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WHATCANZEBRAFISHTELLUSABOUT HUMANSKINCOLOR? Educator Materials OVERVIEW InthefilmTheBiologyofSkinColor,Dr.NinaJablonskiproposedanexplanationforwhyhumansliving indifferentpartsoftheworldhavedifferentnaturalskincolors.Specifically,studentslearnedhow patternsinvariationfortheMC1Rgeneprovideevidencethatdarkskinisfavoredinenvironments thatexperienceintenseUVradiation.Thisactivityfocusesonadifferentgene,SLC24A5,whichplaysa largeroleintheexpressionoflighterskintonescommoninpeopleofEuropeandescent.Students exploredatageneratedfromgeneticstudiesinfishtohelpthemunderstandthefunctionand evolutionofSLC24A5. KEYCONCEPTSANDLEARNINGOBJECTIVES • Studiesofmodelorganismscanprovidevaluableinformationaboutgenefunctioninhumans. • Changestoagene’sDNAsequencecanaffectthetranslationofthegeneintoaminoacids, andultimately,thefunctionofaproteinandtheexpressionofatrait. • Geneticsandtheenvironmentcanaffectexpressionofatrait.Experimentssuggestthedegree towhichdifferencesintraitsareinherited.Differencesinhumanskincoloraremostly controlledbygenetics. • Bycomparinggenesandproteinsamongspecies,scientistscaninfercommonancestryand whenagiventraitevolved. • Patternsinallelefrequenciesamongpopulationscanindicateadaptationandwhetheratrait hasevolvedmorethanonce. Studentswillbeableto • constructexplanations,makepredictions,andproposehypothesesbasedonavailable information;and • userealdatapresentedinscientificfiguresandinformationfromthefilmtomakeevidencebasedclaims. SUGGESTEDAUDIENCE • Thislessonisappropriateforalllevelsofhighschoolbiology,includingAPandIB,aswellas introductorycollegebiology. www.BioInteractive.org PublishedAugust2016 Page 1 of 6 WHATCANZEBRAFISHTELLUSABOUT HUMANSKINCOLOR? Educator Materials CURRICULUMCONNECTIONS Curriculum Standards NGSS(April2013) Practices2,4,7; HS-LS1-1,HS-LS3-3,HS-LS4-1 APBiology(2012–2013) SciencePractices1,5; 1.A.2,1.A.4,3.A.1,3.C.1,4.C.2 IBBiology(2016) 3.1,5.1,5.2,7.2 KEYTERMS adaptation,allele,allelefrequency,gene,melanin,melanosome,phenotype TIMEREQUIREMENTS Thislessonwasdesignedtobecompletedwithinasingle50-minuteclassperiodbutmaytakelonger dependingontheamountofclassdiscussion.Thisdoesnotincludethenecessarytimetowatchthe shortfilm. PRIORKNOWLEDGE • StudentsshouldhaveabasicunderstandingofMendeliangenetics,includingthetermsDNA,gene, andallele,andknowthatvariationsinsometraitsareinherited. • Itwouldbehelpfulforstudentstohavepriorknowledgeofthetermsgenotypeandphenotypeand howtoapplythetermstospecificexamples. • Studentsneedabasicunderstandingofhowadaptationsarisethroughevolutionbynatural selection. TEACHINGTIPS • Havestudentswatchtheshort(19-minute)filmTheBiologyofSkinColorbeforecompletingthis activity.Ifyoudon’thavesufficientin-classtime,considerassigningitforhomework.Have studentswritedownanyquestionstheyhavewhiletheywatch.Runthroughsomeofthese questionsasawarm-uporasaconcludingdiscussion. • Beforebeginningthelesson,considerreviewinggenesandalleleswithstudents.Onewayto accomplishthisistoasktheminanopen-endedclassdiscussiontoeliciteverythingtheyknow