a = factor A is dominant over a, factor B is dominant over b, and there is no gene interaction
b = factor A is epistatic of Bb and bb
c = factor A is equidirectional substituted by B, and factor B is equidirectional substituted by A
d = the dominant alleles A and B show additive interaction in the same direction, i.e. AABb = AaBB, AaBb = AAbb etc
e = factor A can only be phenotypically expressed when A and B are present due to complementary gene action