Nov 14, 2015

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.

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.

Example 2: Bombay Phenotype
The expression of ABO blood type antigens by lA or lB 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 IA or IB 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 IA and IB 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 IA i and the other parent is of Bombay phenotype.