Some disorders represent a non–X-linked recessive trait. To have the disorder, a person usually must receive two abnormal genes, one from each parent. If both parents carry one abnormal gene and one normal gene, neither parent has the disorder (they are unaffected) but each has a 50% chance of passing the abnormal gene to the children. Therefore, each child has
A 25% chance of inheriting two abnormal genes and thus of developing (being affected by) the disorder
A 25% chance of inheriting two normal genes
A 50% chance of inheriting one normal and one abnormal gene, thus becoming a carrier of the disorder (but being unaffected by) the disorder, like the parents
Therefore, among the children, the chance of not developing the disorder (that is, not being a carrier or being an unaffected carrier) is 75%.