aboutgenesandalleles.Writeeverythingstudentssaysoallcanseethelist.Attheendofthe brainstormingsession,highlightthefollowingoverarchingconcepts. www.BioInteractive.org PublishedAugust2016 Page 2 of 6 WHATCANZEBRAFISHTELLUSABOUT HUMANSKINCOLOR? Educator Materials o Genesareinherited.Genesarelocatedonchromosomes.Chromosomesareinheritedin pairs,onefromeachparent.Differentversionsofgenesarecalledalleles.Asinglegenecan havemanyalleles. o Genesaffectphenotypes.Genescodeforproteins,whicharecriticalforthousandsof functionswithincells.Theexpressionandactionofproteinsresultinthedistinguishable traitsofanorganism:itsphenotypes. • Theactivityhasbeenconstructedtominimizecopiesandtopromoteactivelearning.For implementation,consideraninterruptedapproachofinstructionorsmallteam-basedlearning groups.Foradiscussion-basedapproach,figurescouldbeprojectedandstudentscouldbegiven timetolookatthedataindividually,thenpairwithanotherstudent,andthenshareoutviawholegroupdiscussion.Anotherapproachcouldbetoprovidesmallgroupsoftwotofourstudentsa singlesetofreferencefigures(recommendusingcardstockandlaminatingifpossiblesothe referencesetscanbereused)andallowthemtohaveteam-baseddiscussionsaboutthedata. • Someofthedataconsideredinthisactivityistakenfromzebrafish.Forexample,studentsare presentedresultsfromastudythattransplantedahumanskin-colorgeneintoazebrafish.Takethe opportunitytoreinforcetheuniversalityofthegeneticcodeasevidenceforcommondescent. • Makesuretoemphasizethecriticalpointthatwhilemuchofthedataforskincolorpresentedin thisactivityfocusesondifferencesamongpeople,mostoftheothergeneticdatashowhowclosely relatedallhumansaretoeachotherandhowmuchpeoplehaveincommon. • Question4hasstudentsreflectingonwhetherallelefrequencydataisevidenceofselection, specifically,that96percentofindigenousEuropeanshaveoneallelefortheskin-colorgene SLC24A5,whileonly9percentofWestAfricansdo.Ifyourstudentsarefamiliarwithgenetics,you maytellthemthatsimulationsusingmathematicalmodelsshowthatitishighlyimprobable(less thana0.01percentchance)thatthesedifferencesinallelefrequencycouldhavearisenbygenetic driftalone. • Figure4,referencedinquestion8,indicatesthatchimpanzeesandotheranimalsallhavetheG alleleoftheSLC24A5gene,whichwouldsuggestthattheyhavehighlymelanizedskin—but certainlynotallofthemdo(forexample,chimpanzeeshavepaleskinunderneaththeirdarkfur).If itcomesup,pointouttostudentsthatonlyasmallportionoftheproteinisshownandother variationsmayexist.Further,skincolorispolygenic,sovariationsinmanyothergenesmaybe causingthedifferenceinskincolor,andtherecouldbeenvironmentaleffectsaswell. • Questions11through13areaboutdatacontainedintwodifferentmaps.Inquestion13,students areaskedtosynthesizethetwomapstoevaluatetheclaimthatlight-skinnedphenotypesevolved www.BioInteractive.org PublishedAugust2016 Page 3 of 6 WHATCANZEBRAFISHTELLUSABOUT HUMANSKINCOLOR? Educator Materials onlyonceinhumanhistory.Studentsshouldconcludethattheclaimisnotsupportedbythedata, butthatlight-skinnedphenotypesevolvedatleasttwice,onceamongEuropeansandonceamong Asians. VarioushumanpopulationsdispersedfromAfricainwaves.Forexample,somehumansmigrated