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Definition of Heterochromatin, facultative

Heterochromatin, facultative: Heterochromatin that need not always be heterochromatic but which has the faculty to return to the normal euchromatic state. The inactive X chromosome is made up of facultative heterochromatin. When a woman transmits that X to a son, it reverts to euchromatin and genetic activity.

Source: MedTerms™ Medical Dictionary
http://www.medterms.com/script/main/art.asp?articlekey=3738
Last Editorial Review: 6/14/2012

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