Cystic fibrosis is caused by mutations in the CF gene, and there are several different

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%

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