MendelianGeneticsAnOverviewPeaplantshaveseveraladvantagesforgenetics.Peaplantsareavailableinmanyvarietieswithdistinctheritablefeatures(characters)withdifferentvariants(traits).AnotheradvantageofpeasisthatMendelhadstrictcontroloverwhichplantsmatedwithwhich.Eachpeaplanthasmale

(stamens)andfemale

(carpal)sexualorgans.Innature,peaplantstypically

self-fertilize,fertilizingova

withtheirownsperm.However,Mendelcouldalso

movepollenfromoneplant

toanothertocross-pollinate

plants.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsFig.14.1Inatypicalbreedingexperiment,Mendelwouldcross-pollinate(hybridize)twocontrasting,true-breedingpeavarieties.Thetrue-breedingparentsarethePgenerationandtheirhybridoffspringaretheF1generation.MendelwouldthenallowtheF1hybridstoself-pollinatetoproduceanF2generation.ItwasmainlyMendel’squantitativeanalysisofF2plantsthatrevealedthetwofundamentalprinciplesofheredity:thelawofsegregationandthelawofindependentassortment.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsIftheblendingmodelwerecorrect,theF1hybridsfromacrossbetweenpurple-floweredandwhite-floweredpeaplantswouldhavepalepurpleflowers.Instead,theF1hybrids

allhavepurpleflowers,

justapurpleasthe

purple-flowered

parents.2.Bythelawofsegregation,thetwoallelesforacharactersarepackagedintoseparategametesCopyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsFig.14.2WhenMendelallowedtheF1plantstoself-fertilize,theF2generationincludedbothpurple-floweredandwhite-floweredplants.Thewhitetrait,absentintheF1,reappearedintheF2.Basedonalarge

samplesize,Mendel

recorded705

purple-floweredF2

plantsand224

white-floweredF2

plantsfromthe

originalcross.Copyright?2002PearsonEducation,Inc.,publishingasBenjaminCummingsFig.14.2VocabularyCharacter–heritablefeatureTrait–eachvariantforacharacterTrue-breeding–plantsthatself-pollinatealloffspringarethesamevarietyMonohybridcross–acrossthattrackstheinheritanceofasinglecharacterPgeneration–(parental)true-breedingF1-(firstfilial)offspringofPgenerationF2–(secondfilial)offspringfromF1crossVocabulary(continued)Allele-alternateversionofageneDominateallele–expressedintheheterozygoteRecessiveallele–notexpressedintheheterozygoteHomozygote–pairofidenticalallelesforacharacterHomozygousdominant-BBHomozygousrecessive-bbHeterozygote–twodifferentallelesforacharacter(Bb)Genotype–geneticmakeupPhenotype–appearanceofanorganismLawofSegregation-thetwoallelesforeachcharactersegregateduringgameteproduction

LawofIndependentAssortment–Eachsetofallelessegregatesindependently

Testcross–designedtorevealthegenotypeofanorganism

MendelianInheritanceandRulesofProbabilityRuleofMultiplication–theprobabilitythattwoeventswilloccursimultaneouslyistheproductoftheirindividualprobabilitiesProbabilitythataneggfromtheF1(Pp)willreceivep=?ProbabilitythatanspermfromtheF1(Pp)willreceivep=?Probabilitythataofoffspringreceivingtworecessiveallelesduringfertilization?x?=?RuleAppliestodihybridCrossesForadihybridcross,YyRrxYyRr,whatistheprobabilityofanF2havingthegenotypeYYRR?Gopage267andwork#9and#10IncompleteDominancePage267andwork#2CodominanceTwoallelesaffectthephenotypeinseparateanddistinguishableways.Neitherallelecanmasktheotherandbothareexpressedintheoffspringandnotinan“intermediate”form.Example:redflowersthatarecrossedwithwhiteflowersthatyieldredandwhiteflowers.1)Incattle,roancoatcolor(mixedredandwhitehairs)occursintheheterozygous(Rr)offspringofred(RR)andwhite(rr)homozygotes.Whentworoancattlearecrossed,thephenotypesoftheprogenyarefoundtobeintheratioof1red:2roan:1white.Whichofthefollowingcrossescouldproducethehighestpercentageofroancattle?A)roanxroanB)redxwhiteC)whitexroanD)redxroanE)Alloftheabovecrosseswouldgivethesamepercentageofroan.MultipleAllelesPage267andwork#6PleiotropyMostgeneshavemultiplephenotypiceffects.Theabilityofagenetoaffectanorganisminmanywaysiscalledpleiotropy.EpistasisEpistasisoccurswhenageneatonelocusaltersorinfluencestheexpressionofageneatasecondloci.Inthisexample,Cisforcolorandthedominateallelemustbepresentforpigment(color)tobeexpressed.PolygeneticInheritanceQualitativevariationusuallyindicatespolygenicinheritance.Thisoccurswhenthereisanadditiveeffectfromtwoormoregenes.Pigmentationinhumansiscontrolledbyatleastthree(3)separatelyinheritedgenes.