Duchenne muscular dystrophy: the father is unaffected and the mother is a carrier. What is the probability that a female sibling will be a carrier?

Correct answer: C. 50%

• A. 0% is wrong — female children CAN be carriers.
• B. 25% represents the probability of an AFFECTED male child (X-linked recessive), not a female carrier.
• C. Correct. Duchenne MD is X-linked recessive. Mother (X^D X): carrier. Father (X Y): unaffected. For female siblings: they inherit one X from mom (50% chance of X^D — carrier, 50% chance of X — normal) and one X from dad (always normal X). So 50% of daughters will be carriers.
• D. 75% is incorrect — you cannot exceed 50% carrier probability for females in this cross.