Barr body

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A Barr body is an inactivated X chromosome in a female somatic cell. It becomes inactivated by the process of lyonization; the process is there to ensure only one X chromosome is active. It is initiated from a site called the XIC (X inactivation centre)[1]. The key products from this site are the non-coding RNA transcripts Xist and Tsix. Xist work by coating the X outwards leading to it becoming heterochromatin. Not all genes, however, are silenced and about 10-15% escape inactivation[2]. The equation for calculating Barr bodies is Xn-1, where ā€˜nā€™ refers to the number of X chromosomes present on the sex chromosome. For example, Turners Syndrome, XO, results in no Barr bodies whereas people with Klinefelter Syndrome, XXY, have 1 Barr Body.


  1. ā†‘ Augui S, Nora E P, Heard E. Regulation of X-chromosome inactivation by the x-inactivation centre. Nature reviews genetics.2011;12:429-442
  2. ā†‘ Alberts B, Johnson A, Lewis J, Raff M, Roberts K and Walter P (2008) Molecular Biology of The Cell, 5th edition, New York: Garland Science
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