Why does a parent with one autosomal dominant mutation have a 50 percent chance of passing it to each child?

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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.

A parent with one autosomal dominant mutation is a heterozygote, meaning they carry one mutant gene (a) and one normal, or wild-type, gene (A), designated as Aa.1

During the production of germ cells—eggs or sperm—the two copies of the gene segregate. Because of this process, the parent produces two types of germ cells in roughly equal proportions: half will carry the mutant gene (a) and half will carry the normal gene (A). Consequently, for each pregnancy, there is a 50 percent statistical probability that the child will inherit the mutant gene from the affected parent.12

If the child inherits the mutant gene, they may express the disease phenotype, depending on the specific condition's penetrance.1