CHAPTER15

THECHROMOSOMALBASISOFINHERITANCESectionA:RelatingMendelismtoChromosomes1. Mendelianinheritancehasitsphysicalbasisinthebehaviorofchromosomesduringsexuallifecycles2.Morgantracedagenetoaspecificchromosome3.Linkedgenestendtobeinheritedtogetherbecausetheyarelocatedonthesamechromosome4.Independentassortmentofchromosomesandcrossingoverproducegeneticrecombinants5.Geneticistsuserecombinationdatatomapachromosome’sgeneticloci生物醫學暨環境生物學系張學偉助理教授CHAPTER15

THECHROMOSOMALBAIntroductionMendel’shereditaryfactorsarethegeneslocatedonchromosomes.chromosomestheoryofinheritance.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummings1.MendelianinheritancehasitsphysicalbasisinthebehaviorofchromosomesduringsexuallifecyclesIntroductionMendel’shereditarCopyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsChromosomalbasisofMendel’slaws.Fig.15.1Copyright?2002PearsonEduca2.MorgantracedagenetoaspecificchromosomeThomasHuntMorganwasthefirsttoassociateaspecificgenewithaspecificchromosome.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsMorganchoiceanexperimentalanimal,Drosophilamelanogasterfruitflyspeciesthateatsfungionfruitprolificbreedersgenerationtimeoftwoweeks.Fruitflieshavethreepairsofautosomesandapairofsexchromosomes(XXinfemales,XYinmales).3+12.MorgantracedagenetoasThenormalcharacterphenotypeisthewildtype(red).Alternativetraitsaremutantphenotypes.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsFig.15.2Geneslocatedonasexchromosomearecalledsex-linkedgenes.femalemaleThenormalcharacterphenotypeCopyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsFig.15.3Morganconcludedthatafly’seyecolorwaslinkedtoitssex(x).Copyright?2002PearsonEduca3.LinkedgenestendtobeinheritedtogetherbecausetheyarelocatedonthesamechromosomeResultsofcrosseswithlinkedgenes

deviatefromthoseexpectedaccordingtoindependentassortment.Morganreasonedthatbodycolorandwingshapeareusuallyinheritedtogetherbecausetheirgenesareonthesamechromosome.3.LinkedgenestendtobeinhCopyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsFig.15.4(b+vg+Orbvg+)(bvg)Copyright?2002PearsonEduca4.Independentassortmentofchromosomesandcrossingoverproducegeneticrecombinantsresultfrom:(1)independentassortmentofgeneslocatedonnonhomologouschromosomes(2)orcrossingoverofgeneslocatedonhomologouschromosomes.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsTheproductionofoffspringwithnewcombinationsoftraitsinheritedfromtwoparentsisgeneticrecombination.4.IndependentassortmentofcThephysicalbasisofrecombinationbetweenunlinkedgenesistherandomorientationofhomologouschromosomesatmetaphaseIofmeiosis.leadstotheindependentassortmentofalleles.linkedgenes,geneslocatedonthesamechromosome,tendtomovetogetherthroughmeiosisandfertilization.[Don’tassortindependently]UndernormalMendeliangeneticrules,wewouldnotexpectlinkedgenestorecombineintoassortmentsofallelesnotfoundintheparents.Butinfact,recombinationbetweenlinkedgenesdoesoccur.(seeMorgan)ThephysicalbasisofrecombinThisswitchedallelesbetweenhomologouschromosomes.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsFig.15.5aA50%frequencyofrecombinationisobservedforanytwogeneslocatedondifferentchromosomes.ThisswitchedallelesbetweenCopyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsFig.15.5bCopyright?2002PearsonEducaTheresultsofMorgan’stestcrossforbodycolorandwingshapedidnotconformtoeitherindependentassortmentorcompletelinkage.

