Epistasis Definition, Types, Examples

Epistasis refers to the condition in which the impact of a particular gene is dependent upon having one or more genes.

In the beginning, this term was meant to mean that the phenotypic effects of one gene can be disguised by another gene.
In epistasis, the genetic code which performs the masking is known as epistatic genes. The gene that is responsible for masking the effect is hypostatic. Epistatic genes could be dominant or recessive in their results.

Epistasis is a Greek word that means standing over.
It was first introduced during 1909 from Bateson to describe the effect of masking.
An interaction between two pairs of loci in which the phenotypic effects of one locus is dependent on the genotype of the other locus.

Definition of Epistasis

Epistasis is the interplay between genes that influence the character. Genes may mask one to the point that one gene is thought to be “dominant” or they can be combined to create a new characteristic. It’s the relation between two genes that could be the basis for a particular phenotype of certain characteristics. Each locus has two alleles that define the phenotypes. They may affect each other in a manner that regardless of the particular allele of one gene it is recessive another dominant variant of. A different expression for epistasis requires a ratio chart, or table.

Two genes can yield four alleles in total, meaning you have 16 pairings that could be created. The 16 allele pairs translate into 16 genotypes. However, not all combinations are the same because of recessive and dominant characteristics of recessive and dominant alleles. Four alleles are present, and a 4-x4 chart can serve as an illustration of the sixteen different combinations of alleles. The chart below provides the color within one of the species of bee.

It is not a proof of epistasis however, it is intended as an example for introducing the idea of various combinations of alleles that result in various kinds of phenotypes. It is important to remember that certain combinations, even though distinct, can produce the identical characteristic.

Types of Epistasis

There are six types of epistasis-related gene interactions:

1. Dominant epistasis

If a dominant allele at one locus is able to mask of two alleles (dominant as well as recessive) at a different locus it is referred to as epistasis dominant. This means that an expression pattern of one recessive or dominant allele is blocked by a different dominant gene. This is also known as epistasis simple.

Example of Dominant epistasis

A prime example of dominant epistasis can be seen in the colour of fruit during summer squash. There are three kinds of colors for the fruit in this particular cucumber, namely. green, yellow and white. White color will be controlled by the dominant gene, and yellow color is controlled by the dominant gene G. White is dominant over both green and yellow.
They are grown in recessive conditions (wwgg). A cross between two plants that have yellow and white fruits has produced F1 that had white fruits. Inter-mating between F1 plants resulted in plants that have green, yellow, and white colored fruits in F2 in a 12 : 3 : 1 ratio. This is explained by following.

In this case, W is dominant to w and epistatic to alleles G and. This means that it masks the expression of alleles G/g. Thus, in F2 plants, those with W-G-(9/16) and W-gg (3/16) genotypes will yield white fruits. Those that carry wwG-(3/16) will yield yellow fruits, while those with the wwgg (1/16) genotypes will grow green fruits.

So the dihybrid normal ratio 9:3 : 3: 1 is changed to 12:3:1 ratio in the F2 generation. Similar gene interactions is reported for the colour of mice’s skin and the colour of the seed coat in barley.

2. Recessive epistasis

When recessive alleles from one location block their expression by expressing both (dominant as well as recessive) alleles at the other site, it is referred to in the field of recessive epistasis. This kind of gene interaction is also referred to as epistasis with supplementary.

Example of Recessive epistasis

A great example of this gene interaction can be seen in grain color in maize.

There are three colors of maize grain that are. red, purple and white. The purple color develops in when two genes dominate (R as well as P) Red colour is seen when there is an R dominant gene, and white in homozygous resissive disorder (rrpp).
The cross is between the white (RRPP) as well as white (rrpp) grain colouring strains of maize plants that have purple hues in F1. In-mating these F1 plants resulted in progeny that had white, red and purple grains that are in F2 in the ratio 9 : 3: 4.
Here allele r is recessive to R, but epistatic to alleles P and p. In F2, all plants with R-P-(9/16) will have purple grains and those with R-pp genotypes (3/16) have red grain colour. The epistatic allele with homozygous status will yield the plants that have white grains, derived from rrP-(3/16) and the rrpp (1/16) variants.

The normal segregation ratio 9 : 3:3:1 changes to 9:3 4 in the F2 generation. This type of interaction can also be seen for coat color in mice, bulb colour of onions, and for specific characteristics in a variety of other organisms.

3. Dominant inhibitory

In this epistasis type an allele that is dominant at one location can block it’s expression by both (dominant as well as recessive) alleles at the second locus. This is also referred to as an inhibitory gene interaction.

Example of Dominant inhibitory

A prime example of gene interaction is found for anthocyanin pigmentation in rice.

The green color of plants is controlled through the I gene that dominates over the purple colour. The color purple is controlled by the dominant gene called P. If an intercross was made between the green (IIpp) with purple (iiPP) plants, The F1 plant was green. In-mating F1 plants led to the green and purple plants with the ratio 13:3 in F2. This is explained in the following manner.

The allele I is epistatic to alleles P and p. Thus, that in the F2 genotype, plants bearing I-P-(9/16), I-pp (3/16) and iipp (1/16) genotypes are green since I can block the effects of p or P. Plants that have iiP-(3/16) will turn purple as I is not present.
In this manner, the usual dihybrid segregation rate 9 : 3:3:1 is changed to a 13:3 ratio. Similar gene interactions can be observed for grain color on maize and plumage color in poultry, and some characters from other crops.

