Principle of Inheritance and VariationMediumBloom L3

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?

Options

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

Solution

Sol.(a) [Key Concept: Polygenic Inheritance — Dark × Light Skin] Very dark-skinned person: AABBCC (homozygous dominant for all 3 gene pairs). Very light-skinned person: aabbcc (homozygous recessive for all 3 gene pairs). All F1 offspring: AaBbCc (medium/intermediate skin tone). NONE of the F1 offspring will have very dark skin — they will all be heterozygous at every locus and have an intermediate phenotype. Probability of very dark skin (AABBCC) in F1 = 0. (Very dark offspring would only appear in F2 at 1/64 = (1/4)^3 probability).

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