复杂性状遗传课件

2022/12/23InheritanceofComplexTraits1Chapter10

InheritanceofComplexTraits2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits2ChapterOutlineComplexPatternsofInheritanceStatisticsofQuantitativeGeneticsAnalysisofQuantitativetraitsQuantitativeGeneticsofHumanBehavioralTraits2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits3航空报国英模：罗阳2012年11月24日舰载机成功起降“辽宁”号2012年11月25日总指挥罗阳因心肌梗死逝世原因何在？2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits4CardiovascularDisease:ACombinationofGeneticandEnvironmentalFactors2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits5Section1

ComplexPatternsofInheritanceBreedingexperimentsandcomparisonsbetweenrelativesrevealthatcomplexphenotypesmaybeinfluencedbyacombinationofgeneticandenvironmentalfactors.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits61.QuantitativeTraitsNatureisfullofexamplesoftraitsthatvarycontinuously—bodysize,height,weight,theactivityofaparticularenzyme,reproductiveability,andgeneralvigorandfitness,tonamejustafew.Theseareamongthemostinterestingtraitsinbiology,especiallyfromanevolutionarystandpoint.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits7Onephenotypeseemstoblendimperceptiblyintothenext;however,ifweexaminemanyindividualsinapopulation,weusuallyfindsignificantdifferencesamongthem.Oftenthistypeofvariationcanbequantifiedbymeasuringthetraitinasampleofindividualsfromthepopulation.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits8Wemight,forexample,capturemiceinabarnandweightthemorcollectcorncobsfromafieldandcountthenumberofkernelsoneach.Withsuchaquantitativeapproach,thephenotypeofeveryindividualinthesampleisreducedtoanumber.Thesenumberscanbeanalyzedwithavarietyofstatisticaltechniques,enablingustostudythetraitand,inparticulartoinvestigateitsgeneticbasis.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits9Traitsthatareamenabletothiskindoftreatmentarecalledquantitative

traits(数量性状).Theiressentialcharacteristicisthattheycanbemeasured.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits10Ifcanwelinktheevolutionwithgenetics?2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits11AgrowingbodyofevidencesuggestedthatthecomplexinheritanceofquantitativetraitscouldbeexplainedbyMendeliantheory.Twoinsightsmadethispossible:Quantitativetraitsareinfluencedbymanygenes.Quantitativetraitsareinfluencedbymanyfactorsinenvironment.Itisthecombinationofgeneticandenvironmentalinfluencesthatmakesquantitativetraitsdifficulttostudy.Thesetraitsdependonacomplexmixoffactors;thatis,theyaremultifactoral.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits12WilhelmJohannsen

