Mendel performed a dihybrid cross experiment involving two traits - plant height and flower color. He crossed a tall, red-flowered homozygous plant with a dwarf, white-flowered homozygous plant. The F1 generation were all tall with red flowers, showing dominance. In the F2 generation, there were 9 tall red plants, 3 tall white plants, 3 dwarf red plants, and 1 dwarf white plant, in a 9:3:3:1 ratio. This supported Mendel's second law of independent assortment, that two traits are inherited independently. Test crosses on the F1 hybrids confirmed independent assortment of alleles between the two gene pairs.

1. MENDEL’S EXPERIMENT ON HYBRIDIZATION
&
DIHYBRID CROSS, BACK CROSS AND TEST CROSS
By
Dr. ICHHA PURAK
UNIVERSITY PROFESSOR IN BOTANY
RANCHI WOMEN’S COLLEGE,RANCHI
Website : www,dripurak.com
6/15/2013Mendel'sExperiment&DihybridCross
GREGOR JOHANN MENDEL
( 1822-1884 )
IN CONTINUATION OF MENDEL’S EXPERIMENT ON HYBRIDIZATION &
MONOHYBRID CROSS
1

2. Need of Dihybrid Cross
Mendel performed Monohybrid crosses for all the seven traits
independently and was able to differentiate between dominate and
recessive form for each trait. He was then interested to know whether
inheritance of two characters are influenced by each other or are
inherited independently
Mendel began to wonder what would happen if he studied plants that
differed in two traits. Would both traits be transmitted to the offspring
together or would one trait be transmitted independently of the other ?
Steps of Experiment are same as that of monohybrid cross,but two
characters and their alternative forms are considered simultaneously.
For dihybrid cross two parental plants were taken. Male plant was tall
with red(purple ) flowers having TTRR alleles and female plant was
dwarf with white flowers having ttrr alleles.
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DIHYBRID CROSS
Dihybrid cross is a cross when at a time two traits and their
alternative forms are considered during hybridization experiment.
Steps of Experiment are same as that of monohybrid cross,but
two characters and their alternative forms are considered
simultaneously
For Dihybrid cross Homozygous Tall male plants bearing red (
purple ) flowers are crossed with Homozygous Dwarf female
plants bearing white flowers
Mendel performed Monohybrid crosses for all the seven traits
independently and was able to differentiate between dominate and
recessive form for each trait.
He was then interested to know whether inheritance of two
characters are influenced by each other or are inherited
independently.

4. TR Tr tR tr
TR TR
TR 1
Tr
TR 2
tR
TR 3
tr
TR 4
Tr TR
Tr 5
Tr
Tr 6
tR
Tr 7
tr
Tr 8
tR TR
tR 9
Tr
tR 10
tR
tR 11
tr
tR 12
tr TR
tr 13
Tr
tr 14
tR
tr 15
tr
tr 16
F2 progeny of Dihybrid Cross
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Genetic Expression of di hybrid cross
Parents Tall Red (Male ) X Dwarf White ( Female)
TTRR ttrr
Parental gametes TR tr
Hybrid TRtr F1 (All Tall Red )
Hybrid gametes
M F
Mendel'sExperiment&DihybridCross

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DihybridCross
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F2 Generation : 16 possible combinations
(P) TR (Tall Red) : 1,2,3,4, 5,7, 9,10,13 = 9
( R ) Tr (Tall White) : 6,8.14 = 3
( R ) tR ( Dwarf Red) : 11,12.15 =3
( P ) tr ( Dwarf White ) : 16 =1
Dihybrid Phenotypic Ratio - 9:3:3:1
Combinations 1,6,11,16 -------- Homozygous Rest Heterozygous
So Dihybrid Genotypic Ratio ---- 1:2:2:4:1:2:1:2:1
It is very clear from the results that F2 progeny showed parental
types TR(tall red) and tr (dwarf white ) and in addition
recombinants Tr (Tall white) and tR (Dwarf Red ).

