Gene Interaction: Overview, Definition, Types, Importance In Evolution

What Is Gene Interaction?

Gene interaction or gene cooperation is defined as the cooperation of genes to regulate the expression and final manifestation of an organism’s phenotype. This concept is very important in genetics because it aids in depicting how certain traits and diseases are embraced in the human body due to the combination of various genes. They include gene interactions concerning understanding evolution as well as diversification.

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Some of the significant forms of gene interactions include epistasis- one gene dominates or alters the second gene; additive effects- multiple genes contributing to a characteristic/phenotype; and pleiotropy- a single gene influences multiple characteristics/phenotypes. These interact with each other and knowledge of these interactions shows preps into the intricacies of genetic control and evolutionary processes.

Types of Gene Interactions

Gene interactions can be subdivided into two categories:

• Allelic or Non-epistatic Gene Interaction: This gene interaction goes on between the alleles of one gene.

• Nonallelic or Epistatic Gene Interaction: This type of gene interaction involves the interaction between genes on identical or different chromosomes.

Allelic Or Non-Epistatic Gene Interaction

• Allelic interaction is defined as the interaction between alleles at the same gene locus.

• In non-epistatic interactions, one gene does not suppress another.

Types Of Allelic Interactions

The Allelic interactions are classified as:

Complete Dominance

• One allele completely masks the effect of another.

• For example, in garden peas, the tall stem is represented by the allele T, which is dominant to the short allele, t. Thus, the genotypes Tt and TT are both tall in appearance.

Incomplete Dominance

• Here, the heterozygote phenotype is intermediate between the phenotypes of homozygotes.

• Example: In snapdragons, the allele for red flowers (R) is incompletely dominant over the allele for white flowers (r), producing pink flowers (Rr).

Codominance

• Both alleles are expressed in the heterozygote.

• For example: In cattle, the alleles for red hair (R) and white hair (W) are codominant, producing roan cattle (RW) with both red and white hairs.

Overdominance

• Heterozygote has an advantage over both homozygotes.

• Example: Individuals heterozygous for the sickle cell allele (AS) are resistant to malaria compared to those homozygous for normal haemoglobin (AA) or sickle cell disease (SS).

Genotypic And Phenotypic Ratios

• Allele interactions are thus seen to modify Mendelian inheritance patterns that deviate from the genotypic and phenotypic ratios of offspring.

• Incomplete dominance and codominance typically produce a 1:2:1 phenotypic ratio in the F2 generation.

Importance In Evolution

• As a result of allelic interaction, populations increase in their genetic diversity and adaptation.

• Overdominance secures a selective advantage for heterozygotes and thus helps maintain genetic variation within a population.

Research And Applications

• The study of allelic interaction gives insight into genetics, breeding, and medicine.

• It creates awareness of breeding in plants and animals by deducing or foretelling the phenotypic output once genotype combinations are known.

• Under allelic interactions, the study of genetics leading to disorders and the treatment of those disorders through medicine is helped out.

Nonallelic Or Epistatic Gene Interaction

• Epistasis refers to the event of interaction between different genes; the effect of one gene masks or modifies the expression of a second gene.

• Non-allelic interactions refer to interactions between alleles at different loci.

Methods Of Epistatic Interactions

Epistatic interactions are categorised as:

Recessive Epistasis

A recessive allele at one locus masks the effects of alleles at another locus.

Example: In the case of Labrador retrievers, two genes control the coat colour — B and E. Now, the gene E acts as an epistatic for the gene B, such that ee will result in a yellow colour independent of the alleles for the B gene.

Dominant Epistasis

When an allele at one locus is dominant over all alleles at another locus, this can be similar to fruit colour in summer squash.

Example: In summer squash, a dominant allele at one locus acts to mask the expression of the gene for fruit colour. This results in white squash even in the presence of Y.

Duplicate Gene Action

Two genes perform the same function, so a dominant allele at only one of the loci can be sufficient for the expression of the trait.

Example: In some plants, two genes independently act for flower colour. Therefore, in F2 it is due to either A or B that the flower colour appears, which constitutes a 15:1 ratio.

