Regarding X-linked recessive inheritance (e.g., Duchenne muscular dystrophy), if the MOTHER IS A CARRIER and the FATHER IS UNAFFECTED, what is the probability that a DAUGHTER (female sibling) will be a CARRIER?

Correct answer: C. 50% (half the daughters will be carriers, half will be unaffected; sons: 50% affected, 50% normal)

• A. 75% is not a possible probability in simple X-linked recessive inheritance.
• B. 25% would only apply if both parents had different genotypes — for daughters of a carrier mom + unaffected dad, it is 50%.
• C. Correct. X-linked recessive: Mother = X^c X (carrier) × Father = X Y (unaffected). Punnett square for daughters (XX): X^c X (carrier) and X X (unaffected) — 50% of daughters are carriers. Punnett square for sons (XY): X^c Y (affected) and X Y (unaffected) — 50% of sons are affected. Daughters CANNOT be affected (they get X from father which is normal), but 50% will be carriers.
• D. 100% would require the father to also be affected (X^c Y) — the father here is unaffected (X Y).