Barr Bodies

Barr Bodies: heterochromatinized X-chromosomes

    In mammals, males are heterogametic (XY) and females homogametic (XX). Dosage compensation is achieved by inactivation at random of one of the two X chromosomes. The heterochromatized X chromosome appears as a darkly-staining bodies attached to the nuclear membrane. The phenomenon was first described in cats by Dr Murray L. Barr, a Canadian cytogeneticist, and heterochromatin bodies are now called Barr Bodies. [The other dark bodies within the nucleus are nucleoli, which represent repetitive rDNA genes].

    Barr body testing was introduced in the 1966 Olympic games, in an effort to detect male athletes supposedly trying to "pass" as females to gain a competitive advantage.
Teams from eastern Europe were particularly suspect. Such allegations had been made for many years, and a number of athletes were stripped of their medals as a result of ambiguous genital sex. Barr Body testing never detected deliberate fakery. It did however detect a number of cases of testicular feminization syndrome (TFS), a genetic condition in which an XY (male) zygote develops as a phenotypically female adult, due to failure of androgen receptors. Such individuals would test positive for the presence of a Barr body. In most if not all cases, the athletes were themselves unaware of their condition.

Figure after M. Barr (1963) by SM Carr; all text material © 2015 by Steven M. Carr