Barr body

Form taken by the inactive X chromosome in a female somatic cell

A Barr body or X-chromatin is an inactive X chromosome. In species with XY sex-determination, females typically have two X chromosomes, and one is rendered inactive in a process called lyonization. Errors in chromosome separation can also result in male and female individuals with extra X chromosomes. The Lyon hypothesis states that in cells with multiple X chromosomes, all but one are inactivated early in embryonic development in mammals. The X chromosomes that become inactivated are chosen randomly, except in marsupials and in some extra-embryonic tissues of some placental mammals, in which the X chromosome from the sperm is always deactivated.

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