the various alleles. (b) Another cause is the assortment of chromosomes causing certain alleles being lost due to the small percentage of offspring survive to adult and reproduce. (c) A small population is isolated from the rest physically or reproductively. (d) Certain genotypes die due to chance. Certain alleles may just disappear. (e) This also occurs in pioneer population. The various genotypes present at the beginning are purely due to chance. 5. There are variable possible outcome of the effect. (a) A population may just disappear together with certain alleles. (b) Phenotypes or species which do not adapt to the environment too well may survive. (c) A newly evolved species may increase in number very fast. (d) The future generation may decrease in variability and frequency of homozygotes may increase very fast. (e) The allele frequency cannot be predicted, as it varies from one generation to another. 6. Genetic drift is important in evolution. (a) It may result in fast adaptation of a new species in a new environment. (b) It aids in the formation of new species, as shown by finches in the Galapagos Islands. Adaptive Radiation 1. Adaptive radiation is the changing of a homologous structure of organisms in the course of evolution to live in different environments. Exam Tips Remember the meaning of genetic drift. (STPM 2014 and STPM 2015 essay question 2000 2014 Figure 16.12 Genetic drift A a a a a a a a a a a a a A A A A A A A a a a a a A A A A A A Gene pool (original population) Small population (different gene pool) Broken away Info Bio Alleles pass to the next generation by chance. Only a small percentage of offspring survive to adult and reproduce. So, the chances of passing the alleles are known as binomial sampling errors. The chances are high in small population.
113 Biology Term 3 STPM Chapter 16 Selection and Speciation 16 2. Such changing of structure is due to mutation. Though most mutations we know are harmful, many of such changes are of survival value in the course of evolution. 3. The changes were slow. They took many generations and thousands of years to accomplish in the various organisms that we see today (Figure 16.3). Perissodactyla Edentata Pinnipedia Logomorpha Catacea Artiodactyla Primates Carnivora Marsupialia Insectivora Bats Rodentia Proboscidea Ancestral mammal Figure 16.13 Adaptive radiation of pentadactyl limbs 4. The closest evolutionary-related animals show the closest similarities in the homologous structure. They still perform similar functions. 5. Similarly, animals living in closest mode of life also show closest similarities in the homologous structure. 6. There are many examples of such adaptive radiation. (a) One example is the insect mouthparts. The basic insect mouthparts consists of a labrum, a pair of mandibles, a pair of maxillae and a labium, as in the order of Orthopthera such as cockroaches and grasshoppers. The mosquitoes still exhibit the same basic structure but they are modified for piercing and sucking blood or plant juice. Bees retain a pair of mandibles for cutting but the rest of the mouthparts are for sucking. The houseflies and butterflies have completely modified the mouthparts to form proboscis for sucking food.
114 Biology Term 3 STPM Chapter 16 Selection and Speciation 16 (b) Another example is pentadactyl limbs in mammals. The basic five-fingered limbs are for grasping as in primates. The bears have modified them for tearing their food, the anteaters have modified them for digging. The horses have them for running fast, the bats have their forelimbs modified for flying and the whale and sea mammals have modified them into flippers for swimming. 7. Adaptive radiation is very important in evolution. (a) It enables the animals to occupy a specific niche each, even though they are all living together. (b) It brings about divergent evolution when more mutations occur. One species ultimately diversifies to many related species. (c) It enables the organisms to be isolated physically, than reproductively. (d) It finally results in the formation of new species over time when chromosomes become non-homologous. Importance of Speciation in Evolution 1. Speciation is the formation of a new species from an existing species after allopatric or sympatric isolation. Speciation can also occur after hybridisation of two related species to form a sterile hybrid. Chromosome doubling occurs to form a new fertile species. 2. The importance of speciation in evolution is that evolution ultimately forms new species. Thus, speciation is the final process in the course of evolution. Speciation completes the course of evolution for a particular species. For example, from an ancestor finch species in Galapagos Islands, 13 species of finches were formed in the course of evolution by speciation. 3. Speciation is a non-stop process to improve adaptation of organisms for survival. It continues after a new species is formed. However, the environment determines the possibility of the formation of another new species. Speciation takes time, as long as millions of years. If the environment changes drastically, it favours speciation. 4. Speciation produces biodiversity i.e. more species are formed. This is to fill the niche generated if the environment is changed. If the environment is changed, the variation that exists among the members of population may not provide enough substrate for natural selection. In Galapagos Islands, a group of insectfeeding finches feed on parastic insects on large pelican chicks. These finches find the blood of injured chicks tasty and feed on the blood. Can this group evolve into another species of finch? This will need time and more “good” mutations to produce another Exam Tips Remember the meaning, causes and importance of adaptive radiation. Info Bio More evidence revealed that genetic drift played more important roles in evolution than natural selection. This evidence come from molecular and biochemical studies about human revolution. VIDEO Darwin's Discoveries
115 Biology Term 3 STPM Chapter 16 Selection and Speciation 16 species reproductively isolated from the existing insect-feeding finch into a blood-feeding species. 5. Speciation will result in convergent evolution involving adaptive radiation of species in similar but unconnected habitats. Examples of such adaptive radiation is fish in two different lakes in Africa. The ansectors of the two lakes are different, speciation have occured in the two lakes resulting in each lake having more or less the same number of species. Each species seems to have a similar one with the same (analogous) body shape, head structure, mouth part and fin shape with another partner species in the other lake. The fish in one lake are evolutionary related to each other in the same lake but completely unrelated to those with analogous features in the other lake. Quick Check 3 1. Explain the meanings of isolation, hybridisation and adaptive radiation. What are their importances in evolution? STPM PRACTICE 16 Objective Questions 1. The wings of insects and birds which have the same function but do not share the same evolutionary origin are said to be A analogous B homologous C homogenous D heterogenous 2. Which of the following does not contribute to genetic variation? A Hybridisation B Cloning C Chromosome aberrations D Independent assortment 3. Which of the following character is discontinuous variation? A Controlled by one gene B Skin colour in humans C Differences are not discrete D Influenced by environmental factors 4. Which of the following is not true of variation? A The gene pool of a population becomes gradually smaller B Variation enables the production of a new species C Natural selection can raise a species' evolutionary success D A species is more capable of colonising various habitat and niches 5. Which of the following can result in the formation of new species? I Isolation II Mutation III Natural selection IV Formation of a new gene pool A I, II and III B I, III and IV C II, III and IV D I, II, III and IV
