HeredityWhydon’tyoulooklikearhinoceros?Theanswertothisques8onseemssimple:Neitherofyourparentsisarhinoceros.Butthereismoretothisanswerthanmeetstheeye.

HeredityAsitturnsout,heredity,orthepassingoftraitsfromparentstooffspring,ismorecomplicatedthanyoumightthink.Forexample,youmighthavecurlyhair,whilebothofyourparentshavestraighthair.Youmighthaveblueeyeseventhoughbothofyourparentshavebrowneyes.Howdoesthishappen?Peoplehaveinves8gatedthisques8onforalong8me.About150yearsago,GregorMendelperformedimportantexperiments.Hisdiscoverieshelpedscien8stsbegintofindsomeanswerstotheseques8ons.

Gregor Mendel (1822-1884)

The Father of Genetics

GregorJohannMendelWasanAustrianmonkwhostudiedtheinheritanceoftraitsinpeaplants.Hedevelopedthelawsofinheritance,butMendel'sworkwasnotrecognizedun8ltheturnofthe20thcentury–founda8onofgene8cs,scien8ficstudyofheredity.Between1856and1863,Mendelcul8vatedandtestedthousandsofpeaplantsandfoundthattheplants'offspringretainedtraitsoftheparents

WhatisHeredity?Thepassingofphysicalcharacteris8csfromparentstooffspring.Aneworganismsbegintoformwhenaneggandspermjoinintheprocessoffer+liza+on.Par+culateInheritanceMendelstatedthatphysicaltraitsareinheritedas“par+cles” Mendeldidnotknowthatthe“par+cles” wereactuallyChromosomes&DNA.

Mendel’sExperimentsBeforefer8liza8oncanhappeninpeaplants,pollenmustreachthepis8lofapeaflowerthroughpollina8on.Peaplantsareusuallyself-pollina8ng,meaningpollenfromaflowerlandsonthepis8lofthesameflower.Mendeldevelopedamethodbywhichhecross-pollinated,or“crossed,”peaplants.

WhyPeas(Pisumsa)vum)?Peaplantscanbegrownquicklyinasmallareaandhavemanydifferentkindswithlotsofoffspring.Theyareabletoself-pollinateseveralgenera8onsduetohavingbothmaleandfemalereproduc8vestructures.Oneparent.

Whyisitimportant?Becauseeggs(inovule)andsperm(inpollen)fromthesameplantcombinetomakeanewplant,Mendelwasabletogrowtrue-breedingplants.Whenatrue-breedingplantself-pollinates,allofitsoffspringwillhavethesametraitastheparent.

WhyPeas(Pisumsa)vum)?Theycanbear8ficiallypollinated.Peaplantscanalsocross-pollinate.Incross-pollina2on,pollenfromoneplantfer8lizestheovuleofafloweronadifferentplant.Thereareseveralwaysthatthiscanhappen.Pollenmaybecarriedbyinsectstoafloweronadifferentplant.Pollencanalsobecarriedbythewindfromoneflowertoanotherandbyscien8stusingtoolstotransferpollen.

MendelExperimentsMendelstudiedonlyonecharacteris8cata8me.Acharacteris2cisafeaturethathasdifferentformsinapopula8on.Forexample,haircolorisacharacteris8cinhumans.Thedifferentforms,suchasbrownorredhair,arecalledtraits.Mendelusedplantsthathaddifferenttraitsforeachofthecharacteris8cshestudied.Forinstance,forthecharacteris8cofflowercolor,hechoseplantsthathadpurpleflowersandplantsthathadwhiteflowers.Healsolookedatheightandseedshape.

MendelExperimentsHeusedplantsthatweretruebreedingforeachofthetraitshewasstudying.Bydoingso,hewouldknowwhattoexpectifhisplantsweretoself-pollinate.Hedecidedtofindoutwhatwouldhappenifhebred,orcrossed,twoplantsthathaddifferenttraitsofasinglecharacteris8c.

MendelExperimentsTobesureofcross-pollinated,heremovedtheanthersofoneplantsothattheplantcouldnotself-pollinate.Then,heusedpollenfromanotherplanttofer8lizetheplant.ThisallowedMendeltoselectwhichplantswouldbecrossedtoproduceoffspring.

MendelExperimentsMendelcrossedtwopeaplantsthatdifferedinheight.Hecrossedpurebredtallplantswithpurebredshortplants.Theseparentplants,thePgenera8on,werepurebredbecausetheyalwaysproducedoffspringwiththesametraitastheparent.InallofMendel’scrosses,onlyoneformofthetraitappearedintheF1genera+on.However,intheF2genera+on,the“lost”formofthetraitalwaysreappearedinaboutonefourthoftheplants.Fromhisresults,Mendelreasonedthatindividualfactors,onefromeachparent,controltheinheritanceoftraits.Today,scien8stscallthefactorsthatcontroltraitsgenes.Thedifferentormul8pleformsofthesamegeneiscalledalleles.

MendelandHisPeasMendeltested7traits:

1.  Flowercolor2.  Flowerposi8on3.  Seedcolor4.  Seedshape5.  Podshape6.  Podcolor7.  Plantheight

MendelandHisPeasMendelcrossedflowersthatweretrue-breedingforeachcharacteris8c.Hecrossedapurple(PP)floweredplantwithawhite(pp)floweredplant.(ParentGenera8on)

MendelandHisPeasThefirstgenera8on(F1)ofplantsallhadpurpleflowers.Wheredidthewhitecolorgo?

(Pp) (Pp)

(Pp) (Pp)

Purple Parent (PP)

Whi

te P

aren

t (pp

)

MendelandHisPeasMendeltooktwoofhisfirstgenera8on(F1xF1)purplefloweredplantsandcrossedthemtogether.Inthesecondgenera8on(F2)hehad3purplefloweredplants,and1whitefloweredplant.

