Position effect is the effect on the expression

Gene expression

Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...

 of a gene

Gene

A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

 when its location in a chromosome

Chromosome

A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...

 is changed, often by translocation

Chromosomal translocation

In genetics, a chromosome translocation is a chromosome abnormality caused by rearrangement of parts between nonhomologous chromosomes. A gene fusion may be created when the translocation joins two otherwise separated genes, the occurrence of which is common in cancer. It is detected on...

. This has been well described in Drosophila

Drosophila melanogaster

Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...

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

The phenotype

Phenotype

A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...

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

Mutant

In biology and especially genetics, a mutant is an individual, organism, or new genetic character, arising or resulting from an instance of mutation, which is a base-pair sequence change within the DNA of a gene or chromosome of an organism resulting in the creation of a new character or trait not...

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

Mosaic (genetics)

In genetic medicine, a mosaic or mosaicism denotes the presence of two populations of cells with different genotypes in one individual who has developed from a single fertilized egg...

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

Heterochromatin

Heterochromatin is a tightly packed form of DNA, which comes in different varieties. These varieties lie on a continuum between the two extremes of constitutive and facultative heterochromatin...

. The heterochromatin can spread stochastically and switch off the color gene resulting in the white eye sectors.

Position effect is also used to describe the variation of expression exhibited by identical transgene

Transgene

A transgene is a gene or genetic material that has been transferred naturally or by any of a number of genetic engineering techniques from one organism to another....

s that insert into different regions of a genome

Genome

In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....

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

Enhancer (genetics)

In genetics, an enhancer is a short region of DNA that can be bound with proteins to enhance transcription levels of genes in a gene cluster...

 that regulate neighboring genes. These local enhancers can also effect the expression pattern of the transgene. Since each transgenic organism

Genetically modified organism

A genetically modified organism or genetically engineered organism is an organism whose genetic material has been altered using genetic engineering techniques. These techniques, generally known as recombinant DNA technology, use DNA molecules from different sources, which are combined into one...

has the transgene in a different location each transgenic organism has the potential for a unique expression pattern.