In X-linked recessive inheritance, if the mother is a carrier and the father is unaffected, what are the risks per pregnancy for a son to be affected and a daughter to be a carrier?

• A. 25% for a son and 25% for a daughter
• B. 50% for a son to be affected; 50% for a daughter to be a carrier
• C. 50% for a son to be affected; 0% for a daughter
• D. 0% for a son; 100% for a daughter

Answer: (B)

In X-linked recessive inheritance, a carrier mother has one normal X and one mutated X, while an unaffected father contributes a normal X to daughters and a Y to sons. For each pregnancy, the mother passes either her normal or mutated X to the child with roughly equal chance.

• Sons receive the X from their mother and a Y from their father. If the son inherits the mutated X, he will be affected. This happens in about 50% of pregnancies.

• Daughters receive an X from both parents. The father provides a normal X, so a daughter will be a carrier if she inherits the mutated X from the mother, which occurs about 50% of the time. She cannot be affected unless the father also contributed a mutated X, which isn’t possible when he is unaffected.

Thus, about half of pregnancies yield affected sons, and about half of daughters are carriers.