4. Gene interaction - Epistasis - Dominant & Recessive, Non-epistatsis

Basics of Undergraduate/university fellows

Epistasis is a Greek word that means standing over.
 BATESON used term epistasis to describe the masking effect in 1909
 The term epistasis describes a certain relationship between genes, where an allele of
one gene hides or masks the visible output or phenotype of another gene.
 When two different genes which are not alleles, both affect the same character in such
a way that the expression of one masks (inhibits or suppresses) the expression of the
other gene, the phenomenon is said to be epistasis.
 The gene that suppresses other gene expression is known as Epistatic gene.
 The gene that is suppressed or remain obscure is called Hypostatic gene
 The classical phenotypic ratio of 9:3:3:1 F2 ratio becomes modified by epistasis.

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GENE INTERACTION

UNIT II (B): GENE INTERACTIONS

 Mendel and other workers assumed that characters are governed by single genes but
later it was discovered that many characters are governed by two or more genes.
 Such genes affect the development of concerned characters in various ways; this lead
to the modification of the typical Dihybrid ratio (9:3:3:1) or trihybrid (27:9:9:9:3:3:3:1).
 Numerous deviations have been recorded in which different kinds of interactions are
possible between the genes.
 The phenomenon of two or more genes affecting the expression of each other in various
ways in the development of a single character of an organism is known as GENE
INTERACTION.
 Most of the characters of living organisms are controlled/ influenced/ governed by a
collaboration of several different genes.
 BATESON suggested concept of gene interaction and this concept is called Bateson’s
factor hypothesis.

KINDS OF GENE INTERACTIONS
1. INTRA- ALLELIC GENE INTERACTIONS
 The genetic interactions between the alleles of a single gene are referred to intra-
allelic gene interactions.
 Commonly referred as Intragenic interaction

2. INTER-ALLELIC GENE INTERACTIONS
 The genetic interactions between the alleles of one gene with the allele of other gene
are referred to inter- allelic gene interactions.
 Commonly referred as Intergenic interaction
Some of the interallelic gene interactions to be discussed are tabulated below.



Interallelic gene
interaction
Example Parental
generation
F1
generation
F2
Ratio
A Complementary
gene interaction
Flower color in
Lathyrus odoratus
White X White
ccPP X CCpp
Purple
CcPp
9:7
B Supplementary
gene interaction
Grain color in
Maize
Purple X White
RRPP X rrpp
Purple
RrPp
9:3:4
C Epistasis
i Dominant Epistasis Fruit color in
Cucurbita pepo
White X Yellow
WWyy X wwYY
White
WwYy
12:3:1
ii Recessive Epistasis Coat color in Mice

Black X Albino
CCaa X ccAA
Agouti
CcAa
9:3:4
D Non- Epistasis Comb pattern in
Poultry
Rose X Pea
RRpp X rrPP
Walnut
RrPp
9:3:3:1

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GENE INTERACTION


C. EPISTASIS GENE INTERACTION
 Epistasis is a Greek word that means standing over.
 BATESON used term epistasis to describe the masking effect in 1909
 The term epistasis describes a certain relationship between genes, where an allele of
one gene hides or masks the visible output or phenotype of another gene.
 When two different genes which are not alleles, both affect the same character in such
a way that the expression of one masks (inhibits or suppresses) the expression of the
other gene, the phenomenon is said to be epistasis.
 The gene that suppresses other gene expression is known as Epistatic gene.
 The gene that is suppressed or remain obscure is called Hypostatic gene
 The classical phenotypic ratio of 9:3:3:1 F2 ratio becomes modified by epistasis.
Difference between Dominance and Epistasis
DOMINANCE EPISTASIS
1. Involves intra-allelic gene interaction Involves inter-allelic gene interaction
2. One allele hides the effect of other allele
of the same gene
One allele hides the effect of other allele
of the different gene

Epistasis is of two main types
i. Dominant Epistasis
ii. Recessive Epistasis

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GENE INTERACTION

C {i} DOMINANT EPISTASIS (12:3:1)
Ex: Fruit Color in Cucurbita pepo

 When out of the two genes, the dominant allele (Example: A) of one gene masked the
activity of alleles of another gene (Example: B), and expressed itself phenotypically,
then A gene locus is said to be Epistatic to B gene locus.
 Because, the dominant allele A can express itself in the presence of either B or b allele,
therefore, such type of epistasis is termed as dominant epistasis.
 The alleles of hypostatic locus or gene B will be able to express themselves
phenotypically only when gene A locus may contain two recessive allele.
 The dominant epistasis modify the classical ratio of 9:3:3:1 into 12:3:1

Ex: FRUIT COLOR IN Cucurbita pepo (Summer squash)
 In fruit color in Cucurbita Pepo, commonly known as summer squash, is a standard
example of dominant epistasis.
 There are three types of fruit colors in this cucumber, viz., white, yellow and green.

The color in Cucurbita pepo is controlled by two
genes.

