Define Poylgenic Inheritance.
In human population, characters such as height, skin colour and intelligence show gradations (continuous variations) in expression and not only two contrasting expressions. These characters are determined by two or more gene pairs, and they have additive or cumulative effect. These genes are called cumulative genes or polygenes or multiple factors.
Explain polygenic inheritance with examples.
The ratio is 1:4:6:4:1 if there are two genes having additive effect (e.g. Wheat kernel colour)
In case of three such genes (polygenes), the ratio is 1:6:15:20:15:6:1 (e.g. skin colour in human).
Example 1: Wheat kernel colour:
Swedish geneticist H. Nilsson-Ehle ( 1908) discovered the polygenic inheritance. He crossed red kernelled variety of wheat with white kernelled variety. In F, generation all plants had grains with intermediate colour between red and white, and in F2 generation five different phenotypic expressions appeared in the ratio 1:4:6:4:1. (Instead of red, intermediate and white in the ratio, 1:2:1.) Continuous variations in the expression were as follows,
1. The darkest red (as red as the parent plant) - 1/16
2. Medium red (less than parent plant but more than F1, hybrids) - 4/16
3. Intermediate red (as F1 hybrids) - 6/16
4. Light red (less than F1 hybrids) - 4/16
5. White (as white as parent plant) - 1/16
Nilsson Ehle suggested that the kernel colour in wheat is controlled by two pairs of genes, Aa and Bb.
Gene A and B determine the red colour of kernel (responsible for producing red pigment) and are dominant over their recessive allele a and b which do not produce red colour pigment and the expression is white, if dominant gene is not present. Thus genotype of a parent with red kernels is AABB and that of parent with white kernels is aabb. Genotype of F1 hybrids is AaBb. During gamete formation each gene pair shows Mendelian segregation and four types of gametes are produced. Continuous variations (gradations) in expressions is observed as each dominant gene produces specific amount of pigment and the shade of red colour, (dark or light) depends on number of dominant genes present as shown in the following Punnett square. Only one, out of sixteen is with four dominant genes, and is the darkest red. Four, out of sixteen have three dominant genes and are medium red, six have two dominant genes and are intermediate red, four have only one dominant gene and are light red and again only one is without any dominant gene and is therefore with white kernel colour.
