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Definition of Facultative heterochromatin

Facultative heterochromatin: Chromosome material that can be either heterochromatin or euchromatin.

The inactive X chromosome is made up of facultative heterochromatin. When a woman transmits that X chromosome to a son, the chromosome reverts to the euchromatic state and to genetic activity.

Source: MedTerms™ Medical Dictionary
http://www.medterms.com/script/main/art.asp?articlekey=31987
Last Editorial Review: 6/14/2012

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