In autosomal recessive inheritance, if two unaffected parents have an affected child, what does this indicate about their genotypes?

• A. Both are carriers (Aa)
• B. Both are homozygous dominant (AA)
• C. Both are homozygous recessive (aa)
• D. One is AA and the other Aa

Answer: (A)

This situation shows how an autosomal recessive trait is expressed only when two recessive alleles come together. If both parents are unaffected but have an affected child, each parent must pass on a recessive allele. That means both parents carry one recessive and one dominant allele, giving them the genotype Aa. They can pass the recessive allele to their child, resulting in an affected aa child when both contribute it.

Carriers (Aa) don’t show the disease themselves, but they can produce affected offspring when they mate with another carrier. If both parents were AA, they could not produce an affected child; if both were aa, they would be affected themselves; and a cross of AA and Aa would also not yield an affected child. The only compatible parental genotypes here are both Aa.