MendelianGenetics

GregorMendel(1822-1884)wasanAustrianmonkwhodiscoveredthebasicrulesofinheritance.From1858to1866,hebredgardenpeasinhismonasterygardenandanalyzedtheoffspringofthesemating’s.

LawofDominance

WhenMendelcrossedtwotrue-breedingpeaplantsthatshowedcontrastingtraits,onlyoneofthetraitswereevidentintheoffspring.Forexample,ifhecrossedapurplefloweredpeaplantwithawhitefloweredplant,alltheoffspringwerepurple.Thispurpletraitwasdescribedasdominant.Thewhitetraitwasdescribedasrecessive.Thelawofdominancestatesthatwhendominantandrecessiveallelesexisttogetherthedominanttraitisexpressedwhiletherecessiveismaskedandnotexpressed.

Thepure-breedingparentgenerationiscallthePgeneration.TheoffspringofthecrossesbetweenmembersofthePgenerationarecalledthefirstfilialorF1generation.CrossesbetweentheF1generationsproducetheF2generationandsoon.

Dominantalleles(genes)ofatraitareusuallysymbolizedbyacapitalitalicizedletter(P=purple)andrecessiveallelesofthesametraitaresymbolizedbyalowercaseitalicletter(p=white).Inapreviouslessonwelearnedthatfertilizationbringstogethertwohomologouschromosomesthatwereseparatedduringmeiosis.Theresultingzygotewillusuallyhavetwoalleles(genes)foragiventrait.Whenthetwoinheritedallelesarealike(PPorpp)theyaredescribedashomozygous.Whenthetwomembersofanallelicpairaredifferent(Pp)theyaretermedhybridorheterozygous.Thephysicalappearanceoftheoffspringisreferredtoasthephenotypeandthegenecombinationsarereferredtoasthegenotype.

Usethefollowingcrosstoanswerthequestionsthatfollow.

{PPxpp}

a. WhyisthiscrossreferredtoasthePgeneration?

b. Thegenotypeoftheplantswithpurpleflowersmaybe_______________

c. Thegenotypeofthewhiteflowersis_______________________

d. Theoffspringofthecrossmakeupthe_______________generation.

e. ThephenotypeandgenotypeoftheF1generationis___________________________and_____________

f. Theoffspringinthiscrossarealsoreferredtoas____________________________.

g. Ifallthemonohybridoffspringhavepurpleflowers,howdoesillustratethelawof

dominance?________________________________________________________________________________________________

______________________________________________________________________________________________________________

LawofSegregation

Inanotherexperiment,MendelallowedthehybridmembersoftheF1generationofpurple(Pp)flowerstoundergoself-fertilization(PpxPp).Thisisalsoknownasamonohybridcross.TheoffspringofthecrossarereferredtoastheF2generation.

1. PerformtheF1crossandprovidethephenotypesandgenotypesoftheF2offspring.Determinegenotypicandphenotypicratios.PhenotypicRatio

GenotypicRation

Hopefullyyourealizedthatinamonohybridcross,75%oftheF2generationshowedthedominanttrait(purpleflowers)and25%showedtherecessivetrait(whiteflowers).ThisillustratesthatalthoughtheF1generationexpressedthedominanttrait,theystillcarriedtherecessiveallele.Thelawofsegregationstatesthatthetwoallelesforaheritabletraitseparate(segregate)duringgameteformationandendupindifferentingametes.Thereforetheeggandspermreceiveonlyonealleleforagiventrait.

Questions

1.Howdoesthelawofsegregationrelatetohomologouschromosomes?

______________________________________________________________________________________________________________________

______________________________________________________________________________________________________________________

______________________________________________________________________________________________________________________

F1generation(GgxGg)

G=Good

g=Bad

Questions

1. Whatisthephenotypeandgenotypeoftheaboveoffspring?

2. Whichgenerationdoestheoffspringbelongto?

3. Whatisthenamegiventothiscross?

4. UseaPunnettsquaretocrossthefollowing(TTxTt)T=Tall,t=short.Givethephenotypeandgenotypeoftheoffspring.

(TTxTt)

5. WhydoestherecessivetraitNOTshowupintheoffspring?

TestCross

Atestcrossisamethodfordeterminingwhetheranorganismthatshowsadominanttraitishomozygousorheterozygous.Inatestcross,theorganismwiththeunknowngenotypeiscrossedwithapurerecessiveforthattrait.Thecrossisdonewithapurerecessivebecauseweknowthatgenotypeforsure.Ifnoneoftheoffspringexpresstherecessivetraitthentheunknownorganismwashomozygous.

Questions

1.Inguineapigs,blackcoatcolorisdominantoverwhite.Yourfriendgivesyouablackguineapigforyourbirthday.Howcouldyoudiscoverwhethertheguineapigishomozygous(BB)orheterozygous(Bb)?

UsethePunnettsquaresbelowtoillustratehoweachtestcrossmaydiffer.Whichistheheterozygousguineapig?

LawofIndependentAssortment

Mendelderivedthelawofsegregationbyperformingbreedingexperimentsinwhichheonlyfollowedasingletraitsuchasflowercolororplantheight.AlltheF1progenyweremonohybrids,indicatingthattheywereheterozygousforasingletrait(Pp).Crossingthisgenerationisreferredtoasamonohybridcross. Mendelcontinuedhisworkbyfollowingtwotraitsatthesametime.Heinvestigatedthetraitsforseedcolorandseedshape.Peaseedsmaybeeitheryelloworgreenandtheymayberoundorwrinkled.PriorcrossesallowedMendeltoconcludethatyellow(Y)seedsweredominantovergreen(y)androundseeds(R)weredominantoverwrinkled(r).ThereforecrossingYYRRxyyrrwouldresultinalloftheoffspringwith____________________________genotypeand___________________and____________________phenotype.ThisF1generationwouldbereferredtoasdihybrid.Mendelwonderedwhetherthesetwotraitswouldbetransmittedtooffspringasa“packageddeal”.Isseedcolorandseedshapeinheritedindependentlyofoneanother?