116 Biology Term 3 STPM Chapter 16 Selection and Speciation 16 6. Which of the following is true of the distribution of seed weight as shown in the graph below? W Seed weight Numbers of seed I It exhibits a continuous distribution. II It is controlled by many genes. III Its highest frequency has weight of W. A I only B II only C I and II D I, II and III 7. Which is true of polymorphism? A It gives a special modification naturally with attractive characteristics. B It makes organisms better adapted to survive in a certain environment. C It exists in different phenotypic variants within the same species. D It is heritable and can be modified by natural selection. 8. Which of the following examples are post-zygotic isolating mechanisms of the formation of new species? I Two populations of bird live in two different islands. II Two different populations of goat mate and their offspring are sterile. III Two different populations of frog live in the same area but do not mate due to different structure of the reproductive organ. IV Two different populations of snake live in the same area but do not mate due to different structure of the reproductive organ. A I and II C II and III B I and IV D III and IV 9. A coastal area was the habitat of a population of wild mice. When a harbour was built in the area, the mice lost their habitat and moved to the store within the harbour area. After many generations, the mice had adapted to the environment in the store. Their characteristics changed and they became a new species of mice. What type of selection has occurred? A Artificial selection B Directional selection C Disruptive selection D Stabilising selection 10. Which of the following is the most important contributing factor for evolution? A Change in the structure of chromosomes in a somatic cell B Change in the number of chromosomes in a somatic cell C Gene mutation in a somatic cell D Point mutation in a sex cell 11. Which type of selection causes the necks of giraffes to grow longer through the course of evolution? A Artificial selection B Stabilising selection C Discruptive selection D Directional selection 12. A study on lizard found that small lizards have a hard time defending their territories, while large lizards are more likely to be preyed upon by owls. Therefore, the medium-sized lizards have chances to survive. What type of this selection is correct? A Stabilising C Natural B Directional D Disruptive 13. Which is the most important factor for the occurence of evolution? A Point mutation in a sex cell B Gene mutation ia a somatic cell
117 Biology Term 3 STPM Chapter 16 Selection and Speciation 16 C Change in the number of chromosomes in a somatic cell D Change in the structure of chromosomes in a somatic cell 14. The sperm count of human population of developed countries has dropped from 130 million per ml to 50 million per ml in recent years. This is an example of A sexual selection B stabilising selection C directional selection D disruptive selection 15. Crow and pigeon cannot interbreed because A gametic isolation B temporal isolation C ecological isolation D behavioural isolation 16. Which of the following produces the slowest evolution for an interbreeding population? Variation due to mutation Migration Selection pressure A low absent low B low high high C high absent high D high high low 17. Genetic variation can be achieved by A division of cell by mitosis B vegetative propagation of plants C inbreeding practices in animal husbandry D independent assortment of chromosomes during meiosis 18. Which is an example of adaptive radiation? A Beaks of Darwin’s finches in Galapagos Island? B Melanic peppered moths in industrialised area C Marsupial mammals in Australia D Pentadactyl limbs in mammals 19. Which of the following is a unit of evolution? A Species C Family B Population D Individual 20. Three types of natural selection graphs and three statements are given below. The arrows indicate the direction of the elimination of phenotypic characteristics. I Number Phenotype II Number Phenotype III Number Phenotype (a) One extreme phenotype is selected (b) Both extreme phenotypes are selected (c) Extreme phenotype characteristic is not selected. Which types of natural selection graphs correspond correctly to their respective statements of natural selection? I II III A (a) (c) (b) B (b) (a) (c) C (b) (c) (a) D (c) (a) (b)
118 Biology Term 3 STPM Chapter 16 Selection and Speciation 16 Structured Questions 1. The diagram below shows the three types of selection. Frequency Then Later Measurement of character Measurement of character Measurement of character Beginning A B C (a) Explain the three types of selection. [6] (b) Give an example each of the above selection. [3] (c) What is genetic drift? [1] 2. The diagram below shows a summary of Darwin-Wallace’s theory of natural selection. Variation is shown in all populations There is a struggle for existence Natural selection Individuals show great reproductive capacity Population numbers remain constant (a) What are the two main types of variation? [2] (b) Explain what are the causes of the heritable variations. [4] (c) Explain the importance of variation. [2] (d) Give two reasons for the struggle of existence. [2] Essay Questions 1. (a) Explain allopatric speciation? [8] (b) Describe postzygotic isolating mechanisms. [7] 2. (a) What is genetic drift? [3] (b) Explain the sympatric speciation. [4] (c) Give a description on prezygotic isolating mechanisms. [8]
119 Biology Term 3 STPM Chapter 16 Selection and Speciation 16 Quick Check 1 1. Continuous variation Discontinuous variation The variation is of degree The variation is of kind Quantitative characters are involved Qualitative characters are involved It is controlled by polygenes It is controlled by discrete genes It exhibits normal distribution It does not exhibit normal distribution Its analysis is by statistical means Its analysis is by simple counting 2. Alleles cause variation through mutation, usually from dominant to recessive or codominant in which a contrasting character is shown. Hybridisation causes variation, through the combination of characters from two species. Polygenes cause variation in quantitative characters. Quick Check 2 1. Intrasexual selection is the selection of the stronger individual, usually among males to mate with females. Intersexual selection is the selection by the opposite sex, usually the female on the most suitable partner. Sexual dimorphism is the difference in size, colour, morphology and behaviour between the two sexes with the males much bigger, with extra structure, more colourful and aggressive. 2. Polymorphism is the existence of two or more contrasting phenotypes in a population. It signifies a transitional stage during natural selection or a random development of chance of no evolutionary significance. 3. • It increases the mating of female stock. • It allows the easy access of females to a particular stock. • It leaves the upkeep of stock to the expert. • It cuts down the transmission of sexually transmitted diseases. • It prolongs the use of a particular stock long after its death. 4. • It creates ethical controversies that go against religious beliefs. • It creates legal controversies that questions the role of the father. • It creates misuses that lead to the controversies of eugenics. Quick Check 3 1. Isolation is the separation population either physically or reproductively. It cuts gene flow and in the long term results in speciation. Hybridisation is the crossing of two different species. It creates new species straight away after chromosome doubling of the hybrid. Adaptive radiation is the formation of new species based on the changes of homologous structures as a result of mutation. It results in divergent evolution that fill species in different niches. STPM Practice 16 Objective Questions 1. A 2. B 3. A 4. A 5. D 6. D 7. A 8. C 9. D 10. D 11. D 12. C 13. A 14. C 15. A 16. A 17. D 18. B 19. A 20. D Structured Questions 1. (a) Type A is stabilising selection where the moderate phenotypes are selected. There is no change in the environment and the individuals have adapted well to the environment as in most populations. Type B is directional selection where one extreme is selected for and the other is selected against. There is a change in the environment and the individuals are adapting to the change. Type C is disruptive selection where the moderate is selected against and the two extremes are selected for. There is a change in the environment that favours the extremes. (b) An example for type A is king crabs that never change over millions of years since they evolved. An example for type B is animals facing colder weather have longer hair over the generations. An example for type C is finches with moderate-sized beaks selected against as they face competition for seeds from both the big-beaked finches and small-beaked finches. (c) Genetic drift is a fast change of gene frequency in a small population due to chance when broken away from the main population. 2. (a) Genetic variation and environment variation. (b) Mutation, gene mutation provides new allele and chromosomal mutation provides new genetic raw materials. Crossing-over reshuffles the sequence of alleles in the linked genes along the various homologous chromosomes. Independent assortment of homologous chromosomes reshuffles the various alleles in different homologous chromosomes to give combinations equal to ANSWERS
