dihybrid crosses while not being restricted by them. The classical patterns of epistasis were described in the first decade of the 20th century (see e.g. Miko 2008, for a recent didactic summary of the classical ratios). Thus, the 9:7 F2 segregation characteristic of complementary gene interaction is realized when, inter alia, da = db = ha = hb = iab = jab = jba = lab in Table 1. In contrast the 15:1 F2 segregation characteristic of duplicate gene interaction arises, for example, when da = db = ha = hb = −iab = −jab = −jba = −lab. “Complementary” epistasis arises when genes form a series in a biological pathway such that failure of either component leads to failure of the pathway. “Duplicate” epistasis corresponds to systems that are buffered by redundant parallel pathways so that failure of both components is required for system failure and has commonly been associated with a strong linear component of the relationship between phenotype and fitness (see e.g. Mather 1966).
