Cystic fibrosis is caused by the allele f, which is completely recessive to the normal dominant allele f. consider a couple with the genotype ff. what is the chance that their first child will develop cystic fibrosis?

1/4

Explanation:

Given that the disease cystic fibrosis is caused by a recessive allele (f), the individuals with genotype "ff" will express the disease. Both "FF and Ff" genotype will give the individuals a normal phenotype.

Both the parents are carrier (Ff) for the disease. They have 3/4 probability of getting a healthy child and 1/4 probability of getting a child with cystic fibrosis.