Extra Credit Are the b and vg genes on the same chromosome If so how far apart are they Multiple crossovers can be useful for mapping three genes at a time A B C A B C a b c a b c A B C A b C a B c a b c 3 recessive phenotypes in maize corn coded by three linked genes l l lazy or prostrate growth g g glossy leaves s s sugary endosperm To map the genes mate a triple heterozygote to triple recessive homozygote Ll Gg Ss x ll gg ss Gene order is not known so the order shown here is arbitrary Linkage phase is not know How many different kinds of gametes do you get from triple heterozygote Ll Gg Ss L or l 2 2 G or g S or s 2 Wildtpe for all Recomb L G S l g s lazy gloss sugary x l g s l g s Where to begin Parental types will constitute 50 of all progeny so Rule 1 Two most frequent gametes types are the parental types Tells us the linkage phase of heterozygous parent LGS l g s or L g S l Gs or l g S or LG s L g s l G S L G S l g s x l g s l g s Linkage phase in heterozygous parent LGS l g s or LgS l Gs or l g S LG s or L g s l G S Rule 2 The double recombinant gametes will be the two least frequent types A a B C b c L G S l g s x l g s l g s Rule 3 Effect of double crossovers is to interchange the members of the middle pair of alleles between the chromosomes A a B C A b C b c a B c Parental types LGS and lgs Double crossover types LGs and lgS Which gene is in the middle L S G L s G l s g l S g Now you know linkage phase of heterozygous parent and gene order L S G l s g How far apart are the genes Count the crossovers between adjacent genes L S G l s g In parents L allele on same homolog as S and l on same homolog as s So if these get broken up crossover between L and S loci In parents S on same homolog as G and s on same homolog as g If these get broken up recombination between S and G loci L S G l s g Progeny Phenotype Progeny Genotype wildtype LGS lgs laz y l GS lgs g loss y LgS lgs s uga ry LGs lgs laz y gl o ssy lgS lgs laz y su g ary lGs lgs g loss y s uga ry Lgs lgs laz y gl o ssy su g ary l g s l g s To tal 286 33 59 4 2 44 40 272 740 Crossover or NonCrossover Parental NCO si ngl e C O be twee n L and S si ngl e C O be twee n S an d G double CO double CO si ngl e C O be twee n S an d G si ngl e C O be twee n L and S Parental NCO Rule 4 Reciprocal products expected to occur in approximately equal numbers LGS lgs 286 272 LgS lGs 59 44 Lgs lGS 40 33 LGs lgS 4 2 Rule 5 Don t forget to include the double recombinants when calculating recombination frequency Progeny Phenotype Progeny Genotype wildtype LGS lgs laz y l GS lgs g loss y LgS lgs s uga ry LGs lgs laz y gl o ssy lgS lgs laz y su g ary lGs lgs g loss y s uga ry Lgs lgs laz y gl o ssy su g ary l g s l g s To tal 286 33 59 4 2 44 40 272 740 Rec Freq L S l L L l g G S g s G s S 33 40 4 79 2 Crossover or NonCrossover L l L l Parental NCO si ngl e C O be twee n L and S si ngl e C O be twee n S an d G double CO double CO si ngl e C O be twee n S an d G si ngl e C O be twee n L and S Parental NCO Rec Freq S G g S 59 G s 44 G s 4 g S 109 2 Rec Freq L S l L L l g L l L l G S g s G s S 33 40 4 79 Rec Freq S G g S 59 G s 44 G s 4 g S 109 2 2 79 740 or 10 7 of gametes recombinant between L S So map distance between L S 10 7 map units 109 740 or 14 8 of gametes recombinant between S G So map distance between S G 14 8 map units Genetic Map 10 7 mu 14 8 mu L S G Interference Assuming independence expected probability of double crossovers is the probability of recombination in one region times the probability of recombination in other product rule Maize example Probability of recombination between L and S is 10 7 Probability of recombination between S and G is 14 8 If crossovers independent probability of double crossover should then be 0 107 0 148 0 0158 In 740 events the double crossover class should occur 0 0158 740 12 times Expected DCO 12 Observed DCO 6 Typical Result O E Conclusion Crossing over in one region reduces probability of crossing over in adjacent regions This is Interference Why Physical constraints that prevent two chiasmata in close proximity during meiosis Quantifying Interference Coefficient of coincidence Obs DCO Exp DCO cc 6 12 Interference 1 cc 1 0 5 0 5 Recombination is not independent at small distances If distance between genes is small 10 map units in Drosophila no double crossovers occur interference is complete I 1 At large distances 45 map units Interference disappears Obs Exp and I 0 In Drosophila the allele b gives black body wild type is tan at a separate gene the allele wx gives waxy wings nonwaxy is wild type and at a third gene the allele cn gives cinnabar eyes red is wild type A female that is heterozygous for these three genes is testcrossed and 980 progeny are classified as follows for body color wing phenotype and eye color Extra Credit Question for Next Time Phenotype brown nonwaxy red black waxy cinnabar brown waxy cinnabar black nonwaxy red brown nonwaxy cinnabar black waxy red brown waxy red black nonwaxy cinnabar Haploid Genotype b wx cn wx cn b cn b wx wx b cn Number 6 6 79 77 369 369 38 36 a What is the linkage phase of the heterozygous female parent b What is the order of the three genes c Construct a linkage map with the genes in …
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