QuantitativeGenetics假如能夠讓歷史倒轉，讓地球上的生命重新進化，每一次的結果都會大不一樣。--科普作家古爾德新的物種從哪里來？Doesevolutionproceedbysmallstepsorbigjumps?自然選擇讓小變化不斷累積，直到一個類群不能再與它們的親緣類群交配繁衍，從而形成了新的物種。--達爾文的進化論新物種形成的觸發(fā)點是某個單獨的、從進化學角度看無法預料的突發(fā)事件。而自然選擇只對已經(jīng)存在的物種產(chǎn)生緩慢的、可以預見的影響，但物種形成的偶然性意味著進化的大方向是不可預料的。--英國雷丁大學進化生物學家馬克·帕赫爾自然選擇無法解釋的現(xiàn)象非洲馬達加斯加島上生活著16種鼠狐猴，它們都生活在相似的環(huán)境下，做著相似的事情，甚至看上去很像。如果物種形成真是自然選擇導致生物逐漸改變以適應不同生境的結果，這些鼠狐猴的存在就難以解釋。帕赫爾的證據(jù)他和同事從已經(jīng)出版的文獻中收集了130個根據(jù)DNA繪制的進化樹，涵蓋植物、動物和菌類。從中篩選出包括貓、大黃蜂、鷹、玫瑰花等101個物種的進化樹。如果物種形成源于自然選擇和小變化的累積，那么樹枝的長度將符合鐘形曲線。根據(jù)變化累積的速度又可以分為常態(tài)曲線和對數(shù)正態(tài)曲線。然而讓他們吃驚的是，兩種曲線都不符合他們找到的數(shù)據(jù)。對數(shù)正態(tài)曲線只能解釋8%的情況，常態(tài)曲線更找不到一個對應的例子。相反，帕赫爾德小組發(fā)現(xiàn)78%的進化樹的樹枝長度分布模式更符合另一種人們熟悉的曲線-指數(shù)分布。而指數(shù)分布曲線通常用來解釋某些單獨的偶然事件，比如高速公路上動物被車撞死事故地點之間的距離。這無疑說明，并非事件的積累導致了物種形成，而是因為某個單一、罕見事件突然從天而降。所以說，物種形成是隨意的意外事故。比如：地理上的隔離、重大機遇變化、環(huán)境和遺傳學上的變化以及心理變化等。其他科學家的觀點單一偶然事件導致物種形成理論是一種有趣的解釋，只是一種參考。--加拿大西蒙弗雷澤大學阿尼·莫爾斯帕赫爾的分析只強調(diào)了故事的局部。它只告訴你物種形成的一種必要因素，而非全部因素。物種形成需要兩個條件：導致隔離的原因；導致分化的原因。后一個過程-兩個被分割的類群發(fā)生足夠變化，最終被我們視為兩個不同物種-包括可能在自然選擇作用下的逐漸的、適應性的變化。--加州大學伯克利分校丹尼爾·拉博斯基Outline

1.Introductionofquantitativetrait2.Johannsen’sThreeExperiments3.MathematicalBasisofQuantitativeGenetics4.Heritability5.QuantitativeTraitLoci

6.HeterosisQuantitativeTraitsMendelworkedwithtraitsthatwerealldiscrete,either/ortraits:yelloworgreen,roundorwrinkled,etc.Differentallelesgaveclearlydistinguishablephenotypes.However,manytraitsdon’tfallintodiscretecategories:height,forexample,oryieldofcornperacre.Theseare“quantitativetraits”.

