Dihybrid cross


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Dihybrid cross

  2. 2. Need of Dihybrid CrossMendel performed Monohybrid crosses for all the seven traitsindependently and was able to differentiate between dominate andrecessive form for each trait. He was then interested to know whetherinheritance of two characters are influenced by each other or areinherited independentlyMendel began to wonder what would happen if he studied plants thatdiffered in two traits. Would both traits be transmitted to the offspringtogether or would one trait be transmitted independently of the other ?Steps of Experiment are same as that of monohybrid cross,but twocharacters and their alternative forms are considered simultaneously.For dihybrid cross two parental plants were taken. Male plant was tallwith red(purple ) flowers having TTRR alleles and female plant wasdwarf with white flowers having ttrr alleles.6/15/20132MendelsExperiment&DihybridCross
  3. 3. 6/15/2013MendelsExperiment&DihybridCross3DIHYBRID CROSSDihybrid cross is a cross when at a time two traits and theiralternative forms are considered during hybridization experiment.Steps of Experiment are same as that of monohybrid cross,buttwo characters and their alternative forms are consideredsimultaneouslyFor Dihybrid cross Homozygous Tall male plants bearing red (purple ) flowers are crossed with Homozygous Dwarf femaleplants bearing white flowersMendel performed Monohybrid crosses for all the seven traitsindependently and was able to differentiate between dominate andrecessive form for each trait.He was then interested to know whether inheritance of twocharacters are influenced by each other or are inheritedindependently.
  4. 4. TR Tr tR trTR TRTR 1TrTR 2tRTR 3trTR 4Tr TRTr 5TrTr 6tRTr 7trTr 8tR TRtR 9TrtR 10tRtR 11trtR 12tr TRtr 13Trtr 14tRtr 15trtr 16F2 progeny of Dihybrid Cross6/15/20134Genetic Expression of di hybrid crossParents Tall Red (Male ) X Dwarf White ( Female)TTRR ttrrParental gametes TR trHybrid TRtr F1 (All Tall Red )Hybrid gametesM FMendelsExperiment&DihybridCross
  5. 5. 6/15/2013MendelsExperiment&DihybridCross5F2 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 =1Dihybrid Phenotypic Ratio - 9:3:3:1Combinations 1,6,11,16 -------- Homozygous Rest HeterozygousSo Dihybrid Genotypic Ratio ---- 1:2:2:4:1:2:1:2:1It is very clear from the results that F2 progeny showed parentaltypes TR(tall red) and tr (dwarf white ) and in additionrecombinants Tr (Tall white) and tR (Dwarf Red ).
  6. 6. MendelsExperiment&DihybridCrossGametes TR Tr tR trTR TRTR 1TrTR 2tRTR 3trTR 4Tr TRTr 5TrTr 6tRTr 7trTr 8tR TRtR 9TrtR 10tRtR 11trtR 12tr TRtr 13Trtr 14tRtr 15trtr 16Genetic Expression of di hybrid crossParents Tall Red (Male ) X Dwarf White ( Female)TTRR ttrrParental gametes TR trHybrid TRtr F1 (All Tall Red )Hybrid gametesM FTR Tr tR trTR-Tall RedTr- Tall whitetR-Dwarf Redtr- Dwarf white6/15/20136
  7. 7. 6/15/2013MendelsExperiment&DihybridCross7First phenotype is Tall plant with Red Flowers1-TTRR is homozygous for both the genes ---------12,5- TRTr Heterozygous for colour ----------23,9- TRtR- Heterozygous for height ------------24,7,10,13- TtRr- Heterozygous for both traits ----------4Second phenotype is Tall plants with white flowers6- Tr Tr – is homozygous for both genes ---------18,14-Trtr- is heterozygous for height ------- -2Third phenotype is Dwarf plant with Red flowers11- tR tR – is homozygous for both genes ----------112,15- tRtr- is heterozygous for colour of flower ---------2Fourth Phenotype is Dwarf plant with white flowers16- tr tr is homozygous for both genes --------19So the Genotypic Ratio for F2 generation of Dihybrid Ratiobecomes 1:2:2:4 :1 :2 : 1 :2 :1331
  8. 8. 6/15/2013MendelsExperiment&DihybridCross8On the basis of results of Dihybid cross , Mendel proposed hissecond law of inheritance and is called as law of IndependentAssortment of factorsThis law states that in a hybrid union for 2 or many traits , thedifferent traits are inherited independently of each other as genesfor two characteristics are inherited independently.Mendel was lucky that in his studies each character was governedby single gene and its two alleles and the genes responsible forthose characters were probably present on different chromosomesand were not linked. Even if the genes were present on the samechromosome they were present at a distance sufficient for crossingover
  9. 9. Dihybrid cross between Homozygous Tall plant with Purpleflowers(Dominant) and Homozygous Dwarf plant with white flowers(Recessive)Parent-1 AABBDominantParent-2 aabbRecessive6/15/20139MendelsExperiment&DihybridCross
  10. 10. F1 Generation –All plants tall with Purple flowers(AaBb)6/15/201310MendelsExperiment&DihybridCross
