X-inactivation (also called lyonization) is a process by which one of the two copies of the
X chromosome present in
female mammals is inactivated. The inactive X chromosome is silenced by packaging in repressive
heterochromatin. X-inactivation occurs so that the female, with two X chromosomes, does not have twice as many X chromosome
gene products as the
male, which only possess a single copy of the X chromosome (see
dosage compensation). The choice of which X chromosome will be inactivated is random in
placental mammals such as
mice and
humans, but once an X chromosome is inactivated it will remain inactive throughout the lifetime of the cell. Unlike the random X-inactivation in placental mammals, inactivation in
marsupials applies exclusively to the paternally derived X chromosome.
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The proposition by Mary F Lyon that random inactivation of one X chromosome in the somatic cells of mammalian females is responsible for dosage compensation and mosaicism.
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