Themanipulationofquantitativetraitshasallowedmajorincreasesincropyieldduringthepast80years.Thisisanimportantpartofwhytodayfamineisrare,aproductofpoliticalinstabilityratherthanarealshortageoffood.Untilveryrecently,cropimprovementthroughquantitativegeneticswasthemostprofitableaspectofgenetics.EarlyinthehistoryofgeneticsiswasarguedthatquantitativetraitsworkedthroughageneticsystemquitedifferentfromMendeliangenetics.Thisideahasbeendisproved,andthetheoryofquantitativegeneticsisbasedonMendelianprinciples.Whyisquantitativegeneticsimportant?MedicineDisease=variationComplexdisorderscausedbymultiplegeneticandenvironmentalfactorsUnderstandinggeneticvs.environmentalcausespreventiongeneticcounsellinggenetically-tailoredtreatmentsWhyisquantitativegeneticsimportant?AgricultureEconomicallyimportanttraits=quantitativetraitsQuantitativegeneticstheory->basisforbreedingprogramsEnvironmentalvariationreducesefficiencyofselectionWhyisquantitativegeneticsimportant?ConsequencesofinbreedingandoutcrossingAgriculture–inbredlines,hybrids,F1sConservation–endangeredspecies,captivebreedingprogramsWhyisquantitativegeneticsimportant?EvolutionNaturalselectionrequiresheritablevariationfortraitsWhataretheforcesthatmaintainvariationwithinpopulations?Balancebetweenselection,driftandmutationBalancingselection?Whyisquantitativegeneticsimportant?EvolutionDoesevolutionproceedbysmallstepsorbigjumps?Whatistherelativeimportanceofpre-existingvariationvs.newmutations?Dogeneticcorrelationsbetweentraitsposeconstraintsonevolution?TypesofQuantitativeTraitIngeneral,thedistributionofquantitativetraitsvaluesinapopulationfollowsthenormaldistribution(alsoknownasGaussiandistributionorbellcurve).Thesecurvesarecharacterizedbythemean(mid-point)andbythevariance(width).Oftenstandarddeviation,thesquarerootofvariance,isusedasameasureofthecurve’swidth.1.continuoustrait:cantakeonanyvalue:height,forexample.2.countable(meristic):cantakeonintegervaluesonly:numberofbristles,forexample.3.thresholdtrait:hasanunderlyingquantitativedistribution,butthetraitonlyappearsonlyifathresholdiscrossed?.PunchlineandBasicQuestionsThebasictenetofquantitativegenetics:thevariationseeninquantitativetraitsisduetoacombinationofmanygeneseachcontributingasmallamount,plusenvironmentalfactors.Or:phenotype=geneticsplusenvironment.Basicquestions(plusanswers):1.Whatisthegeneticbasisofquantitativetraits?(theyarecausedbynormalgenesfollowingMendel’srules).2.Howcanweseparatetheeffectsofgeneticsfromtheeffectsoftheenvironment?(byinbreedingtoeliminategeneticvariation).3.Howcanwepredictandcontroltheoutcomeofacross?(byartificialselection).QuantitativeTraitsareCausedbyMendelianGenesIn1909HermanNilsson-EhlefromSwedendidaseriesofexperimentswithkernelcolorinwheat.Wheatisa

hexaploid(六倍體),theresultof3differentspeciesproducingastablehybrid,anallopolyploid.Therearethus3similarbutslightlydifferentgenomescontainedinthewheatgenome,calledA,B,andD.Eachgenomehasasinglegenethataffectskernelcolor,andeachoftheselocihasaredalleleandawhiteallele.WewillcalltheredallelesA,B,andD,andthewhiteallelesa,b,andd.Inheritanceoftheseallelesispartiallydominant,or“additive”.Theamountofredpigmentinthekernelisproportionaltothenumberofredallelespresent,from0to6.WheatKernelColorThecross:AABBDDxaabbdd.Redxwhite.F1:AaBbDdphenotype:pink,intermediatebetweentheparents.Nowselfthese.F2:allelesfollowabinomialdistribution(二項式分布):1/64haveall6redalleles=red6/64have5red+1white=lightred10/64have4red+2white=darkpink15/64have3red+3white=pink10/64have2red+4white=lightpink6/64have1red+5white=verypalepink1/64haveall6white=whiteAddabitofenvironmentalvariationandhumaninabilitytodistinguishsimilarshades:yougetaquantitativedistribution.ThisdemonstratesthatasimpleMendeliansystem:3genes,2alleleseach,partialdominance--canleadtoaquantitativetrait.MoreWheatKernelColorSeparatingGeneticsfromEnvironmentThreeexperimentsbyWilhelmJohannsen(約翰森),fromDenmark,usingthecommonbean(刀豆)(Phaseolusvulgaris).Johannsencoinedthewords“gene”,“genotype”and“phenotype”.FirstJohannsenexperiment:heweighedagroupofbeans,thengrewthemupandweighedtheirprogeny(afterselfingthem).Johannsen’sFirstExperimentInterpretationIngeneral,heavyparentsgaveheavyoffspringandlightparentsgavelightoffspring.Thatis,thereisasignificantcorrelationbetweenparentandoffspringweights.However,thereisalsoaconsiderablevariationamongtheoffspringweights.Thisisduetovariationsinbothgeneticsandenvironment.Mostoffspringofextremeparents(veryheavyorverylight)aremoreaveragethantheirparents.Thisisaphenomenoncalled“regression(衰退,回歸)tothemean”.Extrememembersofapopulationbenefitfromveryluckyenvironmentalconditions,whichcan’tbeinherited.Johannsen’sSecondExperimentJohannsenthenworkedonseparatingenvironmentaleffectsfromgeneticeffects.Hedidthisbyinbreedingthebeansfor10generations.Inbreedingmeansdoingtheclosestpossiblecross,inthiscase,