120 Biology Term 3 STPM Chapter 16 Selection and Speciation 16 2n , where n is the haploid number. Sexual reproduction, selects two sets of alleles from two different individuals who can survive long to produce the gametes. (c) Variation gives resilience to the species. If there is a change in the environment, such as an outbreak of diseases, there are some individuals that are resistant to it and able to survive. Variation provides raw material for formation of new species. The accumulation of enough variation, consisting of new alleles, results in new genome. (d) There are far too many young offspring produced. For example, a female fish produces thousands of fries at any one spawning. There are too many young to be sustained by limited environmental resources such as food. Essay Questions 1. (a) • Allopatric isolation isthe formation of a new species among its members after prolong physical isolation. • An example is separation by mountain range though most mountain ranges may take million years to attain the final height but volcanic eruption can hasten the process separating plants, animals and even birds. • Fragmentations of grassland or forest due to river formation or fire may also separate plants or animals. • Sinking of land mass in the formation of island also isolate animals like marsupials in Australia. • The isolated groups are subjected to different natural selections. • Different mutations occur in the groups. • Accumulations of different alleles, physical, behavioural and physiological changes in the groups. • The final formation of two new species occurs when their chromosomes become non-homologous. (b) • Postzygotic mechanisms arewaysto separate isolated groups even if young are produced. • One mechanism is hybrid in viability or lethality i.e. too weak to survive for long. • An example is the mating of northern and southern frogs of America. • Anotheris hybrid sterility i.e. no gametes are produced by the hybrid. • The best example is horse and donkey producing mules which are sterile. • They are many other artificial crossings especial hybrid orchids. • Another mechanism is hybrid emasculation i.e. the amphidiploid is a new species. • A natural example is the central region Spartina grass of America, the hybrid cannot mate back to their two original ancestors. 2. (a) • Genetic drift is the random change in allelic frequencies in a population over time. • This normally occurs in a small population due to chance of survival and reproduction not natural selection. • It is also means ‘bottle effect ’in which a break away population has different allelic frequencies as founder population. (b) • Sympatric speciation is the formation of a new species in a population in which there’s no physical separation of individuals such as mountain range. • The original population is isolated by ecological, behavioural or reproductive means. • For example, the ancestors of eagles and owls might be isolated by being active during the day and night respectively. • After prolonged isolation, the two groups become two species due to their chromosomes are no longer homologous. • The two species formed cannot interbreed to produce fertile offspring due to the formation of hybrid that cannot have meiosis. (c) • Prezygotic isolating mechanisms prevent the formation of viable zygotes. • An example is ecological isolation. • in which the geographic distribution of two species overlap, but their ecological needs differ enough to prevent mating. • Another is temporal isolation. • in which two species whose distribution overlap but have different periods of sexual activity or breeding seasons. • Another is behavioural isolation. • inwhich two specieswith complex courtship rituals during which a female accepts a male before mating. • Another is mechanical isolation. • in which sperm and ova of the two species are chemically incompatible and will not fuse to form a zygote.
CHAPTER Bilingual Keywords INHERITANCE AND GENETIC CONTROL Concept Map 17 Phenotype: Fenotip Genotype: Genotip Allele: Alel Homozygote: Homozigot Heterozygote: Heterozigot Gamete: Gamet Dominant allele: Alel dominan Recessive allele: Alel resesif Selfing: Swakacuk Cross: Kacuk Wild type: Jenis liar Chromosome: kromosom Progeny: Progeni Mutation: Mutasi Monohybrid: Monohibrid Dihybrid: Dihibrid Bivalent: Bivalen Test Cross: Kacuk uji Trichotomy: Trikotomi Dichotomy: Dikotomi Condominant: Kodominan Allelomorph: Alelomorf Recombinant: Rekombinan Coupling phase: Fasa pergandingan Repulsion phase: Fasa penolakan Sexual selection: Pemilihan seks Equilibrium: Keseimbangan DNA replication: Replikasi DNA Conservative replication: Pengulangan konservatif Transcription: Transkripsi Inheritance and Genetic Control Types of Genetic Crosses and Breeding System Mutations Modifications to Mendelian and Non-Mendelian Inheritance Genetic Mapping DNA Replication Gene Expression Regulation of Gene Expression Population Genetics
122 Biology Term 3 STPM Chapter 17 Inheritance and Genetic Control 17 17.1 Types of Genetic Crosses and Breeding System Terminologies Phenotype 1. Phenotype is any measurable trait that can be inherited. It is the result of a gene brought into expression in an environment. A gene is part of the DNA molecule found in the chromosome which encodes a polypeptide through which an inheritable trait is expressed. 2. Phenotype determines individual structure, physiology and behaviour that include the followings: (a) characters that can be observed such as colour (b) characters that can be felt such as texture of the hair (c) characters that can be tested serologically such as blood group (d) quantitative characters that can be measured, including intelligence using an IQ test 3. Environmental factors affect the expression of a character. Examples are as follows: (a) The fur colour of Siamese cats or that of Himalayan rabbits. The enzyme involved is sensitive to temperature and is denatured at 37°C and above. Only at the tips of the body such as the snout, ears, paws and tail does the enzyme function to produce typical black spots. Under temperatures of more than 37°C, the animal becomes completely white as no melanin can be formed. (b) The flower colour of Hydrangea. The pigment produced in the petals is sensitive to pH like that of an acid-base titration indicator. When soil is acidic, the flowers are orange, whereas in alkaline condition, the flowers are blue. Genotype 1. The genotype of an organism is its gene constituent (however, only one or two genes are considered at one time). Genotype can be homozygous or heterozygous. 2. Homozygous genotype is a genotype of an individual which both alleles (alternative forms of a given gene) are identical such as AA, aa, AABB, AAbb, aaBB or aabb. A homozygote may be defined as a diploid individual with two identical alleles at a given locus. (a) Homozygotes are also known as pure breeds or pure lines as each of them only produces a single type of gametes, as shown in Table 17.1. Language Check Language Check Gamete = a haploid cell that is produced by meiosis in higher organisms and fuses with that of opposite sex to form a zygote. Table 17.1 Homozygous genotypes and their gametes Genotypes Gametes AA A aa a AABB AB AAbb Ab aaBB aB aabb ab Learning Outcomes Students should be able to: (a) explain the Mendelian inheritance pertaining to the phenotypic and genotypic ratios; (b) describe the types of crosses (test cross, backcross, reciprocal cross and selfing) and explain their importances; (c) describe pure breeding, outbreeding, inbreeding, selective breeding, and explain their importances.
