You are studying cystic fibrosis (CF). While looking at a pedigree, you notice that the CF phenotype is not present in a set of parents, but one out of their five children has CF. What can you conclude about CF?

Explanation:

1. Cystic fibrosis (CF); it  is an autosomal recessive disease.

2. As given here, that both the parents  are normal but their one out of five children has CF, so we can say that both the parents are carrier for CF,

3. So their genotype is: Cc and Cc.

4. In Cc and Cc; C is dominant over c, and genotype cc is for CF.

cross between both parents; results into progeny: CC, Cc, Cc and cc.

Genotype cc  has CF because it is  homozygous recessive.

if you know your abc's you should be able to get your answer or just use google