genetic interaction (sensu unexpected) MI:0208 A genetic interaction refers to an unexpected phenotype not easily explained by combining the effects of individual genetic variants (7). Fisher epistasis Two genes A and B 'genetically interact' when the phenotype generated as the result of mutations in both genes (double mutant ab) is unexpectedly not just a combination of the phenotypes of the two single mutants a and b. A quantitative genetic interaction definition has two components: a quantitative phenotypic measure and a neutrality function that predicts the phenotype of an organism carrying two noninteracting mutations. Interaction is then defined by deviation of a double-mutant organism's phenotype from the expected neutral phenotype. An effect in which two genetic perturbations, when combined, result in a phenotype that does not appear to be merely explained by the superimposition or addition of effects of the original perturbations. ab (not=) E PSI-MI More generally, a genetic interaction can be defined as the difference between an experimentally measured double-mutant phenotype and an expected double-mutant phenotype, the latter of which is predicted from the combination of the single-mutant effects, assuming the mutations act independently. genetic interaction