Barr body

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A Barr body (named after discoverer Murray Barr)[1] is the inactive X chromosome in a female somatic cell, or the inactive Z in a male[2], rendered inactive in a process called lyonization, in those species (including humans) in which sex is determined by the presence of the Y or W chromosome rather than the diploidy of the X or Z. The Lyon hypothesis states that in cells with multiple X chromosomes, all but one are inactivated during mammalian embryogenesis[3]. This happens early in embryonic development at random in mammals[4], except in marsupials and in some extra-embryonic tissues of some placental mammals, in which the father's X chromosome is always deactivated[5].

In men and women with more than one X chromosome, the number of Barr bodies visible at interphase is always one less than the total number of X chromosomes. For example, men with a 47,XXY karyotype have a single Barr body, whereas women with a 47,XXX karyotype have two Barr bodies. Barr bodies can be seen on the nucleus of neutrophils.

Contents

Mechanism

A normal human female has only one barr body per somatic cell, while a normal human male has none.

Mammalian X-chromosome inactivation is initiated from the X inactivation centre or Xic, usually found near the centromere[6]. The center contains twelve genes, seven of which code for proteins, five for untranslated RNAs, of which only two are known to play an active role in the X inactivation process, Xist and Tsix[7]. The centre also appears to be important in chromosome counting: ensuring that random inactivation only takes place when two X-chromosomes are present. The provision of an extra artificial Xic in early embryogenesis can induce inactivation of the single X found in male cells[8].

The roles of Xist and Tsix appear to be antagonistic. The loss of Tsix expression on the future inactive X chromosome results in an increase in levels of Xist around the Xic. Meanwhile, on the future active X Tsix levels are maintained; thus the levels of Xist remain low[9]. This shift allows Xist to begin coating the future inactive chromosome, spreading out from the Xic[10]. In non-random inactivation this choice appears to be fixed and current evidence suggests that the maternally inherited gene may be imprinted[11].

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