Re: Barr body chromosome selection

X chromosome inactivation in female mammals results from condensation or 
packaging of one of the two copies of the X chromosome into heterochromatin; this 
highly-packaged X chromosome is observable as a darkly stained Barr body in 
interphase cells. The reason that one of the two X chromosomes in females is 
inactivated is to provide "dosage compensation" of gene products encoded on the 
X; since females have two Xs, if both remained transcriptionally active, females 
would have twice the level of X-encoded gene products as males, thus one copy of 
the X chromosme is inactivated by packaging it into heterochromatin.

The "decision" of which of the two X chromosomes is inactivated is made in the 
early stages of female development and occurs at radom in each of the cells of the 
developing embryo. However, subsequent cells that arise by mitosis inherit the 
inactive X from these progenitor cells. Each embryonic cell decides independently 
which X will be inactivated; in some cells it will be the maternal X in others the 
paternal X.  Regardless, every cell that arises after the decision will inherit that 
"decision".

A small number of genes on the inactivated X chromosome remain transcriptionally 
active even after packaging into heterochromatin; most of these genes are active 
on the active copy of X too. One gene that is not transcribed on the unpackaged X, 
but is active on the packaged X chromosome is called Xist (X inactivation specific 
transcript). Deletion of this gene abolishes the ability of that X chromosome from 
being packaged into heterochromatin. The Xist transcript does not appear toleave 
the nucleus and it has been suggested that the Xist transcript may associate with 
the X chromosome that produces it and in association with other proteins lead to 
the inactivation of that X chromosome.