selfingthem.Halfiftheremainingheterozygosity(percentageofheterozygotes)disappearsforeachgenerationofselfing.ThisisbecausewhenaAaheterozygoteisselfed,1/4oftheoffspringareAA,1/4areaa,and1/2areAa.SelfingtheAAandaaoffspringgivesonlyhomozygousoffspringforever.Thusiseachgeneration,1/2theoffspringbecomehomozygotesandalltheiroffspringstayhomozygotes.After10generationsofselfing,thepercentageofheterozygotesislessthan1/1000oftheoriginallevel.ResultsJohannsencreated19inbredlines.Theinbredlineshadsomevariation,butlessthantheoriginalrandom-bredpopulation.Theremainingvariationwasduetoenvironmentalvariations.Themeanweightofeachlinewasdifferent,butitwasstableacrossgenerations.Thereasonisthatthelinesaregeneticallydifferentfromeachother,buttheyaregenetically(moreorless)identical.Thevariancewasalsostablebetweengenerations.Johannsen’sThirdExperimentJohannsenthenstartedtoworkoutthebasisofartificialselection:howtoimprovea

species

asefficientlyaspossible.Startwitharandom-bredpopulation.Takethebestonestobeparentsofthenextgeneration.Thenextgenerationhasameanthatisshiftedinthedesireddirection.Thisproceduredoesn’tworkforinbredpopulations,becausethereisnogeneticvariationtoinherit.Thenextgeneration’smeanisthesameasthepreviousgeneration’smeandespitehavingselectedthebestparents.SelectionEdwardM.EastfromtheUnitedStatesworkedouttheformalbasisformodernartificialselection,followingtheworkofGeorgeShullonmaize.Eastworkedonbothmaizeandtobacco.Eastmeasuredthelengthofthetobaccocorolla(花冠)(thestraightpartoftheflower).Hecrossed2inbredlineswithdifferentlengths,thenselfedtheF1togetandF2,thenselfedtheF2’stogetaseriesofF3lines.Thevariationintheplantscanbeobservedinthewidthofthedistributioncurves.Environmentalvariationisconstantamongallplants.GeneticvariationisminimalinboththeinbredparentallinesandintheF1’s.TheF1’sareheterozygous,buttheyaregeneticallyuniform,becausealloftheirparentswerehomozygous.TheF2displaysthemaximumvariation:foreverygene,allpossiblegenotypes(AA,Aa,andaa)arepresentinthepopulation.TheF3’sshowlessvariationthantheF2’s,asinbreedingstartstoeliminateheterozygosity.HeterosisBoththeparentallinesandtheF1’saregeneticallyuniform.However,theparentallinesarerelativelysmalland

weak,aphenomenoncalled“inbreedingdepression”:Toomuchhomozygosityleadstosmall,sicklyandweakorganisms,atleastamongorganismsthatusuallybreedwithothersinsteadofself-pollinating.Incontrast,theF1hybridsarelarge,healthyandstrong.Thisphenomenoniscalled“heterosis”or“hybridvigor”.ThecornplantedintheUSandotherdevelopedcountriesinnearlyallF1hybridseed,becauseitproduceshighyielding,healthyplants(duetoheterosis)anditisgeneticallyuniform(andthusmaturesatthesametimewithearsinthesamepositiononeveryplant).MathematicalBasisofQuantitativeGenetics(1)

