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 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 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 like the parents)
Therefore, among the children, the chance of not developing the disorder (that is, being normal or a carrier) is 75%.