Question

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?

Answer 1

25% since there's a 25% chance they'd be homozygous dominant(not have CF) and a 50% chance they'd be heterozygous dominant(not have CF) then the last 25% is the potential they'd be homozygous recessive(have CF)

Answer 2

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.