In X-linked recessive inheritance, if the father is affected and the mother is unaffected, which offspring are affected?

• A. All daughters.
• B. All sons.
• C. No offspring.
• D. Sons inherit the X chromosome from their father.

Answer: (A)

In X-linked recessive inheritance, the father’s X chromosome determines what all his daughters inherit. An affected father has X with the mutation and passes that X to every daughter, while sons receive the Y chromosome from him.

If the mother is unaffected and not a carrier, each daughter gets the mutant X from her father and a normal X from her mother, so their genotype is X^a X^A. They become carriers of the trait but typically do not show the disease, because females usually need two copies of the mutant allele to be affected. All sons, meanwhile, receive the normal X from the mother and the Y from the father, so they are unaffected (X^A Y).

If the mother is a carrier (X^A X^a), the daughters have a 50% chance to be affected (X^a X^a) and a 50% chance to be carriers (X^a X^A); the sons have a 50% chance to be affected (X^a Y) and a 50% chance to be unaffected (X^A Y). The key idea is that the father’s mutant X is passed to all daughters, but whether they express the disease depends on the mother’s genotype.