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EPISTASIS. Dr. S. Ramgopal Rao. Epistatic Gene Interactions. Gene interactions occur when two or more different genes influence the outcome of a single trait Most morphological traits (height, weight, color) are affected by multiple genes

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Dr. S. Ramgopal Rao

Epistatic gene interactions
Epistatic Gene Interactions

  • Gene interactions occur when two or more different genes influence the outcome of a single trait

  • Most morphological traits (height, weight, color) are affected by multiple genes

  • Epistasis describes situation between various alleles of two genes

  • Quantitative loci is a term to describe those loci controlling quantitatively measurable traits

  • Pleiotropy describes situations where one gene affects multiple traits


  • Epistasis is a form of gene interaction in which one gene masks the phenotypic expression of another.

  • There are no new phenotypes produced by this type of gene interaction.

Epistatic versus hypostatic
Epistatic versus Hypostatic

  • The alleles that are masking the effect are called epistatic alleles

  • The alleles whose effect is being masked are called the hypostatic alleles.

Recessive or dominant
Recessive or Dominant?

  • Epistasis can be described as either recessive epistasis or dominant epistasis.

  • Let’s look at an example of recessive epistasis….

Labrador retrievers
Labrador Retrievers

  • Fur color in Labrador Retrievers is controlled by two separate genes.

    • Fur color is a polygenic trait!

      Gene 1: Represented by B

      : Controls color

      Gene 2: Represented by E

      : Controls expression of B

Labrador retrievers1
Labrador Retrievers

  • If a Labrador retriever has a dominant B allele, they will have black fur.

  • If they have two recessive alleles (bb) they will have brown fur.

Labrador retrievers2
Labrador Retrievers

  • If a retriever receives at least one dominant “E” allele, they will remain the color that the “B” allele coded for.

    • Either black of brown

  • However, if a dog receives a pair of homozygous recessive “e” alleles, they will be golden regardless of their “B” alleles!

Labrador retrievers3
Labrador Retrievers

  • BBEE and BbEe --> Black retrievers

  • bbEE and bbEe --> Brown retrievers

  • BBee, Bbee, or bbee --> Golden retrievers

Try this cross
Try this cross…

  • You have decided to cross your golden retriever (bbee) with the neighbor’s chocolate retriever (bbEe). What color pups will they have?

Bbee x bbee
bbee x bbEe

  • gametes: be

  • Gametes: bE or be

  • Genotypes of F1 generation: bbEe and bbee

  • Pups phenotypes:Brown and golden

Dominant epistasis
Dominant Epistasis

  • Let’s have a look at dominant epistasis…

  • Squash fruit color is controlled by two genes.

  • Gene 1 is represented by a W

  • Gene 2 is represented by a G

Squash fruit color
Squash Fruit Color

  • Genotypes and Phenotypes:

  • W-/G- white

  • W-/gg white

  • ww/G- green

  • ww/gg yellow

Squash fruit color1
Squash Fruit Color

  • Which allele is epistatic in squash color?

  • How do you know?

The dominant W allele is epistatic

Because every time a dominant W allele

shows up in a squash genotype, the squash

fruit color is white.

Try this cross1
Try this cross….

  • Cross a green squash (wwGg) with a white squash (Wwgg).

  • What color are the offspring?

Wwgg x wwgg
Wwgg x wwGg

  • gametes: Wg or wg

  • gametes: wG or wg

  • F1 generation genotypes:

  • Phenotypes:

Epistatic gene interactions1
Epistatic Gene Interactions

  • examine cases involving 2 loci (genes) that each have 2 alleles

  • Crosses performed can be illustrated in general by

    • AaBb X AaBb

    • Where A is dominant to a and B is dominant to b

  • If these two genes govern two different traits

    • A 9:3:3:1 ratio is predicted among the offspring

    • simple Mendelian dihybrid inheritance pattern

  • If these two genes do affect the same trait the 9:3:3:1 ratio may be altered

    • 9:3:4, or 9:7, or 9:6:1, or 8:6:2 or 12:3:1, or 13:3, or 15:1

    • epistatic ratios

A cross producing a 9 7 ratio
A Cross Producing a 9:7 ratio



9 C_P_ : 3 C_pp :3 ccP_ : 1 ccpp

Epistatic gene interaction
Epistatic Gene Interaction

  • Complementary gene action

    • Enzyme C and enzyme P cooperate to make a product, therefore they complement one another

Enzyme C

Enzyme P







Epistatic gene interaction1
Epistatic Gene Interaction

Enzyme C

Enzyme P







  • Epistasis describes the situation in which a gene masks the phenotypic effects of another gene

  • Epistatic interactions arise because the two genes encode proteins that participate in sequence in a biochemical pathway

  • If either loci is homozygous for a null mutation, none of that enzyme will be made and the pathway is blocked

Enzyme C

Enzyme P







genotype cc

genotype pp