Recessive diseases are caused by changes (called genetic variants) in both copies of a person’s genes. Every person has two copies of each gene, one inherited from each parent. A recessive disease occurs when both copies of the same gene have the same variant. Cystic fibrosis is an example of a recessive disease.
When two parents are carriers of the same genetic disease, their children have a 1 in 4 (or 25%) chance of having that disease.