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Dihybrid cross
The document outlines Gregor Mendel's experiments on hybridization, specifically focusing on dihybrid crosses and their significance in understanding inheritance patterns. Mendel's findings led to the formulation of the law of independent assortment, which states that different traits are inherited independently. The document also discusses the methodologies, ratios obtained from crosses, and the concepts of back crosses and test crosses.
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Dihybrid cross
- 1. MENDEL’S EXPERIMENT ONHYBRIDIZATION & 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 DihybridCross 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. 5/29/2014 2 Mendel'sExperiment&DihybridCross
- 3. 5/29/2014Mendel'sExperiment&DihybridCross 3 DIHYBRID CROSS Dihybrid crossis 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 tRtr 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 5/29/2014 4 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
- 5. 5/29/2014 Mendel'sExperiment& DihybridCross 5 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 TrtR 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 5/29/2014 6
- 7. 5/29/2014Mendel'sExperiment&DihybridCross 7 First phenotype isTall 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
- 8. 5/29/2014Mendel'sExperiment&DihybridCross 8 On the basisof 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 betweenHomozygous Tall plant with Purple flowers(Dominant) and Homozygous Dwarf plant with white flowers (Recessive) Parent-1 AABB Dominant Parent-2 aabb Recessive 5/29/2014 9 Mendel'sExperiment&DihybridCross
- 10. F1 Generation –Allplants tall with Purple flowers(AaBb) 5/29/2014 10 Mendel'sExperiment&DihybridCross
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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) 5/29/2014 12 Mendel'sExperiment&DihybridCross
- 13. It can alsobe 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 5/29/2014 13 Mendel'sExperiment&DihybridCross
- 14. Some other examplesof Dihybrid cross are as follows Cross between plants with Round Yellow seeds and Wrinkled green seeds 5/29/2014 14 Mendel'sExperiment&DihybridCross 1 2
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- 16. Plants with GreenPods and Yellow seeds and plants with Yellow pods and green seeds 5/29/2014 16 Mendel'sExperiment&DihybridCross
- 17. Back Cross Across 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 5/29/2014 17 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 isa 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 5/29/2014 18 Mendel'sExperiment&DihybridCross
- 19. gametes t t TTt 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 5/29/2014 19 Mendel'sExperiment&DihybridCross
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- 21. According to lawof 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. 21 5/29/2014Mendel'sExperiment&DihybridCross
- 22. TEST CROSS OFA 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. 5/29/2014 22 Mendel'sExperiment&DihybridCross
- 23. tr(recessive parent gamete ) F1hybrid 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 ) 5/29/2014 23 Mendel'sExperiment&DihybridCross
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