  11. 11. ParentsF1 PlantsF2 plantsMalegametesFemalegametes6/15/2013MendelsExperiment&DihybridCross11
  12. 12. F2 –Generation ( 9:3 : 3: 1 ) 9 Tall Purple3 Tall white3 Dwarf Purple1 Dwarf white12 Tall 4 dwarf12 Purlple 4 white(3 : 1)6/15/201312MendelsExperiment&DihybridCross
  13. 13. It can also be mentioned that even in dihybrid cross the ratio for alternativeforms of a trait remains 12:4 or 3:1 as in monohybrid cross. As it is very clearfrom the above slide that even in a dihybrid cross the results show 12 tall plantand 4 dwarf plants so the ratio remains 3: 1 and also there are 12 plants withpurple flowers and 4 plants with white flowers so the ratio remains 3:1The results of diybrid cross can also be obtained by combining results of twoMonohybrid crossFor exampleMonohybrid cross between Tall and Dwarf plant F2 generation shows ¾ talland ¼ Dwarf plantMonohybrid cross between Red flowered and white flowered plant shows ¾ Redflowered plants and ¼ white flowered plantBy 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= 1/166/15/201313MendelsExperiment&DihybridCross
  14. 14. Some other examples of Dihybrid cross are asfollowsCross between plants with Round Yellow seeds andWrinkled green seeds6/15/201314MendelsExperiment&DihybridCross1 2
  15. 15. 6/15/201315MendelsExperiment&DihybridCrossContinued
  16. 16. Plants with Green Pods and Yellow seeds and plants with Yellowpods and green seeds6/15/201316MendelsExperiment&DihybridCross
  17. 17. Back Cross A cross between F1 hybrid and one of the parent.There are two possibilities1. Cross of hybrid with dominant parent2. Cross of hybrid with recessive parent1.Tt x TTResult : 100% plants tall (Dominant)T tT TT TtT TT Tt2. Tt x ttResult : 50% plants tall (Dominant) &50% plants dwarf(Recesive) T tt Tt ttt Tt tt6/15/201317MendelsExperiment&DihybridCrossBack cross is very significant as itcan be helpful to the farmers or plantbreeders to design the breeding as perthe demand. For example if purpleflowers are more in demand thenbreeder should try to cross F1 hybrid with the dominant parent and ifwhite flowers are also in demand then breeder should cross F1 hybridwith recessive parent
  18. 18. Test cross:It is a cross used to determine the genotype of an individual withdominant phenotype or to know the genotype of F1 Phenotype(dominant ) whether true breeding / Homozygous (TT ) or Falsebreeding / Heterozygous (Tt) It is a cross between F1 hybrid and astrain having recessive form of character. The purpose of Test crossis to obtain evidence that segregation for the alleles of a single genein the F1 hybrid produces two type of gametes in equal frequencies Iftall of F1 is heterozygous ,then by crossing with recessive parent ratioof tall and Dwarf plants will be 1:1, In the same way red floweredplant and white flowered plant will also be 1:1 and if tall ishomozygous then after test crossing with recessive parent only tallplants (100% ) will be resulted6/15/201318MendelsExperiment&DihybridCross
  19. 19. gametes t tT Tt TtT Tt Ttgametes t tT Tt Ttt tt ttTT x tt Tt x tt100% Tall Phenotype 50% Tall & 50% DwarfTEST CROSSHOMOZYGOUS TALL HETEROZYGOUS TALL6/15/201319MendelsExperiment&DihybridCross
  20. 20. 206/15/2013MendelsExperiment&DihybridCross
  21. 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 inequal frequencies. The strain with recessive traits (t t) would produceonly one type of gamete (t) .Union of gametes from F1 with those oftest cross parent, therefore is expected to produce two types of plantsTt (Tall ) and tt (dwarf ) . The frequencies of these two types of plantsare expected to be equal and ratio of dominant and recessivephenotype would be 1:1.216/15/2013MendelsExperiment&DihybridCross
  22. 22. TEST CROSS OF A DIHYBRIDF1 hybrid TtRr is crossed with a double recessive parent for bothtraits ( ttrr) . The purpose is to verify ratio in which different gametesof F1 hybrid are produced. According to Independent Assortment oftwo 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 doublerecessive parent would produce only one kind of gamete (tr)The union of tr gamete with four different type of gametes producedby F1 hybrid will yield following four kinds of Zygotes/combinationsTRtr, Trtr,tRtr & trtr . These would develop Tall Red, Tall white, dwarfRed and dwarf white each ¼ of the total yielding the ratio 1:1:1:1.6/15/201322MendelsExperiment&DihybridCross
  23. 23. tr(recessive parentgamete )F1 hybrid gamete Zygote genotype PhenotypeTR TRtr Tall RedTr Trtr Tall WhitetR tRtr Dwarf Redtr trtr Dwarf WhiteDIHYBRID TEST CROSS ( 1:1 : 1 : 1 )6/15/201323MendelsExperiment&DihybridCross
  24. 24. 6/15/2013MendelsExperiment&DihybridCross24THANK YOU