DNA is made up of four building blocks called bases. These bases are joined in a specific order for each gene. When a change occurs in the arrangement of the bases, it can cause the gene not to work properly. A structural gene change which can cause a disease or a birth defect is called a mutation. Genes are inherited in pairs, with one gene inherited from each parent to make the pair.
Cystic fibrosis occurs when both genes in the pair have a mutation.
A person with cystic fibrosis inherits one CF gene from each parent. The parents of a child with CF each carry one nonworking copy of the gene and one working copy of the gene. The parents are called CF carriers and because they have one working gene they have no symptoms.