Cystic fibrosis is inherited as an autosomal recessive trait. What does this imply about the parental genotypes?

• A. Both parents must be affected.
• B. One parent must be affected and the other a carrier.
• C. Both parents are carriers.
• D. The trait is X-linked.

Answer: (C)

In autosomal recessive conditions, two copies of the recessive allele are required for disease to manifest. If a child has cystic fibrosis, it means they inherited a recessive allele from each parent. The only way that happens while the parents themselves are not affected is that both parents are carriers (they each have one normal and one recessive allele). Their cross would produce about one quarter affected, one half carriers, and one quarter unaffected non-carriers, assuming typical Mendelian inheritance.

The other possibilities aren’t necessary to explain this scenario: if both parents were affected, they would show disease themselves; if one parent were affected and the other a carrier, that could produce affected offspring as well but isn’t the standard inference from the inheritance pattern alone; and the trait being X-linked would involve a different pattern of inheritance entirely.