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

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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

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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 ).

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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

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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

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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)

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Parents

F1 Plants

F2 plantsMale gametes

Female gametes

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F2 –Generation ( 9:3 : 3: 1 ) 9 Tall Purple 3 Tall white

3 Dwarf Purple

1 Dwarf white

12 Tall 4 dwarf 12Purlple 4 white (3 : 1)

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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

(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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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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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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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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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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