1857-1921

Oneofthefirstpeopletoshowthatvariationinaquantitativetraitisduetogeneticandenvironmentfactors.Hestudiedtheweightofseedsfromthebroadbean,Phaseolusvulgaris(蚕豆).Amongtheplantsavailabletohim,seedweightvariedfrom150mgto900mg.Heestablishedlinesfromindividualseedsacrossthisrangeandmaintainedeachlinebyself-fertilizationforseveralgeneration.丹麦生物学家基因概念的提出者2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits13Johannsenobservedthatseedweightalsovariedwithineachofthepurelines.Thisresidualvariationinthistraitisduetogeneticdifferencesbecauseeachlinehadbeensystematicallyinbredtomakeithomozygousforitsgenes.Rather,itmusthavebeenduetovariationinuncontrolledfactorsintheenvironment.Hisworkledtotherealizationthatphenotypicvariationinaquantitativetraithastwocomponents—onegenetic,theotherenvironmental.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits14H.Nilsson-EhleHeprovidedevidencethatthegeneticcomponentofthisvariationcouldinvolvethecontributionsofseveraldifferentgenes.Nilsson-Ehlestudiedcolorvariationinwheatgrains.Whenhecrossedawhite-grainedvarietywithadarkred-grainedvariety,heobtainedanF1withanintermediateredphenotype.Self-fertilizationoftheF1producedanF2withsevendistinctclasses,rangingfromwhitetodarkred.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits15Inheritanceofgraincolorinwheat.Threeindependentlyassortinggenes(A,B,andC)areassumedtocontrolgraincolor.Eachgenehastwoalleles.Theallelesforpigmentation(uppercaseletter)aresemidominantovertheallelesforlackofpigmentation(lowercaseletters).这就能用多基因的分离和组合来解释2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits16TheAmericangeneticistEastextendedNilsson-Ehle’sstudiestoatraitthatdidnotshowsimpleMendelianratiosintheF2.Eaststudiedthelengthenofthecorollaintobaccoflowers.Inonepureline,thecorollalengthaveraged41mm;inanother,itaveraged93mm.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits172022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits18Withineachpureline,Eastobservedsomephenotypicvariation—preassemblytheresultofenvironmentalinfluences.Bycrossingthetwolines,EastobtainedanF1thathadintermediatecorollalengthandapproximatelythesameamountofvariationthathadseenwithineachoftheparentalstrains.WhenEastintercrossedtheF1plants,heobtainedanF2withaboutsamecorollalength,onaverage,thathesawintheF1;however,theF2plantsweremuchmorevariablethantheF1.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits19Thevariabilitywasduetotwosources:thesegregationandindependentassortmentofdifferentpairsofallelescontrollingcorollalength,andenvironmentalfactors.EachinbredsomeoftheF2plantstoproduceanF3andobservedlessvariationwithinthedifferentF3linesthanintheF2.ThereducedamountofvariationwithintheF2lineswaspresumablyduetothesegregationoffewerallelicdifferences.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits20Thus,thecomplexinheritancepatternthatEastobservedwithcorollalengthcouldbeexplainedbyacombinationofgeneticsegregationandenvironmentalinfluences.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits21HowmanygeneswereinvolvedindeterminingcorollalengthinEast’sstrainsoftobacco?WecanmakeacrudeguessbycomparingtheF2plantswitheachoftheinbredparentalstrains.Let’ssupposethatthestrainwiththeshortercorollaswashomozygousforonesetofallelesandthatthestrainwiththelongercorollaswashomozygousforanothersetofalleles.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits22Furthermore,let’ssupposethatthelong-corollaallelesaresemidominant,thatalllength-controllinggenesassortindependently,andthateachgenemakesanequalcontributiontothephenotype.Ifcorollalengthweredeterminedbyonegene,withallelesa(forshortcorolla)andA(forlongcorolla),wewouldexpect¼ofF2tohaveshortcorollas(liketheshortparentalstrain)and¼tohavelongcorollas(likelongparentalstrain).2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits23Iftwogenesdeterminedcorollalength,wewouldexpect1/16toresemblethelong-corollaparent.Ifthreegeneswereinvolved,thefrequencyofeachparentaltypeinF2wouldbe1/64,andiffourgeneswereinvolved,itwouldbe1/256.Withfivegenes,theparentalfrequenciesintheF2wouldeachbe1/1024.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits24Eaststudied444F2plantsandfailedtofindevenonewitheitheroftheparentalphenotypes.Thisfailurewouldseemtoruleoutthehypothesisoffourorfewergenescontrollingcorollalength.Thus,wecanconcludethatatleastfivegenesareresponsibleforthedifferenceincorollalengthbetweenEast’stwoinbredstrains.至少有5个基因参与了花冠长度的调控！2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits252.ThresholdTraits(阈性状)Continuouslyvaryingtraitssuchascorollalength,graincolor,andbeansizearecontrolledbymultiplefactors,bothgeneticandenvironmental.Geneticistshavefoundthatsomediscontinuoustraitsalsoappeartobemultifactorial.Forexample,somehumansarebornwithcleftlip,aconditionduetoanerrorinembryologicaldevelopment.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits26Althoughfamilystudieshavesuggestedthatcleftlipiscausedbygeneticandenvironmentalfactors,itisnotaquantitativetraitintheusualsense;individualseitherexpressitortheydonot.Nonetheless,thegeneticandenvironmentalfactorsthatpredisposeanindividualtoshowitappeartovaryquantitatively.Thus,although

