Fig. 5.

Transvection was first observed within the bithorax complex as genetic lesions removing promoter and cis-regulatory elements were easily available. Mutations that delete the promoter region are usually homozygous lethal, whereas the cis-regulatory deletions are homozygous viable, displaying hypomorphic phenotypes or lethality. When these two mutations were combined to form a trans-heterozygous genetic complementation was observed. This can be explained only by the ability of the cis-regulatory element to activate the gene in trans (green arrow) present in the homologous chromosome due to pairing. Hence, the genetic complementation of a promoter deletion with a cis-regulatory deletion is termed as transvection