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If skin colour is determined by three genes A, B and C with additive dominant alleles, two parents of genotype AaBbCc each produce a child. What is the probability that the child has the darkest possible phenotype (AABBCC)?
AOne in eight
BOne in sixteen
COne in thirty-two
DOne in sixty-four
Answer & Solution
Correct answer: A. One in eight
For each gene, the probability of AA from Aa x Aa is one quarter. For three independently assorting genes, the joint probability is (1/4) x (1/4) x (1/4) = 1/64. Only this genotype gives the darkest additive phenotype.
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