Define Epistasis-Classes, Types, Examples, Significance

Define Epistasis- Classes, Types, Examples, Significance

Epistasis is the interaction between two non-allelic genes in which the phenotypic expression of one gene is masked or repressed by the expression of one or more other genes.

Mendelian and his collaborators previously assumed that the traits were determined by the expression of a single gene with two alleles, one dominating the other. It was later discovered that the expression of two or more genes could control the development of certain traits. Therefore, Mendelian genetics failed to identify those different modes of inheritance for which their phenotypic ratios differ from Mendelian ratios (monohybrid 3:1 and dihybrid 9:3:3:1 in the F2 generation).

William Bateson and R.C. Punnett proposed the “factor hypothesis” in which they explained the genetic interaction. This states that certain characters are determined by the expression of two or more genes. Gene interaction occurs as a result of interaction between two or more allelic or non-allelic genes of the same genotype. As a result, variations in certain phenotypic traits occur.

The expression of a gene relies on the expression (i.e., presence or absence) of another gene, i.e., H. the expression of genes is not independent of each other. There are two types of genetic interactions, namely:

Allelic/Intraallelic Gene Interaction/Intragenic Interaction

Non-allelic/Interallelic Gene Interaction/Intergenic Interaction

Intragenic interaction is the genetic interaction between the two alleles of a single gene. However, intergenic interaction is the genetic interaction between two or more genes located in the same or in different chromosomes. Two or more gene products are involved in the outcome of a common trait.

What is Epistasis?

William Bateson first coined the term “epistatic” in 1909 to describe the genetic interaction in which one mutation masks or obscures the effects of other mutations. Later, R.A. Fischer used the derived term “epistacy” to describe the statistical deviation from the cumulative addition of loci in phenotypic expression.

It was also defined as a deviation from the expected result in terms of phenotypic traits when combining mutations. Epistasis can occur under the following circumstances:

  • Two or more loci interact to produce new phenotypes.
  • The presence of an allele at a locus masks the effects of alleles at one or more loci.
  • The presence of an allele at a locus alters the effects of alleles at one or more loci.

Epistasis occurs at the phenotypic level of organization. The expression of a particular gene not only affects the phenotype but also masks the output of other genes that interact with each other.

What are the Different classes of Epistasis? 

Epistatic interactions are divided into different classes based on:

Outcome

Based on the result, the epistasis is divided into positive and negative epistasis.

If the result (as a result of mutation(s)) is better (more appropriate) than expected for a given genetic background, it is positive epistasis. If in comparison, the result is worse (less fit) than expected, the epistasis is negative.

Strength and direction of mutation

Magnitude epistasis

It occurs when the magnitude of the effect of mutation changes the genetic background.

Sign epistasis

It occurs when the direction of the effect of mutation changes the genetic background.

Number of mutations

If the epistatic interaction embodies two mutations, it’s called pairwise epistasis. Similarly, the involvement of more than two mutations is called higher-order epistasis

Specificity

Specific epistasis:  In specific epistasis, one mutation modulates the phenotypic effects of a few other mutations. 

Non-specific epistasis: Here, a mutation influences the effects of a relatively large number of other mutations.

Measurement of interaction

When the genetic interaction is measured in terms of specific genetic background, it is known as background-relative epistasis. However, the average of all sets of genetic backgrounds is called background-averaged epistasis.

Types of Epistasis

Generally, there are six types of epistasis gene interaction. Namely:

  1. Dominant epistasis
  2. Dominant inhibitory epistasis
  3. Recessive epistasis
  4. Duplicate dominant epistasis
  5. Duplicate recessive epistasis
  6. Polymeric epistasis

Dominant Epistasis

  • A dominant allele of one gene conceals the phenotypic expression of both recessive and dominant alleles at other loci of other genes.
  • It is also known as simple epistasis.
  • Example: Fruit color in Summer Squash (Cucurbita pepo)
  • It modifies the classical ratio of 9:3:3:1 into 12:3:1 for the F2 hybrid.
  • White, yellow, and green are the three different colors of squash fruits.
  • The allele for white color is dominant over both yellow and green. Similarly, yellow is dominant over green color.
  • If the dominant allele W is present, it masks the phenotypic effect of the yellow and green allele, and thus, the fruit becomes white.
  • In the chart above, allele W is dominant to allele w and epistatic to alleles G and g. A cross made between white fruit and yellow fruit produces F1 white fruits. This F1, when allowed to interbreed, resulted in F2 progeny of green, yellow, and white colored fruits in the ratio of 12:3:1.

