Most of the characters of living organisms are controlled/ influenced/ governed by a collaboration of several different genes. • Numerous deviations have been recorded in which different kinds of interactions are possible between the genes.

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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
Ratio Example Parental
generation
F1
generation
A Complementary
gene interaction
9:7 Flower color in
Lathyrus odoratus
White X White
ccPP X CCpp
Purple
CcPp
B Supplementary
gene interaction
9:3:4 Grain color in
Maize
Purple X White
RRPP X rrpp
Purple
RrPp
C Epistasis
i Dominant Epistasis 12:3:1 Fruit color in
Cucurbita pepo
White X Yellow
WWyy X wwYY
White
WwYy
ii Recessive Epistasis 9:3:4 Coat color in Mice Black X Albino
CCaa X ccAA
Agouti
CcAa
D Non- Epistasis 9:3:3:1 Comb pattern in
Poultry
Rose X Pea
RRpp X rrPP
Walnut
RrPp

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GENE INTERACTION
A. COMPLEMENTARY GENE INTERACTION (9:7)
Ex: Flower color in Lathyrus odoratus (Sweet pea)
 Complementation between two non-allelic genes (C and P) are essential for production
of a particular or special phenotype i.e., complementary factor.
 Two genes involved in a specific pathway and their functional products are required
for gene expression, then one recessive allelic pair at either allelic pair would result in
the mutant phenotype.
 When Dominant alleles are present together, they complement each other to yield
complementary factor resulting in a special phenotype.
 They are called complementary genes.
 When either of gene loci have homozygous recessive alleles (i.e., genotypes of ccPP,
ccPp, CCpp, Ccpp and ccpp), they produce identical phenotypes and change F2 ratio
to 9:7.
Example: Flower color in Lathyrus odoratus (Sweet pea)
 In sweet pea (Lathyrus odoratus) two varieties of white flowering plants were seen.
 Each variety bred true and produced white flowers in successive generations.
 According to Bateson & Punnett, when two such white varieties of sweet pea were
crossed, the offspring were found to have purple-coloured flowers in F1.
 But, in F2 generation 9 were purple and 7 white.
 Anthocyanin pigment synthesis in sweet pea (Lathyrus Odoratus).
PATHWAY OF ANTHOCYANIN PIGMENT SYNTHESIS
 The dominant allele or alleles [CC or Cc] of gene C are responsible for the presence of
chromogen, while the homozygous recessive [cc] alleles of this gene are responsible for
the absence of chromogen.
 Likewise, the dominant alleles of gene P in homozygous [PP] or heterozygous [Pp]
conditions result in the production of an enzyme which is necessary for Anthocyanin
(Complementary factor) from chromogen, while homozygous recessive [pp] condition
does not produce any such enzyme.
 Thus, only the double dominant genotype has both enzymes functional and can make
pigment.
 Blocking either of two steps prevents pigment formation.

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GENE INTERACTION
When a pure line variety of white flowered sweet pea was crossed with another pure
line variety of white flowered sweet pea, in F1 purple-colored flowered plants were
produced. The F1 plants when self- pollinated [crossed among themselves]; the
obtained F2 generation had the phenotypic ratio of 9 purple-colored and 7 white
flowered plants.
Pea Plant Variety : Variety-1 X Variety-2
Parental Phenotype : White-Colorless X White-Colorless
Sweet Pea flower Sweet Pea flower
Parental Genotype : ccPP X CCpp
Parental Gametes : cP X Cp

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GENE INTERACTION
F1 Generation : CcPp
Purple Colored
F1 selfing: F1 X F1 : CcPp X CcPp
Purple-colored Purple-colored
F1 Gametes : CP Cp cP cp X CP Cp cP cp
F2 generation :
F2 phenotypic ratio = 9 Purple-colored : 7 White-colorless
F2 Analysis:
Genotype Flower color Enzyme
Activities
9C_P_ Colored flowers:
Anthocyanin
produced
Functional
enzymes from
both genes
3C_pp White flowers:
No anthocyanin
p enzyme
nonfunctional
3cc P_ White flowers:
No anthocyanin
C enzyme
nonfunctional
1ccpp White flowers:
No anthocyanin
C and p enzymes
nonfunctional
 It is clear in the above example that for the production of the purple flower colour
both complementary (C and P) genes are necessary to remain present.
 In the absence of either genes (C or P) the flowers are white.
 Thus, we can conclude that genes C and P interact and presence of both is essential
for the purple colour in the flower.
 These types of genes in which one gene complements the action of the other gene,
constitute complementary genes or factors.
CP Cp cP Cp
CP CCPP
Purple
colored
CCPp
Purple
colored
CcPP
Purple
colored
CcPp
Purple
colored
Cp CCPp
Purple
colored
CCpp
White
Colorless
CcPp
Purple
colored
Ccpp
White
Colorless
cP CcPP
Purple
colored
CcPp
Purple
colored
ccPP
White
Colorless
ccPp
White
Colorless
cp CcPp
Purple
colored
Ccpp
White
Colorless
ccPp
White
Colorless
Ccpp
White
Colorless

