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Graham Dellaire popa0206 at PO-Box.McGill.CA
Mon Jun 26 15:08:40 EST 1995

>   sbassi at CRIBA.EDU.AR (Sebastian Bassi) writes:
>  Hello,
>  I'm  studing  medicine,  and  I'd  like  to  understand  what  is
>  IMPRINTING.   I   read  books  of  genetics  (like  'Genetics  in
>  medicine', by Thompson & Thompson), but I couldn't understand it.
>  I'd  like  that  someone  could  help,  with  a  'plain   english
>  explanation'.
>  I  know  how is defined, but I think would be nice some  example,
>  with pedigree.
>  Regards,
>  Sebastian Bassi
>  email: sbassi at criba.edu.ar
Okay here goes.... Imprinting as I understand it (by all means this subject 
is greatly open to debate as to the true nature of imprinting)

involves the differential silencing (or expression, depends on point of view of "normal state")
of a gene copy (allele) in an individual organism (zygote) depending on the sex of parent that the
gene copy was recieved from (or as they say in the vernacular "gamete sex dependent").

So lets say you inherit a copy of gene Y from your DAD and a copy of the same gene from your
MOM.  Then lets distinguish each alleles from each other as allele Ya , Yb, Yc etc
*=expressed and {}=silenced

              DAD ------- MOM
            Yb*{Yd} |    Ya*{Yc}

Now this is one of four sets of alleles possible. 
So if you noticed even though the Ya allele was expressed in you MOM it is not expressed in YOU!
This means that when you inherit the gene copy (allele) from your DAD it is expressed
and when you inherit it from your MOM it is silenced.  This is called imprinting...  whether you expressed
the copy of the gene or not was dependent on the sex of the donor of that allele (ie MOM or DAD).

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>!

Now the mechanism of Imprinting is under great debate.  It is safe for now to say it is often 
associated with methylation of nearby CpGp islands (groups of CGCG repeats that often coincide with
the start of coding regions or are within coding regions of the genome).  Whether methylation is the
cause or effect of imprinting is debatable.  Also methylation is not always correlated with silencing of the
gene either (look for the Ig2f and Ig2f-receptor genes in which one is expressed and the other silenced
upon methylation of the same locus, though which is silenced in the presence of methylation escapes me 
right now <grin>).   

Another mode of imprinting may involve the association of heterochromatin-assoc. proteins early in 
development with a given region to be silenced (ie altering the chromosomal structure at that sight
such that perhaps transcription factors no longer have access to the DNA).

I hope this helps a little.....

Imprinting is a tough one : )

G. ______________________________________________________________________ 
Graham Dellaire			    Snail Mail:
                                    Red Cross, Research		
McGill University                   Montreal Blood Services	  	
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