APA Dictionary of Psychology

a condensed mass of chromatin that is observed in the nucleus of nondividing somatic cells of females. It is not observed in the somatic cells of normal males. The substance represents material within the X chromosome that is not involved in somatic-cell metabolism. The presence of sex chromatin in somatic cells is generally regarded as proof of the sexual identity of females. Also called Barr body. See also chromatin negative; chromatin positive.