X-chromosome inactivation refers to the phenomenon in a female by which either the paternal or maternal X chromosome is inactivated randomly in each early embryonic cell, with subsequent inactivation of that same X chromosome in all cells descended from that cell. This ensures that the amount of gene product from the X chromosome is the same in males (who have only one X chromosome, and a Y chromosome) and females (who have two X chromosomes). X inactivation is also called 'lyonization' after the English geneticist, Dr Mary Lyon who first described it.
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