Epistasis: Types and Inferences
Question
Discuss
1.
Epistasis.
2.
The
types
of
epistasis
and
their statistical
inferences
i
n
t
r
o
d
u
c
t
io
n
Epistasis
is
a Greek word
that means
standing
over.
Bateson
used it
to
describe
the masking
effect
in
1909.
Definition of epistasis:
"
An
interaction
between
a
pair of
genes
where an
allele
of
one
gene
hides or
masks the visible
output,
or
phenotypic expression
of
another gene at another
locus”
Genes
whose
phenotypes
are
;
Expressed
are called
epistatic
genes
.
S
uppressed
are called
hypostatic
genes
.
The masking of
the phenotypic
effect
of
alleles
of
one
gene
by
alleles
of
another gene.
A
gene is said to be
epistatic
when its
presence
suppresses
the
effect
of a
gene
at
another
locus. Epistatic
genes are sometimes
called inhibiting genes
because of
their
suppressed
effect
on
other genes
that are
described
as
hypostatic.
Epistatic genes can be either a dominant allele or
homozygous recessive allele.
E
p
i
s
t
a
s
i
s
i
s
t
h
e
p
h
e
n
o
m
e
n
o
n
w
h
e
n
o
n
e
a
l
l
e
l
e
o
f
a
g
e
n
e
m
a
s
k
s
t
h
e
e
x
p
r
e
s
s
i
o
n
o
f
a
l
l
e
l
e
s
o
f
a
n
o
t
h
e
r
g
e
n
e
.
Difference
between
dominance and
epistasis
D
o
m
i
n
a
n
c
e
E
p
i
s
t
a
s
i
s
Involves intra-allelic gene
interaction.
Involves inter-allelic gene
interaction.
One
allele
hides
the
effect
of
One gene hides
the
effect
of
other
allele
at
the
same
gene
other gene
at
different
gene loci.
pair.
Kinds of
Epistatic
Interactions
I
n
e
p
i
s
t
a
s
i
s
l
e
s
s
t
h
a
n
f
o
u
r
p
h
e
n
o
t
y
p
e
s
a
p
p
e
a
r
i
n
F
2
.
(
і
)
D
o
m
i
n
a
n
t
E
p
i
s
t
a
s
i
s
(
1
2
:
3
:
1
)
(ii)
R
e
c
e
s
s
i
v
e
e
p
i
s
t
a
s
i
s
(
S
u
p
p
l
e
m
e
n
t
a
r
y
g
e
n
e
i
n
t
e
r
a
c
t
i
o
n
)
(
9
:
3
:
4
)
.
(iii)
D
u
p
l
i
c
a
t
e
R
e
c
e
s
s
i
v
e
G
e
n
e
s
(
9
:
7
)
(
C
o
m
p
l
e
m
e
n
t
a
r
y
G
e
n
e
s
)
(iv)
D
u
p
l
i
c
a
t
e
D
o
m
i
n
a
n
t
G
e
n
e
s
/
D
u
p
l
i
c
a
t
e
g
e
n
e
s
(
1
5
:
1
)
.
(v)
D
u
p
l
i
c
a
t
e
G
e
n
e
s
w
i
t
h
C
u
m
u
l
a
t
i
v
e
E
f
f
e
c
t
/
A
d
d
i
t
i
v
e
g
e
n
e
s
(
9
:
6
:
1
)
.
(vi)
D
o
m
i
n
a
n
t
R
e
c
e
s
s
i
v
e
I
n
t
e
r
a
c
t
i
o
n
/
I
n
h
i
b
i
t
o
r
y
g
e
n
e
s
(
1
3
:
3
)
.
1.
Dominant
Epistasis.
(12:3:1)
•
When
a
dominant allele
at
one locus
can
mask
the expression of
both alleles (dominant and
recessive) at
another
locus, it
is
known as
dominant epistasis. This
is
also
referred
to as
simple
epistasis.
•
This type
of
dominant epistasis modifies the
classical
ratio
of
9:3:3:1 into
12:3:1
Example:
Studied
in
summer squash
(
Cucurbita
pepo
)
•
An
example
of
dominant epistasis
is
found for
fruit colour
in
summer
squash.
There
are
three
types of
fruit colours
in
this
cucumber,
viz.,
white, yellow
and green.
White colour is
controlled
by
dominant gene
W
and yellow
colour
by
dominant gene
G.
White
is
dominant
over
both yellow and
green.
