8.1 Introduction

Learning Objectives

  • Discuss the concepts of recessive and dominant epistasis.
  • Recall modified dihybrid phenotypic ratios which occur due to gene interactions.

Gene interaction takes place when genes at multiple loci determine a single phenotype — when the effects of genes at one locus depend on the presence of genes at other loci. The specific type of Gene interaction whereby one gene masks the effect of another gene is called Epistasis, and there are two main types of epistasis — dominant and recessive.

Generally, when epistasis is present, the four Mendelian genotypic classes (in a dihybrid cross) produce fewer than four observable phenotypes because one gene masks the phenotypic effects of another. Often, the basis of epistasis is a gene pathway in which the expression of one gene is dependent on the function of a gene that precedes or follows it in the pathway.  In recessive epistasis, the recessive allele of one gene masks the effects of either allele of the second gene, whilst in dominant epistasis, the dominant allele of one gene, masks the effects of either allele of the second gene.

The principles of genetic analysis that we have described for a single locus (dominance/recessiveness) can be extended to the study of alleles at two different loci. While the analysis of two loci concurrently is required for genetic mapping, it can also reveal interactions between genes that affect the phenotype. Understanding these interactions is very useful for both basic and applied research. Before discussing these interactions, we will first revisit Mendelian inheritance for two loci.


Simple graphic showing a person with yellow, red, and no hair
Figure 8.1.1 The Gene for Baldness is Epistatic to Those for Red Hair or Blond Hair. The baldness phenotype supersedes genes for hair colour and so the effects are non-additive.

The video below, Gene Interaction IB Biology, by Alex Lee (2016) on YouTube, provides an introductory discussion to gene interactions (epistasis).

Media Attribution


Lee, A. (2016, January 24). Gene interaction (2016) IB biology (video file). YouTube. https://www.youtube.com/watch?v=CzBOy48AfSQ


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Open Genetics by Natasha Ramroop Singh, Thompson Rivers University is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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