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MCAT Question A Day - 5/7/13 - Answer!

A hemophiliac woman and her husband whose blood clots normally, have a son. What is the probability that this child will have hemophilia? Hemophilia is a sex-linked recessive trait.

A.  25%
B.  50%
C.  75%
D.  100%

The correct answer is (D). If mom has hemophilia, her genotype is XhXh. She can only contribute an X chromosome with the recessive allele for hemophilia to all her sons. (They must receive the Y chromosome from dad in order to be sons in the first place!) Therefore, all sons produced by this couple will have the genotype XhY, and will have hemophilia.

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