The green fruits are
produced
in
recessive
condition (wwgg). A cross between plants
having white
and yellow fruits
produced
F
1
with
white fruits. Inter-mating
of
F
1
plants
produced plants
with
white, yellow
and
green
coloured fruits
in
F
2
in 12
:
3
:
1
ratio
The
effect of
dominant
gene
’Y’ is
masked by the
dominant
gene
’W’
(epistatic
gene)
P
WWYY X
wwyy (whi
t
e
)
↓
(
g
reen)
F
1
WwYy
(white)
(selfed)
F2
White:Yellow:Green
12
:
3
:
1
2.
Recessive
epistasis (9:3:4)
(Supplementary
interaction)
•
When
recessive
alleles
at
one
locus
mask
the
expression of
both (dominant and
recessive)
alleles
at
another
locus, it is known as
recessive epistasis.
This type
of
gene
interaction
is also known as
supplementary
epistasis.
•
In
h
o
r
s
es,
b
r
own
coat
c
o
l
o
r
(
B
)
is
domin
a
nt
ove
r
tan
(
b
).
controls
the deposition
of
pigment in
hair.
•
The
dominant gene
(
C
)
codes
for the
presence of
pigment
in
hair,
whereas
the
recessive gene (
c
) codes
for the
absence of
pigment.
3
.
D
u
p
l
i
c
a
t
e
R
e
c
e
s
s
i
v
e
G
e
n
e
s
(
9
:
7
)
(
C
o
m
p
l
e
m
e
n
t
a
r
y
G
e
n
e
s
)
•
Non allelic dominant genes when present together produces a
phenotype different from that produced by either alone. Both
dominant alleles, when present together, complement each other
and produce a different phenotype.
•
I
n
o
t
h
e
r
w
o
r
d
s
,
h
o
m
o
z
y
g
o
u
s
r
e
c
e
s
s
i
v
e
a
l
l
e
l
e
s
a
t
b
o
t
h
l
o
c
i
e
i
t
h
e
r
a
l
o
n
e
o
r
t
o
g
e
t
h
e
r
p
r
o
d
u
c
e
s
i
d
e
n
t
i
c
a
l
p
h
e
n
o
t
y
p
e
s
,
t
h
e
F
2
r
a
t
i
o
b
e
c
o
m
e
s
9
:
7
.
T
h
e
g
e
n
o
t
y
p
e
s
a
a
B
-
,
A
-
b
b
a
n
d
a
a
b
b
p
r
o
d
u
c
e
o
n
e
p
h
e
n
o
t
y
p
e
.
•
B
a
t
e
s
o
n
a
n
d
P
u
n
n
e
t
t
o
b
s
e
r
v
e
d
t
h
a
t
w
h
e
n
t
w
o
w
h
i
t
e
f
l
o
w
e
r
e
d
v
a
r
i
e
t
i
e
s
o
f
s
w
e
e
t
p
e
a
,
L
a
t
h
y
r
u
s
o
d
o
r
a
t
u
s
w
e
r
e
c
r
o
s
s
e
d
,
F
1
p
r
o
g
e
n
y
h
a
d
p
u
r
p
l
e
f
l
o
w
e
r
s
.
W
h
e
n
F
1
w
a
s
s
e
l
f
e
d
,
t
h
e
F
2
r
a
t
i
o
s
h
o
w
e
d
t
h
e
p
r
e
s
e
n
c
e
o
f
b
o
t
h
p
u
r
p
l
e
a
n
d
w
h
i
t
e
f
l
o
w
e
r
e
d
v
a
r
i
e
t
i
e
s
i
n
t
h
e
r
a
t
i
o
9
:
7
.
•
The purple
colour
of flower in
sweet pea is governed
by two dominant genes say A
and
B. When these
genes are
in separate individuals (AAbb or
aaBB)
or
recessive (aabb)
they
produce
white
flower.
•
Other
examples
are;
Maize
colour
Human
mutism
Etc.
Exa
E
m
x
p
a
le
mple
•
I
n
t
h
i
s
c
a
s
e
d
o
m
i
n
a
n
t
a
l
l
e
l
e
s
o
n
b
o
t
h
l
o
c
u
s
a
r
e
r
e
q
u
i
r
e
d
h
e
n
c
e
w
h
e
r
e
v
e
r
A
a
n
d
B
b
o
t
h
a
r
e
p
r
e
s
e
n
t
t
h
e
y
r
e
s
u
l
t
i
n
t
o
p
u
r
p
l
e
e
f
f
e
c
t
m
a
s
k
i
n
g
t
h
e
w
h
i
t
e
.
