A patient with Duchenne muscular dystrophy's mother is a known carrier, and the father has no disease. What is the probability that a FEMALE sibling will be a CARRIER?

Correct answer: D. 50%

• A. 100% would only apply if the father also had DMD, which he does not.
• B. 25% is the probability that any child (regardless of sex) will be affected (male) — not the carrier probability for females.
• C. 75% is incorrect — there is no mechanism for this probability in X-linked inheritance.
• D. Correct. DMD is X-linked recessive. If the mother (X^d X) has one mutated X and the father (X Y) is normal, each daughter has a 50% chance of inheriting the mother's mutated X and being a carrier (X^d X). This is independent of the male child risk (50% of sons affected).

Note: These are DIFFERENT questions. 25% = chance of having an affected (male) child; 50% = chance a female sibling is a carrier.