Epistasis - Functional or Mechanistic Classification

Functional or Mechanistic Classification

• Genetic suppression

the double mutant has a less severe phenotype than either single mutant.

• Genetic enhancement

the double mutant has a more severe phenotype than one predicted by the additive effects of the single mutants.

• Synthetic lethality or unlinked non-complementation

two mutations fail to complement and yet do not map to the same locus.

• Intragenic complementation, allelic complementation, or interallelic complementation

  two mutations map to the same locus, yet the two alleles complement in the heteroallelic diploid. Causes of intragenic complementation include:

  • homology effects such as transvection, where, for example, an enhancer from one allele acts in trans to activate transcription from the promoter of the second allele.
  • trans-splicing of two mutant RNA molecules to produce a functional RNA.
  • At the protein level, another possibility involves proteins that normally function as dimers. In a heteroallelic diploid, two different abnormal proteins could form a functional dimer if each can compensate for the lack of function in the other.