•
T
h
i
s
i
s
b
e
c
a
u
s
e
A
a
n
d
B
a
l
l
e
l
e
s
m
o
d
i
f
i
e
d
t
h
e
c
o
l
o
r
l
e
s
s
p
r
e
c
u
r
s
o
r
b
y
s
h
o
w
i
n
g
t
h
e
i
r
e
f
f
e
c
t
s
4.
Duplicate
Dominant
Genes.
(15:1)
•
If
a dominant allele of
both
gene
loci
either alone or together
produces the same
phenotype
without
cumulative
effect,
i.e.,
independently the ratio will be
15:1.
It is also called duplicate
gene
inter
action.
•
They are identical genes but are situated on two different pairs
of chromosomes.
•
T
h
e
d
u
p
l
i
c
a
t
e
g
e
n
e
s
a
r
e
a
l
s
o
c
a
l
l
e
d
p
s
e
u
d
o
a
l
l
e
l
e
s
.
Example
•
A
good example
of duplicate
dominant epistasis
is
awn character
in rice.
Development
of
awn in rice
is controlled by
two
dominant duplicate genes
(A
and
B).
•
Presence of any of
these
two
alleles
can
produce
awn.
The awnless
condition develops only when
both these genes are
in
homozygous recessive
state (aabb).
A cross
between
awned
and
awnless
strains produced awned plants in
F
1
.
Inter-mating
of F
1
plants produced awned and
awnless
plants in
15
:
1 ratio in
F
2
generation
The allele A is
epistatic
to B/b alleles and
all plants
having allele A will
develop
awn. Another
dominant
allele
B is epistatic to
alleles
A/a.
Individuals
with this allele
also will
develop
awn
character.
Hence in
F
2
,
plants with
A-B-(9/16), A-bb-(3/16) and aaB-(3/16)
genotypes
will
develop
awn.
The
awnless condition
will develop
only
in
double
recessive
(aabb) genotype (1/16).
In
this
way
only
two classes of
plants
are
developed and
the
normal dihybrid segregation
ratio 9 : 3 : 3 : 1 is
modified
to 15 : 1
ratio
in
F
2
.
Similar gene action
is
found
for
nodulation
in
peanut and non-floating
character in
rice.
5.
Duplicate
Genes
with
Cumulative
Effect.
(9:6:1)
Two dominant genes at both loci have similar effect when
they are alone, but produced enhanced effect when they
come together. Such gene interaction is known as duplicate
genes with cumulative effect. Also called
Additive gene
interaction /Polymeric gene action
I
n
t
h
e
a
b
s
e
n
c
e
o
f
a
n
y
d
o
m
i
n
a
n
t
a
l
l
e
l
e
,
t
h
e
r
e
c
e
s
s
i
v
e
a
l
l
e
l
e
i
s
e
x
p
r
e
s
s
e
d
.
Example
•
A well-known example of polymeric gene
interaction is
fruit
shape in summer
squash.
There are
three types
of fruit shape
in this
plant,
viz., disc,
spherical
and
long.
The
disc
shape is
controlled
by two
dominant
genes (A
and
B), the
spherical
shape is produced by
either dominant
allele (A or B) and long
shaped fruits
develop
in double recessive
(aabb)
plants.
A cross between disc shape (AABB)
and long shape (aabb) strains
produced disc shape fruits in
F
1
.
Inter-
mating
of
F
1
plants produced plants
with
disc,
spherical and long shape
fruits in 9
:
6
:
1
ratio
in
F
2
Here
plants
with
A—B—(9/16)
genotypes
produce disc shape fruits, those with A-bb-
(3/16) and aaB-(3/16) genotypes produce
spherical fruits,
and plants with aabb (1/16)
genotype produce long
fruits.
Thus in
F
2
,
normal
dihybrid segregation
ratio
9:3:3:
1 is
modified to
9
:
6
:
1
ratio. Similar gene
action is also found in barley for awn
length.
6.
Inhibitory gene action
(13:3)
•
In
this
type of epistasis, a
dominant allele
at
one locus
can
mask
the expression of
both (dominant and
recessive)
alleles
at
second
locus.
Here this gene is called inhibitory gene as it is capable of
inhibiting the production of purple colour. Plants are purple, only
if they possess the gene for purple colour, in the absence of the
inhibitory gene.
This is
also
known as
inhibitory gene interaction
.
An
example
of
this
type of
gene interaction
is
found
for
anthocyanin pigmentation
in
rice.
Example
•
The
green
colour of
plants
is
governed
by
the
gene
I
which
is
dominant over purple
colour.
The
purple colour
is
controlled
by
a
dominant
gene
P.
When
a cross was
made between
green (IIpp) and purple (iiPP) colour plants, the
F
1
was green.
