Pattern of inheritance whereby disease results only when an individual inherits two gene mutations for the particular disease. The parents of a child who is affected are said to be “carriers” of the disease, because they have one mutation and do not manifest symptoms. If both members of a couple are carriers, there is a 1 in 4 or 25% chance in each pregnancy for a child to be affected. Also referred to as autosomal recessive inheritance if the gene is not located on the X or Y chromosomes.