Medical 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.

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Last Editorial Review: 9/7/2018