Inter-mating
of
F
1
plants
produced green and purple plants
in 13 : 3
ratio
in
F
2
.
This
can be
explained
as
follows.
Here the allele I isepistatic to alleles P
and p. Hence in
F
2
,
plants with
I-P-
(9/16),
I-pp
(3/16) and iipp (1/16)
genotypes will be green because I will
mask the
effect
of P or p. Plants with
iiP-(3/16) will be purple, because I is
absent.
In this
way
the
normal
dihybrid
segregation
ratio
9 : 3 : 3 : 1 is modified
to
13 : 3
ratio.
Similar
gene
interaction
is
found
for
grain
colour
in
maize,
plumage colour
in poultry and
certain
characters
in
other
crop
species.
Epistasis is the interaction between genes where one gene's allele masks the expression of another gene. Various types of epistasis, such as dominant and recessive epistasis, can be observed, impacting the phenotypic ratios. Statistical inferences can help analyze epistatic interactions in genetic studies.
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Question Discuss 1.Epistasis. 2.The types of epistasis and their statistical inferences
iin nt tr ro od du uc ct tio ion n Epistasis is a Greek word that means standing over. Bateson used it to describe the masking effect in 1909. Definition of epistasis: "An interaction between a pair of genes where an allele of one gene hides or masks the visible output, or phenotypic expression of another gene at another locus Genes whose phenotypes are ; Expressed are called epistatic genes. Suppressed are called hypostatic genes.
Epistasis is the phenomenon when one allele of a gene masks the expression of alleles of another gene. The masking of the phenotypic effect of alleles of one gene by alleles of another gene. A gene is said to be epistatic when its presence suppresses the effect of a gene at another locus. Epistatic genes are sometimes called inhibiting genes because of their suppressed effect on other genes that are described as hypostatic. Epistatic genes can be either a dominant allele or homozygous recessive allele.
Difference between dominance and epistasis Dominance Epistasis Involves intra-allelic gene interaction. Involves inter-allelic gene interaction. One allele hides the effect of One gene hides the effect of other allele at the same gene other gene at different gene loci. pair.
Kinds of EpistaticInteractions In epistasis less than four phenotypes appear in F2. ( ) Dominant Epistasis (12:3:1) (ii) Recessive epistasis (Supplementary gene interaction)(9:3:4). (iii) Duplicate Recessive Genes (9:7) (Complementary Genes) (iv) Duplicate Dominant Genes / Duplicate genes (15:1). (v) Duplicate Genes with Cumulative Effect/Additive genes (9:6:1). (vi) Dominant Recessive Interaction/Inhibitory genes (13:3).
1. Dominant Epistasis. (12:3:1) When a dominant allele at one locus can mask the expression of both alleles (dominant and recessive) at another locus, it is known as dominant epistasis. This is also referred to as simple epistasis. This type of dominant epistasis modifies the classical ratio of 9:3:3:1 into 12:3:1
Example: Studied in summer squash (Cucurbita pepo) An example of dominant epistasis is found for fruit colour in summer squash. There are three types of fruit colours in this cucumber, viz., white, yellow and green. White colour is controlled by dominant gene W and yellow colour by dominant gene G. White is dominant over both yellow and green.
The green fruits are produced in recessive condition (wwgg). A cross between plants having white and yellow fruits produced F1 with white fruits. Inter-mating of F1plants produced plants with white, yellow and green coloured fruits in F2in 12 : 3 : 1 ratio
The effect of dominant gene Y is masked by the dominant gene W (epistatic gene) P WWYY X wwyy (white) (green) F1 (white) (selfed) F2 White:Yellow:Green 12 : 3 : 1 WY Wy wY wy / WY WWYY WWYy WwYY WwY y WwYy Wy WWYy WWyy WwYy Wwy y wY WwYY WwYy wwYY wwY y wy WwYy Wwyy wwYy wwy y
2. Recessive epistasis (9:3:4) (Supplementary interaction) When recessive alleles at one locus mask the expression of both (dominant and recessive) alleles at another locus, it is known as recessive epistasis. interaction is also known as supplementary epistasis. This type of gene
In horses, brown coat color (B) is dominant over tan (b). However, phenotype is dependent on a second gene that controls the deposition of pigment in hair. how that gene is expressed in the The dominant gene (C) codes for the presence of pigment in hair, whereas the recessive gene (c) codes for the absence of pigment.
3. Duplicate Recessive Genes (9:7) (Complementary Genes) Non allelic dominant genes when present together produces a phenotype different from that produced by either alone. Both dominant alleles, when present together, complement each other and produce a different phenotype. In other words, homozygous recessive alleles at both loci either alone or together produces identical phenotypes, the F2 ratio becomes 9:7. The genotypes aa B-, A-bb and aabb produce one phenotype.