MeanThisisthearithmeticmean(算術平均);justtheaverageofthemeasurements.Justsumthemeasurementsanddividebythenumberofsamples.MeanSymbolically:x=Sxi/nThemeanisgoingtoshowuswhattheaverageindividuallookslike.Butweknow,sincethisisafrequencydistribution,thatallindividualsdon’tlookthesame.Weneedastatistictoindicatehowmuchindividualsdiffer,orvary,fromeachother.Thisstatisticisthevariance,symbolizedbys2.Awaywecouldmeasurehowmuchindividualsvaryfromeachotheristomeasurehowmuch,ontheaverage,theyvaryfromthemean.Thatis,wecouldcomputeanaverageamountbywhichindividualsinthepopulationvaryfromthemean.

Thatwouldbeourmeasureofvariation,orvariance.So,symbolically:

s2=S(xi

–x)2/(n-1)Wethenbasicallyfigureoutanaveragevalueforallthesesquareddifferencesbyaddingthemupanddividingbythenumberofsamples(minus1tocorrectforbias).So,ifwegivethemeanandvarianceofphenotypesforsomepopulation,wehaveactuallydescribedalotaboutitsfrequencydistribution.3distributions–samemean,different variancesCorrelationCoefficient(相關系數(shù))Frequently,biologicaltraitsarecorrelatedThatis,asonevaluechangestheotherislikelytochangealso.CorrelationCoefficientExamples:overallbodysizeandthesizeofappendages.Or,heightandweight.CorrelationCoefficientThecorrelationcoefficientisameasureofthestrengthofassociationbetweentwovariables.CorrelationCoefficientItcanrangefrom–1to+1Apositivevaluemeansthatanincreaseinonevalueisassociatedwithanincreaseintheothervalue.CorrelationCoefficientAnegativevaluemeansthatanincreaseinonevalueisassociatedwithadecreaseintheothervalue.CorrelationCoefficientTheabsolutevalueofthenumberisameasureofthestrengthofthecorrelation:thehigherthenumberthestrongertheassociation.CorrelationCoefficientItisimportanttorememberthatcorrelations,evenstrongones,donotimplycause-and-effect(因果關系)!Correlationsmayexistformanyreasons.CorrelationCoefficientExample:thereisapositivecorrelationbetweenthenumberofstorks(鸛鳥)inanareaandthebirthrate.So,dostorksbringbabies?CorrelationCoefficientNo,chimneysincitiesprovidenestingsitesforstorks,andcitiestendtohavehigherbirthratesthanrural(鄉(xiāng)下的)areas.RegressionCorrelationcoefficientstellusaboutthestrengthofassocationbetween2variables,andwhethertherelationshipispositiveornegative.RegressionBut,wemaywanttoknowmorepreciselytherelationshipbetweenthevariables.Ifonechanges,canwepredicthowmuchtheotheronewillchange?RegressionForthispurpose,weuseadifferentstatistic,theregressioncoefficient(回歸系數(shù)).RegressionThenextslideshowsanexample:theregressionofsons’heightsontheirfathers’heights.RegressionThelineisamathematicallycomputedlinethatbestfitsthescatterofpointsfromthedata.RegressionTheslopeofthelinehasthefamiliarformula:y=a+bxWherebistheslopeoftheline.Thisvalue,b,isalsotheregressioncoefficient.RegressionThevalueoftheslopeindicateshowmuchofachangeintheyvariableisassociatedwithachangeinthexvariable.MathematicalBasisofQuantitativeGenetics(2)Recallthebasicpremise(前提)ofquantitativegenetics:phenotype=geneticsplusenvironment.Infactwearelookingatvariationinthetraits,whichismeasuredbythewidthoftheGaussiandistributioncurve.Thiswidthisthevariance(oritssquareroot,thestandarddeviation).Varianceisausefulproperty,becausevariancesfromdifferentsourcescanbeaddedtogethertogettotalvariance.However,theunitsofvariancearethesquaresoftheunitsusedtomeasurethetrait.Thus,iflengthincentimeterswasmeasured,thevariancesofthelengthareincm2.Thisiswhystandarddeviationisusuallyreported:length±s.d.--becausestandarddeviationisinthesameunitsastheoriginalmeasurement.Standarddeviationsfromdifferentsourcesarenotadditive.Quantitativetraitscanthusbeexpressedas:VT=VG+VE

