Extra Credit QuestionAnnouncementsGenetic Linkage and MappingPowerPoint PresentationSlide 5NotationDihybrid CrossSlide 84 gamete types, equally frequentF2 of dihybrid cross when genes not linkedBut what if the genes are on the same chromosome?Two linked genesNo crossing over between markersWhen there is no crossing over between markers, all gametes are parentalCrossing over in 4-strand stageWhen there is a crossover, 50% of gametes are parental and 50% are recombinantNo crossover CrossoverWhen genes are on the same chromosome, but far apart:Slide 19Genes that are so far apart on the same chromosome that crossing over occurs between them in 100% of meioses behave as if they are unlinked: 50% parental combinations 50% recombinantsWhat if genes are close together?Slide 22If crossing over between genes occurs in 20% of meiosisGenes with < 50% recombination are on the same chromosomeThe frequency of crossing over between two genes on the same chromosome is proportional to the distance between the genesSlide 26How would you tell if a gamete produced by a double heterozygote is recombinant or parental?Mate double heterozygote to an individual that is homozygous recessive for both traitsSlide 29Slide 30If 15% of the offspring of the testcross are recombinant, what proportion of meioses had crossover between the brown and the heavy-veined genes?If 15% of the gametes are recombinant, the genes are said to be 15 map units apart on a genetic mapUnit of distance in linkage map is a map unit: 1 map unit is equal to 1 percent recombinationSlide 34Two X-linked genesSlide 3662.8% non-recombinant offspring, 37.2% recombinant offspringIf two genes are 50 map units apart, what proportion of recombinant offspring would you observe?Slide 39Multiple crossovers can lead to inaccuracyMultiple crossovers can be useful for mapping three genes at a time!Extra Credit QuestionCrossing over in Eukaryotes occurs during when there are _____ chromatids present at the metaphase plate in Meiosis I, but the crossover event involes _____ chromatids.AnnouncementsHomework #2 is due on Thursday before 9 am.Midterm: Mar 8, 9:00 am.Genetic Linkage and MappingNotation—•—————A__•_______AaAaDiploidAdultHaploid gametes(single chromatid)—•—————Two homologouschromosomes,four chromatids total__•_______A__•_______a__•_______a1/2 A : 1/2 aDihybrid CrossAA BB Female aa bb Male A B a b—•————— ——•————— —•————— ——•————— P: —•————— ——•————— —•————— ——•————— A B a bGametes: Egg (A B) Sperm (a b) A B a b—•————— ——•———— —•————— ——•————A B—•————— ——•————— F1: —•————— ——•————— a bafter the S phaseF1 adult4 gamete types, equally frequent A B—•————— ——•———— A b—•————— ——•———— a B—•————— ——•———— a b—•————— ——•———— A B—•————— ——•————— F1: —•————— ——•————— a bParentalRecombinantRecombinantParentalF2 of dihybrid cross when genes not linkedWhat proportion of the gametes of the double heterozygote are recombinant?50 %But what if the genes are on the same chromosome? (What if the genes are linked?)They do not assort independently at meiosisTwo linked genes Female Male A B a b—•————————— —•————————— P: —•————————— —•————————— A B a bGametes: Egg (A B) Sperm (a b) A B a b —•————————— —•————————No crossing over between markers A B—•————————— a b—•————————— A B—•————————— F1: —•————————— a bParentalParental A B—•————————— a b—•—————————ParentalParentalWhen there is no crossing over between markers, all gametes are parentalCrossing overin 4-strand stage A B—•————————— a b—•————————— A B—•————————— F1: —•————————— a bParentalParental A b—•————————— a B—•—————————RecombinantRecombinantWhen there is a crossover, 50% of gametes are parental and 50% are recombinantA B—•————————— a b—•————————— A B—•————————— F1: —•————————— a b A b—•————————— a B—•—————————ParentalParentalRecom.Recom. A B—•————————— a b—•————————— A B—•————————— F1: —•————————— a bParentalParental A B—•————————— a b—•—————————ParentalParentalNo crossover CrossoverWhen genes are on the same chromosome, but far apart: Crossing over between them will be common, approaching 100% of the timeIn this case, what proportion of all F1 gametes will be recombinant (have a combination of alleles that was not present in the original true-breeding parents)?Crossing overin 4-strand stage A B—•————————— a b—•————————— A B—•————————— F1: —•————————— a bParentalParental A b—•————————— a B—•—————————RecombinantRecombinantGenes that are so far apart on the same chromosome that crossing over occurs between them in 100% of meioses behave as if they are unlinked:50% parental combinations50% recombinantsWhat if genes are close together?Say that crossing over occurs between the genes in 20% of meiosesWhat proportion of F1 gametes will be recombinant?A B—•————————— a b—•————————— A B—•————————— F1: —•————————— a b A b—•————————— a B—•————————— A B—•————————— a
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