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

A barr body in a mammalian female cell represents