epistasis quantitative epistasis 2013-06-05T20:47:35Z An effect in which individual perturbations of two different genes result in different mutant phenotypes (which are traits measured on the same quantitative scale but each significantly deviating, in any direction, from wild type), and the resulting phenotype of their combination 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 quantitatively different phenotypes. MI:1284 PSI-MI orchard