Epistasis: Definition, Meaning Types, Examples, Causes, Explanation

Epistasis is a phenomenon in which the effect of a gene mutation is dependent on the presence or absence of a mutation in one or other genes. They are termed as modifier genes. The effect of the mutation is determined by the genetic background in which it appears. Epistatic mutations have different effects on their own than when they appear together.

This Story also Contains

1. What is Epistasis?
2. Basic Concepts of Epistasis
3. Types of Epistasis
4. Case Studies of Epistasis
5. Recommended video for "Epistasis"
6. MCQs on Epistasis

The concept of epistasis is also used in biochemistry, computational biology, and evolution. This phenomenon arises owing to interactions, either between genes (like mutations being needed in regulating gene expression), or within them (multiple mutations occurring before the gene loses function), leading to non-linear effects. It has an influence on shaping evolutionary relationships, leading to many consequences for the evolution of phenotypic traits. Epistasis is an important topic in biology subject.

What is Epistasis?

Epistasis, therefore, is a gene interaction that may suppress the effects of one gene by another gene. This concept is critical in dealing with genetics since it aids in understanding how various genes can make organisms develop various characteristics. Such characteristics cannot be predicted based on the features of different genes.

Epistasis is significant in the knowledge of the broader picture of genetics and the ways through which its inheritance occurs. In this article, the subject of gene interactions will be briefly discussed, aiming to give important information to the reader about the epistasis model and how it complicates the expression of certain traits.

Basic Concepts of Epistasis

The genetics related to this topic involves the basics of genes, alleles, genotype, and phenotype. The relationship between these topics explains how traits are inherited and expressed.

Gene: Definition and Function

A gene is a section of DNA that contains information on the production of proteins or RNA, these being essential components of an organism’s formation and operations. These hereditary units influence many characteristics and biological phenomena by controlling the synthesis of molecules, specifically proteins, which execute certain tasks in cells.

Alleles: Dominant and Recessive

Alleles are other forms of a single gene that are brought about by certain types of mutation of the genetic material. The result shows that dominant alleles hide recessive alleles in a heterozygous condition. For instance, if a gene contains a dominant allele for brown-eyed and a recessive allele for blue-eyed, then the characteristic owned by the brown allele will be inherited.

Genotype vs. Phenotype

Genotype can be described as the total of genetic factors occupied in an organism or the sum of allelic components for a given gene. The gene carrying genotype, in combination with a certain environment, produces the manifestation of the phenotype or the physical trait. For instance, the BB or Bb genotype may give a phenotype of brown eye colour.

Types of Epistasis

There are different kinds of epistasis based on the phenotype expressed. They can be dominant epistasis, recessive epistasis, etc. Epistasis is classified as:

Recessive Epistasis

Recessive epistasis is said to occur when a recessive gene in an organism affects the manifestation of genes located at other regions of the same chromosome. This indicates that an organism that is homozygous for a gene can hinder the phenotype that is linked with another gene.

Examples: In Labrador retrievers, coat colour is therefore controlled by two genes, meaning that the colour of the coat you find attractive will be controlled by these two genes. The E gene contributes to colour, and the B gene is related to black/brown. At locus E, the non-expression of the B gene due to the recessive e allele leads to the yellow coat colour.

Dominant Epistasis

Dominant epistasis happens when a dominant allele at a particular locus overrides the expression of alleles at the other locus. The first gene means that the gene submits the other gene’s work and always controls the phenotypes, whether with a dominant or recessive allele.

Examples: In the case of squash fruit colour, the alleles present in one locus W are capable of overwhelming or effectively hiding the wild-type allele gene Y. Any plant that has at least one megasporangium W allele will produce white squash, the colour of the fruit depending on the number of recessive w alleles and dominant Y alleles.

Duplicate Recessive Epistasis

The duplicate recessive epistasis represents a case where one of the two different recessive genes can hide another gene. Since phenylketonuria is dominant, both genes require the presence of at least one dominant allele; otherwise, the recessive alleles at that particular locus will nullify the expression of this trait.

Examples: ATD and TAD can lead to albino conditions in plants, and the effect of recessive alleles at either of the two different loci. If a plant inherits an indicator other than a dominant allele at both loci, then it will be an albino, irrespective of the other alleles being a feature of the plant.

Duplicate Dominant Epistasis

Dominant duplicate epistasis takes place when a dominant allele at one locus can suppress a second gene at another locus. The dominant allele at either of the loci prevents the expression of the trait; the presence of one gene with one of its alleles produces the same phenotype, independent of the other gene’s alleles.

Examples: In fruit shape, in shepherd’s purse, the complete dominance of alleles at one of the two loci causes an absolute expression of the specific trait of that locus, thereby obliterating the effect of alleles at the other locus.

Complementary Gene Interaction

Complementary gene interaction takes place when two different traits are the complex result of several genes that need to act in harmony in the process of expressing a particular characteristic. Homozygous dominant alleles of both genes are required for the phenotype to be expressed; if one of the genes contains recessive alleles, there is no phenotype.

