In mammals, females have two copies of the X chromosome, whereas males have only one copy. Dosage compensation is accomplished in mammals by:

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Females possess a randomly inactivated X chromosome in all cells, a process called X inactivation. This ensures that both males and females have equal expression of X chromosome genes. One of the X chromosomes in each female cell is randomly inactivated in a process called lyonization, which results in the formation of a condensed structure called a Barr body.

In Mammals, Females Have Two Copies of the X Chromosome