positive epistasis maximal epistasis 2013-06-05T20:11:27Z An effect in which individual perturbations of two different genes result in the same mutant phenotype to varying degrees of severity/penetrance and the resulting phenotype of their combination is equal in severity/penetrance to the most severe/penetrant of the individual perturbations. With respect to any single quantifiable phenotype, this may be expressed as an inequality as: ab = a* < b < wt OR wt < b < a* = ab 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:1273 PSI-MI orchard quantitative epistasis