Difference Between Dominance and Epistasis

Main Difference – Dominance vs Epistasis

A gene that determines a particular trait generally comes in two forms known as alleles. The two alleles of a gene occur at the same locus of the homologous chromosomes. The alleles of the two genes, as well as the alleles of separate genes, interact with each other during their expression. Dominance and epistasis are two types of interactions of alleles while they produce the corresponding phenotypes. The main difference between dominance and epistasis is that dominance is a type of interactions between alleles of the same gene whereas epistasis is a type of interactions between alleles of different genes.

Key Areas Covered

1. What is Dominance
      – Definition, Types, Examples
2. What is Epistasis
     – Definition, Types, Examples
3. What are the Similarities Between Dominance and Epistasis
     – Outline of Common Features
4. What is the Difference Between Dominance and Epistasis
    – Comparison of Key Differences

Key Terms: Alleles, Allelic Interactions, Co-dominance, Complete Dominance, Dominant Epistasis, Dominant Inhibitory Epistasis, Duplicate Epistasis, Epistasis, Genes, Incomplete Dominance, Recessive Epistasis

What is Dominance

Dominance refers to a phenomenon of genetics whereby, in an individual containing two allelic forms of a particular gene, one is expressed to the exclusion of the others. Generally, a gene that determines a particular trait exists in more than one form known as alleles. The alleles of a particular gene are located on the same locus of homologous chromosomes. Therefore, a diploid organism consists of two alleles. Each allele is inherited from each parent. Each allele is also passed to the offspring through sexual reproduction. The inheritance of genes over generations was first described by Gregor Mendel in 1890s.

The paired alleles can be either heterozygous or homozygous. Homozygous allele pairs consist of identical alleles whereas heterozygous allele pairs consist of different alleles. Homozygous alleles express their phenotype as it is. But in the heterozygous allele pair, one allele is expressed over the other. This allele is known as the dominant allele. The presence of a dominant allele in the allele pair is identified as dominance. Three types of dominance patterns can be identified as complete dominance, incomplete dominance, and co-dominance.

Complete Dominance

In complete dominance, one allele is completely dominant over the other. The completely masked allele is called the recessive allele. Thus, the phenotype of the gene is completely determined by the dominant allele. A Punnett square that describes the Mendelian inheritance of the flower color of the pea plant is shown in figure 1.

Figure 1: Inheritance of the Color of Pea Plants

Three genotypes, BB, Bb, and bb, determine the color of the flower in pea plants. The dominant phenotype for the flower color is purple; white is the recessive phenotype. Thus, the dominant allele is identified as B whereas the recessive allele is identified as b. 

Incomplete Dominance

In incomplete dominance, one allele is not completely dominant over the other. Thus, neither dominant nor recessive phenotype is expressed. But, a third phenotype, which contains a mixture of both dominant and the recessive phenotype, is expressed. The pink color of the snapdragon flower is an example of incomplete dominance. The two phenotypes are red and white in snapdragon flowers.

Co-dominance

In co-dominance, neither allele is dominant. However, both alleles are completely expressed. Thus, more than one phenotype is expressed in co-dominance. The inheritance of the ABO blood group in humans is an example of co-dominance.

What is Epistasis

Epistasis refers to a phenomenon whereby, the expression of one gene affects the inheritance of one or more independently inherited genes. Thus, the inter-genic interactions between different genes control the expression of them. The gene that consists of a masked effect from another gene is called an epistatic gene. The coat color of the young Labrador retrievers is an example of epistasis. For example, a black Labrador mother can have babies with black, brown or yellow coat colors. Dominant epistasis, dominant inhibitory epistasis, duplicate dominant epistasis, duplicate recessive epistasis, polymeric gene interaction, and recessive epistasis are the six types of epistasis.

Dominant Epistasis

Dominant epistasis occurs when a dominant allele of particular locus masks both dominant and recessive alleles at another locus. It is also called simple epistasis. Dominant epistasis is shown in figure 2.

Figure 2: Dominant Epistasis

Recessive Epistasis

When both dominant and recessive alleles of a particular locus are masked by a recessive allele at another locus, the recessive epistasis occurs.

Dominant Inhibitory Epistasis

When a particular gene serves as a suppressor of another gene, the dominant inhibitory epistasis occurs.

Duplicate Epistasis

In duplicate epistasis, either a dominant or recessive allele masks the expression of recessive alleles or dominant alleles respectively at two separate loci.

Polymeric Gene Interactions

In polymeric gene interactions, two separate, dominant alleles in combination produce a third or median phenotype.

Similarities Between Dominance and Epistasis

• Dominance and Epistasis are two types of interactions between alleles.
• Both dominance and epistasis occur during the gene expression.

Difference Between Dominance and Epistasis

Definition

Dominance: Dominance refers to a phenomenon of genetics whereby, in an individual containing two allelic forms of a particular gene, one is expressed to the exclusion of the others.

Epistasis: Epistasis refers to a phenomenon where the expression of one gene affects the inheritance of one or more independently inherited genes.

Type of Interaction

Dominance: Dominance is a type of interaction between alleles of different genes.

Epistasis: Epistasis is a type of interaction between alleles of the same gene.

Types

Dominance: Complete dominance, incomplete dominance, and co-dominance are the three types of dominance.

Epistasis: Dominant epistasis, dominant inhibitory epistasis, duplicate dominant epistasis, duplicate recessive epistasis, polymeric gene interaction, and recessive epistasis are the six types of epistasis.

Examples

Dominance: Mendelian inheritance of the flower color of the pea plant is an example of dominance.

Epistasis: The coat color of the young Labrador retrievers is an example of epistasis.

Conclusion

Dominance and epistasis are two types of inter-genic interactions involved in the determination of the phenotype. Dominance is the phenomenon in which the alleles of the same locus interact with each other to produce a phenotype. Epistasis is a type of interaction that occurs between alleles of the different loci. This is the main difference between dominance and epistasis.

Reference:

1. Bailey, Regina. “Why We Look Like Our Parents.” ThoughtCo, Available here.
2. “Epistasis – Definition, Types and Examples.” Biology Dictionary, 28 Apr. 2017, Available here.

Image Courtesy:

1. “Punnett square mendel flowers” By Madprime – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Epistatic hair” By Thomas Shafee – Own work (CC BY 4.0) via Commons Wikimedia