Question

hemophilia is a sex - linked trait. which of the following describes the probability of hemophilia in the offspring of a man who does not have hemophilia and a woman whose father is a hemophiliac?
a each of their sons will have hemophilia
b none of their daughters will have hemophilia
c their sons have a 25% chance of having hemophilia
d their daughters have a 50% chance of having hemophilia

Explanation:

Hemophilia is a sex - linked recessive disorder. Let \(X^h\) represent the hemophilia - carrying allele and \(X^H\) represent the normal allele. The man has the genotype \(X^HY\) (since he does not have hemophilia). The woman's father is a hemophiliac (\(X^hY\)), so she must have received an \(X^h\) allele from her father. Her genotype is \(X^HX^h\) (because she is not affected).

• For sons: The man contributes \(Y\) and the woman can contribute either \(X^H\) or \(X^h\). The probability of a son getting \(X^h\) (and thus having hemophilia) is \(\frac{1}{2}\).
• For daughters: The man contributes \(X^H\). Since the daughter will always get \(X^H\) from the father, even if she gets \(X^h\) from the mother (\(X^HX^h\)), she will be a carrier but not affected. So none of the daughters will have hemophilia.

Answer:

B. None of their daughters will have hemophilia