2. Gene interaction - complementary

Basics of Undergraduate/university fellows

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.

1. P a g e | 1


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 he terozygous [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.

2. P a g e | 2


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 w hen 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

3. P a g e | 3


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