Barr body The condensed, single X-chromosome, appearing as a densely staining mass, that is found in the nuclei of somatic cells of female mammals. It is named after its discoverer, Murray Barr, and is derived from one of the two X-chromosomes which becomes inactivated. The number of Barr bodies is thus one less than the number of X-chromosomes. Barr bodies are commonly referred to as sex chromatin. The human abnormalities called Kleinefelter's syndrome and Turner's syndrome both result from an unnatural presence or absence of a Barr body. In the case of the former, the male possesses a Barr body that it would normally not have, and in the latter case the Barr body is absent. See MARY LYON EFFECT.

Barr body A structure consisting of a condensed X chromosome (see sex chromosome) that is found in nondividing nuclei of female mammals. The presence of a Barr body is used to confirm the sex of athletes in sex determination tests. It is named after the Canadian anatomist M. L. Barr (1908–95), who identified it in 1949.

Barr body (bar) n. see sex chromatin. [ M. L. Barr (1908–95), Canadian anatomist]