To facilitate clear understanding of interaction between genes, cases involving only two genes were used as examples to illustrate the concept. However, in actual practice in many cases, several genes may be involved in the expression of same character and may interact in a rather complex
manner. This may be understood if we realize that a phenotypic character is the end product of several steps involved in a biochemical pathway, and each step may depend on a gene. One such example is the study of coat colour in guinea pigs, which involves a number of gene pairs including the following :
- EE, black; ee, yellow; epep, tortoise shell (yellow and black).
- AA, agouti (wild type, black and yellow banded hairs); aa, no agouti, only black.
- BB, black; bb, brown.
- CC, no colour dilution; Ck, Cd, Cr, Ca, various degrees of colour dilution; CkCk, yellow; CaCa, albino.
- PP, no dilute; pp, dilution of black or brown, but not yellow.
- SS, no spotting; ss, spotting; Ss occasional spots.
- SiSi, unsilvered; sisi, silvered effect.
The above different genes act at different stages of pigment development and interact in a complex manner. For a more detailed study, the reader is advised to consult the article by Sewall Wright (1963) on 'Gene Interaction' in the book "Methodology in Mammalian Genetics".