MadSci Network: Genetics

Re: expression of allele in diploid and polyploid organism

Date: Mon Dec 19 09:03:50 2005
Posted By: Paul Szauter, Staff, Mouse Genome Informatics
Area of science: Genetics
ID: 1134545955.Ge

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:

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 

There are good answers on both X-inactivation and imprinting at MadSci:

Please see the MGI Glossary for definitions of terms in genetics:


Paul Szauter
Mouse Genome Informatics

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