1. Gene W :
 Dominant W gives White color
 Recessive w produces colored.

2. Gene Y:
 Dominant Y generates Yellow color
 Recessive w produces green color

 The gene for white-colored squash is dominant to colored squash, and the gene
symbols are W=White and w=colored.
 The gene for yellow-colored squash is dominant to green, and the gene symbols used
are Y= yellow, y= green.
 Gene Y in homozygous or heterozygous condition converts green to yellow fruit color.
 Homozygous recessive yy cannot convert green to yellow, therefore results in green
fruit color.

Parental Phenotype : White X Yellow
fruit color fruit color

Parental Genotype : WWyy X wwYY

Parental Gametes : Wy X wY

F1 Generation : WwYy
White fruit color

F1 selfing: F1 X F1 : WwYy X WwYy
White fruit color White fruit color

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GENE INTERACTION


F1 Gametes : WY Wy wY wy X WY Wy wY wy
F2 generation :









Phenotypic Ratio = 12 White: 3 Yellow: 1 Green
F2 Analysis :






 Gene W codes for inhibitor enzyme that stops conversion white to green.
 So, When Gene is homozygous or heterozygous [WW/Ww] fruit color is white
 Only in homozygous ww (absence of inhibitor) condition, it is colored (green or yellow)
 Therefore, if a Dihybrid is selfed, three phenotypes are produced in the ratio 12:3:1.
 Because the presence of the dominant W allele masks the effects of either the G or g
allele, this type of interaction is called dominant epistasis
Other Examples of Dominant Epistasis are;
 Coat color in dogs
 Color of the hull in oats seeds
 Plumage color in poultry






WY Wy wY Wy
WY WWYY
White
WWYy
White
WwYY
White
WwYy
White
Wy WWYy
White
WWyy
White
WwYy
White
Wwyy
White
wY WwYY
White
WwYy
White
wwYY
Yellow
wwYy
Yellow
Wy WwYy
White
Wwyy
White
wwYy
Yellow
wwyy
Green
Genotype Fruit color Gene Actions
9W_Y_

White

Dominant white allele
suppress effects of Y allele
3W_yy

White

Dominant white allele
Suppress effect of y allele
3wwY_

Yellow

Recessive ‘w’ (color) allele
allow yellow allele expression
1wwyy

Green Recessive ‘wy’ allele allows
green allele expression

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GENE INTERACTION

C {ii} RECESSIVE EPISTASIS (9:3:4)
Ex: Coat Color in Mice

• Sometimes the recessive alleles of one gene locus (aa) masks the action or
phenotypic expressions of alleles of another gene locus (BB, Bb as bb alleles). This
type of epistasis is called recessive epistasis.
• The alleles of B express itself, when epistatic locus has dominant alleles (AA or Aa)
• Due to recessive epistasis, the phenotypic ratio 9:3:3:1 becomes modified into 9:3:4.

EX: COAT COLOR IN MICE
 In mice, various types of Epistatic interactions have been reported.
 The most interesting case is of recessive epistasis in coat colors.
 The common house mouse occurs in a number of coat colors
1. Agouti
2. Black
3. Albino

 The agouti color pattern is commonly occurred one (wild type) and is characterized
by color banded have in which the part nearest the skin is gray, then a yellow band
and finally the distal part is either black or brown.
 The albino mouse lacks totally in pigments and has white hairs and pink eyes.
 Agouti colored coat is dominant over colorless or albino coat.
 Agouti is dominant to black
 Two gene loci ( C and A) are found to be responsible for coat color in mice
 Heterozygous CA alleles give an agouti phenotype, Ca alleles give black phenotype
and cA or ca alleles give an albino phenotype.
 When a homozygous black [CCaa] is crossed with a homozygous albino [ccAA], all F1
progenies are agouti [CcAa].
 When the F1 agouti are selfed, the F2 ratio was found to be 9:3:4.

Parental Phenotype : Black mice X Albino mice
Parental Genotype : CCaa X ccAA

Parental Gametes : Ca X cA
F1 Generation : CcAa
Agouti mice

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GENE INTERACTION

F1 selfing: F1 X F1 : CcAa X CcAa
Agouti mice Agouti mice

F1 Gametes : CA Ca cA ca X CA Ca cA ca

F2 generation :


 From the cross it becomes apparent that two independent pairs of genes (that is C-c
and A-a) have interacted in the production of the phenotypic trait in such a way that
one dominant (C) produces its effect whether or not the second (A) is present.
 But the second (A) gene can produce its effect only in the presence of the first.
 As the recessive cc marks the effect of A, the type of gene interaction is termed as
recessive epistasis.
Epistatic
allele
Hypostatic
allele
Phenotypic
Expression
of allele
F2
Phenotypic
ratio
1. cc AA, Aa, aa C Albino = 4
2. CC, Cc AA, Aa A Agouti = 9
3. CC, Cc Aa A Black = 3