4. Duplicate epistasis

If a dominant allele at one of two loci is able to block recessive alleles that are present at both locations, this is referred to as epistasis with duplicate dominance. It is also known as double gene activity. An excellent illustration of epistasis with a dominant duplicate is the awn character found in rice. The development of awn on rice can be controlled through two duplicate genes that dominate (A as well as B).
The presence of one either of them may result in an awn. Awnless is only asymptomatic when both genes are present in a homozygous recessive states (aabb). The cross of awned as well as unawned strains has resulted in plants that were aw in F1. The inter-mating process of F1 plants resulted in awned and unawned plants in a 15:1 ratios in the F2 generation. This can be explained by following.
The epistatic allele A belongs to B/b alleles, and all plants with allele A are likely to be able to develop awn. A different predominant allele, B, is epistatic alleles to A/a. Alleles carrying this allele will acquire awn like character. Therefore, in F2 the plants carrying A-B-(9/16), A-bb-(3/16) and aaB-(3/16) genotypes will be born with an awn.

The awnless disease will manifest only in the double recessive (aabb) genotype (1/16). In this manner, only two types of plants develop and the dihybrid segregation rate of 9:3:3 1 is altered to a ratio of 15:1 in F2. Similar gene activity is observed for nodulation in peanut as well as the non-floating character of rice.

5. Duplicate recessive epistasis

When recessive alleles from one or both loci may obscure the dominant alleles in the two loci, this is known as epistasis with duplicate recessive. This is also referred to as epistasis complementarity.

Examples of Duplicate recessive epistasis

The most convincing instance of duplicate recessive epistasis is if it occurs for the colour of flowers within sweet pea.

The color purple in sweet peas is controlled through two genes that dominate,, B and A. If these genes are present in distinct individuals (AAbb or AAB) as well as recessive (aabb) they create white flowers.
The cross of white flower (AABB) with white flowers (aabb) cultivars resulted in purple color in F1. The inter-mating between F1 plants resulted in white and purple flowering plants with a 9:7 ratio in the F2 generation. This can be explained by following.

This recessive allele is epistatic to B/b alleles, and it masks their expression. A different recessive allele, b, is epistatic to alleles A/A and conceals their expression.

Therefore, in F2 plants, those that have A-B-(9/16) genotypes produce purple flowers. Plants that carry aaB-(3/16), A-bb-(3/16) and A-bb-(3/16) and (1/16) genotypes grow white flowers. Therefore, only two distinct kinds of phenotypic classes, that is. white and purple are created, and the standard dihybrid segregation ratio 9: 3:3:1 changes to a 9:7 ratio in the F2 generation.

6. Polymeric gene interaction

Two dominant alleles share a similar effects when they are distinct, but they have a greater effects when they are combined. This type of gene interaction is referred to by the term polymeric gene interaction. The combined effect of two alleles is believed to be cumulative or additive however, the two genes shows total dominance, so they can’t be considered additive genes. In the case of an additive effects, genes exhibit absence of dominance.

Example of Polymeric gene interaction

An example of gene interactions that are polymeric is the shape of fruit of summer squash. There are three kinds of fruit shapes in this plant: viz. disc, spherical and lengthy. This shape of disc is created by two dominating genes (A and B) while the spherical form is created via either dominant or dominant variant (A or B) and long-shaped fruits are produced when a two recessive (aabb) plants.

An intermixture of the disc (AABB) and long form (aabb) strains yielded disc-shaped fruit in F1. Inter-mating between F1 plants resulted in plants that had disc, spherical and lengthy shape fruits with 9:6:1 ratio in F2. The explanation for this is in the following manner.

Here , plants that have A–B–(9/16) genotypes yield disc-shaped fruits. Those that have A-bb-(3/16) as well as aaB-(3/16) genotypes yield fruit that are spherical, while plants that have the aabb (1/16) genotype grow long fruit. So in F2 normal dihybrid segregation ratio 9:3 1 is changed to 9:6 ratio of 9:1. Similar gene actions are observed in barley to increase the length of the awn.

Similarities between Epistasis and Dominance

Epistasis	Dominance
Epistasis genes always affect the same character.	Dominant gene also affects a particular character.
Epistasis involves nuclear genes.	Dominance also involves nuclear genes.

Dissimilarities between Epistasis and Dominance

Epistasis	Dominance
Epistasis refers to interaction of two or more genes. Thus it involves two or more loci.	Dominance refers to the interaction of two alleles of the same gene. Thus it involves single locus.
Epistasis may involve both homo and heterozygotes. Hence it is fixable in homozygotes.	Dominancealways refers to heterozygotes and, therefore, it is not fixable
Epistasis is of several types such as dominant, recessive, duplicate, etc.	Dominance is of three types, i.e., incomplete, complete and over dominance.
Epistasis modifies the normal dihybrid phenotypic ratios in F2.	Partial dominance alters the normal segregation ratio of 3 : 1 into 1:2:1.
Epistasis is also known as inter genie or inter-locus gene interaction.	Dominance is known as intragenic or intra-locus gene interaction.
Recessive gene can also exhibit masking effect.	Recessive gene can express only in homozygous condition.

References

Phillips, P. Epistasis — the essential role of gene interactions in the structure and evolution of genetic systems. Nat Rev Genet 9, 855–867 (2008). https://doi.org/10.1038/nrg2452

Churchill, G.A. (2013). Brenner’s Encyclopedia of Genetics || Epistasis. , (), 505–507. doi:10.1016/b978-0-12-374984-0.00482-4
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Heather J. Cordell, Epistasis: what it means, what it doesn’t mean, and statistical methods to detect it in humans, Human Molecular Genetics, Volume 11, Issue 20, 1 October 2002, Pages 2463–2468, https://doi.org/10.1093/hmg/11.20.2463

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