What is Epistasis?

When gene present on one gene locus suppresses expression of gene present on the same or other chromosome, it is called epistasis.

Epistasis is a genetic phenomenon in which the expression of one gene affects the expression of another gene. Specifically, one gene masks or modifies the expression of another gene. The gene that is doing the masking or modifying is called the epistatic gene, while the gene whose expression is being affected is called the hypostatic gene.

There are different types of epistasis, including recessive epistasis, dominant epistasis, and duplicate recessive epistasis. In recessive epistasis, the presence of two recessive alleles of the epistatic gene is required to mask the expression of the hypostatic gene. In dominant epistasis, the presence of at least one dominant allele of the epistatic gene is required to mask the expression of the hypostatic gene. In duplicate recessive epistasis, two recessive alleles of either gene are required to mask the expression of the hypostatic gene.

Epistasis can have significant impacts on the expression of traits in organisms and can result in unexpected ratios of offspring phenotypes in genetic crosses. Epistasis also plays an important role in evolution by allowing for the development of novel traits through changes in gene regulation and interaction.

Types of epistasis

The epistatic gene may be dominant or recessive.

Dominant epistasis

The epistatic gene is dominant over its own allele.

It can exert epistatic effect in homozygous as well as in heterozygous condition.

Recessive epistasis

The recessive epistatic gene can have its epistatic influence only in homozygous condition.

Duplicate Recessive Epistasis

Complementary genes, are two pairs of genes present on separate gene loci that interact jointly to produce only one phenotypic character, neither of them if present alone expresses itself. It means these genes are complementary to each other.

Example 1

In sweet peas, it would appear that there are two genes affecting pigmentation and that being homozygous recessive in either gene results in a lack of color. If the two varieties of white plants are crossed, the have purple flowers but among the F₂, 9/16 have purple flowers. Since the ratio is in sixteenths, the F, plants must have been di-hybrid as shown in figure.

The 9:7 ratio instead of a 9:3:3:1 ratio can be explained assuming that both a dominant A allele and a dominant B allele are required for pigmentation to result. Although the exact details of pigmentation synthesis are known, the metabolic pathway is hypothesized.

If the A allele codes for the first enzyme and the B allele codes for the second enzyme. Then being homozygous recessive for either gene would result in white instead of purple flowers. A similar situation occurs in mammalian animals. If individuals inherit any one of several defects in the metabolic pathway for the synthesis of melanin, the individual is an albino.

Epistasis

Example 2: Bombay Phenotype

The Bombay phenotype is a rare genetic condition that affects the expression of ABO blood group antigens on red blood cells. Individuals with the Bombay phenotype do not produce the H antigen, which is necessary for the formation of the A and B antigens. As a result, they cannot produce A, B, or AB blood types and instead has the O blood type.

The Bombay phenotype is an example of epistasis because it involves the interaction of two different genes. Specifically, the H gene encodes for the production of the H antigen, which is required for the A and B antigens to be produced. However, the ABO gene determines which specific antigen (A or B) is produced on the red blood cells. In individuals with the Bombay phenotype, the mutation in the H gene prevents the formation of the H antigen, so the ABO gene cannot add the A or B antigens.

This means that the expression of the ABO gene is dependent on the presence of the H antigen, which is controlled by a separate gene. This is an example of epistasis, where the expression of one gene is dependent on the expression of another gene. In the case of the Bombay phenotype, the expression of the ABO gene is epistatic to the expression of the H gene.

The expression of ABO blood type antigens by l^A or l^B gene depends upon the presence of another gene H. ABO locus is on chromosome 9, while H locus is on chromosome 19. H gene changes a precursor substance into substance H. It produces an enzyme that inserts a sugar onto a precursor glycoprotein on the surface of RBC. Only then antigen A or antigen B specified by I^A or I^B gene could attach to this sugar of substance H. The recessive allele h cannot insert sugar molecule to glycoprotein. Therefore, hh individuals lack the site of attachment for antigen A or antigen B. A and B antigens cannot adhere to their RBC and fall away. Their RBC lack A and B antigens although they do not lack I^A and I^B genes. They are phenotypically like O, but are not genotypically O. their phenotype is called Bombay Phenotype.

Problem: How type A and AB parents could produce a child of blood type O.

Solution: Either parents have Bombay Phenotype or A is heterozygous I^A i and the other parent is of Bombay phenotype. 

If both parents have type A blood but carry a recessive gene for O blood type, then each parent will contribute one A allele and one O allele to their child. Therefore, the child will have a 25% chance of inheriting two O alleles, making them blood type O.

Similarly, if one parent has type A blood and the other parent has type AB blood but carries a recessive gene for O blood type, then the parent with type A blood will contribute one A allele and one O allele, while the parent with type AB blood will contribute either an A or a B allele and either an A or an O allele. If the parent with type AB blood contributes an O allele, then the child will have two O alleles and be blood type O. This gives a 25% chance of producing a child of blood type O.