Cystic fibrosis results from mutations in a single gene that lies on Chromosome 7. Only homozygous ...

A. The genotypes of the mother and father are the same: Ff. The only way that a child can have the disease is if both parents are carriers of the mutant cystic fibrosis gene.
B. The chance that the second child will have cystic fibrosis is one-quarter. The chance that the mother will transmit her mutant f allele to the offspring is one-half, multiplied by an equal chance that the father will transmit his mutant f allele to the offspring: 1/2  1/2 = 1/4. (Having one child with cystic fibrosis does not change the probability of having another child with the disease.)