Difference Between Complementation and Epistasis

The key difference between complementation and epistasis is that complementation is a genetic interaction in which a pair of genes often work together to create a specific phenotype, while epistasis is a genetic interaction in which one gene’s allele masks the phenotype of the other gene’s alleles.

Complementation and epistasis are two genetic interactions. In complementation, two strains of an organism having different homozygous recessive mutations and the same mutant phenotype produce offspring with wild-type phenotype when they are mated. In epistasis, some genes mask the expression of other genes in the same way a fully dominant allele masks the expression of its recessive counterpart.

CONTENTS

1. Overview and Key Difference
2. What is Complementation 
3. What is Epistasis
4. Similarities Between Complementation and Epistasis
5. Side by Side Comparison – Complementation vs Epistasis in Tabular Form
6. Summary

What is Complementation?

Complementation interaction refers to a relationship between two different strains of an organism having homozygous recessive mutations that produce the same phenotype but not residing on the same gene. When these strains are crossed with each other, some offspring show recovery of the wild-type phenotype. Therefore, this phenomenon is called “genetic complementation”.  Complementation basically occurs if the mutations are in different genes (intergenic complementation interaction). It may also take place if the two mutations are at distinct sites in the same gene (intragenic complementation interaction). But the effect is usually weaker than intergenic complementation.

In the case of mutations in different genes, each strain’s genome contributes to the wild type allele to complement the mutated allele. The offspring will display the wild type phenotype since the mutations are recessive. The complementation test (Cis trans test) was developed by the American geneticist Edward B. Lewis. This test can be used to determine whether the mutations in two strains are in different genes as complementation ordinarily will occur more weakly or not at all if the mutations are in distinct sites of the same gene. The eye colour of Drosophilais is a good model to demonstrate the complementation test.

What is Epistasis?

Epistasis is a genetic interaction where one gene’s allele masks the phenotype of the other gene’s alleles. There are mainly two types of epistatic interactions: recessive and dominant. In the recessive epistasis, the recessive allele of one gene masks the effects of either allele of the second gene. On the other hand, in the dominant epistasis, the dominant allele of one gene masks the effects of either allele of the second gene.

In epistasis, the interaction between genes is antagonistic, such that one gene masks the expression of the other gene. The alleles that are being masked is called hypostatic alleles. The alleles doing masking is known as epistatic alleles.  A well-known example of epistasis is pigmentation in mice. The wild type coat colour, agouti (AA), is dominant to coloured fur (aa). Anyhow, a separate gene (C) is necessary for pigmentation production. A mouse with recessive c allele at this locus is unable to produce pigment and is albino regardless of the allele present in locus A. Therefore, the genotypes: AAcc, Aacc, and aacc, all produce the albino phenotype. In this case, the C gene is epistatic to the A gene. Epistasis can also occur when the dominant allele masks the expression at a separate gene, as mentioned earlier. Fruit colour in summer squash is expressed in this way. The homozygous recessive expression of the W gene (ww) coupled with a homozygous dominant or heterozygous dominant expression of the Y gene (YY or Yy) in summer squash produces yellow fruit, while the wwyy (both genes recessive) genotype produces green fruit. However, if a dominant copy of the W gene is present in the homozygous or heterozygous form, the summer squash will be a white fruit regardless of the Y alleles.

What are the Similarities Between Complementation and Epistasis?

• They are two types of gene interactions.
• Both phenomena depend on alleles of genes.
• They are very important for genetic diversity and evolution.
• Both show variations from Mendel’s laws.
• Both phenomena can be observed in plants as well as animals.

What is the Difference Between Complementation and Epistasis?

The genes of an individual are not expressed isolated from one another, but they function in a common environment. Thus, it expects interactions between genes would occur. Complementation is a form of genetic interaction between non-allelic genes. For example, in complementation, when a normal copy of a gene is introduced into a cell that harbours a mutated copy, it corrects the genetic defect. In epistasis, the effect of gene mutation is dependent on the presence and absence of mutations in one or more other genes, respectively termed as modifier genes. So, this is the key difference between complementation and epistasis.

Summary – Complementation vs Epistasis

Complementation and epistasis are variations involving multiple genes. Complementation is the production of wild type phenotype by a cell or an organism that contains two mutant genes. If complementation occurs, the mutations are almost non-allelic (in different genes). On the other hand, in epistasis, one or more genes cannot be expressed because of another genetic factor hindering their expression. Thus, this the summary of the difference between complementation and epistasis.

Reference:

1. “Complementation (Genetics).” Wikipedia, Wikimedia Foundation, 7 Dec. 2020, Available here.
2. “Biology for Majors I.” Lumen, Available here.

Image Courtesy:

1. “Complementation-bn” By Sumita Roy Dutta – File:Complementation.svg (CC BY-SA 4.0) via Commons Wikimedia
2. “Epistasis” By Sciencia58 – Own work. Source: Neil A. Campbell, Jane B. Reece: Biologie. Spektrum-Verlag Heidelberg-Berlin 2003, ISBN 3-8274-1352-4, page 306 (CC0) via Commons Wikimedia