cleftlipismanifesteddiscontinuously,itsunderlyingvariationseemstobecontinuous.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits27Geneticiststheorizethatwhentheunderlyingvariableexceedsacertainlevel,thetraitappears.Thistypeoftraitisthereforecalledathresholdtrait.当潜在的变量超出一定水平的时候，性状就会表现出来，这就是阈性状Amodelforexpressionofathresholdtrait.Whentheunderlyingvariable,the“liability(易患性),”reachesathresholdvalue,thetraitisexpressed.Thisvariableisassumed

tobecontinuouslydistributedinthepopulation.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits28Thereis,ineffect,agenetic“liability”todevelopthecondition.However,onlythoseindividualswhoseliabilitycrossesthethresholdareactuallyatrisk.Theunderlyingvariableisafunctionoffactorsinthegenotypeaswellasfactorsintheenvironment.Someindividualscarryallelesthatpredisposethemtodevelopcleftlip.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits29Inhumans,theevidencethattraitssuchascleftlipareinfluencedbygeneticfactorscomesfromcomparisonsbetweenrelatives,especiallytwins.Occasionallyafertilizedhumaneggsplitsandformstwogeneticallyidenticalzygotes.Theindividualswhodevelopfromthesezygotesarereferredtoasone-egg,ormonozygotic(MZ),twins;theyshare100%oftheirgenes.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits30单羊膜腔2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits31Morefrequently,twoindependentlyfertilizedeggsdevelopatthesametimeinthemotherwombs.Thesetwo-egg,ordizygotic(DZ),twinsarecloselyrelatedasordinarysiblings;thus,theyshare50%oftheirgenes.Becauseoftheirgeneticidentity,wewouldexpectMZtwinstobephenotypicallysimilarthanDZtwins.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits32Similaritywithrespecttoatraitlikecleftlipisassessedbydeterminingtheconcordancerate(一致率)—thefractionoftwinpairsinwhichbothtwinsshowthetraitamongpairsinwhichatleastoneofthemdoes.Forcleftlip,theconcordanceratehasbeenestimatedtobeabout40%forMZtwinsandabout4%forDZtwins.ThemuchgreaterconcordancerateforMZtwinsstronglysuggeststhatgeneticfactorsinfluenceanindividual’slikelihoodofbeingbornwithcleftlip.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits33Mentalillnessessuchasschizophreniaandmanic-depressivedisordercanalsoberegardedasthresholdtraits.Forschizophrenia,theconcordanceraterangesfrom30to60%forMZtwinsandfrom6to18%forDZtwins;formanic-depressivedisorder,theconcordancerateis70-80%forMZtwinsandabout20%forDZtwins.Thus,twinstudiessuggestthatbothofthesementalillnessareinfluencedbygeneticfactors.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits34KeyPointsResemblancesbetweenrelativesandresponsestoselectivebreedingindicatethatcomplextraitshaveageneticbasis.SomecomplextraitscanbequantifiedtopermitgeneticanalysisbasedonMendelianprinciples.Manygeneticandenvironmentalfactorsinfluencethevariationobservedinquantitativetraits.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits35KeyPointsPhenotypicsegregationsmayprovideawaytoestimatethenumberofgenesthatinfluenceaquantitativetrait.Traitsthataremanifestedwhenanunderlyingcontinuousvariable(theliability)reachesathresholdvaluemaybeinfluencedbygeneticfactors.Inhumans,evidencethatathresholdtraithasageneticbasiscomesfromstudieswithtwins.Theconcordancerateisthefractionoftwinpairsinwhichbothtwinsshowatraitamongpairsinwhichatleastoneofthemdoes.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits36Section2

