DOSAGE COMPENSATION

Although the female has two X chromosomes, compared to only one in the male, in very early embryogenesis of the female, one of the X chromosomes is inactivated allowing only one to stay active. In a female with more than two X chromosomes, all X chromosomes excepting one are inactivated. The nett effect being that females have only one functional X chromosome. The inactivated X may be maternally or paternally derived, the selection being purely random. Once inactivated the same X will continue to be inactivated in all the cells which are derived from it. This phenomenon of inactivation of X chromosomes was first described by Mary Lyon and was known as the Lyon Hypothesis, now considered as the Lyon Principle.

It has now been shown, that only parts of the inactivated X are in fact inactive, and that certain areas crucial to development remain active. This X is usually late replicating, highly coiled and condensed and can be visualised microscopically as a clear dense mass against the inner aspect of the nuclear membrane. This mass is referred to as the X chromatin body or the Barr body. The identification of this structure formed the basis of the Gender Verification Tests carried out in sporting events in the past. In these tests a buccal smear was taken from the individual and the cells were stained and examined for these X chromatin bodies. Their presence confirmed the female gender, (X chromatin positive). This test has now been abandoned owing to many ambiguities or inaccuracies in the interpretation of their results. Males are thus X chromatin negative.