Fig 1. Models for transvection via enhancer action in trans and pairing-sensitive silencing.

A, mutant alleles of yellow can be placed into at least three classes based on their molecular lesions: Class A includes deletions of enhancers and insertions of insulator elements between the enhancers and the promoter; Class B includes point mutations and deletions within the core promoter and transposon insertions in either the promoter or the 5’ UTR; Class C includes point mutations and indels in the coding region. Class A and Class B alleles complement when paired, indicating robust enhancer action in trans, whereas complementation between Class A and Class C is either non-existent or too weak to be detected, indicating poor enhancer action in trans. B, a PRE fused to a mini-white transgene typically permits expression in a hemizygous transgenic insertion, but silences expression when insertions are homozygous, reflecting pairing-sensitive silencing.