In humans, cystic fibrosis is a recessive genetic disorder that results in mucus clogging major organs. if both parents are genetically heterozygous for cystic fibrosis, then what are their chances of having a child with the disease?

The right answer is 25%.

Here are the different cross between the gametes of parents:

A: healthy allele

a: sick allele.

Aa   x     Aa

AA, Aa, Aa, aa

Since cystic fibrosis is an autosomal recessive disease (chromosome 7): The disease is expressed only in children with two sick alleles (aa) (which represents one in four children). Those with only one sick allele (Aa) will carry the disease.