outofAfricaandwenttoAsiaabout35,000yearsagoandevolvedlessmelanizedskininresponse tolowerlevelsofUVradiation.AdifferentgroupofhumanssettledinEuropeabout40,000years agoandalsoevolvedlessmelanizedskin.Thus,bothpopulationsevolvedsimilartraits(less melanizedskin)inresponsetoasimilarenvironmentalinfluence. BoththeEuropeanandAsiandispersalstookplacelessthan50,000yearsago.Bothpopulations camefromancestorsinAfrica.Thus,themovenorthinfluencednaturalselectiononbothEuropean andAsianpopulationsinarelativelyshortamountoftime.IntheEuropeanpopulation,theGallele ofSLC24A5wasstronglyselectedagainst,resultinginselectionoftheAalleleandlesspigmentin skin.BecausethisalleleremainsinhighfrequencyinAsianpopulations,someothergeneorgenes mustberesponsibleforthelowermelanincontentinAsianskin.EvidencefromtheOCA2Gallele frequencysupportsthisclaimbecauseitisassociatedwithlighterskinbutisessentiallyrestricted toindividualsindigenoustoEastAsia,orofEastAsianancestry. ANSWERKEY 1. Thecells(melanophores)inthewild-typezebrafisharelargerandcontainlargerandmore abundantmelanosomesandmuchmoremelanincomparedtothemelanophoresofthegolden zebrafish. 2. Answersmayvary.Anacceptableanswerwouldbethatthemelanincolorinthegolden zebrafishmelanosomeslookslikethemelaninfromthewomanwithlightskin. 3. TheimagesinFigure2showthatthesizeandnumberofmelanosomes,andtheamountof melanin,aredifferentinpeoplewithdifferentskincolors.Remindstudentsthatthefilm mentionedthataperson’sskincolorisdeterminedbythetypeandamountofmelaninintheir skin.“Thereddish-yellowpheomelaninismoreabundantinlightlypigmentedpeople.More darklypigmentedpeoplehavemoreofthebrown-blackeumelanin,andthemoreeumelanin, thedarkertheskin.” 4. DNAcodesforamessengerRNA(mRNA),whichcanbetranslatedintoaprotein.ThemRNAis readingroupsofthreetocodeforaspecificaminoacid.AchangeinaDNAsequencewill changethemRNAthatgetsformed,whichcouldresultinacodeforadifferentaminoacidand ultimatelyadifferentprotein. www.BioInteractive.org PublishedAugust2016 Page 4 of 6 WHATCANZEBRAFISHTELLUSABOUT HUMANSKINCOLOR? Educator Materials 5. Yes.ThedifferencesinallelefrequencyfortheSLC24A5AallelearesodramaticbetweenWest Africans(0.09)andEuropeans(0.96)thatselectionmustbeatplay.AlmostallEuropeanshave theAallele,whileitisextremelyrareamongWestAfricans.ThisisverysimilartotheMC1R variationseen(ornotseen!)amongindividualsinthesepopulations,whichwascausedby selection. 6. Sampleanswer:Notreally;althoughpeoplewhoarehomozygousfortheGallelehavethe highestamountofmelaninandthedarkestskincolorandpeoplewhoarehomozygousforthe Aallelehavethelowestamountofmelaninandthelightestskincolor,thereisalargeamount ofoverlapamongthegroups.SomeonewithaΔmelaninindexvalueof0,forexample,could belongtoanyofthethreegenotypes. 7. a.othergenesandenvironmentalfactors b.Thereissignificantoverlapamongthegroups.Ifitaccountedforallofthedifferences,then therewouldbenooverlapatall;eachgenotypewouldresultinauniquephenotype. 8. Thehighdegreeofsimilarityinsequencessuggeststhattheproteinshaveasimilarfunction acrossthedifferentspecies,andthisfunctioniscriticalforsurvivaland/orreproduction.The simplestexplanationforthesimilarityinthesequencesisthatthegenewasinheritedfroma commonancestorinallthespecies. 