123 Biology Term 3 STPM Chapter 17 Inheritance and Genetic Control 17 (b) Two identical homozygotes, when crossed or selfed, breeds pure or only produces one type of offspring. (c) Homozygote is formed when two fusing gametes of identical allele form a zygote. (d) Homozygotes can be artificially produced in organisms (pure breeding), depending on whether self-fertilisation is possible. (e) In plants, pure breeding can be carried out by selfing or self-pollination. The procedure to produce homozygote is shortened by selfing a plant with a chosen character. If the progeny produced are all of the chosen type, then the chosen plant and its progeny are pure-breeds. If not, another plant of the same phenotype or any one of the next generation of that chosen type is selfed until one type of progeny is obtained. (f) In animals, it is done by crossing the chosen animal with the character to its closest relative (inbreeding). The offspring with the chosen character is further mated among its siblings until all offsprings are pure-breed. This may take many generations. 3. Heterozygous genotype is a genotype of an individual with one or more pairs of different alleles, such as Aa, AABb, aaBb, AaBB, Aabb or AaBb. A heterozygote may be defined as a diploid individual with two different alleles at a given locus. (a) Heterozygotes are also known as hybrids or carriers of recessive alleles. They produce more than one type of gametes, as shown in Table 17.2. (b) Heterozygotes are formed when fusing gametes contain different alleles. Most human beings are heterozygous for many genes because we seldom inbreed. Allelic relationship 1. Dominant allele (a) A dominant allele is an allele that expresses itself in the phenotype while present in a homozygous or heterozygous state. It is the allele that cannot be masked by another allele. Therefore, a dominant allele dictates the appearance of a heterozygote, even in the presence of an alternative allele. (b) Usually, a dominant allele controls the production of a protein, such as an enzyme that catalyses a reaction to produce a certain phenotype. So, the phenotype is the same if despite being a homozygote or a heterozygote. The dominant allele does not suppress the effect of the recessive allele but masks it in such a way that the effect is not expressed or is observable in a heterozygote. (c) An example of dominant allele is the ability to form melanin, a black pigment in the skin or hair. Allele A can form melanin as it controls the formation of an enzyme that brings Language Check Language Check Selfing = a cross with oneself by plants and certain animal hermaphrodites Table 17.2 Heterozygous genotypes and their gametes Genotypes Gametes Aa 1 2 A : 1 2 a AaBB 1 2 AB : 1 2 aB Aabb 1 2 Ab : 1 2 ab AaBb 1 4 AB : 1 4 Ab : 1 4 aB : 1 4 ab VIDEO Allele
124 Biology Term 3 STPM Chapter 17 Inheritance and Genetic Control 17 about the conversion of a precursor to melanin. This allele A expresses itself in a homozygous state i.e. two genes function at the same time to produce the enzyme or in heterozygous state where one gene functions to produce the same amount of enzyme. (d) The wild-type phenotype is usually dominant. This is also the normal or common type. (e) Dominant allele can be obtained from the recessive allele through mutation, though it seldom occurs. For example, the dominant allele of Huntington disease is mutated from a normal recessive allele. This allele produces a harmful protein that causes death. 2. Recessive allele (a) A recessive allele is an allele that expresses itself only in the homozygous state. (b) The gene is usually not functional. It determines the production of a protein that is not functional or it produces no such protein. (c) Example of a recessive allele is the albino allele, a that results in albinism when in homozygous state. (d) Recessive alleles are usually formed from dominant alleles through mutation. Test cross 1. It is a cross that is used to test if a dominant phenotype is homozygous or heterozygous. 2. The individual is crossed with a recessive phenotype, as shown below: F1: Yy ?y Phenotype: yellow yellow / green? Ratio: : ? P1: Y? Phenotype: Gametes: yy green – yellow Y ? 1 2 – y – 1 2 If the offsprings produced are all yellow, this means that the yellow parent is homozygous (YY). If the offspring produced is 1 2 yellow : 1 2 green, this means that the yellow parent is heterozygous (Yy). 3. The test cross is important in selecteive breeding especially in qualitative characters. Dominant homozygote will breed pure. Thus, test crosses help to fix and maintain a character in an offspring. Language Check Language Check Wild-type = phenotype of the typical form of a species as it occurs in nature.
125 Biology Term 3 STPM Chapter 17 Inheritance and Genetic Control 17 Back cross 1. It is a cross carried out between an offspring with either of the parents, as shown below: P1 : YY × yy Phenotype: yellow green F1 : Yy 2. Two back crosses can be carried out; one is F1 with the yellow parent and the other is F1 with the green parent. 3. The purpose of the cross is used in inbreeding to obtain homozygosity in the alleles. 4. It can be used as a test cross when there is a difficulty of getting a homozygous recessive individual. Mendel’s Laws of Inheritance Mendel's first law of inheritance (the law of segregation) 1. Mendel’s law of segregation states that from one parent, only one factor (allele) is passed from the parent to the offspring through the gamete. 2. This law can be explained by meiosis. 3. In the garden pea that is diploid, a heterozygous yellow seed (Yy) can only transmit one of the alleles to each of its offspring. (Y is a dominant allele for yellow seed coat whereas y is a recessive allele for green seed coat). Parent (P1): Yy (Yellow) Gametes: Y y 4. Each gamete can only obtain one allele from the parent because meiosis reduces a diploid gamete parent cell to haploid gamete. 5. Mendel used garden pea plants for his experiments. One of the characters was seed colour. He started by crossing two pure breeding strains; one with yellow seeds and the other with green seeds. He then allowed the offspring (F1 generation or first filial generation) to self-fertilise and the results were always the same. 6. The F2 generation (second filial generation obtained by random crossing or selfing of the F1 generation) had a ratio of 3 4 of one character and 1 4 of the contrasting character, the classical Mendelian ratio of 3 : 1. Summary Mendel’s first law:: 1. = law of segregation. 2. One allele is passed from one parent to offspring. 3. Due to meiosis. 4. Heterozygote cross with hetetozygote produces 3 : 1 phenotypic ratio.
