epistatis true An effect in which individual perturbations of two different genes result in different mutant phenotypes, and the resulting phenotype of their combination (the double mutant) is equal to that of only one of the perturbations. With respect to any single quantifiable phenotype, this may be expressed as an inequality as: (a != wt AND b != wt AND a != b) AND (ab = a OR ab = b) where 'a' and 'b' are the observed phenotype values of the individual perturbations, 'ab' is the observed phenotype value of the double perturbation, 'wt' is the wild type phenotype value and 'a != b' indicates qualitatively or quantitatively different phenotypes. epistatic genetic interaction MI:0930 PSI-MI