Dominant Inhibitory Epistasis

  • A dominant allele at one location blocks the expression of both dominant and recessive alleles at the second locus.
  • It is also known as inhibitory gene interaction.
  • Example: Anthocyanin pigmentation in rice
  • The standard dihybrid segregation ratio of 9:3:3:1 is changed to 13:3 in the F2 generation.
  • In the above figure, a dominant allele, I controls the green color while P controls the purple coloration in rice. A cross between the green and purple plants produced green plants in the F1 generation. Again, in-breeding of F1 progeny resulted in the production of green and purple plants in the ratio of 13:3 in the F2 generation.
  • Here, allele I is dominant to i and epistatic to alleles P and p. Thus, the presence of allele I conceals the effects of P and p and produces green color; however, the absence of allele I in iiPP and iiPp produces purple plants.

Recessive Epistasis

  • The presence of recessive alleles of one gene hides the phenotypic expression of both dominant and recessive alleles at other loci of other genes. 
  • It is also known as supplementary epistasis.
  • Example: Coat color in Labrador retrievers and grain color in maize
  • It modifies the classical ratio 9:3:3:1 into 9:3:4 for the F2 hybrid.
  • The above figure illustrates recessive epistasis for grain color in maize. Typically, there are three different colors of grains in maize. Namely: Purple, Red, and White grains. The presence of two dominant alleles (R and P together) leads to the development of the purple color. Similarly, the occurrence of only R as the dominant gene produces red color and white in homozygous conditions. 
  • Initially, the cross between white and white grains produced purple grains in F1. Moreover, when F1 progenies were crossed with each other, it resulted in white, red, and purple grains in F2 generation in the ratio of 9:3:4.
  • Allele r is epistatic to alleles P and p but recessive to R.

Duplicate epistasis

  • The presence of a dominant allele at either of two loci conceals the expression of complementary recessive alleles at the two loci.
  • It is also known as duplicate gene interaction or duplicate dominant epistasis.
  • Examples: Awn character in rice, Seed capsules of Shepherd’s purse (Capsella spp.)
  • The normal dihybrid ratio of 9:3:3:1 is modified to 15:1 in the F2 generation.
  • Duplicate alleles A and B control the awn character in rice. The presence of one either of them is essential for developing awn character, which becomes asymptomatic when present in a homozygous recessive condition. 
  • Cross between awned and awnless rice produces awned rice in F1 progeny. In-breeding between F1 progenies resulted in awned and unwanted plants in a 15:1 ratio in the F2 generation.
  • A dominant allele A is epistatic to alleles B and b. Similarly, dominant allele B is epistatic to alleles A and a. The presence of either or both of these dominant alleles is responsible for the awn character in rice. However, the occurrence of the double homozygous recessive gene (aabb) results in awnless in rice.

Duplicate recessive epistasis

  • The presence of recessive alleles at either of the two loci masks the expression of dominant alleles in the two loci.
  • It is also known as complementary gene action because the two genes function together to produce outcomes.
  • Example: Flower color in sweet pea, Lathyrus odoratus.
  • The normal dihybrid ratio of 9:3:3:1 is altered to 9:7 in F2 progeny.
  • The purple color of the flower in sweet peas is due to the presence of two dominant genes A and B, as represented above. Similarly, whenever these genes are present in recessive form (aabb) or separate individuals (AAbb or aaBB), a white flower is produced.
  • The above figure depicts the cross between purple and white flower, which produced purple color in F1 progeny. Likewise, interbreeding of F1 progenies produced purple and white flowers in the ratio of 9:7 in the F2 generation.
  • The recessive allele a is epistatic to B and b alleles, and recessive allele b is epistatic to alleles A and a. 

Polymeric gene interaction

  • The presence of a dominant allele at two or more loci acts synergistically to produce a unique phenotype with greater effects than it would produce when present individually.
  • It is also known as duplicate gene interaction with cumulative effect.
  • Example: Fruit shape in Summer Squash.
  • The normal dihybrid ratio of 9:3:3:1 is modified to 9:6:1 in F2 progeny.
  • Three types of fruit shapes can be observed in summer squash, namely disc, spherical and long, in which two dominant genes A and B, control disc shape. Either A or B produces the spherical shape, and the presence of a double recessive gene (aabb) develops long-shaped fruits.
  • The above figure portrays the polymeric gene interaction for fruit shape in summer squash in which a cross between disc shape and long shape fruit produced disc shape fruit in the F1 generation, which on interbreeding produced disc shape, spherical and long shape fruit in the ratio of 9:6:1 in the F2 generation.

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Define Epistasis-Classes, Types, Examples, Significance

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