This Power Point Presentation is designed to explain Mendel's experiment on hybridization and dihybrid cross which considers inheritance of two traits at a time and to know whether they are inherited independently or are influenced by each other and also about Law of Independent assortment

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
5/29/2014Mendel'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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10. F1 Generation –All plants tall with Purple flowers(AaBb)
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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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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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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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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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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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