Complementary Gene Action

Two genes complement each other— each contributing what the other lacks—for an organism to produce that phenotype.

Example: Here, two dominant alleles are needed at two loci to have purple flowers in sweet peas. This constitutes an F2 generation ratio of 9:7.

Genotypic/Phenotypic Ratios

Epistasis can shift typical genotypic and phenotypic ratios for a 9:3:3:1 ratio in dihybrid inheritance to any other ratio, including 9:3:4, 12:3:1, 15:1, or even 9:7, pending the type of epistasis.

Significance Of Epistatic Genes in Evolution

• Epistatic interactions contribute to genetic complexity and diversity in populations.

• They have the power to influence traits under natural selection, hence the evolutionary pathways and adaptation processes.

Research And Applications

• Epistasis is of importance in genetic research, in particular, in the study of complex traits and diseases.

• It is important in breeding programs for predicting phenotypic outcomes and improving desirable traits in plants and animals.

• In medicine, such epistatic interactions are of key importance for understanding the genetic basis of complex diseases and for the development of better diagnostic and therapeutic strategies.

Classification Of Epistatic Gene Interaction

Epistatic gene interaction is categorised based on how the involved genes affect one another’s expression:

• Supplementary Gene Interaction

• Complementary Gene Interaction

• Inhibitory Gene Interaction

• Duplicate Gene Interaction

• Masking Gene Interaction

• Polymeric Gene Interaction

Supplementary Gene Interaction

• In supplementary gene interaction, the phenotypic effect is produced by the dominant allele of any one of the two genes controlling a character.

• The dominant allele of the second gene has no phenotypic effect alone but it can modify the effect of the first gene when both the dominant alleles occur together.

• Example: Agouti (grey) coat development in mice.

• F2 Generation:

• Phenotypic Ratio: 9 Agouti : 3 Coloured: 4 Albino.

Complementary Gene Interaction

• Complementary gene interaction occurs when two genes producing the same phenotype when they are homozygous result in only one phenotype in the heterozygous state.

• Interaction occurs in the presence of both kinds of dominant alleles to produce a different kind of phenotype.

• Example: Flower colour in sweet peas.

• F2 Ratio: 9:7 instead of 9:3:3:1.

• Mechanism: Both dominant alleles must be present for the expression of phenotype; their absence does not produce the phenotype.

Inhibitory Gene Interaction

• Inhibitory gene interaction occurs when a dominant allele at one locus inhibits the expression of alleles at another locus.

• There is no distinction in phenotype between the heterozygous and homozygous dominant forms of the inhibitory gene.

• Example: Feather color in chickens.

• F2 Ratio: Changes from 9:3:3:1 to 13:3.

• Mechanism: A dominant allele is an inhibitor of another gene, producing a unique phenotype only in the homozygous recessive condition.

Duplicate Gene Interaction

• Duplicate gene interaction is an interaction between two gene loci, either of which, when homozygous dominant, produces the same phenotype and there is no cumulative effect.

• Example: The shape of the seed capsule of Shepherd's purse

• Ratio in F2: Modified into 15:1 ratio from 9:3:3:1

• Mechanism: Both genes produce the same phenotype except when both are homozygous recessive and the phenotype is the ovoid capsule.

Masking Gene Interaction

• Masking gene interaction occurs when the dominant allele of one gene masks the activity of the allele of another gene.

• Example: Fruit colour in summer squash.

• F2 Ratio: Dominant epistatic relationships where the dominant allele expresses itself no matter what the alleles are for the other gene.

• Mechanism: The allele of the epistatic locus expresses itself unless the epistatic locus is homozygous recessive.

Polymeric Gene Interaction

• Polymeric gene interaction is when two dominant alleles in genes act together to enhance a phenotype or create a medium-type phenotype.

• Example: Color of kernels in wheat.

• The ratio in F2: Altered into 9:6:1 and other ratios showing the combined effect.

• Mechanism: Both dominant alleles are of an equal degree in the expression of phenotype; therefore, it enhances or leads to a medium-type phenotype.

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