StatisticsofQuantitativeGeneticsThefrequencydistributionsofquantitativetraitscanbecharacterizedbysummarystatistics.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits37Thehallmarkofquantitativetraitsisthattheyvarycontinuouslyinapopulationofindividuals.Thistypeofvariationposesaformidableproblemforthegeneticist.Segregationratiosaredifficult,ifnotimpossible,todiscernbecausethenumberofphenotypesislargeandonephenotypeblendsimperceptiblyintothenext.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits38Forquantitativelyvaryingtraits,routinegeneticanalysesofthesortthatwehavedonewithseedcolorinpeasandwithhumandisorderssuchasalbinismareoutofthequestion.Forthesetypesoftraitswemustresorttoadifferentkindofanalysis,onethatisbasedonstatisticaldescriptionsofphenotypeinapopulation.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits391.FrequencyDistributionThefirststepinthestudyofanyquantitativetraitistocollectmeasurementsofthetraitfromindividualsinapopulation.Usually,onlyasmallfractionofalltheindividualsinthepopulationcanbemeasured.Wecallthisgroupthesample(样本).Thedatafromthesamplecanbepresentedgraphicallyasafrequencydistribution(频数分布).2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits40Frequencydistributionsanddescriptivestatisticsoftimetomaturityinfourpopulationsofwheat.AandBareinbredstrainsthatwerecrossedtoproduceF1hybrids.TheF1plantswereintercrossedtoproduceanF2.Seedfromallfourpopulationswasplantedinthesameseasontodeterminethetimetomaturity.Ineachcase,datawereobtainedfrom40plants.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits412.TheMeanandtheModalClassTheessentialcharacteristicsofafrequencydistributioncanbesummarizedbysimplestatisticscalculatedfromthedata.Oneofthesesummarystatisticsiscalledthemeanoraverage=(Xk)/n2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits42Themodalclass(众数组)inasamplethatcontainsthemostobservations.Ineachofthedistributionsthemeaniswithinorveryclosetothemodelclass.Likethemean,italsocapturesthe“center”ofthedistribution.Thiscoincidencereflectsthesymmetryofthedistributions;ineachcase,roughlyequalnumbersareaboveandbelowthemeanandthemodalclass.Notalldistributionshavethisfeature.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits43Someareskewed(偏离的),withmostoftheobservationsclusteredatoneendandonlyafewattheotherendformingalongtail.Statisticianshavedevelopedanextensivetheoryaboutaparticulartypeofsymmetricaldistributioncalledanormaldistribution(正态分布).Inthisbell-shapeddistribution,themeanandthemodalclassarelocatedexactlyinthecenter.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits443.ThevarianceandtheStandardDeviationThedatainafrequencydistributioncouldbedispersed,ortheycouldbeclustered.Tomeasurethespreadofdatainafrequencydistribution,weuseastatisticcalledvariance(方差).Datathatarewidelydispersedproducealargevalueforthevariance,whereasdatathataretightlyclusteredproduceasmallvalue.s2=(X-)2/(n-1)s2=(X-

2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits45Weshouldnotetwofeaturesofthevariance.Firstly,itmeasuresthedispersionofthedataaroundthemean.Whenwecalculatethevariance,wetakethemeantobecentralvalueofthedistributionsinthesample.Second,thevarianceisalwayspositive.Whenwecalculatethevariance,wesquarethedifferencebetweeneachobservationandthemean,andthensumthesquareddifferences.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits46Althoughthevariancehasdesirablemathematicalproperties,itisdifficulttointerpretbecausetheunitsofmeasurementaresquared.Consequently,anotherstatistic,calledthestandarddeviation,isoftenusedtobedescribethevariabilityofasample.Thestandarddeviation(s)isthesquarerootofthesamplevariance.S=2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits47KeyPointsThemean(=(X)/n)andmodalclasspointtobecenterofafrequencydistribution.Thevariance(s2=(X-)2/(n-1)andstandarddeviations=arestatisticsthatindicatetheextenttowhichdataarescatteredaroundthemeaninafrequencydistribution.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits48Section3

