Mendel described every character is influenced by the particular expression of the gene, but when the characters are controlled by the combination of more than one gene, called gene interaction.

Complementary gene interaction is first discovered by William Bateson and Reginald Punette in 1906. 9:7 ratio shows the 9 offspring have both dominant genes, while 7 is having either one dominant or both recessive.

Codominance is the phenomenon where both the alleles of the gene pair in a heterozygote are expressed and resulting in offspring which is neither dominant nor recessive. Example: AB blood group.

It is a non-allelic suppression which involves two pairs of alleles. It can suppress both dominant or recessive alleles.

Epistasis can be of many types based on the ability of an allele in masking and modification of gene effects. It can be of dominant, recessive, complementary or supplementary.