Barr Bodies

Barr Bodies: heterochromatized X-chromosomes

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

    Barr body testing was used in the 1968 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 instances of Androgen Insensitivity Syndrome (AIS, formerly TFS), a genetic condition in which an XY zygote develops as a phenotypically female adult, due to failure of androgen receptors. Such individuals would test negative 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 © 2016 by Steven M. Carr