126 Biology Term 3 STPM Chapter 17 Inheritance and Genetic Control 17 YY yellow P1: Phenotype: F1: Phenotype: Gametes: Yy yellow x yy green Y y Yy yellow Y y Yy yellow Y y YY Yy Yy yy Gamet: Genotypic ratio = 1/4 YY : 2/4 Yy : 1/4 yy Phenotypic ratio = 3/4 yellow : 1/4 green P2 (selfing): Phenotype: yellow yellow yellow green F2: Phenotype: 7. He repeated his experiments using several contrasting characteristics, which included tall and dwarf plants; round and wrinkled seeds; inflated and constricted pods; and red and white flowers. 8. He concluded that each plant carried two factors, though only one factor was exhibited in F1 , the dominant phenotype. When selfed, the F1 segregated the factors and produced the characteristic ratio, 3 dominant : 1 recessive phenotype. Monohybrid crosses and their progeny ratios There are six different crosses. 1. YY yellow P1: Phenotype: F1: Phenotype: Genotypic ratio = all YY Phenotypic ratio = all yellow Gametes: YY yellow YY yellow Y Y 2. F1: Phenotype: yellow yellow Genotypic ratio = 1/2 YY : 1/2 Yy Phenotypic ratio = all yellow YY yellow P1: Phenotype: Gametes: YY Yy Yy yellow Y Y y
127 Biology Term 3 STPM Chapter 17 Inheritance and Genetic Control 17 Summary 3. YY yellow P1: Phenotype: F1: Phenotype: Genotypic ratio = all Yy Phenotypic ratio = all yellow Gametes: yy green Y y Yy yellow 4. F1: Phenotype: yellow yellow yellow green Genotypic ratio =1/4 YY : 2/4 Yy : 1/4 yy Phenotypic ratio = 3/4 yellow : 1/4 green Yy yellow P1: Phenotype: Gametes: YY Yy Y y Yy yellow Yy yy Y y 5. Phenotype: Genotypic ratio = 1/2 Yy : 1/2 yy Phenotypic ratio = 1/2 yellow : 1/2 green Yy yellow P1: Phenotype: Gametes: Yy yellow yy green yy green Y y y 6. F1: yy Phenotype: green Genotypic ratio = all yy Phenotypic ratio = all green yy green P1: Phenotype: Gametes: yy green y y Types Parents F1 genotypic ratios F1 phenotypic ratios 1. YY × YY All YY All yellow 2. YY × Yy 1 2 YY : 1 2 Yy All yellow 3. YY × yy All Yy All yellow 4. Yy × Yy 1 4 YY : 1 2 Yy : 1 2 yy 3 4 yellow : 1 4 green 5. Yy × yy 1 2 Yy : 1 2 yy 1 2 yellow : 1 2 green 6. yy × yy All yy All green Mendel's second law (the law of independent assortment) Dihybrid crosses 1. Mendel's second law states that the assortment of one pair of alleles is independent of that of another pair. 2. This is only true if the two pairs of alleles are located at different chromosomes. 3. The law can be explained using meiosis of a dihybrid genotype of YyRr (where R is a dominant allele for round seed and r is a resessive allele for wrinkled seed). There are two types of gametes formed, as shown below:
128 Biology Term 3 STPM Chapter 17 Inheritance and Genetic Control 17 Y y R Y R Y R Y R r y r y r y r Meiosis I Meiosis II 4. The gametes are YR and yr but the orientation of the bivalents may be different, as shown below: Y y r Y r Y r Y r R y R y R y R Meiosis I Meiosis II 5. Because the two orientations are of equal chance (50%), the gametes produced are 1 4 YR : 1 4 Yr : 1 4 yR : 1 4 yr. 6. Therefore, when two pairs of alleles assort, they assort independently of the other i.e. Y can move upwards with R or r whereas y can move down with R or r to the other pole. Genotypes and their gametes There are nine different genotypes when we consider two independently assorting genes, as shown below: Genotypes Ratios of gametes YYRR All YR YYRr 1 2 YR : 1 2 Yr YyRR 1 2 YR : 1 2 yR YyRr 1 4 YR : 1 4 Yr : 1 4 yR : 1 4 yr YYrr All Yr Yyrr 1 2 Yr : 1 2 yr yyRR All yR yyRr 1 2 yR: 1 2 yr yyrr All yr Summary Mendel’s second law: 1. = law of independent assortment. 2. one pair of allele is passed on to offspring independent of another pair. 3. Due to random orientation of bivalents in metaphase I of meiosis. 4. Dihybrid cross with dhybrid produces 9 : 3 : 3 : 1 phenotypic ratio.
129 Biology Term 3 STPM Chapter 17 Inheritance and Genetic Control 17 Examples of dihybrid crosses 1. There are two ways of getting dihybrids. (a) YYRR yellow round P1: Phenotypes: Gametes: F1: YyRr yyrr green wrinkled YR yr (b) YYrr yellow wrinkled P1: Phenotypes: Gametes: YyRr F1: yyRR green round Yr yR 2. When the dihybrid crosses with one another or selfs, the following is obtained: YyRr yellow round P2 (F1 x F1) : Phenotypes: Gametes: YyRr yellow round YR Yr yR yr YR Yr yR yr F2 (Punnet square): P2 P1 YR Yr yR yr YR YYRR (yellow round) YYRr (yellow round) YyRR (yellow round) YyRr (yellow round) Yr YYRr (yellow round) YYrr (yellow wrinkled) YyRr (yellow round) Yyrr (yellow wrinkled) yR YyRR (yellow round) YyRr (yellow round) yyRR (green round) yyRr (green round) yr YyRr (yellow round) Yyrr (yellow wrinkled) yyRr (green round) yyrr (green wrinkled) F2 (summary): Genotypes Ratios Phenotypes Ratios YYRR YYRr YyRR YyRr YYrr Yyrr yyRR yyRr yyrr 1 2 2 4 1 2 1 2 1 yellow round (Y_R_) yellow wrinkled (Y_rr) green round (yyR_) green wrinkled (yyrr) 9 16 3 16 3 16 1 16 A classic Mendelian ratio of 9 : 3 : 3 : 1 is obtained. 14243 123 123
130 Biology Term 3 STPM Chapter 17 Inheritance and Genetic Control 17 3. Another way of only getting the F2 genotypes is to use genotypic trichotomy method. Consider only the Y/y gene, Yy × Yy will give gametes = 1 4 YY: 2 4 Yy : 1 4 yy Consider only R/r gene, Rr × Rr will give gametes = 1 4 RR : 2 4 Rr : 1 4 rr To get the F2 genotypes = YyRr × YyRr 1 4 YY 1 4 RR 1 16 YYRR 1 2 Rr 2 16 YYRr 1 4 rr 1 16 YYrr 2 4 Yy 1 4 RR 2 16 YyRR 1 2 Rr 4 16 YyRr 1 4 rr 2 16 YYrr 1 4 yy 1 4 RR 1 16 yyRR 1 2 Rr 2 16 yyRr 1 4 rr 1 16 yyrr 4. To only get the F2 phenotypes, a phenotypic dichotomy is used. Consider only the Y/y gene, Yy × Yy will give = 3 4 yellow : 1 4 green Consider only R/r gene, Rr × Rr will give = 3 4 round : 1 4 wrinkled So, when the YyRr yellow round crosses with YyRr yellow round, we will get 9 16 yellow round : 3 16 yellow wrinkled: 3 16 green round: 1 16 green wrinkled as shown below. 3 4 YY yellow 3 4 round 9 16 yellow round 1 4 wrinkled 3 16 yellow wrinkled 1 4 yy green 3 4 round 3 16 green round 1 4 wrinkled 1 16 green wrinkled Exam Tips It is better to memorise the six monohybrid and dihybrid crosses as it would save time when doing objective questions. Dihybrid crosses can be separated and treated as monohybrid crosses if the offspring ratios are given and the parental genotypes or phenotypes need to be determined. Language Check gg• Trichotomy = a splitting into three parts. • Dichotomy = a splitting into two parts. Quick Check 1 1. Distinguish the following pairs of terminologies: (a) phenotype and genotype (b) gene and allele (c) dominant and recessive alleles
