Question

Cystic fibrosis is a genetic disorder. It is a recessive trait that is determined by two alleles. Suppose two parents each carry at least one recessive allele, but neither of them has cystic fibrosis. Which statement reflects the likelihood of a child of these parents having cystic fibrosis?
The child cannot have the disease because only the dominant alleles will be passed on.
The child will have the disease because only recessive alleles will be passed on.
The child may or may not have the disease because a recessive gene from each parent may be passed on.
The child may or may not have the disease because the disease does not depend on the alleles of the parents.

Answer 1

The correct choice is: "The child may or may not have the disease because a recessive gene from each parent may be passed on."

Reason: If both parents are carriers (genotype Aa), a Punnett square shows offspring genotypes 25% AA (unaffected, not a carrier), 50% Aa (unaffected carrier), and 25% aa (affected). Thus there's a 25% chance the child will have cystic fibrosis.