Pairing-dependent interallelic complementation was first described for the 
Ultrabithorax gene of the bithorax-complex in Drosophila by Lewis and cited as 
an example of a new phenomenon that Lewis called the "trans-vection effect." 
Several different kinds of pairing-dependent gene expression have been observed 
in Drosophila, and it is now clear that a variety of different molecular 
mechanisms probably underlie the changes in gene expression that are observed 
after disrupting chromosome pairing. Transvection in the bithorax-complex 
appears to result from the ability of cis-regulatory elements to regulate 
transcription of the promoter on the homologous chromosome. The same phenomenon 
appears to be responsible for pairing-dependent interallelic complementation at 
numerous other genes in Drosophila. Some transvection effects are dependent on 
the presence of wild-type or specific mutant forms of the protein encoded by the 
zeste trans-regulatory gene, but other transvection effects are 
zeste-independent. The ease with which chromosome aberrations can disrupt 
transvection also varies widely among different genes.