This document discusses epistasis, a phenomenon where one gene masks the effects of another gene at a different locus. It provides two examples of epistasis - recessive epistasis seen in determining coat color in Labrador retrievers, and dominant epistasis seen in determining fruit color in summer squash. Recessive epistasis in Labradors involves two loci, one for pigment type and one for pigment deposition, where the recessive e allele at the second locus masks the effects of the B and b alleles at the first locus. Dominant epistasis in squash involves two loci, where the dominant W allele at one locus inhibits pigment production and masks the second locus.

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Dr Ruqia Mehmood Baig
EPISTASIS

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Sometimes the effect of gene interaction is that one gene masks (hides)
the effect of another gene at a different locus, a phenomenon known as
epistasis.
In epistasis, the gene that does the masking is called the epistatic gene;
the gene whose effect is masked is a hypostatic gene. Epistatic genes may
be recessive or dominant in their effects.
Recessive epistasis Recessive epistasis is seen in the genes that determine
coat color in Labrador retrievers.
Gene Interaction with
Epistasis
These dogs may be black,
brown, or yellow; their
different coat colors are
determined by interactions
between genes at two loci.

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One locus determines the type of pigment produced by the skin cells:
a dominant allele B codes for black pigment, whereas a recessive allele
b codes for brown pigment.
Alleles at a second locus affect the deposition of the pigment in the
shaft of the hair;
allele E allows dark pigment (black or brown) to be deposited,
whereas a recessive allele e prevents the deposition of dark pigment,
causing the hair to be yellow.
The presence of genotype ee at the second locus therefore masks the
expression of the black and brown alleles at the first locus.
The genotypes that determine coat color and their phenotypes are:
Gene Interaction with
Epistasis

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If we cross a black Labrador
homozygous for the dominant
alleles with a yellow Labrador
homozygous for the recessive
alleles and then intercross the F1,
progeny is obtained in the F2 in a
9 : 3 : 4 ratio:
Gene Interaction with
Epistasis

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Gene Interaction with
Epistasis
Notice that yellow dogs can carry
alleles for either black or brown
pigment, but these alleles are not
expressed in their coat color.
In this example of gene
interaction, allele e is epistatic to B
and b, because e masks the
expression of the alleles for black
and brown pigments, and alleles B
and b are hypostatic to e.
In this case, e is a recessive
epistatic allele, because two copies
of e must be present to mask of
the black and brown pigments

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Gene Interaction with
Epistasis

7. Gene Interaction with
Epistasis
Dominant epistasis
Dominant epistasis is seen in the
interaction of two loci that determine
fruit color in summer squash, which is
commonly found in one of three
colors: yellow, white, or green.
If use the symbol W to represent the
dominant allele that inhibits pigment
production, then genotype W_ inhibits
pigment production and produces
white squash, whereas ww allows
pigment and results in colored squash.
W_Y_ white squash
W_yy white squash
wwY_ yellow squash
wwyy green squash

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Gene Interaction with
Epistasis
Dominant epistasis

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Duplicate recessive epistasis
Let’s consider one more
detailed example of epistasis. Albinism is the
absence of
pigment and is a common genetic trait in many
plants and
animals. Pigment is almost always produced
through a
multistep biochemical pathway; thus, albinism may
entail
gene interaction.
Gene Interaction with
Epistasis

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Duplicate recessive epistasis
Gene Interaction with
Epistasis