whereVT=totalvariance,VG-varianceduetogenetics,andVE=varianceduetoenvironmental(non-inherited)causes.Thisequationisoftenwrittenwithanadditionalcovarianceterm:thedegreetowhichgeneticandenvironmentalvariancedependoneachother.Wearejustgoingtoassumethistermequalszeroinourdiscussions.Heritability(遺傳力,遺傳率)Onepropertyofinterestis“heritability”,theproportionofatrait’svariationthatisduetogenetics(withtherestofitdueto“environmental”factors).Thisseemslikeasimpleconcept,butitisloadedwithproblems.Thebroad-senseheritability,symbolizedasH(sometimesH2toindicatethattheunitsofvariancearesquared).Hisasimpletranslationofthestatementfromaboveintomathematics:

H=VG/VT

Thismeasure,thebroad-senseheritability,isfairlyeasytomeasure,especiallyinhumanpopulationswhereidenticaltwinsareavailable.However,differentstudiesshowwidevariationsinHvaluesforthesametraits,andplantbreedershavefoundthatitdoesn’taccurately

reflecttheresultsofselectionexperiments.Thus,

Hisgenerallyonlyusedinsocialsciencework.Additivevs.DominanceGeneticVarianceThebiggestproblemwithbroadsenseheritabilitycomesfromlumpingallgeneticphenomenaintoasingleVgfactor.Paradoxically,notallvariationduetogeneticdifferencescanbedirectlyinheritedbyanoffspringfromtheparents.Geneticvariancecanbesplitinto2maincomponents,additivegenetic(累加遺傳方差)variance(VA)anddominancegeneticvariance(VD).

VG=VA+VDAdditivevarianceisthevarianceinatraitthatisduetotheeffectsofeachindividualallelebeingaddedtogether,withoutanyinteractionswithotherallelesorgenes.Dominancevarianceisthevariancethatisduetointeractionsbetweenalleles:synergy

effectsduetotwoallelesinteractingtomakethetraitgreater(orlesser)thanthesumofthetwoallelesactingalone.Weareusingdominancevariancetoincludebothinteractionsbetweenallelesofthesamegeneandinteractionsbetweendifferencegenes,whichissometimesaseparatecomponentcalledepistasisvariance(上位方差).Theimportantpoint:dominancevarianceisnotdirectlyinheritedfromparenttooffspring.?Itisduetotheinteractionofgenesfrombothparentswithintheindividual,andofcourseonlyonealleleispassedfromeachparenttotheoffspring.NarrowSenseHeritabilityForapracticalbreeder,dominancevariancecan’tbepredicted,anditdoesn’taffectthemeanorvarianceoftheoffspringofaselectioncrossinasystematicfashion.Thus,onlyadditivegeneticvarianceisuseful.Breedersandotherscientistsuse“narrowsenseheritability”,h,asameasureofheritability.

