A patient with Duchenne muscular dystrophy (DMD) has an unaffected father and a carrier mother. What is the probability that the patient's sister will be phenotypically affected?

Correct answer: A. 0% (sisters cannot be phenotypically affected)

• A. CORRECT. DMD is X-linked recessive. The mother is a carrier (X^DMD X), the father is unaffected (X Y). For a daughter: she inherits one X from father (always normal, since father is unaffected) and one X from mother (50% chance of X^DMD). Therefore, daughters can be: 50% carriers (X^DMD X) or 50% normal (X X). Since DMD is X-linked recessive, females need BOTH X chromosomes to carry the mutation to be phenotypically affected. With a normal X guaranteed from the unaffected father, NO daughter can be phenotypically affected. Sisters have a 50% chance of being carriers, but 0% chance of being phenotypically affected.
• B. INCORRECT. 25% would apply to an autosomal recessive condition where both parents are carriers. This is X-linked recessive, and the father is unaffected (providing a normal X to all daughters).
• C. INCORRECT. 50% is the probability that a sister will be a CARRIER (not phenotypically affected). Carriers of X-linked recessive conditions are almost always phenotypically normal females.
• D. INCORRECT. 100% would mean all sisters are affected, which is impossible given the genetics of this X-linked condition when the father is unaffected.