131 Biology Term 3 STPM Chapter 17 Inheritance and Genetic Control 17 17.2 Modifications to Mendelian and Non-Mendelian Inheritance Terminologies Incomplete dominant allele 1. Incomplete dominant allele determines a phenotype of a heterozygote in which it is intermediate between the two homozygotes. 2. It usually determines colour intensity or degree of a quantitative character. It expresses itself in both the homozygous or heterozygous states. 3. For example, homozygous dominant produces a darker colour whereas homozygous recessive is white. Heterozygous state is lighter than the homozygous dominant or of intermediate colour. 4. This is because homozygous dominant has two functional alleles that produce more proteins or enzymes that in turn produce more colour. Heterozygote has only one functional allele that produces enzyme and therefore produces a lesser colour substance. 5. Unlike a normal dominant allele, an incomplete dominant allele produces a fixed amount of enzyme that in turn produces a fixed amount of pigment. A normal dominant allele produces the same amount of pigment from one or two of the alleles regulating the production of the amount of pigment. 6. An example is the flower colour of Snapdragon (Antirrhinum). Rr pink Rr pink R r R r Rr pink fi fi R r RR Rr Rr rr Genotypic ratio : Phenotypic ratio : Selfing(F1 fi F1) : rr white RR red P1: red pink pink white 1 – RR 4 2 – Rr 4 1 – rr 4 : : 1 – red 4 2 – pink 4 1 – white 4 : : F2: F1: G2: G1: Figure 17.1 Inheritance of incomplete dominance Allele R that is incomplete dominant produces red pigment whereas its recessive allele r produces white or no pigment. However, heterozygous Rr produces pink pigment, RR produces red pigment and rr produces white or no pigment. Therefore, incomplete dominant allele produces three types of phenotypes, unlike dominant alleles that produce only two types. Learning Outcomes Students should be able to: (a) explain • incomplete dominance (flower colour in snapdragon), • codominance (MN blood group in humans), • multiple alleles (ABO blood group in humans), and calculate the genotypic and phenotypic ratios; (b) explain lethal genes (sickle-cell in human colour in mice/ chlorophyll production in maize), polygenes (height in human), linked and sex-linked genes (Drosophila eye colour and haemophilia in human), and epistasis (coat colour in dog and capsule shape in shepherd's-purse plant); (c) explain the pedigree analysis. Selfing occurs in flowering plants, rarely in termaphroditic animals. The pollens from one flower in a plant are transferred to the stigma of the same flower or different flower but on the same plant. Selfing occurs naturally in many flowering plants. It can be carried out artificially during breeding. Info Bio
132 Biology Term 3 STPM Chapter 17 Inheritance and Genetic Control 17 Codominant allele 1. Codominant alleles do not exhibit recessiveness, both alleles involved are equally dominant and both alleles express themselves in the heterozygous condition. 2. Like incomplete dominant allele, three types of phenotypes are produced. However, the heterozygous phenotype has a phenotype of both homozygotes. 3. An example is human MN blood group controlled by LM and LN alleles, as shown in Table 17.3 and Figure 17.2. Table 17.3 Genotypes and phenotypes of MN blood groups Genotype Reaction with Phenotype, blood group Anti – M Anti – N LMLM + – M LNLN – + N LMLN + + MN LMLN group MN LMLN group MN LN LM LN LM LMLN group MN fi fi LN LM LN LN LMLN LMLN LMLM Genotypic ratio : Phenotypic ratio : F1 fi MN : LN LN group N LMLM group M P1: 1 – LMLM 4 2 – LMLN 4 1 – LN LN 4 : : 1 – group M 4 2 – group MN 4 1 – group N 4 : : F2: F1: G2: G1: Figure 17.2 Inheritance of MN blood group A marriage between a blood group M woman with a group N man produced one daughter with a group MN who married a boy with a group MN. 4. Another example is the fur colour of short horn cattle controlled by CR for red and CW for white. Therefore, CRCR produces red fur, CWCW produces white fur and CRCW produces roan. On close examination of the roan fur, there are two types of hair i.e. a mixture of red and white. 5. In radish, the shape and colour may also be considered as codominant, though they may also be considered as incomplete dominant alleles. Thus, SRSR produces round, SRSL produces oval and SL SL produces long radishes. CRCR produces red, CRCW produces purple and CWCW produces white radishes.
133 Biology Term 3 STPM Chapter 17 Inheritance and Genetic Control 17 Multiple alleles 1. Multiple alleles are alleles that have more than the two usual alternative forms of dominant and recessive alleles found in a particular locus. 2. The different alternatives form an allelomorph series. 3. The alleles may be one dominant over another or they are all codominant. 4. Multiple alleles produce more phenotypes as the number of genotype also increases. 5. However, diploid organisms can only have two of the alternatives as the chromosomes exist in homologous pairs i.e. there are two alleles with one allele from the mother and another allele from the father, for a particular locus. 6. An example is the ABO blood group with three alleles IA, IB or I O/i, as shown in Table 17.4. Table 17.4 The alleles, genotypes and phenotypes of ABO blood group Genotype Reaction with Phenotype, blood group Anti – M Anti – N IAIA, IAIO + – A IBIB, IBIO – + B IAIB + + AB IOIO – – O I A controls the synthesis of antigen A in the membrane of red blood cell. IB controls the synthesis of antigen B in the membrane of red blood cell. I O/i does not control the synthesis of antigens, A or B in the membrane of red blood cell. 7. Another example of multiple alleles is fur colour of rabbits. The fur colour is controlled by four alleles, as shown in Table 17.5. Table 17.5 The genotypes and phenotypes of fur colour of rabbits and mice Genotypes Phenotypes of rabbits Phenotypes of mice CC, Ccch, Cch , Cc Bright grey Brown (agouti) cchcch “Chinchila” “Chinchila” cch ch , cchc Light grey “Chinchila” ch ch , ch c “Himalaya” “Himalaya” cc Albino Albino Language Check Language Check Allelomorph = one of number of alternative forms of the same gene occupying a given position on a chromosome
134 Biology Term 3 STPM Chapter 17 Inheritance and Genetic Control 17 C – full colour of the wild type, which is grey. cch – “chinchila” colour, silver-tipped grey. ch – “Himalaya” colour, white with patches of black in the snout, ears, paws and the tail. c – albino, white with no pigmentation. 8. In mice, the fur colour is slightly different from that of the rabbits. “Chinchila” is produced in both homozygous and heterozygous states. Lethal alleles 1. Lethal alleles are alleles that express themselves and result in death of individuals before birth, at birth or after birth. 2. This is the result of an essential protein (e.g. cytochrome oxidase) which is not produced or the production of an abnormal protein that affects an essential process (e.g. Huntington's disease). 3. In the history of human genetics, certain alleles like those of sickle-celled anaemia and haemophilia that were once lethal are not lethal anymore because of medical advancement. 4. The alleles can be dominant, incomplete dominant, codominant or recessive. 5. Truly dominant lethal alleles are common but may not be easily identified. This is because a single dominant allele formed by mutation may result in death of the gametes, zygote, embryo or foetus, baby at birth and after birth. This is a result of an abnormal protein produced. 6. Huntington's disease in human is controlled by a dominant allele, and the effect may appear after 40 years. The abnormal protein is called ‘Huntingtin’. This protein affects the brain and finally causes death. 7. Incomplete dominant lethal alleles have more examples. These alleles bring death only when they are in homozygous states and not in heterozygous states. This is the result of the production of an abnormal protein. 8. A common example is Y allele in mice that controls the yellow coat colour and the recessive allele that controls the wild type agouti (dark brown) coat. In terms of lethal effects, the allele exhibits incomplete dominance whereas it is complete dominance for the yellow coat colour. A feature of lethal allele is its deviation from the normal Mendelian ratio. A class of progeny is conspicuously absent as in the cross between two yellow mice. A ratio of 2 3 yellow(Yy) to 1 3 agouti(yy) is obtained instead of the expected 1 : 2 : 1 ratio because YY dies.
