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.
The fruit fly is famously extensively studied in genetics and much of the understanding of how gene interactions works comes from working with the fruit fly in lab environments. In organisms like humans, where genetic experimentation is viewed as unethical, geneticists are forced to rely on data from the existing population to learn about dominant and recessive traits and to see how groups of genetic traits can interact. A gene interaction is the result of inheriting genes that conflict in some way, making it impossible for all of them to express as coded, or of inheriting a set of interrelated genes that interact with each other to express a phenotype.
Sometimes a gene interaction limits production of certain proteins, often quite early in fetal development. In other instances, it can interfere with the coding of proteins to result in a garbled expression of a physical trait. Hybrids often show a variety of interesting results of gene interactions. In some cases, the interactions are beneficial and may develop into their own genetic traits, while in others, the gene interaction may create a disadvantage and those traits will eventually die out.