Epistasis – Introduction, Types & Explanation

Epistasis

Introduction

We have already studied that the interaction of genes presents at different loci that affect the expression of the same character is known as gene interaction. 

It is the influence of alleles and non-alleles on the normal phenotypic expression of the genes. 

Gene interactions are of two types: 

1. Inter-allelic or intra-genic gene interaction: 

In such cases, two alleles located on the same gene locus on two homologous chromosomes of a gene interact in such a fashion to produce phenotypic expression. 

Example: co-dominance, incomplete dominance, multiple alleles. 

2. Non-allelic or inter-genic gene interaction: 

In such cases, two or more independent genes present on the same or different chromosomes interact to produce a new expression. 

Example: Epistasis, complementary genes, supplementary genes, duplicate genes, inhibitory genes, lethal genes, etc. 

Explanation

Mendel’s observations in pea plants suggested that the sum of an individual’s phenotype was regulated by genes or unit factors, such that every characteristic was distinctly and completely controlled by a single gene.  

However, it is observed that single characteristics are mostly under the influence of multiple genes (each with two or more alleles) functioning in unison.  

For example, at least eight genes contribute to eye color in humans. 

Epistasis: 

In epistasis, the interactions between genes are antagonistic, such that one gene masks or interferes with the expression of another. 

“Epistasis” is a word derived from Greek roots that mean “standing upon”. The “masked or silenced” alleles are said to be hypostatic, whereas the alleles that are doing the masking are called epistatic. 

A gene pathway wherein the expression of one gene is dependent on the function of a gene that prior or after it in the pathway is often the biochemical basis of epistasis. 

Types of epistasis: 

There are many ways in which epistasis can occur, which are as follows: 

1. Recessive Epistasis  
2. Dominant Epistasis  
3. Dominant (Inhibitory) Epistasis  
4. Duplicate Recessive Epistasis  
5. Duplicate Dominant Epistasis  
6. Polymeric Gene Interaction 

Recessive epistasis (supplementary gene): 

Recessive epistasis takes place when recessive alleles at one locus hide the expression of both (dominant and recessive) alleles at another locus. This type of gene interaction is also known as supplementary epistasis.  

Recessive epistasis is seen in the genes that determine the coat color in Labrador retrievers. These dogs may be black, brown or yellow; their different coat colors are determined by interactions between genes at two loci. 

One locus determines the type of pigment produced by the skin cells: a dominant allele B codes for black pigment, whereas a recessive allele b codes for a brown pigment. Alleles at a second locus affect the deposition of the pigment in the shaft of the hair; allele E allows the dark pigment to be deposited (black or brown), whereas a recessive allele prevents the deposition of dark pigment, causing the hair to be yellow. 

As a result, the presence of genotype ee at the second locus masks the expression of the black and brown allele at the first locus. 

The genotypes that determine the coat color and their phenotypes are: 

If we cross a black Labrador homozygous (BBEE) for the dominant alleles with a yellow Labrador homozygous (bbee) for the recessive alleles and then intercross the F1, we obtain progeny in the F2 in a 9 (black): 3 (brown): 4 (yellow) ratios. 

Dominant epistasis: 

Dominant epistasis occurs when the expression of both alleles (dominant and recessive) at another locus is masked by a dominant allele at one location. In other words, the expression of one dominant or recessive allele is masked by another dominant gene.  

An example of dominant epistasis is seen in summer squash (Cucurbita pepo). Fruit color in summer squash is commonly found in one of the three colors: yellow, white or green. 

When a homozygous plant that produces white squash is crossed with a plant that produces green squash and the F₁ plants are crossed with each other, the following results are obtained.  

The white color is controlled by dominant gene W and yellow color by dominant gene G. White color is dominant over both yellow and green. The green fruits are formed in recessive conditions, i.e., wwgg.  

A cross involving plants that have white and yellow fruits produced F₁ generation with white fruits. Inter-mating of F₁ progeny produced plants with white, yellow and green colored fruits in the F₂ generation in a 12 : 3 : 1 ratio. 

Here, allele W is dominant to w and epistatic to alleles G and g. Hence it will mask the expression of G/g alleles.  

As a result, in F2, plants with W_G_ (9/16) and W_gg (3/16) genotypes will produce white fruits; plants with wwG_ (3/16) will produce yellow fruits, and those with wwgg (1/16) genotype will produce green fruits. 

Thus, the normal dihybrid ratio 9 : 3 : 3 : 1 is modified to 12 : 3 : 1 ratio in the F2 generation. A similar type of gene interaction has been reported for skin color in mice and seed coat color in barley. 

