Davenport and Davenport (1910) studied the inheritance of skin colour in negro and white populations in U.S.A. Populations derived from marriages between negro and white show intermediate skin colour 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 negro parent is AABBCC, and that of albino (pure white, melanin is not produced at all) is aabbcc. Genotype of offspring (mulatto) is AaBbCc. Mulattoes (F1 offspring) produce eight different types of gametes, and total sixty four combinations are possible in the population pf next generation (F2); but there are seven different phenotypes due to the cumulative effect of each dominant gene as follows,
1. Pure black (negro) -6 dominant genes - 1/64
2. Black (less dark than negro parent) -5 dominant genes - 6/64
3. Lesser black or brown -4 dominant - 15/64
4. Mulatto (intermediate- 'sanwla') - 3 dominant genes - 20/64
5. Fair - 2 dominant genes - 15/64
6. Very fair - 1 dominant - 6/64
7. Pure white (albino) - No dominant gene - 1/64