Re: expression of allele in diploid and polyploid organism

There are a number of examples of expression of one allele only in a diploid organism.

First, there is the phenomenon of X-inactivation in female mammals. Female mammals are XX 
and males are XY. The X chromosome is larger than chromosomal segments consistent with 
viability in aneuploids (for example, human chromosome 21, survives in trisomy 21). In order not 
to have genic imbalance between sex-linked and autosomal genes, female mammals inactivate 
most genes on one of the two X chromosomes in most cells. The X chromosome chosen for 
inactivation is chosen randomly in each cell early in development. Thereafter, inheritance of the 
inactive X is clonal. Please see the following reviews:
 http://hmg.oxfordjournals.org/cgi/content/full/10/20/2225
 http://www.nature.com/nrg/journal/v6/n12/full/nrg1755.html

Second, there is the phenomenon of "imprinting," in which some only one allele of a pair is 
expressed. There is quite a bit of speculation on the reason that there is imprinting. Biologists 
are usually better at answering questions about mechanisms rather than about purpose. Please 
see:
 http://www.ucalgary.ca/UofC/eduweb/virtualembryo/imprinting.html
 http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/I/Imprinting.html

There are good answers on both X-inactivation and imprinting at MadSci:
 http://www.madsci.org/posts/archives/jan2002/1012165313.Ge.r.html
 http://www.madsci.org/posts/archives/2003-11/1067964711.Ge.r.html
 http://www.madsci.org/posts/archives/May2003/1053093747.Ge.r.html
 http://www.madsci.org/posts/archives/may2000/959783843.Ge.r.html
 http://www.madsci.org/posts/archives/mar2001/985199953.Dv.r.html
 http://www.madsci.org/posts/archives/jul2000/963508146.Dv.r.html
 http://www.madsci.org/posts/archives/2004-09/1095791597.Ge.r.html

Please see the MGI Glossary for definitions of terms in genetics:
 http://www.informatics.jax.org/javawi2/servlet/WIFetch?page=glossaryIndex&print=no

Yours,

Paul Szauter
Mouse Genome Informatics