Unit – III B
Deviations from Mendel’s Experiments (Contd.)
- More than two alternative forms (alleles) of a gene in a population occupying the same locus on a chromosome or its homologue are known as multiple alleles.
- e.g. In Drosophila, a large number of multiple alleles are known. One of them is the series of wing abnormally ranging in size from normal wings to no wings.
Blood Groups in Human Beings
- The gene I controls ABO blood groups it has three alleles; IA , IB and i. The allele IA and IB produce slightly different types of sugar and allele i does not produce any sugar.
- As human are diploid organisms, each person possesses any two of the three I genes. IA and IB are co-dominant and completely dominant on I.
Blood Group of Progeny (Children):
- The interaction, in which one gene masks or inhibit the expression of another nonallelic gene, is called epistasis. The gene that masks the effect of another is said to be epistatic to other and gene whose expression is prevented is said to be hypostatic.
- When a single gene controls two (or more) different traits, it is called pleiotropic gene and this phenomenon is called pleiotropy or pleiotropism. The ratio is 2:1 instead 3:1
- e.g. 1: In pea plant, the same gene that affects colour of flower also influences the colour of seed coat and the colour of leaf axil.
- e.g. 2: The gene that determines the size of the wings in Drosophila also affects its eye colour, positoion of dorsal bristles, the shape of the spermotheca, fertility and length of life.
- e.g. 3: gene Hbs (recessive) of disease sickle cell anaemia.
Normal or healthy gene is HbA .which is dominant. Thus disease carrier having heterozygotes HbA / Hbs show signs of mild anaemia as their R.B.C.s become sickle shaped (half moon) and their oxygen carrying capacity decreases. But can live normal life. But the homozygotes with recessive gene HbA die of fatal anaemia. A gene which causes a death of the bearer is called a lethal gene. Two carrier parents will produce normal, carriers and sickel cell anemic children in 1:2:1 ratio.
- In human population characters such as height, skin colour and intelligence show continuous variations in expression and not only two contrasting expressions. These characters are determined by two or more gene pairs and they have an additive or cumulative effect. These genes are called cumulative genes or polygenes or multiple factors.
- The ratio is 1:4:6:4:1 if there are two genes having an additive effect.
- This inheritance is also called as quantitative inheritance.
- Note that Mendel studied qualitative inheritance, where complete dominance is observed.
Wheat Kernel Colour:
Swedish geneticist H. Nilsson-Ehle discovered polygenic inheritance. He crossed red kernelled variety of wheat with white kernelled variety. In F1 generation all plants have grains with intermediate colour between red and white. In F2 generation five different phenotypic expressions (the darkest red, medium red, intermediate red, light red, white) appeared in the ratio 1:4:6:4:1. Nilson Ehle suggested that the kernel colour in wheat is controlled by two pairs of genes, Aa and Bb. Genes A and B determines red colour. and b which do not produce red colour pigment and their expression is a white colour of the kernel.
Human Skin Colour:
- The presence of melanin pigment is responsible for the colour of the skin in a human being. Each dominant gene is responsible for the synthesis of fixed amount of melanin. The amount of melanin synthesized is directly proportional to the number of dominant genes.
- A genotype of a pure black parent in which melanin is produced is the highest is AABBCC, while that of pure white also called albino no melanin is formed is aabbcc.
- Mulattoes i.e. F1 offspring produce (23 = 8) different types of gametes. Let us consider mulatto intermediate whose genotype is AaBbCc. By doing cross among two mulatto intermediate we get (26 = 64) combinations in F2 generation. But there only 7 phenotypes due to a cumulative effect of each dominant gene.
- When we analyse all possible combinations and plot the probability graph, in Polygenic inheritance often we get a bell-shaped curve as shown below.
- This means that most people fall in the middle of the phenotypic range, such as skin colour or average height, while very few people are at the extremes, such as pure white or pure dark. At one end of the curve will be individuals who are recessive for all the alleles (for example, aabbcc); at the other end will be individuals who are dominant for all the alleles (for example, AABBCC). In the middle of the curve will be individuals who have a combination of dominant and recessive alleles (for example, AaBbCc or AaBBcc).