# Position effect

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{{short description|Effect on the expression of a gene when its location in a chromosome is changed}}
'''Position effect''' is the effect on the [expression](/source/Gene_expression) of a [gene](/source/gene) when its location in a [chromosome](/source/chromosome) is changed, often by [translocation](/source/Chromosomal_translocation).  This has been well described in ''[Drosophila](/source/Drosophila_melanogaster)'' with respect to eye color and is known as [position effect variegation](/source/position_effect_variegation) (PEV).<ref>{{cite journal |vauthors=Weiler K, Wakimoto B |title=Heterochromatin and gene expression in Drosophila |journal=Annu Rev Genet |volume=29 |issue= 1|pages=577–605 |year= 1995|pmid=8825487 |doi=10.1146/annurev.ge.29.120195.003045}}</ref>

The [phenotype](/source/phenotype) is well characterised by unstable expression of a gene that results in the red eye coloration.  In the [mutant](/source/mutant) flies the eyes typically have a mottled appearance of white and red [sectors](/source/Mosaic_(genetics)).  These phenotypes are often due to a chromosomal translocation such that the color gene is now close to a region of [heterochromatin](/source/heterochromatin).  Regions of heterochromatin can spread and influence transcription, which may result in the cessation of gene expression and subsequently, white eye sectors.

''Position effect'' is also used to describe the variation of expression exhibited by identical [transgene](/source/transgene)s that insert into different regions of a [genome](/source/genome).  In this case the difference in expression is often due to [enhancers](/source/Enhancer_(genetics)) that regulate neighboring genes.  These local enhancers can also affect the expression pattern of the transgene.  Since each [transgenic organism](/source/Genetically_modified_organism) has the transgene in a different location each transgenic organism has the potential for a unique expression pattern.

==References==
<references />

==External links==
*{{cite book |chapter=Figure 4-45. Position effects on gene expression in two different eucaryotic organisms |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.figgrp.665 |author=Alberts, Bruce |title=Molecular biology of the cell |publisher=Garland Science |location=New York |year=2002 |isbn=0-8153-4072-9 |url=https://www.ncbi.nlm.nih.gov/books/NBK21054/ |edition=4th}}

Category:Epigenetics

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