Dosage Compensation – Barr Body

Because females in an XY system carry two X chromosomes while males carry only one, there is a built-in dosage problem: without some correction, females would produce twice as much of every X-linked gene product as males. Mammals solve this through dosage compensation — permanently inactivating one of the two X chromosomes in every female somatic cell, so that both sexes end up with only one functional X chromosome per cell. Barr and Bertram first observed the physical signature of this in 1949, as a small, densely condensed body in the nerve cells of female cats that was simply absent from male cells; they called it sex chromatin, and it is now known as the Barr body.

Mary Lyon's hypothesis (Lyonisation) explained what that condensed body actually is: an inactivated X chromosome that has coiled itself tightly into heterochromatin, making it visible under the microscope as a distinct, stainable clump. Because inactivation always leaves exactly one X chromosome active per cell, the number of Barr bodies follows a simple rule — the N-1 rule, where N is the total number of X chromosomes present in the cell. A normal XX female therefore shows one Barr body ($2-1=1$); an XO female (Turner syndrome) shows none ($1-1=0$); and an XXY male (Klinefelter syndrome) shows one ($2-1=1$), exactly like a normal female, because Lyonisation counts only X chromosomes and does not care whether a Y is present. …