Alleles, both of which interact to produce a trait, which may or may not resemble either of the parental types.
Alleles, each of which produces an independent effect in a heterozygous condition.
A gene expresses itself, suppressing the phenotypic effect of its alleles.
Genes that are similar in phenotypic effect when present separately, but when together, interact to produce a different trait.
Co-dominance is the phenomenon when the two genes neither show dominant-recessive relationship nor show intermediate condition, but both of them express themselves simultaneously. This has been reported in roan character of cattle (i.e., patches of 2 different colours on the skin).