Codominance is a condition in which both alleles of a gene pair in heterozygote are fully expressed, with neither one being dominant or recessive to the other.
Epistasis is the suppression of the normal phenotypic of the gene by a non-allelic gene.
Complimentary genes are two independent genes present on different gene loci which assist each other in expressing a trait.
Supplementary genes are two independent genes present on different gene loci, each of may produce its own trait and interact when present together in dominant state to produce a new trait.