Do you know that the discovery of the Barr body was actually through serendipity? Yes, it was. Murray Llewellyn Barr was actually working on the effects of fatigue on the nerve cells of cats. However, he did not find any changes in the nerve cells of fatigued animals. Instead, he noticed a mass of chromatin material on the nuclear membrane of some nerve cells but not all cells. When he crossed checked the sources of those cells, he discovered that they actually all came from female cats. When he examined cells coming from other mammals including human, the same chromatin mass was also observed only in females. He later discovered that this mass of chromatin material is actually a sex chromatin.
This sex chromatin is now referred to as the “Barr body”. It represents an inactivated X chromosome. It is now known that in mammalian and human females, one of the X chromosomes becomes inactivated during development and it appears as a dark mass (Barr body) near the nuclear membrane of their cells. So a female with XX sex chromosome will always show one Barr body, while a male who has XY sex chromosome should have no Barr body on his cells.
The number of Barr bodies observed in cells is always a good indicator of the number of X chromosomes in an individual. Individuals with multiple X chromosomes will have all the X chromosome inactivated (will appear as Barr bodies) except one. Thus, as mentioned earlier, females with normal number of sex chromosomes will always show one Barr body. Some females however have XXX sex chromosomes. Their cells will thus show 2 Barr bodies. Males showing a Barr body in their cells therefore have XXY sex chromosome.
The discovery of the Barr body thus launched a new era of research on genetic disorders.