Type of epistasis where the dominant alleles A and B have the same effect and reinforce each others effect. In other words: a higher amount of dominant alleles (either A or B) in the genotype is visible in the phenotype, like in a dose-response curve. Hence, a backcross of A.B. x aabb segregates in the offspring as A.B. / {aaB. + A.bb} / aabb in a ratio of 9:[3+3]:1. This type of inheritance may occur for grain colour of cereals, for example.
RT: cumulative isomery ↔ dominant epistasis ↔ recessive epistasis ↔ reciprocal dominant epistasis ↔ reciprocal recessive epistasis
BT: isomery