Duplicate Recessive Epistasis: 

Duplicate recessive epistasis occurs when recessive alleles at either of the two loci can hide the expression of dominant alleles at the two loci. This is also known as complementary epistasis.  

The best example of duplicate recessive epistasis is found for flower color in sweet pea plants. The purple color of the sweet pea flower is controlled by two dominant genes, consider A and B. When these genes are present in separate individuals (AAbb or aaBB) or recessive (aabb), they produce white flowers. 

A cross between a purple flower (AABB) and a white flower (aabb) strain produced purple color in the F₁ generation. Inter-mating of the F1 plants produced purple and white flower plants in 9 : 7 ratio in the F₂ generation.  

Here, recessive allele a is epistatic to B/b alleles and hides or masks the expression of these alleles. Another recessive allele, b, act as an epistatic allele to A/a alleles and mask their expression. 

Hence in F₂ generation, plants with A_B_ (9/16) genotypes will have purple flowers, and plants with aaB_ (3/16), A_bb (3/16), and aabb (1/16) genotypes produce white flowers.  

As a result, only two phenotypic classes, viz., purple and white, are produced, and the normal dihybrid ratio of 9 : 3 : 3 : 1 is changed to 9 : 7 in the F₂ generation. 

Duplicate Dominant Epistasis: 

Duplicate dominant epistasis occurs when a dominant allele at one of two loci can hide the expression of recessive alleles at both loci. It is referred to as duplicate dominant epistasis.  

This is also known as duplicate gene action. An example of duplicate dominant epistasis is the awn character in rice.  

Development of awn in rice is governed by the two dominant duplicate genes (A and B). Awn can be produced by the presence of any of these two alleles. The awnless condition appears only when both these genes are in a homozygous recessive state (aabb).  

A cross between awned and awnless strains produced awned plants in the F₁ generation. Inter-mating of F₁ plants produced awned and awnless plants in 15 : 1 ratio in F₂ generation. This can be explained as follows. 

Allele A is epistatic to alleles B/b, and all plants having allele A will develop awn character. Another dominant allele, B, is epistatic to A/a alleles. Individuals with this allele will also develop awn character. 

Thus, in F₂, plants with A_B_ (9/16), A_bb (3/16) and aaB_(3/16) genotypes will develop awn character. 

The awnless condition will develop only in the double recessive (aabb) genotype (1/16).  As a result, only two classes of plants are obtained, and the normal dihybrid segregation ratio 9 : 3 : 3 : 1 is modified to 15 : 1 ratio in the F₂ generation. 

Summary

• The interaction of genes present at different loci that affect the expression of the same character is known as gene interaction.
• In inter-allelic or intra-genic gene interaction, two alleles located on the same gene locus on two homologous chromosomes of a gene interact in such a fashion to produce
  phenotypic expression.
• In non-allelic or inter-genic gene interaction, two or more independent genes present
  on the same or different chromosomes interact to produce a new expression.
• In epistasis, the interactions between genes are antagonistic, such that one gene masks
  or interferes with the expression of another.
• The “masked or silenced” alleles are said to be hypostatic, whereas the alleles that are
  doing the masking are called epistatic.
• Recessive epistasis takes place when recessive alleles at one locus hide the expression of both (dominant and recessive) alleles at another locus. This type of gene interaction is also known as supplementary epistasis.
• Recessive epistasis is seen in the genes that determine the coat color in Labrador retrievers. The normal dihybrid ratio 9:3:3:1 is modified to 9:3:4 ratio in the F2 generation.
• Dominant epistasis occurs when the expression of both alleles (dominant and recessive) at another locus is masked by a dominant allele at one location.
• An example of dominant epistasis is seen in summer squash (Cucurbita pepo). The normal dihybrid ratio 9:3:3:1 is modified to 12:3:1 ratio in the F2 generation.
• Duplicate recessive epistasis occurs when recessive alleles at either of the two loci can hide the expression of dominant alleles at the two loci. This is also known as complementary epistasis.
• An example of duplicate recessive epistasis is found for flower color in sweet pea plants. The normal dihybrid ratio of 9:3:3:1 is changed to 9:7 in the F2 generation.
• Duplicate dominant epistasis occurs when a dominant allele at one of two loci can hide the expression of recessive alleles at both the loci. It is referred to as duplicate dominant epistasis.
• This is also known as duplicate gene action. An example of duplicate dominant epistasis is the awn character in rice, and the normal dihybrid segregation ratio 9:3:3:1 is modified to 15:1 ratio in the F2 generation.