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Jo Fay jmf at sanger.ac.uk
Tue Jun 27 11:25:11 EST 1995

Graham Dellaire <popa0206 at PO-Box.McGill.CA> wrote:
>Most likely imprinting (much like X chromosome inactivation in females) is a mode of limiting
>gene dosage in the cell.  Sometimes there can be such a thing as too much of a good thing <grin>!

X chromosome inactivation is not determined by imprinting, it is a random 
process, which is why females show mosaicism for X-linked diseases - some
cells have the 'good' X chromosome inactivated, and some have the 'bad' X
chromsome inactivated.

I can't explain what imprinting is ;-)  but one possible example are diseases
caused by expansion of simple tandem DNA repeats.  In some diseases, such as
Fragile X syndrome, the expanded repeat is transmitted differently by males
and females - in males the length of repeat is restricted to the premutation
range, while during female transmission the repeat can increase in size, so
that offspring of a woman with a certain size repeat are more likely to
have fragile X syndrom than the offspring of a male carrying the same size
repeat. (see Sutherland and Richards in PNAS 92 pg3636 for the review I got
this info from).


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