Question

Cystic fibrosis is a deadly disease of the lungs and digestive system. It is inherited as a recessive trait. How can two healthy parents have a child with cystic fibrosis?

Answer 1

They are both heterozygous for the gene. If you do a pun net square, you will find that they would have a 25% chance of having a child with Costco fibrosis.

Answer 2

We'll make the alleles: Cc.

C is a healthy parent, and c is the cystic fibrosis (recessive).

If you make a punnett square and the parents are both Cc (healthy), then you would get CC, Cc, Cc, and cc. That cc is a 25% chance that the offspring will have that, which means there is a 25% chance the child will have cystic fibrosis.

Two healthy parents can have a child with cystic fibrosis if both the parents have the dominant allele and recessive allele, if both the parents have heterozygous alleles.