A gene interaction is an interplay between multiple genes that has an impact on the expression of an organism’s phenotype. While the expression of physical traits is often described as the result of inheriting two genes, one at each allele from each parent, it is actually much more complicated. Groups of genes interact with each other, explaining why phenotypes are so variable between individual members of a species. Understanding gene interactions is an important aspect of understanding inheritance, particularly inheritance of deleterious traits.
Gene interactions can result in the alteration or suppression of a phenotype. This can occur when an organism inherits two different dominant genes, for example, resulting in incomplete dominance. This is commonly seen in flowers, where breeding two flowers that pass down dominant genes can result in a flower of an unusual color caused by incomplete dominance. If red and white are dominant, for example, the offspring might be pinkish or striped in color as the result of a gene interaction.
Sometimes, genetic traits are entirely suppressed. People with albinism may carry genes for traits that are not expressed in their phenotypes because the albinism acts to turn those genes off. This is also seen in coloration patterns in animals such as tortoiseshell cats, where the unusual hair color is the result of selective gene interactions, with genes being turned off at some locations and turned on in others.
Below you will find epistasis slideshow:
More theory about epistasis:
Big thanks to www.austincc.edu/asession/biol2316/CH13d2notes&HW.pdf, www.tnau.ac.in/eagri/eagri50/GBPR111/lec03.pdf, www-personal.une.edu.au/~jvanderw/gene251-351-lec3.pdf and www.clfs.umd.edu/classroom/bsci410-liu/BSCI410-S09/Lecture5.pdf
Finally, a summary of genetic interactions: