Position effect is the effect on the expression of a

gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

when its location in a

chromosome A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...

is changed, often by translocation. This has been well described in ''

Drosophila ''Drosophila'' (), from Ancient Greek δρόσος (''drósos''), meaning "dew", and φίλος (''phílos''), meaning "loving", is a genus of fly, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or p ...

'' with respect to eye color and is known as position effect variegation (PEV). The

phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology (physical form and structure), its developmental processes, its biochemical and physiological propert ...

is well characterised by unstable expression of a gene that results in the red eye coloration. In the

mutant In biology, and especially in genetics, a mutant is an organism or a new genetic character arising or resulting from an instance of mutation, which is generally an alteration of the DNA sequence of the genome or chromosome of an organism. It i ...

flies the eyes typically have a mottled appearance of white and red

sectors Sector may refer to: Places * Sector, West Virginia, U.S. Geometry * Circular sector, the portion of a disc enclosed by two radii and a circular arc * Hyperbolic sector, a region enclosed by two radii and a hyperbolic arc * Spherical sector, a ...

. These phenotypes are often due to a chromosomal translocation such that the color gene is now close to a region of

heterochromatin Heterochromatin is a tightly packed form of DNA or '' condensed DNA'', which comes in multiple varieties. These varieties lie on a continuum between the two extremes of constitutive heterochromatin and facultative heterochromatin. Both play a rol ...

. 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 A transgene is a gene that has been transferred naturally, or by any of a number of genetic engineering techniques, from one organism to another. The introduction of a transgene, in a process known as transgenesis, has the potential to change the ...

s that insert into different regions of a

genome A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...

. In this case the difference in expression is often due to

enhancers In genetics, an enhancer is a short (50–1500 bp) region of DNA that can be bound by proteins ( activators) to increase the likelihood that transcription of a particular gene will occur. These proteins are usually referred to as transcriptio ...

that regulate neighboring genes. These local enhancers can also affect the expression pattern of the transgene. Since each transgenic organism has the transgene in a different location each transgenic organism has the potential for a unique expression pattern.

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/bv.fcgi?rid=mboc4 , edition=4th Epigenetics