Mostoftheoffspringhadparentalphenotypes,suggestinglinkagebetweenthegenes.However,17%oftheflieswererecombinants,suggestingincompletelinkage.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsTheresultsofMorgan’stestcr5.Geneticistscanuserecombinationdatatomapachromosome’sgeneticlociOneofMorgan’sstudents,AlfredSturtevant,usedcrossingoveroflinkedgenestodevelopamethodforconstructingachromosomemap.Thismapisanorderedlistofthegeneticlocialongaparticularchromosome.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummings5.GeneticistscanuserecombiSturtevanthypothesized:thatthefrequencyofrecombinantoffspringreflectedthedistancesbetweengenesonachromosome.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsThefartheraparttwogenesare,thehighertheprobabilitythatacrossoverwilloccurbetweenthemandthereforeahigherrecombinationfrequency.Sturtevantusedrecombinationfrequenciesfromfruitflycrossestomaptherelativepositionofgenesalongchromosomes,alinkagemap.Sturtevanthypothesized:CopyrCopyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsFig.15.6UsingrecombinationfrequenciestoconstructageneticmapOnemapunit(sometimescalledacentimorgan)isequivalenttoa1%recombinationfrequency.Copyright?2002PearsonEducaSomegenesonachromosomearesofarapart,thefrequencyofrecombinationreachesisitsmaximumvalueof50%andthegenesactasiffoundonseparatechromosomesandareinheritedindependently.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsSomegenesonachromosomeareCopyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsFig.15.7Morethanonegenecanaffectagivenphenotypiccharacteristic(eyecolor)PartialgeneticmapCopyright?2002PearsonEducaMapunits(linkagemap)indicaterelativedistanceandorder,notpreciselocationsofgenes.Thefrequencyofcrossingoverisnotactuallyuniformoverthelengthofachromosome.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsCombinedwithothermethodslikechromosomalbanding,geneticistscandevelopcytologicalmaps.thepositionsofgeneswithrespecttochromosomalfeatures.MorerecenttechniquesshowtheabsolutedistancesbetweengenelociinDNAnucleotides.Mapunits(linkagemap)indicaCHAPTER15

THECHROMOSOMALBASISOFINHERITANCESectionB:SexChromosomes1. Thechromosomalbasisofsexvarieswiththeorganism2.Sex-linkedgeneshaveuniquepatternsofinheritance1.ThechromosomalbasisofsexvarieswiththeorganismInhumanandothermammals,therearetwovarietiesofsexchromosomes,XandY.CHAPTER15

THECHROMOSOMALBACopyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsFig.15.8AvoidconfusionwithXYsystemMaledevelopfromunfertilizedeggs(theyhavenofather)Inhumans,theanatomicalsignsofsexfirstappearwhentheembryoisabouttwomonthsold.Copyright?2002PearsonEducaInindividualswiththeSRYgene(sexdeterminingregionoftheYchromosome),thegenericembryonicgonadsaremodifiedintotestes.lackingtheSRYgene,thegenericembryonicgonadsdevelopintoovaries.ActivityoftheSRYgenetriggersacascadeofbiochemical,physiological,andanatomicalfeaturesbecauseitregulatesmanyothergenes.Inaddition,othergenesontheYchromosomearenecessaryfortheproductionoffunctionalsperm.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsInindividualsCopyright?2002.Sex-linkedgeneshaveuniquepatternsofinheritancethesexchromosomes,especiallytheXchromosome,havegenesformanycharactersunrelatedtosex.

