From: Robert J. Bradbury (bradbury@aeiveos.com)
Date: Sun Jun 22 2003 - 10:09:40 MDT
On Sat, 21 Jun 2003, Phil Osborn wrote:
> Altho, I wonder if it's true. Is one of the X's
> turned off in women? [snip]
> ... while women were able to mask a bad gene on the spare X.
> [snip]
> Is one X turned off? Or is it random from cell to cell,
> so that half of the cells get a bad gene, while the other
> half get the good one turned on?
One X chromosome is turned off by the XIST gene at random
during female development. This ends up becoming a "Barr body".
So in a technical sense femailes are a "mosaic" (a mixture of cells
with different genomes). If the X that is turned off happens
to contain an "essential" gene that is defective on the remaining
X chromosome that is on, then the cell may be at a replicative
disadvantage or may even undergo apoptosis. So there may be
a slight bias for more cells with the functional X than a
dysfunctional X in the female body.
This is different from "imprinting", where specific genes are
"stamped" to be on or off during the production of the oocytes
and sperm. This has to do, at least in one case (probably),
the the preference for the males to have large offspring
at birth with the preference for the females to have smaller
offspring at birth.
Robert
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