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Epistasis is defined as "the interaction between multiple genes to produce a single phenotype (Wikipedia). In a simple model displayed in the picture below one can see that the phenotype (in this case dog coat color) is affected differently by the varying interactions between the two different genes, 1 and 2, encoding the alleles E/e and B/b respectively. For example if both copies of the recessive alleles ee are present for gene 1 then the fur coat is yellow regardless of the alleles present on gene 2. However if a single copy of the dominant allele "E" is present than coat color varies based on the alleles present for gene 2.

Epistasis plays an important role in the process of continuing the evolution of biological traits within organisms and can have an impact on species divergence, the evolution of sexual reproduction, and the evolution of foundation of individual genetic systems (Mclean). This is possible through various models or theories of epistasis including:

  • Suppressive Epistasis: Suppressive epistasis occurs when the combination of alleles of one gene results in the suppression or masking of the phenotypic effects of the alleles of the second gene (Phillips, 855). In an evolutionary sense if  two deleterious genes produce a less harmful or severe phenotype than one of the two genes alone, this results in an advantageous phenotype and increased fitness for an individual.
  • Magnitude Epistasis: Magnitude epistasis occurs when the combination of alleles of both of the genes results in more effective phenotype than one of the genes alone (Wikipedia). For example if both genes were mutated produced a more viable phenotype than either of the mutated genes alone that would increase the organisms fitness and be consid
    Fig-6-16-Epistasis 0

    Example of Epistasis in Dogs

    ered advantageous.



References:

1.WIkipedia, Epistasis Paragraphs 7-9

2. Patrick Phillips, Epistasis-the essential role of gene interactions in the structure and evolution of genetic systems, NIHPA Author Manuscripts PMC2689140, pg 855

3. Phillip Mclean, Mendelian Genetics: Gene Interactions, 2000, http://www.ndsu.edu/pubweb/~mcclean/plsc431/mendel/mendel6.htm

Image: https://online.science.psu.edu/biol011_active002/node/4230