Cystic fibrosis is caused by mutations in the CF gene, and there are several different mutations that are known to result in CF disease. The CF mutations behave as recessive alleles to the WT CF allele. If two carriers that have different mutations in their CF genes have children what is the probability that one of their children will have CF disease?

A. 25%
B. 50%
C. 100%
D. 75%

25%

Explanation:

Let's assume that the allele "c" causes cystic fibrosis in homozygous conditions. The genotype of the carrier parent would be "Cc". A cross between two carrier parents, each with a genotype "Cc" would produce progeny in the following phenotype ratio=

Cc x Cc= 3/4 normal (1/4 CC: 1/2 Cc) : 1/4 affected (1/4 cc).

Therefore, the probability that one of their children will have CF disease is= 25%