|MadSci Network: Genetics|
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
Try the links in the MadSci Library for more information on Genetics.