Barr body
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Barr body X chromosome inactivation definition authoritative source
A Barr body is the condensed, inactive X chromosome seen as a small dark-staining mass of sex chromatin in the nucleus during interphase.
- It results from X-chromosome inactivation (lyonization), which equalizes X-linked gene dosage between individuals with different numbers of X chromosomes.
- It is usually near the nuclear membrane.
- Number of Barr bodies = number of X chromosomes - 1.
Examples:
- 46,XX: 1 Barr body
- 46,XY: 0
- 45,X (Turner syndrome): 0
- 47,XXY (Klinefelter syndrome): 1
- 47,XXX: 2
In people with two X chromosomes, inactivation is generally random early in embryonic life, producing cellular mosaicism. Some genes on the inactive X can escape inactivation. NCBI overview
Source: Ganong's Review of Medical Physiology, Sex Chromatin; Histology: A Text and Atlas, section on chromatin.
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