A family study describing second cousins with cystic fibrosis and no common ancestor who is a carrier.

We describe an extended pedigree in which second cousins are affected with cystic fibrosis (CF). Since the degree of relationship of the affected subjects is 1/32, common descent of one CF mutation was expected when the DNA laboratory undertook mutation analysis. Mutation testing showed that each CF mutation was introduced into the family by random mating and not by descent through a common ancestor. Implications for pedigree analysis and DNA testing are discussed.