Answer
Lyonization of X chromosomes.
In human females one of the two X chomosomes is randomly silenced while the other is "selected" to be active. This random silencing of X chromosomes occurs in the epiblast of the blastocyst about the time of gastrulation.
Females receive one X chromosome from mother and one from father. . Maternal and apternal chromosomes have the smae probability of being silenced
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The reason for inactivation seems to be "biological justice". Since the Y chromosome has very few gens ( apart from the SRY complement), if all the genes on the two female X chromosomes remained active, that would give females an "unfair" genetic advantage over males . The inactivation of one of the feamale's X chromosomes balances thinngs out genetically. and compensates for the difference in the number of active genes in a nucleus with an X/X genotype and one with an X/Y genotype.
The inactivated female X chromosome usually appears in histological examination of cells as what is called a Barr body. The inactivation of the X chromosome ((XIC) also goes by the name of Lyonization.