Examples: In flower colour, in sweet peas, therefore the C gene, which determines colour and the P gene, which determines pigment, must both be dominant. Flowers will be white if both genes are recessive or if only one of them is recessive because the other will be dominant.

Case Studies of Epistasis

The concept of epistasis is difficult ot understand just by theory; therefore, case studies are required to understand this concept. A few detailed examples to understand epistasis are:

Detailed Case Study 1: Coat Colour in Mice

The coat colour in mice is polygenically determined, which proves the presence of various forms of gene interactions. There is, for instance, recessive epistasis that includes the Agouti and Extension genes or loci. The A gene controls where the pigment is delivered, and the E gene controls the production of the pigment.

If the mouse were created from a mating of two heterozygous mice that had the recessive e allele, the mouse would still have a yellow coat even if it had two dominant non-yellow genes at the A locus. On the other hand, the dominant E allele enables the A locus to produce black/brown pigments. Such an interaction provides an example of how recessive epistasis can hide the other genes and their impact on the final phenotype.

Detailed Case Study 2: Human Eye Colour

Eye colour in human beings is a classic example of a polygenic trait, or one that is controlled by multiple genes. The diagnostically most imperative gene groups for eye colour are OCA2 and HERC2. Changes in these genes range from blue-eyed to green-eyed to even brown-eyed individuals is thus due to this belief.

These genes work synergistically and, therefore, their allelic variations are additive; formerly, different sets of alleles of these genes contribute to the existing variety of eye colour. For instance, multiple dominant alleles for brown eyes will cause the frequency of brown eyes, plus multiple recessive alleles for light-coloured eyes will also lead to light-coloured eyes.

Detailed Case Study 3: Flower Colour in Morning Glory

That means that in Morning Glory, flower colour is brought about by complementary genes. Thereby, to have part or full-coloured flowers, the ‘C’ gene for colour and the ‘P’ gene for pigment production must possess at least one dominant allele. If either of the genes has the recessive alleles, that i, cc or pp, the flowers will be white. This often-used interaction shows how two different genes can combine to give one characteristic. This case study brings out the factor of complementary gene interaction in which the phenotypic character is only observed when both genes possess dominant alleles.

Recommended video for "Epistasis"

MCQs on Epistasis

Q1. If a plant produces green coloured fruit in summer squash, its genotype would be:

Option 1: WwYy

Option 2: wwyy

Option 3: WWYY

Option 4: WwYY

Correct answer: (2) wwyy

Explanation:

In summer squash, the fruit colour is governed by two genes. The dominant gene W for the white colour suppresses the expres­sion of the gene Y which controls the yellow colour. So yellow colour appears only in the absence of W. Thus W is epistatic to Y. In the absence of both W and Y, a green colour develops.

Hence, the correct answer is option (2) wwyy.

Q2. Dominant epistasis in summer squash was observed in

Option 1: Fruit shape

Option 2: Fruit colour

Option 3: Flower shape

Option 4: Flower colour

Correct answer: (2) Fruit colour

Explanation:

When a dominant allele of one gene suppresses the expression of all other alleles of another gene, this is known as dominant epistasis. This indicates that if the second allele has even one dominant allele, it has total influence over the first allele.

Colour of squash: Squash can be yellow, green, or white. The influence of the yellow and green colour genes is obscured by the white colour gene, which is epistatic. Compared to the green colour gene, the yellow colour gene is dominant.

Hence, the correct option is (2) Fruit colour

Q3. Dominant epistasis is a type of genetic interaction where the presence of a dominant allele at one locus masks the expression of alleles at another locus.

Reason: Dominant epistasis can result in a 9:3:4 ratio in the F2 generation.

Option 1: Both assertion and reason are true, and the reason is the correct explanation of the assertion.

Option 2: Both assertion and reason are true, but the reason is not the correct explanation of the assertion.

Option 3: The assertion is true, but the reason is false.

Option 4: The assertion is false, but the reason is true.

Correct answer: (1) Both assertion and reason are true, and the reason is the correct explanation of the assertion.

Explanation:

Dominant epistasis is a type of genetic interaction where the presence of a dominant allele at one locus (the epistatic gene) masks the expression of alleles at another locus (the hypostatic gene). This means that the presence of the dominant allele at the epistatic gene can override the effects of the alleles at the hypostatic gene, resulting in a modified phenotype.

The reason is also correct in stating that dominant epistasis can result in a 9:3:4 ratio in the F2 generation. This is because the modified phenotype resulting from dominant epistasis can produce a modified dihybrid ratio in the F2 generation. Specifically, the modified phenotype produced by the dominant epistatic gene will be expressed in the 9:3 ratio, while the other two phenotypes will be expressed in the 3:3:1 ratio, resulting in an overall 9:3:4 ratio.

Therefore, the correct answer is Option (1) Both assertion and reason are true, and the reason is the correct explanation of the assertion.

Also Read:

• Practice MCQs on Dominant and Recessive Epistasis
• Complementary genes
• Polygenic inheritance