epistasis positive genetic interaction positive epistasis 2013-06-05T20:05:15Z 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 the same 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* < b < wt) OR (wt < b < a*)) AND (ab = a OR ab = b) where 'a' and 'b' are the observed phenotype values of the individual perturbations ('a*' being the most severe of the two), 'ab' is the observed phenotype value of the double perturbation, and 'wt' is the wild type phenotype value. MI:1272 PSI-MI orchard