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Why do two carriers of an autosomal recessive disorder have a 25 percent chance of an affected child in each pregnancy?

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BRCA2 tumor suppressor gene on chromosome 13 — human genetic disease Photomicrograph of the *BRCA2* tumor suppressor gene on chromosome 13 of the human genome. Inactivation of this growth-regulating gene is associated with a higher risk of developing breast cancer.

In an autosomal recessive disorder, an individual must inherit two mutant alleles—one from each parent—to manifest the disease.12

When both parents are carriers, they are heterozygotes, meaning each possesses one normal gene (A) and one mutant gene (a), represented as Aa. During reproduction, each parent has a 50 percent chance of passing either the normal gene or the mutant gene to their offspring. According to the principles of Mendelian inheritance, the possible genetic combinations for their children are:12

AA: A child who inherits a normal gene from both parents (25% chance).
Aa: A child who inherits one normal and one mutant gene, becoming a carrier like the parents (50% chance).
aa: A child who inherits the mutant gene from both parents and is therefore affected by the disorder (25% chance).

Because each pregnancy is an independent event, there is a 25 percent probability that the child will be homozygous for the mutant gene (aa) and thus affected by the condition.123