E.g.,white-eyelocusinDrosophila.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsFig.15.92.Sex-linkedgeneshaveuniquBecausemaleshaveonlyoneXchromosome(hemizygous),anymalereceivingtherecessiveallelefromhismotherwillexpressthetrait.malesarefarmorelikelytoinheritsex-linkedrecessivedisordersthanarefemales.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsBecausemaleshaveonlyoneXhumansex-linkeddisordersDuchennemusculardystrophy.duetotheabsenceofanX-linkedgeneforakeymuscleprotein,calleddystrophin.characterizedbyaprogressiveweakeningofthemusclesandlossofcoordination.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsHemophiliaisasex-linkedrecessivetraitdefinedbytheabsenceofoneormoreclottingfactors.Individualscanbetreatedwithintravenousinjectionsofthemissingprotein.humansex-linkeddisordersCopy2.Sex-linkedgeneshaveuniquepatternsofinheritanceAlthoughfemalemammalsinherittwoXchromosomes,onlyoneXchromosomeisactiveandoneXchromosomecondensesintoacompactobject,aBarrbodyAfterBarrbodyformation,alldescendentcellshavethesameinactiveX.ThecondensedBarrbodychromosomeisreactivatedinovariancellsthatproduceova.involvestheattachmentofmethyl(CH3)groupstocytosinenucleotidesontheXchromosome

(inactivatesmostofitsgenes).2.Sex-linkedgeneshaveuniquFig.15.10BarrbodyoccursrandomlyandindependentlyinembryoniccellsatthetimeofXinactivation.Asaconsequence,femalesconsistofamosaicofcells,somewithanactivepaternalX,otherswithanactivematernalX.Ifafemalehumanisheterozygousforasex-linkedtrait,willhavepatchesofnormalskinandskinpatcheslackingsweatglands.Fig.15.10BarrbodyoccursranCHAPTER15

THECHROMOSOMALBASISOFINHERITANCESectionC:ErrorsandExceptionsinChromosomalInheritance1. Alterationsofchromosomenumberorstructurecausesomegeneticdisorders2.Thephenotypiceffectsofsomemammaliangenesdependonwhethertheyareinheritedfromthemotherorthefather(imprinting)3.Extranucleargenesexhibitanon-MendelianpatternofinheritanceCHAPTER15

THECHROMOSOMALBA1.AlterationsofchromosomenumberorstructurecausesomegeneticdisordersNondisjunctionproblemswiththemeioticspindleFig.15.11Meioticnondisjunction1.Alterationsofchromosomenafternondisjunctioncellswillhaveanabnormalchromosomenumberoraneuploidy.Trisomic=2n+1;Monosomic=2n-1.Althoughthefrequencyofaneuploidzygotesmaybequitehighinhumans,mostofthesealterationsaresodisastrousthattheembryosarespontaneouslyabortedlongbeforebirth.Organismswithmorethantwocompletesetsofchromosomespolypoidy.(nondisjunction)e.g,triploid(3n).tetraploid(4n)e.g.,bothfishesandamphibianshavepolyploidspecies.Polyploidyisrelativelycommonamongplantsandmuchlesscommonamonganimals.Polyploidsaremorenearlynormalinphenotypethananeuploids.afternondisjunctioncellswilDownsyndrome:threecopiesofchromosome21aneuploid(nondisjunction)Fig.15.14ThefrequencyofDownsyndromecorrelateswiththeageofthemother.age-dependentabnormalityDownsyndrome:threecopiesofFig.15.13BmoninmeiosisTypicallyharmfulchronicmyelogenousleukemia

(CML).[ch9/ch22]Fig.15.13Breakageofachromo2.Thephenotypiceffectsofsomemammaliangenesdependonwhethertheywereinheritedfromthemotherorthefather(imprinting)Formostgenesitisareasonableassumptionthataspecificallelewillhavethesameeffectregardlessofwhetheritwasinheritedfromthemotherorfather.However,forsometraitsinmammals,itdoesdependonwhichparentpassedalongtheallelesforthosetraits.TheimprintinggenesinvolvedarenotsexlinkedandmayormaynotlieontheXchromosome.2.Thephenotypiceffectsofs.adeletionofaspecificsegmentofchromosome15Prader-Willisyndrome

