Cystic fibrosis is caused by the allele f, which is completely recessive to the normal dominant allele f. consider a couple who both have the genotype ff. their first child did not get cystic fibrosis. what is the chance that their second child will develop cystic fibrosis?

%

Explanation:

Given

Cystic fibrosis is caused by the allele f, which is completely recessive to the normal dominant allele F. Thus if "F" and "f" occur together then the  characteristics associated with allele "F" will be expressed.

Or a person having cystic fibrosis must have genotype "ff"

A punnet square cross between the parents having genotype "Ff" is as given below-

Ff

FFFFf

fFfff

Thus, out of four offspring only one is having the "ff" genotype. Thus the

chance that their second child will have cystic fibrosis is

%