AnalysisofQuantitativetraitsQuantitativegeneticistsfocustheiranalysesonphenotypicvariabilityasmeasuredbythevariance.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits49Thekeyideainquantitativegeneticsisthattraitsarecontrolledbymanydifferentfactorsintheenvironmentandinthegenotype.Fisherhypothesizedthataparticularvalueofaquantitativetrait,T,istheresultofthecombinedinfluenceofgeneticandenvironmentalfactors.1TheMultipleFactorHypothesis2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits50Herepresentedtheeffectsofthesefactorsasdeviationsfromtheoverallpopulationmean:T=+g+eInthisequation,therepresentsthepopulationmean,

grepresentsthedeviationfromthegeneticfactors,anderepresentsthedeviationfromthemeanthatisduetoenvironmentalfactors.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits51Quantitativephenotypesandthedeviationsofindividualmeasurementsfromthepopulationmean.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits52SomefactorsproducelargevaluesofT,andsomeproducesmallvalueofT.Foreachindividual,thesefactorsaredifferent.Furthermore,Fisheremphasizedthatamultitudeoffactorsareinvolved.Hehypothesizedthatmanygenescontributetoaquantitativetrait,andheassumedthatmanygenescontributetoaquantitativetrait,andheassumedthatmanyaspectoftheenvironmentalsomakecontributions.Atraitthatiscontrolledbymanygenesispolygenic(多基因的).2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits532PartitioningthePhenotypicVariance(表型方差的分割)VT=Vg+VeVe=(VA+VB+VC)/3=(1.92days2+2.05days2+2.88days2)/3=2.28days2Vg=VT–Ve=14.26days2–2.28days2=11.98days22022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits54VT=VG+Ve14.26days2=11.98days2+2.28days2Formthispartition,weseethatmostofthevarianceinmaturationtimeintheF2wheatpopulationisduetogeneticdifferencesamongtheindividuals.ThisgeneticvariabilityarosefromthesegregationandassortmentofgeneswhentheF1plantsreproduced.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits553.Broad-SenseHeritability

(广义遗传力)Oftenitisinformativetocalculatetheproportionofthetotalphenotypicvariancethatisduetogeneticdifferencesamongindividualsinapopulation.Thisproportioniscalledthebroad-senseheritability,symbolizedH2.IntermsofFisher’svariancecomponents,H2

+Vg/VT

=Vg/(Tg+Ve)2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits56Broad-senseheritability:Inquantitativegenetics,theproportionofthetotalphenotypicvariancethatisduetogeneticfactors(因遗传因素导致的全部表型方差的部分).Becauseofthewayitiscalculated,thebroad-senseheritabilitymustliebetween0and1.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits57Ifitiscloseto0,littleoftheobservedvariabilityinthepopulationisattributabletogeneticdifferencesamongindividuals.Ifitiscloseto1,mostoftheobservedvariabilityisattributabletogeneticdifferences.Thebroad-senseheritabilitythereforesummarizestherelativecontributionsofgeneticandenvironmentalfactorstotheobservedvariabilityinapopulation.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits58However,itisimportanttonotethatthisstatisticispopulation-specific.Foragiventrait,differentpopulationsmayhavedifferentvaluesofthebroad-senseheritability.Thus,thebroad-senseheritabilityofonepopulationcannotautomaticallybeassumedtorepresentthebroad-senseheritabilityofanotherpopulation.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits59IntheF2wheatpopulation,H2=11.98/14.26=0.84.Thisresulttellsusthatinthispopulation84%ofobservedvariabilityinwheatmaturationtimeisduetogeneticdifferencesamongindividuals.However,itdoesnottelluswhatthesedifferencesare.Thegeneticvarianceuponwhichthebroad-senseheritabilitydependsincludesallthefactorsthatcausegenotypestohavedifferentphenotypes:theeffectsofindividualalleles,thedominancerelationshipsbetweenalleles,andtheepistaticinteractionsamongdifferentgenes.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits604.Narrow-SenseHeritability