ExaEmxpalemple Bateson and Punnett observed that when two white flowered varieties of sweet pea, Lathyrus odoratus were crossed, F1progeny had purple flowers. When F1was selfed, the F2ratio showed the presence of both purple and white flowered varieties in the ratio 9:7. The purple colour of flower in sweet pea is governed by two dominant genes say A and B. When these genes are in separate individuals (AAbb or aaBB) or recessive (aabb) they produce white flower. Other examples are; Maize colour Human mutism Etc.
In this case dominant alleles on both locus are required hence wherever A and B both are present they result into purple effect masking the white. This is because A and B alleles modified the colorless precursor by showing their effects
4. Duplicate Dominant Genes. (15:1) If a dominant allele of both gene loci either alone or together produces the same phenotype without cumulative effect, i.e., independently the ratio will be 15:1. It is also called duplicate gene interaction. They are identical genes but are situated on two different pairs of chromosomes. The duplicate genes are also called pseudoalleles.
Example A good example of duplicate dominant epistasisis awn character in rice. Development of awn in rice is controlled by two dominant duplicate genes (A and B). Presence of any of these two alleles can produce awn. The awnless condition develops only when both these genes are in homozygous recessive state (aabb). A cross between awned and awnless strains produced awned plants in F1. Inter-mating of F1 plants produced awned and awnless plants in 15 : 1 ratio in F2generation
The allele A is epistatic to B/b alleles and all plants having allele A will develop awn. Another dominantallele B is epistatic to alleles A/a. Individuals with this allele also will develop awn character. Hence in F2, plants with A-B-(9/16), A-bb-(3/16) and aaB-(3/16) genotypes will develop awn.
The awnless condition will develop only in double recessive (aabb) genotype (1/16). In this way only two classes of plants are developed and the normal dihybrid segregation ratio 9 : 3 : 3 : 1 is modified to 15 : 1 ratio in F2. Similar gene action is found for nodulation in peanut and non-floating character in rice.
5. Duplicate Genes with Cumulative Effect. (9:6:1) Two dominant genes at both loci have similar effect when they are alone, but produced enhanced effect when they come together. Such gene interaction is known as duplicate genes with cumulative effect. Also called Additive gene interaction /Polymeric gene action In the absence of any dominant recessive alleleis expressed. allele, the
Example A well-known example of polymeric gene interaction is fruit shape in summer squash. There are three types of fruit shape in this plant, viz., disc, spherical and long. The disc shape is controlled by two dominant genes (A and B), the spherical shape is produced by either dominant allele (A or B) and long shaped fruits develop in double recessive (aabb) plants.
A cross between disc shape (AABB) and long shape (aabb) strains produced disc shape fruits in F1. Inter- mating of F1plants produced plants with disc, spherical and long shape fruits in 9 : 6 : 1 ratio in F2
Here plants with AB(9/16) genotypes produce disc shape fruits, those with A-bb- (3/16) and aaB-(3/16) genotypes produce spherical fruits, and plants with aabb (1/16) genotype produce long fruits. Thus in F2, normal dihybrid segregation ratio 9:3:3: 1 is modified to 9 : 6 : 1 ratio. Similar gene action is also found in barley for awn length.
6. Inhibitory gene action (13:3) In this type of epistasis, a dominant allele at one locus can mask the expression of both (dominant and recessive) alleles at second locus. Here this gene is called inhibitory gene as it is capable of inhibiting the production of purple colour. Plants are purple, only if they possess the gene for purple colour, in the absence of the inhibitory gene. This is also known as inhibitory gene interaction. An example of this type of gene interaction is found for anthocyanin pigmentation in rice.
Example The green colour of plants is governed by the gene I which is dominant over purple colour. The purple colour is controlled by a dominant gene P. When a cross was made between green (IIpp) and purple (iiPP) colour plants, the F1 was green. Inter-mating of F1 plants produced green and purple plants in 13 : 3 ratio in F2. This can be explained as follows.
Here the allele I isepistatic to alleles P and p. Hence in F2, plants with I-P- (9/16), I-pp (3/16) and iipp (1/16) genotypes will be green because I will mask the effect of P or p. Plants with iiP-(3/16) will be purple, because I is absent.
In this way the normal dihybrid segregation ratio 9 : 3 : 3 : 1 is modified to 13 : 3 ratio. Similar gene interaction is found for grain colour in maize, plumage colour in poultry and certain characters in other crop species.
