In the case of cystic fibrosis, two parents don't suffer from the disease but both carry the recessive cystic fibrosis allele. What is the probability that a child of the parents will suffer from cystic fibrosis.

25%. Draw a punnet square to find this:

Write in the alleles for the parents along the top and side of the square, then match up the possible allele combinations for the children inside the square. 

Cystic fibrosis is a recessive allele, only people who carry the two recessive alleles (ff) will suffer from the disease. On the punnet square one of the four possible combinations for the child has two recessive alleles. The for the probabilty of a child getting the disease (as a percentage) is (1/4) x 100 % = 25 %