(狭义遗传力)Theabilitytomakepredictionsinquantitativegeneticsdependsontheamountofgeneticvariationthatisduetotheeffectsofindividualalleles.Geneticvariationthatisduetotheeffectsofdominanceandepistasishaslittlepredictivepower(由于显性和上位性的遗传方差很少具可预期的能力).2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits61Narrow-senseheritability:Inquantitativegenetics,theproportionofthephenotypicvariancethatisduetotheadditiveeffectsofalleles(表型方差是由于等位基因的附加效应所致的部分).2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits62Toseehowdominancelimitstheabilitytomakepredictions,considertheABObloodtypesinhuman.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits63Thistraitisdeterminedstrictlybythegenotype;environmentalvariationhasessentiallynoeffectonthephenotype.However,becauseofdominance,twoindividualswiththesamephenotypecanhavedifferentgenotypes.Thus,althoughwecanmakesomekindofpredictionaboutthechild’sphenotype,dominancepreventsusfrommakingapreciseprediction.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits64Ourabilitytomakepredictionsaboutanoffspring’sphenotypeisimprovedinsituationswherethegenotypesarenotconfusedbydominance.Consider,forexample,theinheritanceofflowercolorinthesnapdragon.环境因素对金鱼草的花色也没有影响影响花色的原因是门德尔的分离律2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits65Quantitativegeneticistsdistinguishbetweengeneticvariancethatisduetoallelesthatactadditivelyandgeneticvariancethatisduetodominance.Thesedifferentvariancecomponentsaresymbolizedas:Va=additivegeneticvarianceVd=dominancevarianceVi=epistaticvarianceEpistaticinteractions,likedominance,areoflittlehelpinpredictingphenotype.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits66Altogether,thesethreevariancecomponentsconstitutethetotalgeneticvariance:Vg=Va+Vd+ViIfwerecallthatVT=Vg+Ve,wecanexpressthetotalphenotypicvarianceasthesumoffourcomponents:VT=Va+Vd+Vi+VeOfthesefourvariancecomponents,onlytheadditivegeneticvariance,Va,isusefulinpredictingthephenotypesofoffspringfromthephenotypesoftheirparents.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits67Thisvariance,asafractionofthetotalphenotypicvariance,iscalledthenarrow-senseheritability,symbolizedh2.Thus,h2=Va/VTLikethebroad-senseheritability,h2liesbetween0and1.Thecloseritistoone,thegreateristheproportionofthetotalphenotypicvariancethatisadditivegeneticvariance,andthegreaterisourabilitytopredictanoffspring’sphenotype.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits68Humanstatureishighlyheritable,butlittersize(每窝的产子数)inpigsisnot.Thus,ifweknewtheparentalphenotypes,wewouldbebetterabletopredicttheheightofahuman’soffspringthanlittersizeofapig’soffspring.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits695.PredictingPhenotypesPredictinganoffspring’sphenotypebasedonthephenotypesofitsparentsandthenarrow-senseheritabilityoftrait.Onlyaportionofthedeviationofthemidparent(Tp)valuefromthepopulationmeanisheritable.Themagnitudeofthisportionisdeterminedbythenarrow-senseinheritability.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexTraits70Michael(M)andFrances(F)havetakenastandardizedintelligencetest,andtheirIntelligenceQuotients(IQs)havebeendetermined.Michael’sscoreis110andFrances’scoreis120.ThemeanIQscoreinthepopulationis100.MichaelandFranceshasaninfantsonOswald(O),whowasgivenupforadoptionwhenhewasborn,andadoptiveparents(养父母)wishtopredictOswald’sIQ.2022/12/17InheritanceofCompl2022/12/23InheritanceofComplexT