9. ThefactthatthemessengerRNA(mRNA)fromthehumanSLC24A5generestoredthehigh melaninlevels(anddarkcolor)inthezebrafishlarvaestronglysuggeststhattheproteinshavea similarfunctioninthedifferentspecies. 10. Thegenemusthaveevolvedinthemostrecentcommonancestorofallthegroupsonthetree, whichexistedatleast400millionyearsago.Asaresult,thegenemusthavebeenpresentat least400millionyearsago(thoughitmaybemucholder). 11. Asia,NorthernNorthAmerica,Australia,Europe,SouthernSouthAmerica.Thesearetheareas withthelightestnativeskintones. 12.Answerswillvary,butmoststudentswillnoticethatEuropehasahighfrequencyoftheAallele butAsiadoesnot;thiswillnotmatchtheirpredictions. 13.Thedatacontradicttheclaimbecauseclearlythereismorethanone“way”togetlightskin. TheAalleleevolvedandspreadamongEuropeanpopulations,butnotinAsianpopulations,so someothergeneticpathwayforforminglightskinmusthaveevolved. 14.Answerswillvary,butstudentsshouldmentionforexamplethatFigure4showsthatzebrafish andhumansshareacommongeneticcodeandaremadeupofthesameconservedaminoacid sequences.InFigure5,scientistswereabletotakeamessengerRNA(mRNA)fragmentfrom thehumanDNAsequencefortheSLC24A5Galleleandinjectitintogoldenzebrafishembryos www.BioInteractive.org PublishedAugust2016 Page 5 of 6 WHATCANZEBRAFISHTELLUSABOUT HUMANSKINCOLOR? Educator Materials andthezebrafishwasabletotranslatethehumanmRNAintoaprotein.Thecladogramin Figure6showsthatzebrafishandhumansshareacommonancestry. REFERENCES ThisactivityisadaptedfromanactivityintheEvolutionofHumanSkinColorcurriculumunitforAP BiologythatisapartoftheSmithsonianInstitution’sNationalScienceFoundation-fundedTeaching EvolutionthroughHumanExamplesproject(GrantNo.1119468).See http://humanorigins.si.edu/education/teaching-evolution-through-human-examplestoexplorethefull curriculumunit. KlugW.S.,CummingsM.R.,SpencerC.A.(2006).ConceptsofGenetics.Pearson:NY. LamasonR.L.,MohideenM.-A.P.K.,MestJ.R.,WongA.C.,NortonH.L.,ArosM.C.,JurynecM.J.,MaoX., HumphrevilleV.R.,HumbertJ.E.,SinhaS.,MooreJ.L.,JagadeeswaranP.,ZhaoW.,NingG.,Makalowska I.,McKeigueP.M.,O’donnellD.,KittlesR.,ParraE.J.,ManginiN.J.,GrunwaldD.J.,ShriverM.D.,Canfield V.A.,ChengK.C.(2005).SLC24A5,aputativecationexchanger,affectspigmentationinzebrafishand humans.Science310(5755):1782–1786. Norton,H.L.,Kittles,R.A.,Parra,E.,McKeigue,P.,Mao,X.,Cheng,K.,Canfield,V.A.,Bradley,D.G., McEvoy,B.andShriver,M.D.GeneticEvidencefortheConvergentEvolutionofLightSkininEuropeans andEastAsians.(2007).Mol.Biol.Evol.24(3):710–722. SturmR.A.,DuffyD.L.(2012).Humanpigmentationgenesunderenvironmentalselection.Genome Biology13(9):248.doi:10.1186/gb-2012-13-9-248. AUTHORS WrittenbyPaulBeardsley,CaliforniaStatePolytechnicUniversity.EditedbyK.DavidPinkerton, consultant;MelissaCsikari,HHMI;andStephanieKeep,consultant.CopyeditedbyLindaFelaco. SCIENTIFICREVIEWERS VinceBuonaccorsi,JuniataCollege;andRebeccaLamason,UniversityofCalifornia,Berkeley. www.BioInteractive.org PublishedAugust2016 Page 6 of 6