X-linked recessive: son of an obligate carrier
A woman's father has hemophilia A, an X-linked recessive disorder. Her husband is unaffected. What is the probability that their son will have hemophilia?
Answer B: explanation for every choice
A
1/4 is the probability that any child, of either sex, will be an affected son.
B
Correct. She must have inherited her father's affected X, so she is a carrier. Each son has a 1/2 chance of receiving that X.
C
2/3 applies to an unaffected sibling of a person with an autosomal recessive disorder.
D
Sons get one of the mother's two X chromosomes, so not all are affected.
TakeawayDaughters of affected fathers are obligate carriers. Each of their sons has a 50% risk.
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Reviewed 2026-11-05 by a medical student. Spot an error? Report it with question ID bb165.