Consequently, assortment of genes was presumed from the assortment of characters. It is, however, not necessary that a single character is controlled by one gene only, instead it may be controlled by more than one gene. In such a situation two or more than two genes may interact to give rise to a particular phenotype. For instance if gene A is responsible for phenotype A, and gene B is responsible for phenotype B, both A and B when present together may give rise to a phenotype C. Nevertheless, these genes still obey the principles of segregation and independent assortment.
If A and B are two genes, both dominant over their respective recessive alleles a and b, then the interaction will depend upon (i) the presence of both dominant alleles A and B, (ii) the absence of A, (iii) the absence of B or (iv) the absence of both A and B.
As indicated above the interaction is possible between more than two genes also, but due to the complexity involved in those cases, interactions involving only two genes will be considered in detail in this section, and only a summary of interactions involving more than tw.o genes will be presented.