135 Biology Term 3 STPM Chapter 17 Inheritance and Genetic Control 17 Yy yellow Y y Yy yellow fi Y y yy Agouti Yy Yellow Yy Yellow YY Yellow (die) Genotypic ratio : Phenotypic ratio : P1: 2 – Yy 3 1 – yy 3 : 2 – yellow 3 1 – agouti 3 : F1: G1: Figure 17.3 Inheritance of incomplete lethal allele in mice 9. Another lethal allele can be considered codominant lethal such as the human brachydactyly gene. The SB allele affects the skeleton of an individual whereas the normal allele is SN. SBSB individual dies at birth or earlier as there are massive abnormalities in the skeleton. A heterozygous individual of SBSN is brachydactyly with a few abnormalities, including shorter fingers with only two joints. 10. Recessive lethal is most common but is difficult to identify. Individuals are unaware and may be a carrier of a few such lethal recessive alleles. An example is the CC and Cc genotypes which can survive, but cc genotype dies due to the lack of cytochrome oxidase. Modified Dihybrid Ratios The classic Mendelian ratio of 9 : 3 : 3 : 1 is only obtained if the two genes involved have dominant-recessive allelic relationship and located on non-homologous chromosomes. If the allelic relationship of the loci involved is not dominant-recessive, the ratio will be modified as below: 1. 3 : 6 : 3 : 1 : 2 : 1 If the first gene has dominant-recessive relationship but the second gene has codominant or incomplete dominant relationship i.e. first locus: T – dominant allele for tall and t – allele recessive for dwarf; second locus: R – allele for red r – allele for white but Rr is for pink. P2 : Tt Rr × Tt Rr F2 : TTRR – 1 tall, red TTRr – 2 tall, pink 3 TtRR – 2 tall, red 6 TtRr – 4 tall, pink TTrr – 1 tall, white 3 Ttrr – 2 tall, white ttRR – 1 dwarf, red – 1 ttRr – 2 dwarf, pink – 2 ttrr – 1 dwarf, white – 1 Exam Tips Remember the meanings of phenotype, genotype, gene, allele including dominant, recessive, incomplete dominant, codominant, multiple and lethal alleles. Remember the genetics of ABO blood group.
136 Biology Term 3 STPM Chapter 17 Inheritance and Genetic Control 17 2. 1 : 2 : 2 : 4 : 1 : 2 : 1 : 2 : 1 The first and second genes have codominant or incomplete dominant relationship such as in radish. The first locus is the shape i.e. SL – long, SR – round but SL SR is oval and second locus is colour i.e. CR – red, CW – white but CRCW is purple. P2 : SL SRCRCW × SL SRCRCW F2 : SL SL CRCR long, red – 1 SL SL CRCW long, purple – 2 SL SRCRCR oval, red – 2 SL SRCRCW oval, purple – 4 SL SL CWCW long, white – 1 SL SRCWCW oval, white – 2 SRSRCRCR round, red – 1 SRSRCRCW round, purple – 2 SRSRCWCW round, white – 1 3. 3 : 6 : 1 : 2 The first gene has dominant-recessive relationship; the second gene has codominant or incomplete dominant lethal relationship. The first locus has T – tall, t – dwarf and second locus CG – green and CY – yellow but CGCY is variegated and CYCY dies because of the lack of chlorophyll. P2 : Tt CGCY × Tt CGCY F2 : TTCGCG – 1 tall, green TTCGCY – 2 tall, variegated 3 TtCGCG – 2 tall, green 6 TtCGCY – 4 tall, variegated TTCYCY – 1 tall, yellow (died) TtCYCY – 2 tall, yellow (died) ttCGCG – 1 dwarf, green – 1 ttCGCY – 2 dwarf, variegated – 2 ttCYCY – 1 dwarf, yellow (died) 4. 2 : 4 : 1 : 2 : 2 : 1 The first gene has codominant MN blood group alleles whereas the second gene has incomplete dominant lethal yellow alleles as found in mice. P2 : LM LNYy × LMLNYy F2 : LMLMYY – 1 died LMLMYy – 2 MN yellow – 2 LMLNYY – 2 died LMLNYy – 4 MN yellow – 4 LMLMyy – 1 M agouti – 1 LMLNyy – 2 MN agouti – 2 LNLNYY – 1 died LNLyy – 2 N agouti – 2 LNLNyy – 1 N agouti – 1 5. 4 : 2 : 2 : 1 The first gene is codominant lethal allele such as brachydactyly allele whereas the second gene is that of incomplete dominant lethal yellow allele such as that of mice.
137 Biology Term 3 STPM Chapter 17 Inheritance and Genetic Control 17 P2 : SB SNYy × SBSNYy F2 : SBSBYY – 1 died SBSBYy – 2 died SBSNYY – 2 died SBSNYy – 4 brachydactyl, yellow – 4 SBSByy – 1 died SBSNyy – 2 brachydactyl, yellow – 2 SNSNYY – 1 died SNSNYy – 2 yellow – 2 SNSNyy – 1 agouti – 1 Gene Interactions Gene interactions are divided into two types: epistasis and non-epistasis. Epistasis 1. Epistasis is an interaction in which an allele of one gene locus suppresses the expression of an allele of another gene locus or two gene loci are involved in the expression of one trait, which is usually that of a pigment colour. 2. In epistasis, it can be sub-divided into a suppressive (inhibitory) type and duplicate gene type. 3. Epistasis still follows the Mendelian inheritance but the ratios are modified. 4. There are six types of epistasis as follows: (a) Dominant epistasis (12 : 3 : 1) A dominant allele at one gene locus suppresses the expression of another allele at the second locus. This can be explained by a dominant allele that codes for an enzyme that produces an inhibitor. The inhibitor inhibits the enzyme coded by the second locus. An example is found in dogs, in which a dominant I allele produces an inhibitor whereas a recessive i allele does not. In another locus, a dominant B allele produces a black coat and a recessive allele produces a brown coat. A F1 dihybrid cross produces a 12 : 3 : 1 phenotypic ratio in F2 . P2 : Ii Bb × Ii Bb F2 : 9 16 I _B_ white – 12 3 16 I _bb 3 16 ii B_ black – 3 1 16 ii bb brown – 1 123 A test cross of IiBb × iibb produces a ratio of 2 4 white : 1 4 black : 1 4 brown. 2014
138 Biology Term 3 STPM Chapter 17 Inheritance and Genetic Control 17 (b) Recessive epistasis (9 : 3 : 4) A homozygous recessive genotype at one gene locus suppresses the production of colour in another. Homozygous recessive alleles at one gene locus cannot produce a substrate for the production of a pigment by the enzymes coded by the second gene locus. An example is the albino gene of mice in which a dominant A allele produces normal pigment whereas homozygous recessive albino alleles aa produce no pigment. So, it affects another locus, in which a dominant B allele produces brown coat and recessive b produces black coat. P2 : Aa Bb × Aa Bb F2 : 9 A _B_ brown – 9 3 A _bb black – 3 3 aa B_ 1 aa bb 123 albino – 4 A test cross of AaBb × aabb produces 1 4 brown : 1 4 black : 2 4 albino. (c) Dominant-recessive interaction (13 : 3) A dominant inhibitor allele at one gene locus and homozygous recessive albino alleles at another gene locus produce no pigment or white feather, as seen in chickens. The white chickens have both the dominant inhibitor genes and/or albino homozygous alleles and only when both loci produce no inhibitor (iiA_) are the chickens coloured. P2 : Ii Aa × Ii Aa F2 : 9 I _A_ white 3 I _aa white 13 3 ii A_ coloured –3 1 ii aa white A test cross of IiAa × iiaa produces 3 4 white : 1 4 coloured. (d) Duplicate dominant genes (15 : 1) In certain flowers, there are two identical genes located on different chromosomes that control the same flower colour red. Let gene R1 /R2 for red flowers and r1 /r2 for white flowers. P2 : R1 r1 R2 r 2 × R1 r 1 R2 r2 F2 : 9 R1 _R2 _ red 3 R1 _r2 r2 red 15 3 r 1 r 1 R2 _ red 1 r 1 r 1 r 2 r 2 white – 1 123 A test cross of R1 r1 R2 r2 × r1 r1 r2 r2 produces 3 4 red : 1 4 white flowers.
