Merck Manual

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Non–X-Linked (Autosomal) Recessive Disorders
Non–X-Linked (Autosomal) Recessive Disorders

Some disorders represent a non–X-linked (autosomal) 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%.