12.4 Polygenic Inheritance and Epistasis | Inheritance | Biology 11

Polygenic Inheritance

Polygenic inheritance (also called quantitative inheritance) is a type of inheritance in which a single quantitative trait is controlled by the cumulative effect of two or more genes (called polygenes) located at different loci on the same or different chromosomes.

Key Features

Each gene contributes an additive (cumulative) effect to the phenotype.
Polygenic traits show continuous variation — a range of phenotypes rather than distinct classes.
When plotted, the frequency distribution of phenotypes follows a bell-shaped curve (normal distribution).
The environment also influences the expression of polygenic traits.

Examples in Humans

Trait	Nature
Skin colour	Controlled by multiple gene pairs (estimated 3–6 loci)
Height	Multiple genes + nutrition
Intelligence	Multiple genes + environment
Eye colour	Multiple genes (OCA2, HERC2, etc.)

Wheat Kernel Colour — A Classic Example

Nilsson-Ehle studied kernel colour in wheat, controlled by three gene pairs (A, B, C). Each dominant allele adds colour; recessive alleles add none.

For $n$ pairs of polygenes, the number of phenotypic classes = $2 n + 1$
For $n = 3$ : $2 (3) + 1 = 7$ phenotypic classes
The number of genotypic combinations = $4^{n}$

Example: A trihybrid cross $A a B b C c \times A a B b C c$ :

Produces 7 phenotypic classes based on the number of dominant alleles (0 through 6).
The class with 3 dominant alleles (intermediate colour) is the most frequent.

$Phenotypic classes = 2 n + 1$

Epistasis

Epistasis is a form of gene interaction in which one gene (the epistatic gene) masks or suppresses the expression of another gene (the hypostatic gene) at a different locus.

Epistasis differs from dominance: dominance involves alleles at the same locus, while epistasis involves genes at different loci.

Types of Epistasis

1. Dominant Epistasis

A dominant allele at one locus masks the expression of the gene at the other locus.
Modified dihybrid ratio: 12 : 3 : 1
Example: Fruit colour in squash — a dominant allele at locus A suppresses colour expression at locus B.

2. Recessive Epistasis

The homozygous recessive condition ( $aa$ ) at one locus masks the expression of the gene at the other locus.
Modified dihybrid ratio: 9 : 3 : 4
Example: Coat colour in Labrador retrievers — the $ee$ genotype prevents pigment deposition regardless of the B locus.

Type	Epistatic condition	Phenotypic ratio
Dominant epistasis	Dominant allele masks other gene	12 : 3 : 1
Recessive epistasis	Homozygous recessive masks other gene	9 : 3 : 4
Duplicate recessive epistasis	Either locus homozygous recessive masks	9 : 7

Epistasis vs. Normal Dihybrid Ratio

A normal dihybrid cross ( $A a B b \times A a B b$ ) gives a 9 : 3 : 3 : 1 ratio. Epistasis modifies this ratio because one gene interferes with the expression of another.

Summary Comparison

Feature	Polygenic Inheritance	Epistasis
Number of genes	Two or more	At least two
Gene interaction	Additive/cumulative	One masks the other
Variation type	Continuous	Discontinuous (modified ratios)
Example	Human skin colour, height	Labrador coat colour, squash colour

Polygenic Inheritance

Key Features

Each gene contributes an additive (cumulative) effect to the phenotype.
Polygenic traits show continuous variation — a range of phenotypes rather than distinct classes.
When plotted, the frequency distribution of phenotypes follows a bell-shaped curve (normal distribution).
The environment also influences the expression of polygenic traits.

Examples in Humans

Trait	Nature
Skin colour	Controlled by multiple gene pairs (estimated 3–6 loci)
Height	Multiple genes + nutrition
Intelligence	Multiple genes + environment
Eye colour	Multiple genes (OCA2, HERC2, etc.)

Wheat Kernel Colour — A Classic Example

Nilsson-Ehle studied kernel colour in wheat, controlled by three gene pairs (A, B, C). Each dominant allele adds colour; recessive alleles add none.

For $n$ pairs of polygenes, the number of phenotypic classes = $2 n + 1$
For $n = 3$ : $2 (3) + 1 = 7$ phenotypic classes
The number of genotypic combinations = $4^{n}$

Example: A trihybrid cross $A a B b C c \times A a B b C c$ :

Produces 7 phenotypic classes based on the number of dominant alleles (0 through 6).
The class with 3 dominant alleles (intermediate colour) is the most frequent.

$Phenotypic classes = 2 n + 1$

A dominant allele at one locus masks the expression of the gene at the other locus.
Modified dihybrid ratio: 12 : 3 : 1
Example: Fruit colour in squash — a dominant allele at locus A suppresses colour expression at locus B.

2. Recessive Epistasis

The homozygous recessive condition ( $aa$ ) at one locus masks the expression of the gene at the other locus.
Modified dihybrid ratio: 9 : 3 : 4
Example: Coat colour in Labrador retrievers — the $ee$ genotype prevents pigment deposition regardless of the B locus.

Type	Epistatic condition	Phenotypic ratio
Dominant epistasis	Dominant allele masks other gene	12 : 3 : 1
Recessive epistasis	Homozygous recessive masks other gene	9 : 3 : 4
Duplicate recessive epistasis	Either locus homozygous recessive masks	9 : 7

Epistasis vs. Normal Dihybrid Ratio

A normal dihybrid cross ( $A a B b \times A a B b$ ) gives a 9 : 3 : 3 : 1 ratio. Epistasis modifies this ratio because one gene interferes with the expression of another.

Summary Comparison

Feature	Polygenic Inheritance	Epistasis
Number of genes	Two or more	At least two
Gene interaction	Additive/cumulative	One masks the other
Variation type	Continuous	Discontinuous (modified ratios)
Example	Human skin colour, height	Labrador coat colour, squash colour