6. Mendel'sExperiment&DihybridCross
Gametes TR Tr tR tr
TR TR
TR 1
Tr
TR 2
tR
TR 3
tr
TR 4
Tr TR
Tr 5
Tr
Tr 6
tR
Tr 7
tr
Tr 8
tR TR
tR 9
Tr
tR 10
tR
tR 11
tr
tR 12
tr TR
tr 13
Tr
tr 14
tR
tr 15
tr
tr 16
Genetic Expression of di hybrid cross
Parents Tall Red (Male ) X Dwarf White ( Female)
TTRR ttrr
Parental gametes TR tr
Hybrid TRtr F1 (All Tall Red )
Hybrid gametes
M F
TR Tr tR tr
TR-Tall Red
Tr- Tall white
tR-Dwarf Red
tr- Dwarf white
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First phenotype is Tall plant with Red Flowers
1-TTRR is homozygous for both the genes ---------1
2,5- TRTr Heterozygous for colour ----------2
3,9- TRtR- Heterozygous for height ------------2
4,7,10,13- TtRr- Heterozygous for both traits ----------4
Second phenotype is Tall plants with white flowers
6- Tr Tr – is homozygous for both genes ---------1
8,14-Trtr- is heterozygous for height ------- -2
Third phenotype is Dwarf plant with Red flowers
11- tR tR – is homozygous for both genes ----------1
12,15- tRtr- is heterozygous for colour of flower ---------2
Fourth Phenotype is Dwarf plant with white flowers
16- tr tr is homozygous for both genes --------1
9
So the Genotypic Ratio for F2 generation of Dihybrid Ratio
becomes 1:2:2:4 :1 :2 : 1 :2 :1
3
3
1

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On the basis of results of Dihybid cross , Mendel proposed his
second law of inheritance and is called as law of Independent
Assortment of factors
This law states that in a hybrid union for 2 or many traits , the
different traits are inherited independently of each other as genes
for two characteristics are inherited independently.
Mendel was lucky that in his studies each character was governed
by single gene and its two alleles and the genes responsible for
those characters were probably present on different chromosomes
and were not linked. Even if the genes were present on the same
chromosome they were present at a distance sufficient for crossing
over

9. Dihybrid cross between Homozygous Tall plant with Purple
flowers(Dominant) and Homozygous Dwarf plant with white flowers
(Recessive)
Parent-1 AABB
Dominant
Parent-2 aabb
Recessive
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Mendel'sExperiment&DihybridCross

10. F1 Generation –All plants tall with Purple flowers(AaBb)
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Mendel'sExperiment&DihybridCross

11. Parents
F1 Plants
F2 plantsMale
gametes
Female
gametes
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12. F2 –Generation ( 9:3 : 3: 1 ) 9 Tall Purple
3 Tall white
3 Dwarf Purple
1 Dwarf white
12 Tall 4 dwarf
12 Purlple 4 white
(3 : 1)
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Mendel'sExperiment&DihybridCross

13. It can also be mentioned that even in dihybrid cross the ratio for alternative
forms of a trait remains 12:4 or 3:1 as in monohybrid cross. As it is very clear
from the above slide that even in a dihybrid cross the results show 12 tall plant
and 4 dwarf plants so the ratio remains 3: 1 and also there are 12 plants with
purple flowers and 4 plants with white flowers so the ratio remains 3:1
The results of diybrid cross can also be obtained by combining results of two
Monohybrid cross
For example
Monohybrid cross between Tall and Dwarf plant F2 generation shows ¾ tall
and ¼ Dwarf plant
Monohybrid cross between Red flowered and white flowered plant shows ¾ Red
flowered plants and ¼ white flowered plant
By combining cross 1 and 2 Tall plants with red flowers will be equal to 3/4X3/4=
9/16 And dwarf plants with white flowers shall be 1/4x1/4= 1/16
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Mendel'sExperiment&DihybridCross

