This document discusses epistasis, which is when the expression of one gene is affected by other independently inherited genes. It provides examples of different types of epistatic interactions including recessive epistasis. Recessive epistasis is when a recessive gene suppresses the expression of a non-allelic gene. The document uses pigmentation in onions as an example, where a recessive inhibitor gene (ii) causes white color regardless of other pigmentation genes when in the homozygous recessive state. A Punnett square is provided to show the expected phenotypic ratios in the F2 generation.

1. Presented by
Rahul Naskar
Pranab Karmakar
Date :- 01/10/2020

2. What is Epistasis ?
Epistasis is a circumstance where the expression of one gene is affected by
the expression of one or more independently inherited genes.
Type of Epistasis :
1. Recessive Epistasis
2. Dominant Epistasis
3. Dominant [Inhibitory] Epistasis
4. Duplicate Recessive Epistasis
5. Duplicate Dominant Epistasis
6. Polymeric Gene Interaction

3. What is Recessive Epistasis ?
A recessive gene (Recessive Homozygous genotype) suppresses
the expression of a non allelic gene.
Epistatic Gene : Dominant nature of a single gene will be suppressed
due to epistatic gene. Also called regulatory gene.
Hypostatic Gene : The gene which’s phenotypic expression is
regulated by Epistatic gene.

4. Example of recessive Epistasis :
Pigmentation in onion bulb :
There are three type of pigmentation may be possible in onion bulb.
• Red
• Yellow
• White (No pigment)
Red and yellow appears because of some pigmentation
genes but white colour appears due to presence of pigment inhibition
gene which is epistatic only in recessive state.

5. So lets inhibitor gene denotes with I,I
But this gene shows its epistatic effect only in
homozygous recessive condition ( ii ) as a result
pigmentation of onion bulb will be white.
Now Red ( R ) is dominant over yellow ( r ).
Let’s us look at some examples of genotype and
what would be it’s phenotype :
1. IiRR – Red
2. iiRr – White
3. Iirr –Yellow
4. IIRr – Red

6. P1 ------- IIRR X iirr
Red White
F1 ------- IiRr x IiRr
IR Ir iR iR
IR
IIRR
Red
IIRr
Red
IiRR
Red
IiRr
Red
Ir
IIRr
Red
Iirr
Yellow
IiRr
Red
Iirr
Yellow
iR
IiRR
Red
IiRr
Red
iiRR
White
iiRr
White
ir
IiRr
Red
Iirr
Yellow
iiRr
White
iiRr
White
Red = 9/16
White = 4/16
Yellow = 3/16
So Phenotypic ratio is = 9:4:3
Red Onion
Yellow Onion
White Onion
F2 -------

7. ThankYou