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GENE INTERACTION
B. SUPPLEMENTARY GENE INTERACTION (9:3:4)
Ex: Grain color in Maize
 In supplementary gene action, the dominant allele of one gene is essential for the
development of the concerned phenotype, while the other gene modifies the expression
of the first gene.
EX: GRAIN COLOR IN MAIZE
 For example, the development of grain colour in maize is governed by 2 dominant
genes ‘R’ and ‘P’.
 The dominant allele ‘R’ is essential for red colour production; homozygous state of the
recessive allele ‘r’ (rr) prevents the production of red colour.
 However, the dominant allele of the other gene does not produce a phenotypic effect on
its own. But when it is present with the dominant allele of the first gene, it modifies
the phenotypic effect produced by that gene.
 Therefore, gene ‘P’ is unable to produce any colour on its own but it modifies the
colour produced by the gene ‘R’ from red to purple.
 The recessive allele ‘p’ has no effect on grain colour.
1. Genotype RRPP / RrPP / RRPp / RrPp
Enzyme R Enzyme P
SIMPLIED-PATHWAY FOR DEVELOPMENT OF GRAIN-COLOR IN MAIZE
2. Genotype RRpp / Rrpp
NO
Enzyme R Enzyme P
Red
3. Genotype rrPP / rrPp
NO
Enzyme R Enzyme P
White White
White
Grain color
Red
Grain color
Purple
Grain color
Gene R Gene P
White
Grain color
Red
Grain color
NO Purple
Grain color
Gene R Gene p
White
Grain color
NO Red
Grain color
NO Purple
Grain color
Gene Gene P

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GENE INTERACTION
4. Genotype rrpp
NO
Enzyme R Enzyme P
White White
When a maize inbred - purple grains (RRPP) is crossed with an inbred - white grains
(rrpp). The F1 plants produce purple grains (RrPp). In the F2, 9 zygotic combinations
will have both the dominant alleles R and P
Inbred Maize Plant : Inbred-1 X Inbred-2
Parental Phenotype : Purple X White
Grain color Grain color
Parental Genotype : RRPP X rrpp
Parental Gametes : RP X rp
F1 Generation : RrPp
Purple Grain color
F1 selfing: F1 X F1 : RrPp X RrPp
Purple-colored Purple-colored
F1 Gametes : RP Rp RP Rp X RP Rp RP Rp
F2 generation :
RP Rp rP Rp
RP RRPP
Purple
RRPp
Purple
RrPP
Purple
RrPp
Purple
Rp RRPp
Purple
RRpp
Red
RrPp
Purple
Rrpp
Red
rP RrPP
Purple
RrPp
Purple
rrPP
White
rrPp
White
rp RrPp
Purple
Rrpp
Red
rrPp
White
rrpp
White
F2 phenotypic ratio = 9 Purple : 3 Red : 4 White
White
Grain color
NO Red
Grain color
NO Purple
Grain color
Gene Gene P

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GENE INTERACTION
F2 Analysis:
Genotype Grain
color
Explanation
9R_P_ Purple  Dominant R produces red color
 P modifies it to purple
3R_pp Red  Dominant R produces red
 pp cannot modify
 hence red
3rrP_ White  rr cannot convert white to red
 hence P cannot act
1rrpp White  rr cannot convert white to red
In the F2 generation;
 Nine zygotic combinations will have both the dominant alleles R and P. therefore; they
will develop into purple grains.
 Three zygotes will have R, but will be homozygous for p; these grains will develop red
color.
 Three other zygotes will be homozygous rr, but will have the dominant allele of P;
 The remaining one zygote will be homozygous recessive for both the genes and will
these seeds will be white. produce white grains.
 As a result, the 9:3:3:1 ratio is modified into 9:3:4 ratios.
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.
White colour is
controlled by
dominant gene W
• Yellow colour by
dominant gene Y.
White is dominant
over both yellow
and green
 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.
 Two gene loci are found to be responsible for coat color in mice.
 Agouti colored coat is dominant over colorless or albino coat.
 Agouti is dominant to black and the symbols used as A=agouti, a=black
 Heterozygous CA alleles gives 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 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