Explain Polygenic inheritance with human skin color as example.

Example 2: Human skin colour:

Davenport and Davenport (1910) studied the inheritance of skin colour in black and white populations in U.S.A. Populations derived from marriages between black and white show intermediate skin colour and are called mulattoes. When such individuals marry each other, all shades of colour (continuous variation) are observed in the population in the ratio, 1:6:15:20:15:6:1.

From this it can be concluded that skin colour in humans is controlled by three pairs of genes, Aa, Bb, and Cc.

The presence of melanin pigment in the skin determines the skin colour. Each dominant gene is responsible for the synthesis of fixed amount of melanin. The effect of all the genes is additive and the amount of melanin synthesized is always proportional to the number of dominant genes.

Genotype of black parent is AABBCC, and that of pure white, (melanin is not produced at all) is aabbcc.

Genotype of offspring (mulatto) is AaBbCc. Mulattoes (F, offspring) produce eight different types of gametes, and total sixty four combinations are possible in the population of next generation (F2); but there are seven different phenotypes due to the cumulative effect of each dominant gene as follows,

1. Pure black 6 dominant - 1/64 genes

2. Very dark brown 5 dominant genes - 6/64

3. Dark brown 4 dominant genes - 15/64

4. Mulatto 3 dominant (intermediate-'sanwla') genes - 20/64

5. Fair 2 dominant - 15/64 genes

6. Very fair 1 dominant - 6/64 gene

7. Pure white No dominant - 1/64 gene