Question

Human skin colour is controlled by several gene pairs. Let us assume here, that there are just three gene pairs on different chromosomes and that for each pair there are two alleles - an incomplete dominant one that codes for melanin deposition and an incompletely recessive one that codes for no melanin deposition. If a very dark skinned person marries a very light skinned woman, what will be the chance that their offspring will have very dark skin?
A. 0
B. $\dfrac{1}{4}$
C. $\dfrac{5}{8}$
D. $\dfrac{9}{64}$

Answer 1

Human skin is a polygenic trait which is generally controlled by two or more genes. The inheritance of polygenic traits is called polygenic or quantitative inheritance. Examples of human polygenic inheritance are height, skin color, eye color and weight.

Complete Answer:
Let us take the dominant allele for the three genes controlling the skin color to be represented as A, B and C and their respective recessive forms can be represented as a, b and c.
Since these alleles exhibit incomplete dominance, a very dark skinned man should have all the alleles in dominant form and must have the genotype AABBCC. Similarly, a very light skinned woman should have all the alleles in recessive form aabbcc.
P generation: AABBCC X aabbcc
(very dark) (very light)

Gametes: ABC x abc

| ABC| ABC
---|---|---
abc| AaBbCc| AaBbCc
abc| AaBbCc| AaBbCc

The F1 progeny between very dark skinned and very light skinned individuals called mulatto produces intermediate skin colour. Therefore, none of the offsprings have very dark skin because the allele for melanin deposition is incompletely dominant.

Thus, the correct option is A) 0 (zero).

Note: The presence of melanin pigment in the skin determines the skin colour. The amount of melanin in developing individuals is determined by 3 pairs of genes. The effect of all the genes is additive and the amount of melanin produced is always proportional to the number of dominant genes. Six genes may be involved in controlling the skin colour in human beings.