The Barr body, " the inactive X chromosome in a female somatic cell,^[2]  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.^[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] (Wikipedia article on Murray Barr, accessed 3-2020).} See also Anat. Rec., 1952, 112, 709-12, and Surg. Gynec. Obstet., 1953, 96, 641-8.

Sex chromatin demonstrated in humans.