sex chromatin, a densely staining mass within the nucleus of all nondividing cells of normal mammalian females. It represents the heterochromatin of the inactivated X chromosome. Examination of cells obtained by amniocentesis for the presence of sex chromatin is a technique used for determining the sex of a baby before birth. Sex chromatin is also found as a drumstick-shaped mass attached to one of the nuclear lobes in polymorphonuclear leukocytes in normal females. Also called Barr body. See also Lyon hypothesis.
