Epistasis is a term used in the field of genetics and is defined as an occurrence where one gene or more somehow changes or “masks” the expression of another gene. In these cases, the gene whose traits are not expressed is called a “hypostatic” gene, while the other genes whose traits have surfaced are called “epistatic” genes. In the theory of evolution and natural selection, epistasis is often considered a mutation and is included as an important factor in measuring a species’ biological “fitness” or its capability to survive. The word is made up of two Greek words, “epi” and “stasis,” and literally means “a stoppage.”
The concepts of epistasis and dominance may sometimes be used interchangeably, as both are similar in masking a phenotype, or a certain physical characteristic of a species. The difference, however, is that the latter concept is an interaction between two alleles, which are located in the same gene, but the former concept is an interaction of two altogether different mutant genes. Another significant difference is that dominance is not really a mutation, but is a natural process of how an offspring inherits genes from its parent. Epistasis, however, is a genetic interaction where all involved genes are considered “mutants,” thus a “double mutation” occurs.
A simple example of how epistasis modifies a genetic trait can be seen in a horse’s red coat color, which is actually produced by a mutation in a certain gene. If the horse inherits two of these mutated genes, the red color will be maintained but the shade will either be lighter or darker. A masking effect of an epistatic interaction, on the other hand, can be observed when a person inherits both albinism and redheadedness, both of which are also mutations. A person may not experience having red hair as the albinism would “mask” the color and the body would not produce any pigment, including in the hair.
The fact that epistasis is a double mutation does not necessarily mean that it does not happen as often to humans; in fact, some geneticists believe that the phenomena may cause some diseases, such as Alzheimer’s disease. In the 1990s, scientists discovered that the gene “apolipoprotein E4” increased a person’s risk of having the disease, but not all carriers acquire it, leading scientists to believe there may be other genes responsible. In the late 2000s, scientists found that apolipoprotein E4 was strongly interacting with three other genes, resulting in worse effects from Alzheimer’s. Epistasis has also been observed in patients with adult-onset diabetes, as well as in autism and cardiovascular diseases. As a result, several studies have suggested that treatments specifically targeting associated genes may effectively prevent and inhibit some diseases.