One form of hemophilia is caused by a sex-linked recessive gene.Assume that a man with hemophilia marries a phenotypically normal woman whose father had hemophilia.What is the probability that their first son will have hemophilia?

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Quizplus · Accepted Answer

Since hemophilia is a sex-linked recessive trait, it is carried on the X chromosome. The man with hemophilia has the genotype X^hY, where X^h represents the hemophilia allele. The woman, being phenotypically normal but with a father who had hemophilia, must be a carrier, with genotype X^HX^h (where X^H represents the normal allele). For their son to have hemophilia, he must inherit the X^h allele from his mother and the Y chromosome from his father. The probability of inheriting the X^h allele from the mother is 1/2, and since sons always inherit the Y chromosome from their fathers, the probability that their first son will have hemophilia is 1/2.

One Form of Hemophilia Is Caused by a Sex-Linked Recessive