Pop_Gen_II__Activity_2009

Pop_Gen_II__Activity_2009 - Presectionreading:,Ch7,p232236...

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Unformatted text preview: GeneticDriftSimulation Presectionreading:EvolutionaryAnalysis,Ch7,p232236 Geneticdriftischangeinthefrequencyofallelesinapopulationthatoccursdueto randomevents.Aprimarycauseofgeneticdriftinpopulationsissamplingerror. Samplingerroristhedifferenceinallelefrequencybetweentwogenerationsthat occursbecausegenesintheoffspringgenepoolarearandomsampleofgenesfrom theparentalgenepool.Thus,changesinallelefrequency(andphenotypefrequency) inapopulationcanoccurevenwhenthereisnonaturalselection. Overlongperiodsoftime,geneticdriftcausessomeallelestodrifttoafrequencyof 1inthepopulation(meaningallofthecopiesofthatgeneinthepopulationarethe same).Thisisfixation.Geneticdriftalsocausescompletelossofsomealleles, meaningtheygettoafrequencyofzeroandarenolongerpresentinthegenepool. Boththesizeofapopulationandthestartingallelefrequenciesinapopulationhavea strongaffectonhowfastdriftcausesfixationandloss. Todaywewillsimulatetheeffectofgeneticdriftinmouse populationsofdifferentsizesandwithdifferentstarting allelefrequencies.Wewilltesthowthesefactorsinfluence thepowerofdrifttocauseallelestofixorbecomelostfrom thepopulation.Eachteamwillsimulatereproductionina populationofmicebyfollowingthestepsbelow.Youwill trackthefrequencyofallelesthatcausecoatcolor.There aretwoallelesforcoatcolorinthemousepopulationgene pool,theDarkalleleandtheLightallele.Theseare representedbydarkbrownbeadsandwhite(orlightbrown)beadsinour simulation.(Thereisvariationinourpopulationsforothertraits,andyoumay noticeotherdifferencesbesidescolorbetweenthebeads,butinourpopulations thesedifferencesarenotcontrolledbytheDarkorLightallele.) YourTAwillassignyourteamapopulationtypetosimulate: PopulationtypeA:Small(8alleles)equalfrequencies(4white,4brown) PopulationtypeB:Small(8alleles)unequalfrequencies(2white,6brown) PopulationtypeC:Large(16alleles)equalfrequencies(8white,8brown) PopulationtypeD:Large(16alleles)unequalfrequencies(4white,12brown) (Beforeyougetstarted,eachpersoninthegrouptrytakingaguessastohowmany generationsyouthinkfixationinyourpopulationtypewilltake!) Simulation: 1) Createyourstartingpopulationgenepoolbyplacingalleles(beads)intothe bluecup.Recordyourstartingallelefrequenciesinthetablebelow,under generationzero. 2) Thefirstpersoninthegroupsimulatesthefirstbreedingseasoninthe population.Randomlychooseonebeadfromthestartingpopulation(blue cup).Thecolorofthisbeadrepresentsthecolorofthefirstallelethatmakes itintothenextgeneration'sgenepool.Thesecondpersoninthegroupaddsa beadofthatcolortotheredcupfromthepileofextrabeads. 3) ReplacetheoriginalbeadbackinthestartingpopulationandrepeatStep2. Continuedrawingandreplacingbeadsuntilthenextgeneration(redcup) hasthesamenumberofallelesasthepreviousgeneration. 4) StartagainfromStep2tosimulatethenextbreedingseason,butthistime, drawyourallelesfromthenewgenepoolyoujustcreated.Eachpersonin theteamtaketurnsdrawingallelestocreatethenextgeneration.Recordthe frequencyofeachalleleforeachgenerationofmiceyousimulateinthe tablebelow. 5) ContinuewiththesimulationuntileithertheDarkortheLightalleleforcoat colorfixesinthepopulation. 6) Ifoneallelegoestofixationafteronlyafewgenerations,repeatthe simulationafewtimesandseeifyougetthesameresults. Generation #DarkAlleles #LightAlleles 0(startingpopulation) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 (continuetableonbackifnecessary) Attheendofsection,wewillcompareaverageresultsforallfourtypesof populations.Recordtheresultsforoursectioninthetablebelow: Average#ofgenerationsforonealleletofix: Smallpopulation Largepopulation TypeA TypeC Equalallelefrequencies TypeB TypeD Unequalallelefrequencies Wewilldiscussourresultsasagroup.Usethisinformationtoanswerquestions fromthehomeworkassignmentduenextweek. ...
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This note was uploaded on 09/08/2009 for the course BIOEE 2780 taught by Professor Geber during the Spring '07 term at Cornell.

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