abnormalchromosomefromfather.Angelmansyndrome

fromthemother.Ingenomicimprintingprocess,ageneononehomologouschromosomeissilenced,whiletheotherisexpressed.cp:barrbodyisforxchromosomeinactivation.sameallelesmayhavedifferenteffectsonoffspring,dependingonwhethertheyarriveinthezygoteviaovumorsperm.Theimprintingstatusofagivengene.adeletionofaspecificsegCopyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsFig.15.15Copyright?2002PearsonEducaInmanycases,genomicimprintingoccurswhenmethylgroupsareaddedtocytosinenucleotidesononeofthealleles.FragileXsyndrome,whichleadstovariousdegreesofmentalretardation,alsoappearstobesubjecttogenomicimprinting.(inheritedfromthemother)ThisdisorderisnamedforanabnormalXchromosomewithextraCGGrepeat.Inmanycases,genomicimprint3.Extranucleargenesexhibitanon-MendelianpatternofinheritanceNotallofaeukaryotecell’sgenesarelocatedinthenucleus.phenotypeoftheoffspringwasdeterminedonlybythematernalparent(maternalinheritance).ExtranucleargenesarefoundonsmallcirclesofDNAinmitochondriaandchloroplasts.Theseorganellesreproducethemselves.TheircytoplasmicgenesdonotdisplayMendelianinheritance.Theyarenotdistributedtooffspringduringmeiosis.3.ExtranucleargenesexhibitCHAPTER15

THECHROMOSOMALBASISOFINHERITANCESectionA:RelatingMendelismtoChromosomes1. Mendelianinheritancehasitsphysicalbasisinthebehaviorofchromosomesduringsexuallifecycles2.Morgantracedagenetoaspecificchromosome3.Linkedgenestendtobeinheritedtogetherbecausetheyarelocatedonthesamechromosome4.Independentassortmentofchromosomesandcrossingoverproducegeneticrecombinants5.Geneticistsuserecombinationdatatomapachromosome’sgeneticloci生物醫學暨環境生物學系張學偉助理教授CHAPTER15

THECHROMOSOMALBAIntroductionMendel’shereditaryfactorsarethegeneslocatedonchromosomes.chromosomestheoryofinheritance.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummings1.MendelianinheritancehasitsphysicalbasisinthebehaviorofchromosomesduringsexuallifecyclesIntroductionMendel’shereditarCopyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsChromosomalbasisofMendel’slaws.Fig.15.1Copyright?2002PearsonEduca2.MorgantracedagenetoaspecificchromosomeThomasHuntMorganwasthefirsttoassociateaspecificgenewithaspecificchromosome.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsMorganchoiceanexperimentalanimal,Drosophilamelanogasterfruitflyspeciesthateatsfungionfruitprolificbreedersgenerationtimeoftwoweeks.Fruitflieshavethreepairsofautosomesandapairofsexchromosomes(XXinfemales,XYinmales).3+12.MorgantracedagenetoasThenormalcharacterphenotypeisthewildtype(red).Alternativetraitsaremutantphenotypes.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsFig.15.2Geneslocatedonasexchromosomearecalledsex-linkedgenes.femalemaleThenormalcharacterphenotypeCopyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsFig.15.3Morganconcludedthatafly’seyecolorwaslinkedtoitssex(x).Copyright?2002PearsonEduca3.LinkedgenestendtobeinheritedtogetherbecausetheyarelocatedonthesamechromosomeResultsofcrosseswithlinkedgenes