14. Some other examples of Dihybrid cross are as
follows
Cross between plants with Round Yellow seeds and
Wrinkled green seeds
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Mendel'sExperiment&DihybridCross
1 2

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Mendel'sExperiment&DihybridCross
Continued

16. Plants with Green Pods and Yellow seeds and plants with Yellow
pods and green seeds
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Mendel'sExperiment&DihybridCross

17. Back Cross A cross between F1 hybrid and one of the parent.
There are two possibilities
1. Cross of hybrid with dominant parent
2. Cross of hybrid with recessive parent
1.Tt x TT
Result : 100% plants tall (Dominant)
T t
T TT Tt
T TT Tt
2. Tt x tt
Result : 50% plants tall (Dominant) &
50% plants dwarf(Recesive) T t
t Tt tt
t Tt tt
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Mendel'sExperiment&DihybridCross
Back cross is very significant as it
can be helpful to the farmers or plant
breeders to design the breeding as per
the demand. For example if purple
flowers are more in demand then
breeder should try to cross F1 hybrid with the dominant parent and if
white flowers are also in demand then breeder should cross F1 hybrid
with recessive parent

18. Test cross:
It is a cross used to determine the genotype of an individual with
dominant phenotype or to know the genotype of F1 Phenotype
(dominant ) whether true breeding / Homozygous (TT ) or False
breeding / Heterozygous (Tt) It is a cross between F1 hybrid and a
strain having recessive form of character. The purpose of Test cross
is to obtain evidence that segregation for the alleles of a single gene
in the F1 hybrid produces two type of gametes in equal frequencies If
tall of F1 is heterozygous ,then by crossing with recessive parent ratio
of tall and Dwarf plants will be 1:1, In the same way red flowered
plant and white flowered plant will also be 1:1 and if tall is
homozygous then after test crossing with recessive parent only tall
plants (100% ) will be resulted
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Mendel'sExperiment&DihybridCross

19. gametes t t
T Tt Tt
T Tt Tt
gametes t t
T Tt Tt
t tt tt
TT x tt Tt x tt
100% Tall Phenotype 50% Tall & 50% Dwarf
TEST CROSS
HOMOZYGOUS TALL HETEROZYGOUS TALL
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21. According to law of segregation, the F1 (Tt) from a cross between (TT)
tall and (tt) dwarf pea varieties would produce gametes of T and t in
equal frequencies. The strain with recessive traits (t t) would produce
only one type of gamete (t) .Union of gametes from F1 with those of
test cross parent, therefore is expected to produce two types of plants
Tt (Tall ) and tt (dwarf ) . The frequencies of these two types of plants
are expected to be equal and ratio of dominant and recessive
phenotype would be 1:1.
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22. TEST CROSS OF A DIHYBRID
F1 hybrid TtRr is crossed with a double recessive parent for both
traits ( ttrr) . The purpose is to verify ratio in which different gametes
of F1 hybrid are produced. According to Independent Assortment of
two genes ( Tt & Rr ) , F1 should produce 4 types of gametes TR, Tr,
tR & tr in equal frequencies or in the ratio 1:1:1:1 and double
recessive parent would produce only one kind of gamete (tr)
The union of tr gamete with four different type of gametes produced
by F1 hybrid will yield following four kinds of Zygotes/combinations
TRtr, Trtr,tRtr & trtr . These would develop Tall Red, Tall white, dwarf
Red and dwarf white each ¼ of the total yielding the ratio 1:1:1:1.
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Mendel'sExperiment&DihybridCross

23. tr(recessive parent
gamete )
F1 hybrid gamete Zygote genotype Phenotype
TR TRtr Tall Red
Tr Trtr Tall White
tR tRtr Dwarf Red
tr trtr Dwarf White
DIHYBRID TEST CROSS ( 1:1 : 1 : 1 )
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