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Epistasis
Definition
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Epistasis is a form of gene
interaction in which one pair of
alleles (gene) masks the phenotypic
expression of another.
Epistasis is a form of polygenics
The new phenotype is really the
“absence” of the usual phenotype. It
can result in albinism.
Epistatic versus Hypostatic
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The alleles that are interferring or
masking the effect are called
epistatic alleles
The alleles whose effect is being
interferred with or masked are called
the hypostatic alleles.
Recessive or Dominant?
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Epistasis can be described as either
recessive epistasis or dominant
epistasis.
Let’s look at an example of recessive
epistasis….
Labs (the dogs!)
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Fur color in Labs is controlled by two
separate genes.
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Fur color is a polygenic trait!
Gene 1: Represented by B
: Controls color
Gene 2: Represented by E
: Controls expression of B
Labs
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If a Lab has a dominant
B allele, they will have
black fur.
If they have two
recessive alleles (bb)
they will have brown
fur.
Labs
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If a lab 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!
This would be considered “Recessive
Epistasis”
Labs
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BBEE and BbEe --> Black lab
bbEE and bbEe --> Brown lab
BBee, Bbee, or bbee --> Golden lab
Recessive Epistasis
Try this cross…
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You have decided to cross your golden
lab (bbee) with the neighbor’s black lab
(BbEe). What color pups will they
have? (Shortcut?)
Yup…..you have to do a dihybrid cross.
bbee x BbEe
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FOIL: be
FOIL: BE, Be, bE or be
Genotypes of F1 generation:
BbEe; Bbee; bbEe; bbee
Pups phenotypes:
Black (25%); brown (25%)
and golden (50%)
Dominant Epistasis
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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
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Genotypes and
Phenotypes:
W_/G_
W_/gg
ww/G_
ww/gg
white
white
green
yellow
Squash Fruit Color
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Which type of epistasis is represented
here?
Dominant Epistasis
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How do you know?
Because every time a dominant W allele
shows up in a squash genotype, the squash
fruit color is white.
Dominant Epistasis
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Epistatic Alleles?
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WW
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Hypostatic Alleles?
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G alleles
Try this cross….
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Cross a green squash (wwGg) with a
white squash (Wwgg).
What color are the offspring?
Wwgg x wwGg
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FOIL: Wg or wg
FOIL: wG or wg
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F1 generation genotypes:
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Phenotypes:
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