Transvection is an epigenetic phenomenon that results from an interaction between an allele on one chromosome and the corresponding allele on the homologous chromosome. Transvection can lead to either gene activation or repression. It can also occur between nonallelic regions of the genome as well as regions of the genome that are not transcribed.
The first observation of mitotic (i.e. non-meiotic) chromosome pairing was discovered via microscopy in 1908 by Nettie Stevens. Edward B. Lewis at Caltech discovered transvection at the bithorax complex in Drosophila in the 1950s. Since then, transvection has been observed at a number of additional loci in Drosophila, including white, decapentaplegic, eyes absent, vestigial, and yellow.  As stated by Ed Lewis, "Operationally, transvection is occurring if the phenotype of a given genotype can be altered solely by disruption of somatic (or meiotic) pairing. Such disruption can generally be accomplished by introduction of a heterozygous rearrangement that disrupts pairing in the relevant region but has no position effect of its own on the phenotype" (cited by Ting Wu and Jim Morris 1999). Recently, pairing-mediated phenomena have been observed in species other than Drosophila, including mice, humans, plants, nematodes, insects, and fungi. In light of these findings, transvection may represent a potent and widespread form of gene regulation.
Interestingly, transvection appears to be dependent upon chromosome pairing. In some cases, if one allele is placed on a different chromosome by a translocation, transvection does not occur. Transvection can sometimes be restored in a translocation homozygote, where both alleles may once again be able to pair. Restoration of phenotype has been observed at bithorax, decapentaplegic, eyes absent, and vestigial, and with transgenes of white. In some cases, transvection between two alleles leads to intragenic complementation while disruption of transvection disrupts the complementation.
Transvection is believed to occur through a variety of mechanisms. In one mechanism, the enhancers of one allele activate the promoter of a paired second allele. Other mechanisms include pairing-sensitive silencing and enhancer bypass of a chromatin insulator through pairing-mediated changes in gene structure.
- Stevens NM (1908). "A study of the germ cells of certain Diptera with reference to the heterochromosomes and the phenomena of synapsis". J Exp Zool 5 (3): 359–374. doi:10.1002/jez.1400050304.
- Lewis EB (1954). "The theory and application of a new method of detecting chromosomal rearrangements in Drosophila melanogaster". The American Naturalist 88 (841): 225–239. doi:10.1086/281833.
- Gelbart WM, Wu CT (1982 Oct). "Interactions of zeste mutations with loci exhibiting transvection effects in Drosophila melanogaster". Genetics 102 (2): 179–89.
- Duncan IW (2002). "Transvection effects in Drosophila". Annu. Rev. Genet. 36: 521–56. doi:10.1146/annurev.genet.36.060402.100441. PMID 12429702.
- Kennison JA, Southworth JW (2002). "Transvection in Drosophila". Adv. Genet. Advances in Genetics 46: 399–420. doi:10.1016/S0065-2660(02)46014-2. ISBN 978-0-12-017646-5. PMID 11931232.
- Pirrotta V (July 1999). "Transvection and chromosomal trans-interaction effects". Biochim. Biophys. Acta 1424 (1): M1–8. doi:10.1016/S0304-419X(99)00019-0. PMID 10456029.
- McKee BD (March 2004). "Homologous pairing and chromosome dynamics in meiosis and mitosis". Biochim. Biophys. Acta 1677 (1–3): 165–80. doi:10.1016/j.bbaexp.2003.11.017. PMID 15020057.
- Wu CT, Morris JR (April 1999). "Transvection and other homology effects". Curr. Opin. Genet. Dev. 9 (2): 237–46. doi:10.1016/S0959-437X(99)80035-5. PMID 10322135.
- Sandhu KS, Shi C, Sjölinder M, Zhao Z, Göndör A, Liu L, Tiwari VK, Guibert S, Emilsson L, Imreh MP, Ohlsson R. Genes Dev. (2009-11-15). Nonallelic transvection of multiple imprinted loci is organized by the H19 imprinting control region during germline development 23 (22). pp. 2598–603. PMID 19933149.
- Rassoulzadegan M, Magliano M, Cuzin F (2002 Feb 1). "Transvection effects involving DNA methylation during meiosis in the mouse". EMBO J. 21 (3): 440–50. PMID 11823436.
- Lee AM, Wu CT (2006 Dec). "Enhancer-promoter communication at the yellow gene of Drosophila melanogaster: diverse promoters participate in and regulate trans interactions". Genetics 174 (4): 1867–80. PMID 17057235.
- Ou SA, Chang E, Lee S, So K, Wu CT, Morris JR (2009 Oct). "Effects of chromosomal rearrangements on transvection at the yellow gene of Drosophila melanogaster". Genetics 183 (2): 483–96. PMID 19667134.