- Discuss the concepts of recessive and dominant epistasis.
- Recall modified dihybrid phenotypic ratios which occur due to gene interactions.
Gene interaction occurs 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. 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 depends 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, while 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 useful for both basic and applied research. Before discussing these interactions, we will first revisit Mendelian inheritance for two loci.
The video below, Gene Interaction IB Biology, by Alex Lee (2016) on YouTube, provides an introductory discussion to gene interactions (epistasis).
Lee, A. (2016, January 24). Gene interaction (2016) IB biology (video file). YouTube. https://www.youtube.com/watch?v=CzBOy48AfSQ
- Figure 8.1.1 Gene interactions, whereby genes at multiple loci determine a single phenotype. Therefore, the effects of genes at one locus depend on the presence of genes at other loci. 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. [Back to Figure 8.1.1]