position effect

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The change in the expression of a gene accompanying a change in the position of the gene with respect to neighboring genes. The change in position may result from crossing over or from a chromosomal aberration. Position effects are of two types: the stable (S) type and the variegated (V) type. S-type position effects are also called cis-trans position effects. S-type position effects involve cistrons that possess at least two mutated sites separable by intragenic recombination. In the cis configuration (m 1 m 2/++) a normal phenotype is observed, whereas in the trans configuration (m 1+/+m 2) a mutant phenotype is produced. A reasonable explanation for such an observation would be that the mRNA transcribed from a (++) chromatid would function normally, whereas the mRNAs transcribed from (m 1 m 2), (m 1+), or (+m 2) chromatids would not. V-type position effects generally involve the suppression of activity of a wild-type gene when it is placed in contact with heterochromatin because of a chromosome aberration. Under some conditions, the gene may escape suppression, and consequently the final phenotype may be variegated, with patches of normal and mutant tissues. See Chronology, 1925, Sturtevant; 1936, Schultz; 1945, Lewis; heterochromatization, transvection.

Subjects: Genetics and Genomics.

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