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How can X chromosome inactivation cause symptoms in female carriers of X-linked disorders?

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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 females, X chromosome inactivation—a process also known as lyonization—is a normal developmental mechanism where one X chromosome is randomly silenced in each somatic cell to ensure proper gene dosage.12

Because this process is random, a female who carries a mutation for an X-linked disorder typically possesses two cell populations: one where the normal X chromosome is active and one where the mutant X chromosome is active. While this mosaicism often leaves carriers asymptomatic or only mildly affected, symptoms can arise in individuals known as manifesting heterozygotes.2

This occurs when the random inactivation process results in a skewed distribution, where a higher proportion of cells have the normal X chromosome inactivated. Consequently, the majority of the individual's cells express the mutant gene, leading to the clinical manifestation of the disorder in varying degrees of severity. Additionally, some genes on the X chromosome escape inactivation entirely, which may also contribute to phenotypic abnormalities in certain genetic conditions.21