How can a female carry an X-linked recessive trait without fully showing it?

A female can carry an X-linked recessive trait without fully expressing it because she possesses two X chromosomes, whereas a male has only one.1

In females, the presence of a second X chromosome typically provides a functional, dominant allele that masks the effect of the recessive mutation. Furthermore, females undergo a process called X inactivation (or "lyonization") during early development. In this process, one of the two X chromosomes in each somatic cell is randomly inactivated and condensed into a Barr body.12

Because this inactivation is random, a female who is a carrier for an X-linked recessive trait is a mosaic: roughly half of her cells will express the X chromosome carrying the normal allele, while the other half will express the X chromosome carrying the recessive mutation. This cellular mosaicism often prevents the full clinical expression of the trait, as the functional gene products from the active, normal X chromosomes are usually sufficient to maintain normal biological function.2