In autosomal recessive inheritance, if two unaffected parents have an affected child, what is the probability that their other child is a carrier?

• A. 0%
• B. 50%
• C. 25%
• D. 75%

Answer: (B)

When two unaffected parents have an affected child, it means both parents are carriers of the recessive allele. For each pregnancy from two carriers ( Aa × Aa ), the possible outcomes are: 25% non-carrier (AA), 50% carrier (Aa), and 25% affected (aa). So the chance the next child is a carrier is 50%. This probability is independent of the first child's outcome. (If the question had specified the other child is unaffected, the probability would be 2/3.)