The inheritance of both a sex-linked trait and an autosomal trait in humans Red-

Question

The inheritance of both a sex-linked trait and an autosomal trait in humans Red-green color blindness is due to an X-linked recessive allele in humans. A widow?s peak (a hairline that comes to a peak in the middle of the forehead) is due to an autosomal dominant allele. Consider the following family history: A man with a widow?s peak and normal color vision marries a color-blind woman with a straight hairline. The man?s father had a straight hairline, as did both of the woman?s parents. Use the family history to make predictions about the couple?s children.

Explanation / Answer

1) Son with widow's peak: 1/2 (probability of having a son) x 1/2 (probability of having Widow's Peak) = 1/4

2) Son, colour blind, straight hairline = 1 (probability of colour blind son) x 1/2 (probability of straight hairline)

3) Daughter, colour blind, widow's peak = 0 (all daughters will NOT be colour blind)

4) Daughter, normal vision, widow's peak = definitely heterozygous for normal vision (since she is a carrier) and definitely heterozygous for widow's peak (Ww), so chance she is heterozygous for both genes is 1.

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