A family has an autosomal recessive disease. Both parents are phenotypically normal. They have one affected child (aa) and one normal child. Assuming no mutation, what are the most likely genotypes of the parents and what is the probability that their next child will be affected?

Step-by-step solution

1. Since one child is aa, each parent must contribute an a allele; hence both parents are heterozygous (Aa).
2. Confirm that other combinations (AA x Aa or AA x AA) cannot produce aa offspring.
3. With parents Aa x Aa, the Punnett square gives AA (1/4), Aa (1/2), aa (1/4).
4. Therefore the probability that the next child is affected (aa) is 1/4.

Concept tested: Autosomal recessive inheritance

Published 4 October 2026 · How this solution was prepared and checked