Dosage compensation

Dosage compensation is a genetic regulatory mechanism which operates to equalize the phenotypic expression of characteristics determined by genes on the X chromosome so that they are equally expressed in the human XY male and the XX female. Dosage compensation also occurs in other organisms like the fruit fly Drosophila melanogaster and the round worm Caenorhabditis elegans.
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The phenomenon in women, who have two copies of genes on the X chromosome, of having the same level of the products of those genes as males (who have a single X chromosome). This is due to the process of inactivation of one of the X chromosomes in females (Lyonization). See Lyon's hypothesis.