139 Biology Term 3 STPM Chapter 17 Inheritance and Genetic Control 17 (e) Duplicate recessive gene (9 : 7) Homozygous recessive alleles at either one of the gene loci produce the same phenotypes, as in sweet pea. The recessive alleles can be assumed to be the albino gene, i.e. duplicate albino gene. Let P1 /P2 for purple and p1 /p2 for white. P2 : P1 p1 P2 p2 × P1 p1 P2 p2 F2 : 9 P1 _P2 _ purple – 9 3 P1 _p2 p2 white 3 p1 p1 P2 _ white 7 1 p1 p1 p2 p2 white 123 A test cross of P1 p1 P2 p2 × p1 p1 p2 p2 produces 1 4 purple : 3 4 white. (f) Duplicate dominant genes with cumulative effect (9 : 6 : 1) A dominant allele at either gene locus produces the same phenotype but at both loci, produces a cumulative darker colour. This occurs in wheat kernel colour, let B1 /B2 be the dominant brown allele and b1 /b2 be the recessive white allele. A B1 _B2 _ genotype produces a darker red coloured kernel whereas a b1 b1 b2 b2 produces white kernel. P2 : B1 b1 B2 b2 × B1 b1 B2 b2 F2 : 9 – B1 _B2 _ red – 9 3 – B1 _b2 b2 brown 3 – b1 b1 B2 _ brown 6 1 – b1 b1 b2 b2 white 1 123 A test cross of B1 b1 B2 b2 × b1 b1 b2 b2 produces 1 4 red : 2 4 brown : 1 4 white. 5. Therefore, a test cross of epistasis dihybrid (AaBb) with double homozygous recessive (aabb) does not produce 1 : 1 : 1 : 1 ratio. Those of 9 : 7, 13 : 3, 15 : 1 produce 3 : 1 ratios while those of 12 : 3 : 1, 9 : 3 : 4 and 9 : 6 : 1 produce 1 : 2 : 1 ratios. 6. As a summary, the six types of epistasis can be easily recognised, with the example AaBb × AaBb, as shown in Table 17.6. Exam Tips If the phenotypic ratio is a combination of 9 : 3 : 3 : 1, it is epistasis interaction involving two genes. Remember the six combinations and understand the reasons behind them. You need not remember the name of each type but do remember the ratios. Table 17.6 Summary of the six types of epistasis interaction Genotype Usual Type 1 Type 2 Type 3 Type 4 Type 5 Type 6 A_B_ 9 12 9 13 15 9 9 A_bb 3 3 7 6 aaB_ 3 3 4 3 aabb 1 1 1 1 AaBb × aabb 1 : 1 : 1 : 1 2 : 1 : 1 1 : 1 : 2 3 : 1 3 : 1 1 : 3 1 : 2 : 1 123 123 123123
140 Biology Term 3 STPM Chapter 17 Inheritance and Genetic Control 17 Non-epistasis interaction 1. It is an interaction that occurs at the substrate level i.e. the products of the enzyme actions interact to control a trait. 2. If the two genes involved are located at different chromosomes, it follows the Mendelian laws and a classical 9 : 3 : 3 : 1 dihybrid ratio is obtained. 3. An example is the flower colour of pea plants. 4. Let R be the dominant allele for red and r be the recessive allele for white and for the other locus, let B be the dominant allele for blue and b be recessive allele for white. 5. As a result of interaction, R_B_ genotypes produce purple coloured pigment formed from the mixing of the red and blue pigments. 6. Therefore, an F1 dihybrid cross produces the following phenotypes and ratio: P2 : RrBb × RrBb F2 : 9 R _B_ purple 3 R_bb red 3 rrB_ blue 1 rrbb white 7. Another example is the comb shape in chickens (figure 17.4). There are two genes involved in the expression of the comb shapes. Let R be the dominant allele for rose comb and r be the recessive allele for single comb and for the other locus, let P be the dominant allele for pea-shaped comb and p be the recessive allele for single comb. As a result of interaction, R_P_ genotypes produce a walnut comb. P2 : RrPp × RrPpBb F2 : 9 R _P_ walnut 3 R_pp rose 3 rrP_ pea 1 rrpp single Polygenes 1. Polygenes are genes that individually contribute little to the phenotype but together in groups control a quantitative trait. 2. Polygenes usually exhibit a dominant-recessive relationship in which only the dominant alleles contribute to the phenotype whereas the recessive alleles do not. 3. They are duplicate genes. Each dominant allele contributes equally to the overall phenotype. Walnut Rose Pea Single Figure 17.4 The different shapes of chicken combs
141 Biology Term 3 STPM Chapter 17 Inheritance and Genetic Control 17 4. If the genes are located on different chromosomes, they follow the Mendelian law of inheritance. 5. Some of the genes may be linked and others may be sex-linked. 6. A normal quantitative character may be controlled by ten or more of such genes. 7. Such quantitative characters include colour intensity, height, length, weight, volume and character that can be measured such as intelligence. 8. Quantitative characters can be easily influenced by environmental factors, such as food and soil fertility. 9. Variations of the characters are continuous and exhibit normal distribution as shown below. Frequency (f) Measurement 10. Data analysis is by statistical means using the mean and standard deviation values. 11. The human skin colour intensity can be assumed to be controlled by two pairs of polygenes B1 /b1 and B2 /b2 , which are located on different chromosomes. Therefore, black Africans can be assumed to be B1 B1 B2 B2 and white Europeans can be assumed to be b1 b1 b2 b2 . So, when there is a marriage between black and white, the following is obtained. P1 : B1 B1 B2 B2 × b1 b1 b2 b2 Black White F1 : B1 b1 B2 b2 Brown P2 : B1 b1 B2 b2 × B1 b1 B2 b2 F2 : B1 B1 B2 B2 – 1 black – 1 16 B1 B1 B2 b2 – 2 dark brown – 4 16 B1 b1 B2 b2 – 2 dark brown B1 b1 B2 b2 – 4 brown B1 B1 b2 b2 – 1 brown – 6 16 B1 b1 b2 b2 – 2 light brown b1 b1 B2 B2 – 1 brown – 4 16 b1 b1 B2 b2 – 2 light brown b1 b1 b2 b2 – 1 white – 4 16 123 Therefore, the phenotypic ratio is 1 : 4 : 6 : 4 : 1.