deviatefromthoseexpectedaccordingtoindependentassortment.Morganreasonedthatbodycolorandwingshapeareusuallyinheritedtogetherbecausetheirgenesareonthesamechromosome.3.LinkedgenestendtobeinhCopyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsFig.15.4(b+vg+Orbvg+)(bvg)Copyright?2002PearsonEduca4.Independentassortmentofchromosomesandcrossingoverproducegeneticrecombinantsresultfrom:(1)independentassortmentofgeneslocatedonnonhomologouschromosomes(2)orcrossingoverofgeneslocatedonhomologouschromosomes.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsTheproductionofoffspringwithnewcombinationsoftraitsinheritedfromtwoparentsisgeneticrecombination.4.IndependentassortmentofcThephysicalbasisofrecombinationbetweenunlinkedgenesistherandomorientationofhomologouschromosomesatmetaphaseIofmeiosis.leadstotheindependentassortmentofalleles.linkedgenes,geneslocatedonthesamechromosome,tendtomovetogetherthroughmeiosisandfertilization.[Don’tassortindependently]UndernormalMendeliangeneticrules,wewouldnotexpectlinkedgenestorecombineintoassortmentsofallelesnotfoundintheparents.Butinfact,recombinationbetweenlinkedgenesdoesoccur.(seeMorgan)ThephysicalbasisofrecombinThisswitchedallelesbetweenhomologouschromosomes.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsFig.15.5aA50%frequencyofrecombinationisobservedforanytwogeneslocatedondifferentchromosomes.ThisswitchedallelesbetweenCopyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsFig.15.5bCopyright?2002PearsonEducaTheresultsofMorgan’stestcrossforbodycolorandwingshapedidnotconformtoeitherindependentassortmentorcompletelinkage.

Mostoftheoffspringhadparentalphenotypes,suggestinglinkagebetweenthegenes.However,17%oftheflieswererecombinants,suggestingincompletelinkage.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsTheresultsofMorgan’stestcr5.Geneticistscanuserecombinationdatatomapachromosome’sgeneticlociOneofMorgan’sstudents,AlfredSturtevant,usedcrossingoveroflinkedgenestodevelopamethodforconstructingachromosomemap.Thismapisanorderedlistofthegeneticlocialongaparticularchromosome.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummings5.GeneticistscanuserecombiSturtevanthypothesized:thatthefrequencyofrecombinantoffspringreflectedthedistancesbetweengenesonachromosome.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsThefartheraparttwogenesare,thehighertheprobabilitythatacrossoverwilloccurbetweenthemandthereforeahigherrecombinationfrequency.Sturtevantusedrecombinationfrequenciesfromfruitflycrossestomaptherelativepositionofgenesalongchromosomes,alinkagemap.Sturtevanthypothesized:CopyrCopyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsFig.15.6UsingrecombinationfrequenciestoconstructageneticmapOnemapunit(sometimescalledacentimorgan)isequivalenttoa1%recombinationfrequency.Copyright?2002PearsonEducaSomegenesonachromosomearesofarapart,thefrequencyofrecombinationreachesisitsmaximumvalueof50%andthegenesactasiffoundonseparatechromosomesandareinheritedindependently.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsSomegenesonachromosomeareCopyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsFig.15.7Morethanonegenecanaffectagivenphenotypiccharacteristic(eyecolor)PartialgeneticmapCopyright?2002PearsonEducaMapunits(linkagemap)indicaterelativedistanceandorder,notpreciselocationsofgenes.Thefrequencyofcrossingoverisnotactuallyuniformoverthelengthofachromosome.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsCombinedwithothermethodslikechromosomalbanding,geneticistscandevelopcytologicalmaps.thepositionsofgeneswithrespecttochromosomalfeatures.MorerecenttechniquesshowtheabsolutedistancesbetweengenelociinDNAnucleotides.Mapunits(linkagemap)indicaCHAPTER15

THECHROMOSOMALBASISOFINHERITANCESectionB:SexChromosomes1. Thechromosomalbasisofsexvarieswiththeorganism2.Sex-linkedgeneshaveuniquepatternsofinheritance1.ThechromosomalbasisofsexvarieswiththeorganismInhumanandothermammals,therearetwovarietiesofsexchromosomes,XandY.CHAPTER15

THECHROMOSOMALBACopyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsFig.15.8AvoidconfusionwithXYsystemMaledevelopfromunfertilizedeggs(theyhavenofather)Inhumans,theanatomicalsignsofsexfirstappearwhentheembryoisabouttwomonthsold.Copyright?2002PearsonEducaInindividualswiththeSRYgene(sexdeterminingregionoftheYchromosome),thegenericembryonicgonadsaremodifiedintotestes.lackingtheSRYgene,thegenericembryonicgonadsdevelopintoovaries.ActivityoftheSRYgenetriggersacascadeofbiochemical,physiological,andanatomicalfeaturesbecauseitregulatesmanyothergenes.Inaddition,othergenesontheYchromosomearenecessaryfortheproductionoffunctionalsperm.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsInindividualsCopyright?2002.Sex-linkedgeneshaveuniquepatternsofinheritancethesexchromosomes,especiallytheXchromosome,havegenesformanycharactersunrelatedtosex.

E.g.,white-eyelocusinDrosophila.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsFig.15.92.Sex-linkedgeneshaveuniquBecausemaleshaveonlyoneXchromosome(hemizygous),anymalereceivingtherecessiveallelefromhismotherwillexpressthetrait.malesarefarmorelikelytoinheritsex-linkedrecessivedisordersthanarefemales.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsBecausemaleshaveonlyoneXhumansex-linkeddisordersDuchennemusculardystrophy.duetotheabsenceofanX-linkedgeneforakeymuscleprotein,calleddystrophin.characterizedbyaprogressiveweakeningofthemusclesandlossofcoordination.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsHemophiliaisasex-linkedrecessivetraitdefinedbytheabsenceofoneormoreclottingfactors.Individualscanbetreatedwithintravenousinjectionsofthemissingprotein.humansex-linkeddisordersCopy2.Sex-linkedgeneshaveuniquepatternsofinheritanceAlthoughfemalemammalsinherittwoXchromosomes,onlyoneXchromosomeisactiveandoneXchromosomecondensesintoacompactobject,aBarrbodyAfterBarrbodyformation,alldescendentcellshavethesameinactiveX.ThecondensedBarrbodychromosomeisreactivatedinovariancellsthatproduceova.involvestheattachmentofmethyl(CH3)groupstocytosinenucleotidesontheXchromosome

(inactivatesmostofitsgenes).2.Sex-linkedgeneshaveuniquFig.15.10BarrbodyoccursrandomlyandindependentlyinembryoniccellsatthetimeofXinactivation.Asaconsequence,femalesconsistofamosaicofcells,somewithanactivepaternalX,otherswithanactivematernalX.Ifafemalehumanisheterozygousforasex-linkedtrait,willhavepatchesofnormalskinandskinpatcheslackingsweatglands.Fig.15.10BarrbodyoccursranCHAPTER15

THECHROMOSOMALBASISOFINHERITANCESectionC:ErrorsandExceptionsinChromosomalInheritance1. Alterationsofchromosomenumberorstructurecausesomegeneticdisorders2.Thephenotypiceffectsofsomemammaliangenesdependonwhethertheyareinheritedfromthemotherorthefather(imprinting)3.Extranucleargenesexhibitanon-MendelianpatternofinheritanceCHAPTER15

THECHROMOSOMALBA1.AlterationsofchromosomenumberorstructurecausesomegeneticdisordersNondisjunctionproblemswiththemeioticspindleFig.15.11Meioticnondisjunction1.Alterationsofchromosomenafternondisjunctioncellswillhaveanabnormalchromosomenumberoraneuploidy.Trisomic=2n+1;Monosomic=2n-1.Althoughthefrequencyofaneuploidzygotesmaybequitehighinhumans,mostofthesealterationsaresodisastrousthattheembryosarespontaneouslyabortedlongbeforebirth.Organismswithmorethantwocompletesetsofchromosomespolypoidy.(nondisjunction)e.g,triploid(3n).tetraploid(4n)e.g