Cystic Fibrosis and fertilization
Each individual carries two copies of the CFTR gene. For someone to suffer from cystic fibrosis they should carry mutations in both copies of the gene, that is to be homozygous. If someone carries a mutation in only one copy, i.e. heterozygote, is called a carrier of the disease.
To prevent a child to be born with cystic fibrosis, both parents should not carry mutations or at least only one of the two. If both parents are carriers, then there is a 25% chance for a child to be born with cystic fibrosis.