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UMBC BIOL 100 - POST MENDELIAN GENETICS

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POST MENDELIAN GENETICSEXTENDING MENDELMORGAN & DrosophilaSlide 4Slide 5Slide 6THE DISCOVERY OF SEX CHROMOSOMESSlide 8NORMAL SEX CHROMOSOMESBACK TO MORGAN…Slide 11PRACTICE SEX LINKED PROBLEMSlide 13GENE LINKAGEMORGAN & LINKED GENESSlide 17LINKAGE HYPOTHESISSlide 19RECOMBINATION PROBABILITYRECOMBINATION PROBABILITYGENE MAPPINGSlide 23Slide 24POST MENDELIAN GENETICSEXTENDING MENDEL•Thomas Hunt Morgan:MORGAN & DrosophilaMorgan Revealed Extensions of Mendel’s Rules:MORGAN’S WORK WITH DrosophilaRedEyesIn FliesIs Normal(wild)White Eyes Is Rare (mutation)MORGAN & Drosophila•To Explore How Eye Color is Inherited in Flies Morgan crossed: Red-eyed Female White-eyed Male XMORGAN & Drosophila•Next, Morgan crossed F1:Red-eyed Female XRed-eyed MaleTHE DISCOVERY OF SEX CHROMOSOMESNettie Stevens:Observed differences in chromosomesbetween male & female beetles•Chromosomes named X and Y:–Half male gametes contain Y, other half X–All female gametes contain XTHE DISCOVERY OF SEX CHROMOSOMESNettie Stevens:•Developed hypothesis about sex determination:•Male is formed when •Female is formed when XXYNORMAL SEX CHROMOSOMESA MaleBACK TO MORGAN…Morgan guessed that D. melanogaster (like T.molitor) had chromosomesthat differ between sexes:sperm spermGamete Formation In a Male FlyMORGAN & DrosophilaThe X Linked Hypothesis: •Morgan hypothesized that the gene controlling eye colorPRACTICE SEX LINKED PROBLEMCross a heterozygous red-eyed female with a red-eyed male•R = red eyes, r = white eyes•Genotype of female parent = •Genotype of male parent =PRACTICE SEX LINKED PROBLEM PARENT: XR / Xr PARENT: XR / Y ** hint: gametes along top, “whole people” on inside• Red eyed female crossed with Red eyed male•R = red •r = whiteMorgan’s work on Drosophila provided evidence that:•The X chromosome contains genes the Y doesn’t:•Inheritance patterns of sex-linked genes vary between sexes •Genes are located on chromosomesMORGAN & DrosophilaGENE LINKAGELinkage:–Autosomal Genes: –Reside on the autosomal chromosomes –In humans: genes located on chromosome #–Sex-Linked Genes: –Found on sex chromosomes –In humans: genes found on #MORGAN & LINKED GENES•First examples of linked genes were found on X chromosome of Drosophila:•Morgan established that eye color & body color are linked traits»Both found on X chromosome of fruit flyMORGAN & LINKED GENES•Morgan re-evaluated Mendel’s Principle of Independent Assortment–Morgan predicted:•Linked genes should be transmitted together during gamete formationLINKAGE HYPOTHESISR R g g r r G G g gR RggGGrrHypothesis (Morgan): When two genes occur on ONEchromosome, INDEPENDENTASSORTMENT DOES NOT OCCURAFemale(2X chromo)White eyesGray bodyRed eyesYellow bodyGametesrG RgMeiosis IIMeiosis IONLY 2 gamete typesEye Color: Red = wild type (R) White = mutant (r)Body Color:Gray body = wild type (G)Yellow body = mutant (g)r r G G R Rr RG g G gRRrrG ggGCrossing overduring meiosis IMeiosis IIR Rr rGG ggrGrg RG RgRecombinant chromosomesGametesGENETIC RECOMBINATION: PRODUCTION OF NEW COMBINATION OF TRAITS Linked genes can become unlinked through cross overRECOMBINATION PROBABILITY•The farther apart two genes, the higher the probability they will be separated during crossover:RECOMBINATION PROBABILITY•A & D are more likely than B & C to become separated (unlinked)•B & C more likely to be inherited together (stay linked)GENE MAPPING•Maps of genes can be constructed from recombination dataLinkage map: genetic map based on recombination frequenciesCrossing OverGene 1Gene 2Gene 3Gene 4Gene 5Gene 6Gene 7Gene 8Gene 9Gene 10Gene 11Gene 12Crossing over rarelyoccurs betweenadjacent loci, recombinationsare rare.Crossing overalmost alwaysoccurs betweendistant loci, recombinationsare frequent.0 : Yellow body0 : Yellow body1.4 : White eyesMap units0 : Yellow body1.4 : White eyes20 : Cut wingsChromosomes are composed of genesThe physical distancebetween loci determines the frequency of crossing overFrequency of crossover can be used to map physical between lociLinkage mapGENE MAPPING% recombinant gametes Reflects distance between 2 lociGENE MAPPING•If % of recombinant gametes is high, 2 genes are assumed to be far apart on a chromosome–Map Unit refers to distance between 2 loci•Function of %


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