Epistasis
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Epistasis. 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.

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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.


How do we solve epistatic problems
How do we solve epistatic problems?

  • We are dealing with polygenic traits as in the previous section, however we now have one pair of alleles masking the other.

  • This means we will still be using dihybrid crosses!


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

  • FOIL: be

  • FOIL: 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 epistasis

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

  • FOIL: Wg or wg

  • FOIL: wG or wg

  • F1 generation genotypes:

  • Phenotypes:


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