Dictionary of Human Evolution and Biology

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Paternal X Inactivation

Nonrandom dosage compensation found in marsupials, in which it is always the paternal X chromosome that is inactivated in females. X inactivation in eutherians is a two-stage process:
(1) preferential inactivation by genomic imprinting of the paternal X chromosome prior to fertilization via XIST (and which remains inactivated in blastocyst cells destined to become placental tissues, i.e, the trophoblast); but then
(2) the imprinted paternal X chromosome is reactivated in cells within the inner cell mass (epiblast), and in these subsequent random X inactivation occurs.

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