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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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EPISTASIS

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
definition
Definition
  • 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

purple

white

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

Purple

pigment

Colorless

precursor

Colorless

intermediate

epistatic gene interaction1
Epistatic Gene Interaction

Enzyme C

Enzyme P

Colorless

precursor

Colorless

intermediate

Purple

pigment

  • 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

Colorless

precursor

Colorless

intermediate

Purple

pigment

genotype cc

genotype pp