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 incompletely 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. 1/4
• B. 5/8
• C. 0
• D. 9/64

Answer 1

Answer: (C)

Human skin color is a polygenic trait, which is controlled by a number of genes. According to the question, there are just three gene pairs on different chromosomes and that for each pair there are two alleles - an incompletely dominant one that codes for melanin deposition and an incompletely recessive one that codes for no melanin deposition.
Let the dominant allele for the three genes controlling the skin color be represented as M, N and O and their respective recessive forms can be represented as m, n and o.
Since, these alleles exhibit incomplete dominance, a very dark skinned man should have all the alleles in dominant form and must have the genotype MMNNOO. Similarly, a very light skinned woman should have all the alleles in recessive form mmnnoo.
P generation: MMNNOO X mmnnoo
Gametes: MNO mno
F${}_1$ generation: MmNnOo
None of the offsprings have very dark skin because the allele for melanin deposition is incompletely dominant.