The inactivated form of the X chromosome present in such a cell that constitutes more than one X chromosome is known to be a Barr body. It is named after the scientist Murray Barr who discovered it. This X chromosome is made inactive in those species who have XY sex determination patterns and this process is termed as lyonization. The Lyon hypothesis explains that in the case of mammalian cells which are composed of multiple X chromosomes, one of the X chromosomes gets inactivated during the formation of the embryo.
This phenomenon occurs in almost all mammals except the marsupials. The humans in which there is more than one X chromosome, the total number of these small, well-defined bodies is always one less than the total number of X chromosome. These chromosomes are clearly visible during the interphase of the cell cycle.
The people which have Klinefelter syndrome with 47, XXY karyotype have 1 Barr body whereas those with 47, XXX karyotype have 2 Barr bodies.
