A mother is a carrier of Duchenne muscular dystrophy (X-linked recessive) and the father is healthy. What is the chance that any given child is a carrier DAUGHTER?

Correct answer: A. 25%

• A. Correct. Carrier mother (XX') × normal father (XY): offspring are 1/4 carrier daughter, 1/4 normal daughter, 1/4 affected son, 1/4 normal son. So 25% of all children are carrier daughters.
• B. 50% is the chance among DAUGHTERS only of being a carrier (or the chance any child is a daughter).
• C. 75% does not correspond to any standard outcome here.
• D. 100% would require the father to be affected and is not the case.

Note: If the question is read as 'given that the child is a daughter,' the carrier probability is 50%. As a fraction of ALL offspring, it is 25%.