n :  material of the inactivated X chromosome present in each somatic cell of most female mammals that is used as a test of genetic femaleness (as in a fetus or an athlete) - called also sex chromatin 
 
Barr, Murray Llewellyn (b 1908), Canadian anatomist. A professor of microanatomy, Barr specialized in cytological research, especially as it applies to genetically determined anomalies of sex determination and development, cancer, and mental disease. His most notable work dealt with sex chromatin, now known as the Barr body. In 1949 he noted that females had in the nuclei of their nerve cells a mass of chromatin that males did not. Further study revealed that this sex difference in the nuclei of resting cells is found in most mammals.
 
 
 
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