What does Gene Interaction mean?

When the expression of a single trait is influenced by two or more different non-allelic genes, it is termed as genetic interaction. According to Mendel's law of inheritance, each gene functions in its own way and does not depend on the function of another gene, i.e., a single gene controls each of seven characteristics considered, but the complex contribution of many different genes determine many traits of an organism.

How does it work?

In 1906, William Bateson and Reginald Punnett discovered an association study that a single trait is affected by the interaction of two different genes. They discovered an unexpected higher-order interaction of genes in biological processes when they studied crosses between the sweet pea plants, Lathyrus Odoratus.

1. A true-breeding purple-flowered plant was crossed to a true-breeding white-flowered plant.

2. In the F1 generation, production of all the plants with purple flowers occurred.

3. When plants of the F1 generation were self-fertilized, F2 generation was produced.

4. In the F2 generation, both purple and white-flowered plants were produced in 3:1 ratios, but when crossing between two different varieties of white-flowered plants produced all the purple-flowered plants in the F1 generation.

5. In the F2 generation, self-fertilization of these plants produced both purple and white-flowered plants in a 9:7 (Purple: White) ratio (non-additive).

6. This unexpected result was explained by Bateson and Punnett. They considered the involvement of two different genes because the ratio of the F2 generation (9:7) was simply a variation of the 9:3:3:1 ratios.

Epistasis

This phenomenon is the case in which the expression of one of the genes is masked or modified by the expression of another gene at a distinct locus. A gene that masks the expression of another gene (present in a different allele) is termed as epistatic to the first gene. The suppressed gene is hypostatic. In the pathway for the formation of purple color, homozygous recessive genes are epistatic to each other.

Dominant Epistasis

In this case, the effect of any gene is modified or masked by the dominant allele of one gene, it is termed as Dominant Epistasis. In Dominant epistasis, a certain phenotype at one locus is produced by the dominant allele (A), regardless of the condition of the other locus (B). Since the dominant A expresses itself in the presence of dominant B or recessive b, this is a case of this phenomenon. B or b (Hypostatic) is expressed when the individual's genotype is homozygous recessive (aa), thus the same phenotype is produced by genotype A-B- and A-bb, but two extra physical make-ups are produced by aaB- and aabb. The classical 9:3:3:1 ratio gets modified to the 12:3:1 ratio.

Recessive Epistasis

In this case, in a pair of genes, one individual produces phenotypic effect dominantly in an independent manner, whereas another does not produce this effect in an independent manner. When the gene pairs are expressed in a dominant state together, the latter will produce its main effects. For example, for a pair of genes, A and B, A can produce a physical makeup independently in the dominant state, but the second gene B cannot produce the same independently. In this case, the recessive genetic makeup of aa suppresses the expression of B. In the presence of dominant A, B can be expressed, thus the A-B- and A-bb produce two additional physical makeup. Thus, 9:3:3:1 ratios become 9:3:4 ratio.

Duplicate Recessive Epistasis

If a specific pathway involves a pair of genes that are present in different positions, on the whole genome, both require functional products to express them. In dihybrid crosses, at either pair, one homozygous recessive trait results in the mutant expression. In such a case, genetic make-up aaBB, aaBb, AAbb and aabb produces AABB, AaBB, AABb, AaBb (9:7 ratio). Because both dominant traits complement each other for the correct phenotype, these genes are called Complementary genes and the genetic interaction is called Complementary interaction of genes.

Duplicate Dominant Interaction

If both the genes (present in different regions on the chromosome) produce the same physical make-up without cumulative effect, the 9:3:3:1 ratios are modified into 15:1 ratios (genetic variance). Duplicate genetic interaction allows either duplicate gene to produce the wild-type phenotype. Only organisms with homozygous recessive of both genes have a single-mutant.

An Explanation of Duplicate Dominant Genetic Interaction

The mechanism by which wheat kernel color is determined is an example of duplicate gene-gene interactions. In the genome-wide association studies in wheat, kernel color depends on the biochemical reaction which makes a colorless precursor into a colored product. This reaction can be performed with the product of either gene A or B. Either A or B produces a colored kernel, whereas a lack of A or B will produce a non-colored white kernel. So, when two plants with genotype AaBb are crossed with each other, then AABB, AABb, AaBb, AAbb, Aabb, aaBB, aaBb produce the color, but aabb produces no color. In this cross, whenever a dominant allele is present at either space on the chromosome, the biochemical conversion occurs resulting in a colored kernel. Thus, only the double homozygous recessive one produces no color. The resulting ratio of color to non-color is 15:1.

Dominant and Recessive Genetic Interaction

Dominant and recessive gene-gene interaction is similar to dominant epistasis but occurs when a dominant trait of one gene completely suppresses the expression of another gene. This is sometimes called Dominant suppression because the deviation from 9:3:3:1 is caused by a single trait that produces a dominant physical makeup.

Example of Dominant Suppression

The Primula plant, the pigment of malvidin creates blue-colored flowers. The synthesis of malvidin is controlled by gene A, yet the production of this pigment can be suppressed by B (present in a different part). In this case, the B gene is dominant to the A gene, so plants with the AaBb genetic make-up will not produce malvidin because of the presence of the B gene. So, if two plants with the genetic make-up AaBb cross with each other, AABB, AABb, AaBB, AaBb, aaBB, aaBb, and aabb will produce the white color, and AAbb and Aabb will produce the blue color. In this case, the expression of the B gene suppresses malvidin production.

Genetic Dissection to Investigate Gene Action

• A genetic dissection is an experimental approach that
• Determines the number of intermediate steps within a biosynthetic pathway.
• Determines the order of steps in the pathway.
• Identifies the step affected by each mutation.

Genes that contribute to different steps of a multi-step pathway, work together to produce the end product of that pathway. Because of this genetic interaction, mutation of one gene may prevent completion of the pathway and production of the end product. Beadle and Tatum's experiments opened the way to investigate the roles of individual gene mutations in gene-environment in the biosynthetic pathways. These investigations began with three assumptions that have been proven to be correct.

• Biosynthetic pathways consist of sequential steps.
• When one step in the pathway is completed, it generates the substrate for the next step.
• Completion of every step is necessary for the production of the end product of the pathway.

Pleiotropy

Most of the biochemical pathways in living organisms are interconnected with each other. Products and intermediates of one pathway may be used in several other metabolic processes. Hence, the gene expression (co-expression) usually affects more than one character. This term refers to the effect of a single gene on more than one trait. Sometimes one trait is very evident and others will be less evident.

Examples of Pleiotropic effects

Numerous examples exist in which the gene appears to have pleiotropic effects. For example, a gene for white eyes in Drosophila also affects the shape of organs in the female responsible for sperm storage as well as other structure. Another example is the frizzle-trait in chickens. The primary result of the expression of this gene includes defective feathers production. Secondary results are both good and bad. Good includes increased adaptation to warm temperatures. The bad effects are high-throughput in metabolic rate, decreased tendency of egg-laying and changes in heart, kidney, and spleen.

Context and Applications

This topic is significant in the professional exams for both graduate and post-graduate courses, especially for

• B.Sc. in Zoology
• B.Sc. in Biological Science
• M.Sc. in Zoology
• M.Sc. in Biological Sciences
• M.Sc. in Genetics

Related Concepts

• Mendelian genetics
• Epistatic interactions
• Heritability
• Penetrance
• Expressivity
• Human disease
• Interaction network