Pleiotropy occurs when one gene will code and control the phenotype or expression of several different and unrelated traits. Pleiotropy is most often a bad thing, as many diseases are due to a gene controlling so many traits at once. 

For example, phenylketonuria is a disease caused by pleiotropy.

Due to the phenomenon of dominance a recessive allele remains obscure in the hybrid. But when two different genes which are not alleles, both affect the same character in such a way that the expression of one masks, inhibits or suppresses the expression of the other gene, it is called epistasis. The gene that suppresses is said to be epistatic, and the gene which remains obscure is hypostatic.

Complementary genes are the genes which are present on different genetic loci but interact with each other to express a single character in combination. That is they both together produce a particular phenotypic trait in an individual. Supplementary genes are the genes which include two pairs of non-allelic genes.