genetic epistasis (sensu Bateson) qualitative genetic epistasis orchard An effect in which individual perturbations of two different genes result in different mutant phenotypes (which are traits that cannot be measured on the same scale and, hence, qualitatively different), and the resulting phenotype of their combination is equal to that of only one of the perturbations. 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 phenotypes of the individual perturbations, 'ab' is the observed phenotype of the double perturbation, 'wt' is the wild type phenotype and 'a != b' indicates qualitatively different phenotypes. MI:1277 2013-06-05T20:26:38Z PSI-MI