<P> In humans with more than one X chromosome, the number of Barr bodies visible at interphase is always one fewer than the total number of X chromosomes . For example, men with Klinefelter syndrome (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 . </P> <P> A genotypical human female has only one Barr body per somatic cell, while a genotypical human male has none . </P> <P> Mammalian X-chromosome inactivation is initiated from the X inactivation centre or Xic, usually found near the centromere . 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 . The centre also appears to be important in chromosome counting: ensuring that random inactivation only takes place when two or more 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 . </P> <P> 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 . This shift allows Xist to begin coating the future inactive chromosome, spreading out from the Xic . In non-random inactivation this choice appears to be fixed and current evidence suggests that the maternally inherited gene may be imprinted . </P>

What is the composition of a barr body