Lyonization
Lyonization occurs whereby only a single X chromosome is left active in the nucleus of a cell as a result of dosage compensation,So, in females (posess 2 X chromosomes) only X chromosome is active in any one cell at a time.The inactive X chromosome becomes a Barr body[1]
Which X chromosome becomes inactive is entirely random, and not all cells will have the same inactive X chromosome.[2] During development, one of the X chromosomes is randomly made inactive, and so the progeny cells also have the same X chromosome inactive. As a result of this, defferent alleles that are carried on the X chromosome are expressed in different areas of the female body.
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