Mendelallowedtheseheterozygousplantstoundergoself-fertilization,referredtoasa“dihybridcross”.Mendeldiscoveredthateachpairofallelessegregatesindependentlyofotherpairsofallelesduringgameteformation.*Thislawonlyappliestogenesonnon-homologouschromosomes.

Questions

1.Explainthelawofindependentassortmentinyourownwords?

______________________________________________________________________________________________________________________

____________________________________________________________________________________________________________________

1. Roundseeds(R)aredominantoverwrinkled(r)seedsandyellowseedsaredominant(Y)overgreenseeds(y).Aplantthatishybridforbothtraitsisallowedtoundergoself-fertilization(RrYyxRrYy).Rememberthatthegametesmayonlyhave1versionofeachallele.(i.e.RY).Ifyouusetheequation2n,wheren=#heterozygoustraitineachparent.Inthisexamplethereare_________possibleoutcomes.LetsFOILtodeterminethepossibleoutcomes.a. Thefourtypesofgametesformedare_________,_________,________,________b. CompletethePunnettsquarebelow.Givethephenotypesoftheoffspringandthepercentageof

each.Rememberthateachoutcomewillgooverornexttoeachbox.Thepotentialoffspringwillbeineachbox.Iliketousesymbolsforeachoutcome.

_____:_______:__________________and____________________

_____:_______:__________________and____________________

_____:_______:__________________and____________________

_____:_______:__________________and____________________

2.Ifthegenesforagiventraitwerelocatedonthesamechromosomes,wouldMendelhaveobservedtraitsassortingindependently?Whyorwhynot?______________________________________________________________________________________________________________________

______________________________________________________________________________________________________________________

______________________________________________________________________________________________________________________

______________________________________________________________________________________________________________________

ComplexGeneticsProblemswithRulesofProbability

Considerthefollowingdihybridcrosswithyellowseedsthatareround.Therecessiveconditionwouldbegreenseedsthatarewrinkled.(YyRrxYyRr).

1.WhatistheprobabilitythattheF2offspringwouldexhibittherecessivecondition?(yyrr)

YY= RR=

Yy= Rr=

yy= rr=

Onceyouhavecalculatedtheprobabilityofeach,multiplytheprobabilitiesofrrxyytoobtaintheanswer.Whatresultdidyouobtain?

1b.WhatistheprobabilityofhavingaplantintheF2generationwithYyRRgenotype?

1.Whatistheprobabilitythateachofthefollowingpairsofparentswillproducetheindicatedoffspring?

Ø AABBCCxaabbcc---------àAaBbCc

Ø AABbCcxAaBbCc---------àAAbbCC

Ø aaBbCCxAABbcc---------àAaBbCc

BeyondMendelianGeneticsforaSingleGene

Codominanceoccurswhenbothallelesarebothexpressedinthephenotype.TypeABbloodisanexampleofcodominancewherebothAandBantigensareexpressed.Anotherexampleisroancoatcattlewhereblackandwhitehairsarevisible.

Incompletedominanceoccurswhenthephenotypeisablendingofbothallelesintheparentgeneration.Forexample,crossingpurered(RR)andpurewhite(WW)four-o-clockflowersproduceflowersthatarepink.

Questions

1.Showacrossbetweenapurered(RR)snapdragonandapurewhite(WW)snapdragoniftheyshowincompletedominance.

2. WhataretheresultinggenotypesandphenotypesintheF1generation?Genotype=_____________Phenotype=_______________

3.WhataretheresultinggenotypesandphenotypesintheF2generation?Genotypes:Phenotypes:4.Showacrossbetweenapureredcow(RR)andapurewhitecow(WW)iftheyshowcodominance.

5.WhataretheresultinggenotypesandphenotypesintheF1generation?

Genotype=_____________Phenotype=_______________

6.WhataretheresultinggenotypesandphenotypesintheF2generation?Genotypes:Phenotypes:7.Howarecodominanceandincompletedominancedifferent?

MultipleAlleles

Alltraitsdiscussedsofarhavebeencontrolledbyasinglepairofalleles.However,therearesometraitssuchasbloodtypeinhumansthatinvolvemorethantwogenes.Humanbloodtypeinvolvesthreegenes-A,B,andO.GenesAandBaredominantwhileOIrecessive.ThefourhumanbloodtypesareA,B,ABandO.GenotypesformultipleallelesareshownusingthecapitalletterIfordominantandlowercaseiforrecessive.

Questions

1.Whatarethealleliccombinationsforthefollowingbloodtypes?

TypeA- TypeO -

TypeB- TypeAB-

2.AmanwithtypeABbloodmarriesawomanwith typeOblood.UsingaPunnettsquare,showall possiblegenotypesoftheiroffspring.

3.CouldamotherwithtypeBbloodandafatherwithtypeObloodproduceoffspringwithtypeABblood?Whyorwhynot?

4.AwomanwithtypeBbloodwhosemotherhad typeObloodmarriesamanwhoisheterozygousfor typeB.Whatistheprobabilitythattheiroffspring willhavetypeOblood?