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!3 recessive phenotypes in maize (corn), coded by three linked genesTo map the genes, mate a triple heterozygote to triple recessive homozygoteHow many different kinds of gametes do you get from triple heterozygote?PowerPoint PresentationWhere to begin?Rule 1Slide 9Linkage phase in heterozygous parent?Rule 2Slide 12Rule 3Double-crossover types:Now you know linkage phase of heterozygous parent and gene orderCount the crossovers between adjacent genesSlide 17Rule 4: Reciprocal products expected to occur in approximately equal numbersRule 5Slide 20Slide 21Genetic MapInterferenceMaize exampleSlide 25Why?Quantifying InterferenceRecombination is not independent at small distancesIn 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 Timea) 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 their correct order and indicate the map distances between the genes. d) Calculate the Interference.Slide 32Three-locus mappingSlide 34Multiple crossovers can lead to inaccuracyExtra CreditAre the b and vg genes on the same chromosome? If so, how far apart are they?A B C—•————————— —•————————— a b cMultiple crossovers can be useful for mapping three genes at a time!A B Ca b c A B C—•————————— —•————————— a b cA b Ca B c3 recessive phenotypes in maize (corn), coded by three linked genesl l lazy or prostrate growthg g glossy leavess s sugary endospermTo map the genes, mate a triple heterozygote to triple recessive homozygoteLl Gg Ss x ll gg ss Gene order is not known,so the order shown here is arbitrary.Linkage phase is not knowHow many different kinds of gametes do you get from triple heterozygote? L or l G or g S or s 2 * 2 * 2Ll Gg SsL G S / l g s x l g s / l g sRecomb.Wildtpe for all lazy, gloss, sugaryWhere to begin?Parental types will constitute ≥ 50% of all progeny, so…Rule 1Two most-frequent gametes types are the parental typesTells us the linkage phase of heterozygous parent:L G S or L g S or l g S or L g sl g s l G s L G s l G SL G S // l g s x l g s // l g sLinkage phase in heterozygous parent?L G S or L g S or l g S or L g sl g s l G s L G s l G SRule 2The double-recombinant gametes will be the two least frequent typesA B Ca b cL G S / l g s x l g s / l g sRule 3Effect of double crossovers is to interchange the members of the middle pair of alleles between the chromosomesA B Ca b cA b Ca B cDouble-crossover types:L G s and l g S Which gene is in the middle? L s Gl S gParental types:L G S and l g s L S Gl s gNow you know linkage phase of heterozygous parent and gene orderL S Gl s gHow far apart are the genes?Count the crossovers between adjacent genes•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 lociL S Gl s gProgenyPhenotypeProgenyGenotype#Crossover or Non-Crossover?wildtypeL G S / l g s286Parental (NCO)lazyl G S / l g s 33si ngl e C O be twee n L and Sgloss yL g S / l g s 59si ngl e C O be twee n S and Gs uga ryL G s / l g s 4double COlazy,gl o ssyl g S / l g s 2double COlazy,su garyl G s / l g s 44si ngl e C O be twee n S and Ggloss y,s uga ryL g s / l g s 40si ngl e C O be twee n L and Slazy,gl o ssy ,su g aryl g s / l g s272Parental (NCO)To tal740L S Gl s gRule 4: Reciprocal products expected to occur in approximately equal numbersLGS ≈ lgs (286 ≈ 272) LgS ≈ lGs (59 ≈ 44) Lgs ≈ lGS (40 ≈ 33)LGs ≈ lgS (4 ≈ 2)Rule 5Don't forget to include the double recombinants when calculating recombination frequency!l G S 33L g s 40L G s 4l g S 2 79Rec Freq L-SRec Freq S-GL g S 59l G s 44L G s 4l g S 2 109ProgenyPhenotypeProgenyGenotype#Crossover or Non-Crossover?wildtypeL G S / l g s286Parental (NCO)lazyl G S / l g s 33si ngl e C O be twee n L and Sgloss yL g S / l g s 59si ngl e C O be twee n S and Gs uga ryL G s / l g s 4double COlazy,gl o ssyl g S / l g s 2double COlazy,su garyl G s / l g s 44si ngl e C O be twee n S and Ggloss y,s uga ryL g s / l g s 40si ngl e C O be twee n L and Slazy,gl o ssy ,su g aryl g s / l g s272Parental (NCO)To tal74079/740 or 10.7% of gametes recombinant between L & S. So, map distance between L & S = 10.7 map units109/740 or 14.8 % of gametes recombinant between S & G. So, map distance between S & G=14.8 map unitsl G S 33L g s 40L G s 4l g S 2 79Rec Freq L-SRec Freq S-GL g S 59l G s 44L G s 4l g S 2 109Genetic Map10.7 mu 14.8 mu_____________________________L S GInterferenceAssuming independence, expected probability of double crossovers is the probability of recombination in one region times the probability of recombination in other (product rule).Maize exampleProbability 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 = 0.0158In 740 events, the double crossover class should occur0.0158 * 740 = 12 timesExpected DCO = 12Observed DCO = 6 Typical Result: O < EConclusion: Crossing over in one region reduces probability of crossing over in adjacent regions This is InterferenceWhy?Physical constraints that prevent two chiasmata in close proximity during meiosis?Quantifying InterferenceCoefficient of coincidence = Obs DCOExp DCOcc
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