RTM Nagpur University, Nagpur, B. Sc. Semester III, Paper II, 1.1: Interaction of genes: Epistasis, Codominance and incomplete dominance

1. Interaction of genes
Epistasis, Codominance and Incomplete dominance
By,
Dr. Manoj K. Bangadkar

2. • Phenotypic expression of a gene may be altered by the
influence of other gene
• The modification of the normal phenotype of certain genes is
known as gene interaction.
• Gene interaction is of two types
– Intergenic or non allelic gene : Occurs between alleles of
different gene to alter normal phenotype
– Ex. Complementary gene, Supplementary gene, Polygenes,
Epistasis, Pleotropy and Lethal genes
– Intragenic or allelic gene : Occurs between alleles of same
gene to alter normal phenotype
– Ex. Incomplete dominance, Codominance and Multiple
alleles
Gene Interaction

3. Epistasis
• Phenomenon of masking or supressing the expression of geneby
another non allelic gene
• The gene which supressed by the presence of non allelic gene is
termed as hypostatic gene
• The term epistssis is coined by Bateson in 1909
• Various types of epistatic gene interaction are
– Recessive epistasis
– Dominant epistasis

4. Recessive Epistasis (Supplementary genes)
• Form of gene interaction, in which homozygosity for a recessive
allele at one locus can mask the phenotypic expression of a
second gene
• It has the characteristic 9:3:4 ratio of phenotype
• Example: Coat color in Mice
– If allele, C = Black coat color
c= Albino (Colorless coat) i.e. it has no effect
A= Supplementary gene
- When black coat mice is crossed with albino mice, the F1
generation is Agouti. It mean here the effect of non allelic
gene is changed

6. Dominant Epistasis
• The dominant allele at one locus mask (supress) the expression of
alleles at a second locus
• The F2 ratio is generally 12:3:1
• Example: Coat color in Dog
– Three types of pigmentation occurs in dog- white, black and
brown

7. Codominance
• Sometimes when neither alle is dominant, both alleles are
expressed independently in the heterozygotes
• Example-
• AB Blood group: In the human AB blood type allele for type A (IA)
blood and type B (IB) blood group are codominant so when they
come together in an individual they produce blood group AB

8. Incomplete dominance
• Sometimes two genes of allelomorphic pair do not show
dominant recessive relationship, but when present
simultaneously (or come together) they show intermediate
condition which is called incomplete dominance
• Example- Incomplete dominance in snapdragon