In autosomal recessive inheritance, if both parents are carriers, what is the risk per pregnancy that a sibling is affected?

• A. 0% risk of being affected
• B. 50% affected
• C. 25% affected, 50% carrier, 25% unaffected non-carrier
• D. 75% affected

Answer: (C)

When both parents are carriers for an autosomal recessive trait, each pregnancy has the same genotype possibilities: AA (unaffected non-carrier), Aa (carrier), and aa (affected). The cross Aa x Aa yields 1/4 AA, 1/2 Aa, and 1/4 aa. So the chance a pregnancy results in an affected child is 25%, while 50% are carriers and 25% are unaffected non-carriers. The option that gives this full distribution matches the expected probabilities and thus is the best answer.