Other Example for Recessive Epistasis;
 Wing type in Drosophila







CA Ca cA Ca
CA CCAA
Agouti
CCAa
Agouti
CcAA
Agouti
CcAa
Agouti
Ca CCAa
Agouti
CCaa
Black
CcAa
Agouti
Ccaa
Black
cA CcAA
Agouti
CcAa
Agouti
ccAA
albino
ccAa
Abino
ca CcAa
Agouti
Ccaa
Black
ccAa
Albino
ccaa
Albino

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GENE INTERACTION

D. NON- EPISTASIS GENE INTERACTION (9:3:3:1)
Ex: Comb pattern in Poultry

 In certain cases, two pairs of genes exhibiting full dominance determine a same
phenotype.
 When the dominant allele of one gene is present with the homozygous recessive allele
of the other locus, the dominant alleles of each of the two genes produce separate
forms of the character (phenotype),
 But, when the dominant alleles of both the genes are present together, they produce a
different phenotype.
 The homozygous recessive state at both genes gives rise to another phenotype.
 As two genes are assort independently, they produce new phenotypes and the F2
Phenotypic ratio remains unaltered (9:3:3:1)
EXAMPLE: COMB PATTERN IN FOWL (9:3:3:1)
 The classical case of genetic interaction of two genes is discovered by BATESON and
PUNNETT (1905-1908) in fowls.
 There are many different breeds of domestic chicken.
 Each breed possesses a characteristic type of comb.

Foundational Experiments
A cross of chicken with a rose comb to one
with a single comb produces ¾ rose and ¼
single, showing dominance of rose over
single.
P : Rose X Single
F1 : 3 Rose : 1 Single

Rose is dominant over single

Another cross between pea combed and
single combed chickens produces pea and
single combed chickens in the ratio of 3:1
showing dominance of pea over single.
P : Pea X single
F1 : 3 Pea : 1 Single

Pea dominant over single

But, when a rose combed chicken crossed
with that of pea combed. The F1 Progeny
was found with a different type of comb
known as ‘Walnut’ (Malay breed).

P : Rose X Pea
F1 : Walnut
When the F1 walnut combed chickens were
bred together, in F2 all four types of combs,
that is 9 walnut, 3 rose, 3 pea and 1 single
appeared. 1

F1 : Walnut X Walnut
F2: 9 Walnut:3 Rose: 3 Pea: 1 Single

These peculiar results were interpreted by Bateson and Punnett as follows:
 The rose comb is caused by the combination of homozygous recessive genes pp and
homozygous or heterozygous dominant genes RR and Rr.
 Rose > RRpp | Rrpp

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GENE INTERACTION

 The pea comb is produced by combination of a homozygous recessive condition (rr)
and homozygous or heterozygous dominant condition (PP or Pp)
 Pea > rrPP | rrPp
 While, the single type comb is produced by double recessive, rrpp genes.
 Single > rrpp
 Thus, R gene determine the shape of rose comb and P gene determines the shape of
pea comb, but when both recessive genes happens to come together in a single
individual (due to cross between rose and pea combed chickens ), they interact to
produce a walnut comb in F1.
 In the cross of two walnut chickens, two genes interact variously to produce four types
of offsprings in F2.
 During the inheritance of combs in fowls, the genes themselves do not determine the
development of a character (presence or absence of comb) and simply modify a
character determined by a basic gene and therefore, known as Non-epistatic Genes.

Chicken
variety
Wyandotte breed

Brahma breed Leghorn breed Malay Breed
Comb
pattern
Rose comb Pea comb Single comb

Walnut Comb

Comb
Picture



Genotype RRpp
Rrpp
rrPP
rrPp
rrpp RRPP
RrPP
RRPP
RrPp

Parental Breed : Wyandotte breed X Brahma breed
Parental Phenotype : Rose-comb X Pea-comb
Parental Genotype : RRpp X rrPP
Parental Gametes : Rp X rP
F1 Generation : RrPp
Walnut-Comb

F1 selfing: F1 X F1 : RrPp X RrPp
Walnut-Comb Walnut-Comb

F1 Gametes : RP Rp rP rp X RP Rp rP rp

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GENE INTERACTION

F2 Generation :









F2 Analysis:
Phenotypes Walnut Rose Pea Single
Genotypes RRPP
RRPp
RrPp
RrPP

RRpp
Rrpp

rrPP
rrPp

rrpp
Ratio 9 3 3 1
Explanation Supplemental
interaction of
R and P
Dominance
of R over r
and pp
Dominance
of P over p
and r
Recessiveness
of rr and PP











RP Rp rP Rp
RP RRPP
Walnut
RRPp
Walnut
RrPP
Walnut
RrPp
Walnut
RP RRPp
Walnut
RRpp
Rose
RrPp
Walnut
Rrpp
Rose
rP RrPP
Walnut
RrPp
Walnut
rrPP
Pea
rrPp
Pea
rp RrPp
Walnut
RrPP
Rose
rrPp
Pea
rrpp
Single