h=VA/VTNarrowsenseheritabilitycanalsobecalculateddirectlyfrombreedingexperiments.Forthisreasonitisalsocalled“realizedheritability”.HeritabilityinaSelectionExperimentThereare3easilymeasuredparametersinaselectionexperiment:themeanoftheoriginalrandom-bredpopulation,themeanoftheindividualsselectedtobetheparents,andthemeanofthenextgeneration.Thesefactorsarerelatedbythenarrowsenseheritability:Thedenominator(分母)issometimescalledthe“selectiondifferential”(選擇微分),thedifferencebetweenthetotalpopulationandtheindividualsselectedtobeparentsofthenextgeneration.Thenumerator(分子)issometimescalledthe“selectionresponse”,thedifferencebetweentheoffspringandtheoriginalpopulation,theamountthepopulationshiftedduetotheselection.ExampleInDrosophila,themeannumberofbristlesonthethorax(胸部)(topsurfaceonly)is6.4.Fromthispopulation,agroupwaschosenwhichhadanaveragebristlenumberof7.2.Theoffspringofthechosengrouphadanaverageof6.6bristles.h=(nextgen-original)/(selected-original)h=(6.6-6.4)/(7.2-6.4)h=0.2/0.8h=0.25RealizedHeritabilityThevalueofhasmeasuredbyselectionexperimentsisremarkablyconstantovermanygenerations,andselectioncanbecontinuedforaverylongtimewithoutapparentlyrunningoutofgeneticvariation.InoneexperimentusingTribolium(flourbeetles粉甲蟲),theoriginalpopulationhadameanweightof2.4mg,witharangeof1.8to3.0mg.After125generationsofselection,themeanweightwas5.1mg,morethantwicetheoriginalweightandfaroutsidetheoriginalrange.SimilarexperimentsattheUniversityofIllinoisonmaizeforhighproteinandhighoilcontenthaveshownconsistentimprovementformorethan100generations(i.e.100years,since1896).SummaryofEquationsYouwillneedtoknowtheseequations,andthendemonstratetheminsolvingproblems.VT=VG+VEforanINBREDpopulation,VT=VE,becauseVG=0H=VG/VTVG=VA+VDh=VA/VTh=(nextgen-original)/(selected-original)Notethathcanbecalculatedfromeitherofthe2equations.ExampleProblemInaquesttomakebiggerfrogs,scientistsstartedwitharandombredpopulationoffrogswithanaverageweightof500g.Theychoseagroupwithaverageweight600gtobetheparentsofthenextgeneration.Afewotherfacts:VE=1340,VA=870,VD=410.Whatisthegeneticvariance?VG=VA+VD=1280Whatisthetotalvariance?VT=VG+VE=2620Whatisthebroadsenseheritability?H=VG/VT=0.49Whatisthenarrowsenseheritability?h=VA/VT=0.33Whatisthemeanweightofthenextgeneration?

h=(nextgen-original)/(selected-original)0.33=(next_gen-500)/(600-500)Sothemeanweightofthenextgeneration=533gUnderstandingHeritability

Heritabilityestimateshavelimitationsthatareoftenignored,leadingtomisunderstandingandabuse.Importantqualificationsandlimitationsofheritability:a. Broad-senseheritabilitydoesnotindicatetheextenttowhichatraitisgenetic.Rather,itmeasurestheproportionofthephenotypicvarianceinapopulationresultingfromgeneticfactors.i.Forexample,knowledgeoffootballrequiresanervoussystem,producedbygenes,butdifferencesinfootballknowledgeareusuallynotduetogeneticfactors,sobroad-senseheritability(VG)forfootballknowledgeiszero.ii.Ifallindividualshavethesamegenesatlocicontrollingatrait(e.g.,eyeorearnumber),VG=0,eventhoughgenesareclearlyinvolvedinproducingthetrait.iii.Highheritabilityalsodoesnotmeanthatenvironmentisunimportant.Itmaymeaninsteadthatenvironmentalfactorsinfluencingthetraitarerelativelyuniformacrossthepopulation.b. Heritabilitydoesnotindicatewhatproportionofanindividual’sphenotypeisgenetic.Heritabilityisacharacteristicofapopulation,notanindividual.c. Heritabilityisnotfixedforatrait.Itdependsonthegeneticmakeupandenvironmentofapopulation,andsoacalculationforonegroupmaynotholdtrueinanother.Anexampleishumanheight.i.Inapopulationwithauniformlyhighqualitydiet,differencesinheightarelikelytobeduetogeneticfactors,especiallyifthereishighethnicdiversity.ii.Inapopulationwherequalityofdietvarieswidely,heightdifferenceswillbeduelesstogeneticfactors,andrelativelymoretoenvironmentalfactors.d.Highheritabilityforatraitdoesnotimplythatapopulation’sdifferencesinthesametraitaregeneticallydetermined.Anexampleisdietinmice.i.Geneticallydiversemicewererandomlysplitintotwogroups.Bothgroupsreceivedthesamespace,waterandotherenvironmentalnecessities.Theonlydifferencewasdiet.Miceinthegroupreceivingnutritionallyrichfoodgrewlarge.Heritabilityofadultbodyweightwascalculatedat0.93.(2)Miceinthegroupreceivinganimpoverisheddietlackingcaloriesandessentialnutrientsweresmaller.Heritabilityofadultbodyweightwas0.93.(3)Bothlargesizeandsmallsizewerecalculatedtobehighlyheritableinthesemice.Thisisacontradiction,sincebothgroupsarefromthesamegeneticstock.QuantitativeTraitLoci

