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 0 5 / 2 1 / 2 0 2 2 M e n d e l ' s E x p e r i m e n t & D i h y b r i d C r o s s GREGOR JOHANN MENDEL ( 1822-1884 ) IN CONTINUATION OF MENDEL’S EXPERIMENT ON HYBRIDIZATION & MONOHYBRID CROSS 1
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MENDEL’S EXPERIMENT ON HYBRIDIZATION&DIHYBRID CROSS, BACK CROSS AND TEST CROSS
ByDr. ICHHA PURAKUNIVERSITY PROFESSOR IN BOTANYRANCHI WOMEN’S COLLEGE,RANCHI
Website : www,dripurak.com
Mendel's E
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GREGOR JOHANN MENDEL ( 1822-1884 )
IN CONTINUATION OF MENDEL’S EXPERIMENT ON
HYBRIDIZATION & MONOHYBRID CROSS
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.
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.
TR Tr tR tr
TR TRTR 1
Tr TR 2
tR TR 3
tr TR 4
Tr TRTr 5
TrTr 6
tRTr 7
trTr 8
tR TRtR 9
TrtR 10
tRtR 11
trtR 12
tr TRtr 13
Trtr 14
tRtr 15
trtr 16F2 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 trHybrid TRtr F1 (All Tall Red )Hybrid gametes M F
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 ).
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Gametes
TR Tr tR tr
TR TRTR
1
TrTR 2
tRTR 3
trTR 4
Tr TRTr 5
TrTr 6
tRTr 7
trTr 8
tR TRtR 9
TrtR 10
tRtR 11
trtR 12
tr TRtr 13
Trtr 14
tRtr 15
trtr 16
Genetic Expression of di hybrid crossParents Tall Red (Male ) X Dwarf White ( Female) TTRR ttrr Parental gametes TR trHybrid TRtr F1 (All Tall Red )Hybrid gametes M F
TR Tr tR tr
TR-Tall RedTr- Tall whitetR-Dwarf Redtr- Dwarf white
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
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1
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
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Dihybrid cross between Homozygous Tall plant with Purple
flowers(Dominant) and Homozygous Dwarf plant with white
flowers (Recessive)
Parent-1 AABBDominant
Parent-2 aabbRecessive
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F1 Generation –All plants tall with Purple flowers(AaBb)
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
Some other examples of Dihybrid cross are as followsCross between plants with Round Yellow seeds and Wrinkled green seeds
1 2
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Continued
Plants with Green Pods and Yellow seeds and plants with Yellow pods and green seeds
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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 parent1.Tt x TT Result : 100% plants tall (Dominant)
T t
T TT Tt
T TT Tt2. Tt x ttResult : 50% plants tall (Dominant) & 50% plants dwarf(Recesive) T t
t Tt tt
t Tt tt
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 thenbreeder 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
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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