Nancy and Cliff had four children who did not have cystic fibrosis, but their fifth child was born with the disease. Cystic fibrosis is carried on a recessive gene. Given what you've learned in this chapter, how could this happen?
a. Their fifth child must have been a girl, which makes it more probable that she would have the illness
b. Their fifth child must have been a boy, which makes it more probable that he would have the illness
c. It wasn't until the fifth child that both parents passed on the allele for this illness. It wasn't until the fifth child that both parents passed on the allele for this illness.
d. Recessive genes normally skip a generation, but after the parents had five children, the gene was expressed

The most likely hypothesis is that both parents are heterozygous, carrying the gene for cystic fibrosis (Aa), but not manifesting it, because it is a disease caused by a recessive allele (a), so only recessive homozygous individuals (aa) can have the disease.

AA = Homozygous dominant - Does not manifest cystic fibrosis;

Aa = Heterozygous - Carrier of the trait for cystic fibrosis but do not manifest the disease;

aa = Recessive homozygous - Manifest cystic fibrosis.

Being the parents heterozygous, children with the following allele genes can be generated according to their percentages:

Parent alleles: Aa x Aa

Possible alleles of children:

A x A = AA = 25%

A x a = Aa = 25%

a x A = Aa = 25%

a x a = aa = 25%

According to the scheme above, it is possible to state that in this case there is a 25% chance of a recessive homozygous (aa) child being born, which explains the fact that only the fifth child was born with cystic fibrosis, while the others may not have the trait (AA) or may just be carriers of the gene (Aa), which also do not manifest cystic fibrosis.