1. Theapplicationofstatisticalanalysistopolygenicinheritancehasbeenpowerful,requiresspecificknowledgeofthequantitativetraitloci(QTLs)thataffectthem.(對數(shù)量性狀的遺傳變異起作用的基因稱為QTL)2. QTLsareidentifiedbycorrelationwithphenotypicdifferencesbetweenindividuals,andworkbestwhenanalyzingapopulationwith:a. Adetailedlinkagemapb. Significantphenotypicvariation.c. Largenumbersofindividuals.3. Typically,inbredlineswithdifferentphenotypes(homozygotesfordifferentalleles)arecrossed,andthenbackcrossed,intercrossed,or

selfedtoproduceaseriesofrecombinantinbredstrains.a. Theresultingrecombinantinbredstrainsareanalyzedformarkergenotypesthatcorrelatewithphenotypicvariation. i. Ifthephenotypemeanvalueisthesameforallclasses,themarkerlocusisunlinkedtoaQTL. ii. Ifthemeanvalueisdifferentfordifferentclasses,themarkerlocusislinkedtoaQTL.4. QTLsresponsibleforagronomictraits(農(nóng)藝性狀)andadaptivedifferencesbetweencloselyrelatedspecieshaveemergedfromthiswork.a. Inplants,importantspeciesdifferencesincludethesuitesoftraitsthatattractpollinators(color,shape,odor,nectarrewards).Monkeyflowersareanexample,withpollinatorsrangingfromhummingbirdstobees,andcorrespondingchangesin

traits(Figure14.14). i.Mimuluscardinalishasredcolor,deepnectar,tubeswithabundantnectar,andreflexedpetals.Itisahummingbird(蜂鳥)-pollinatedspecies. ii.M.lewisiihaspinkflowers,broadpetals,andlittlenectarreward.Itisabee-pollinatedspecies.b. CrossesbetweenthesespeciesshowthatatleastoneQTLcontrols25%ofphenotypicvariationinpollinatorattractionandefficiencytraits(Figure14.15).c. Insomecases,importantphenotypicshiftscorrelatewithmutationsinsingleloci.

Fig.14.14Mimuluslewisii(A,C)andM.cardinalis(B,D)flowers

Fig.14.15QTLmapsfor12floraltraitsinmonkeyflowersPeterJ.Russell,iGenetics:Copyright?PearsonEducation,Inc.,publishingasBenjaminCummings.5. QTLapproacheshavebeenusedextensivelyincorn,tomato,andthefruitfly.Examples:a. Incorn,thetb1(teosintebranched1)QTLcontrolsthenumberof

axillarybranches,distinguishingcornfromitswildrelative,teosinte.tb1isaDNA-bindingtranscriptionalregulatorthatsuppressesgrowthinspecifictissues.b. Intomatoes,thefw2.2QTLcontrolsupto30%ofthefruitweightdifferencebetweencultivatedandwildspecies. i.Expressedearlyinfruitdevelopment,variationinfw2.2allelesaffectstimingandlevelsofgeneexpression,butnotmajordifferencesinproteinsequence. ii.Thesmallfruitallelealsocausesapleiotropiceffect,asisogeniclineshomozygousforthealleleproducemorefruitsthandoplantshomozygousforthelargefruitallele.6. QTLmaysometimesbefoundbytestingforassociationsbetweenphenotypicdifferencesandallelicvariation