(Pp) (Pp)

(Pp) (pp)

Purple Parent (Pp)

Purp

le P

aren

t (Pp

)

Mendel’sExperimentsSymbolsforallelesDominantallelesareshownwithacapitallefer.Recessiveallelesareshownwithalowercaselefer.•  Dominantexamplefortall=T•  Recessiveexampleforshort=t•  Twodominantsallelesfortall=TT•  Tworecessivesallelesforshort=f•  Inheri8ngbothalleles=Tt

Mendel’sExperimentsAnorganism’straitsarecontrolledbytheallelesitinheritsfromitsparents.Someallelesaredominant,whileotherallelesarerecessive.Adominantalleleisonewhosetraitalwaysshowsupintheorganismwhenthealleleispresent.Arecessivealleleishiddenwheneverthedominantalleleispresent.Atraitcontrolledbyarecessiveallelewillonlyshowupiftheorganismdoesnothavethedominantallele

MendelandHisPeasMendelno8cedinthefirstgenera8on,allofthewhiteflowersseemedtodisappear.Hecalledthisarecessivetrait(torecedemeans“togoawayorbackoff.”).Thistraitreappearsinthe2ndgenera8onaierdisappearinginthe1stgenera8onwhenparentswithdifferenttraitsarebred.Thewhitecolorfadedintothebackgroundatfirst.Itshowedupinthenextgenera8onwhenhepollinatedtheflowers.

MendelandHisPeasThecolor(purple)thatseemedtomaskovertherecessivecolorwasnamedthedominanttrait.Thistraitisobservedinthe1stgenera8onwhenparentsthathavedifferenttraitsarebred.Mendelwasresponsibleforfiguringoutthateachplantcarriedtwosetsofinstruc8onsforeachcharacteris8c(onefromthe“mom”andonefromthe“dad”).

Mendel’sExperiments•  InMendel’scross,thepurebredtallplanthastwoallelesfortallstems.Thepurebredshortplanthastwoallelesforshortstems.TheF1plantsareallhybrids:theyhavetwodifferentallelesforthetrait—oneallelefortallstemsandoneforshortstems.Gene8cistsuseacapitallefertorepresentadominantalleleandalowercaseversionofthesameleferfortherecessiveallele.

•  Likemanyscien8sts,hisworkwasnotacceptedun8laierhisdeath.

Let’sReview1.  Ifyoucrossedatrue-breedingblackrabbitwitha

true-breedingwhiterabbit,alloftheoffspringwouldbeblack.Whichtraitisdominantinrabbits:blackfurorwhitefur?

2.  Whichtraitisrecessive?

AnswerThetraitforblackfurisdominantoverthetraitforwhitefur.Thewhitefurtraitisrecessive. (Bb)

(Bb)

(Bb) (Bb)

Black Rabbit (BB)

Whi

te R

abbi

t (bb

)

TraitsandInheritanceMendelcalculatedthera8oofdominanttraitstorecessivetraits.Hefoundara8oof3:1.Whatdidthistellhimabouthowtraitsarepassedfromparentstooffspring?

TraitsandInheritanceMendelknewfromhisexperimentswithpeaplantsthattheremustbetwosetsofinstruc8onsforeachcharacteris8c.Thefirst-genera8onplantscarriedtheinstruc8onsforboththedominanttraitandtherecessivetrait.Scien8stsnowcalltheseinstruc8onsforaninheritedtraitgenes.(asegmentofDNAthatcarrieshereditaryinstruc)onsandispassedfromparenttooffspring)Eachparentgivesonesetofgenestotheoffspring.Theoffspringthenhastwoformsofthesamegeneforeverycharacteris8c—onefromeachparent.Thedifferentormul8pleforms(oiendominantandrecessive)ofageneareknownasalleles(uhLEELZ).

PhysicalTraitsCanbeseenbyothersEyecolor,haircolor,height,leihanded

AcquiredTraitsLearnedskillsPlayingsports,ridingabike,playingmusicalinstruments

BehavioralTraitsIns8nctualac8onsNestbuildingandmigra8on

WhatareTraits?

Genesaffectthetraitsofoffspring.Anorganism’sappearanceisknownasitsphenotype(FEEnohTIEP).Thisisanobservabletraitthatweseewhenyoulookatsomeone.Inpeaplants,possiblephenotypesforthecharacteris8cofflowercolorwouldbepurpleflowersorwhiteflowers.Forseedcolor,yellowandgreenseedsarethedifferentphenotypes.Phenotypesofhumansaremuchmorecomplicatedthanthoseofpeas.Themanhasaninheritedcondi8oncalledalbinism(ALbuhNIZuhm).Albinismpreventshair,skin,andeyesfromhavingnormalcoloring.

Phenotype

Bothinheritedallelesfromyourbiologicalparentstogetherformanorganism’sgenotype.Genescontrolyourtraits.Becausethealleleforpurpleflowers(P)isdominant,onlyonePalleleisneededfortheplanttohavepurpleflowers.Whatisthedifferencebetweenhomozygousandheterozygous?Aplantorindividualthathasiden8calallelesofagenetwodominantortworecessiveallelesishomozygous(HOHmohZIEguhs).Aplantorindividualthathasnoniden8calallelesofageneorgenotypePpisheterozygous(HETuhrOHZIEguhs).

Genotype

Phenotype&GenotypeAnalleleisdominantifitseffectmaskstheeffectofarecessiveallelepairedwithit

–  Capitallefers(A)signifydominantalleles;lowercaselefers(a)signifyrecessivealleles

–  Homozygousdominant(AA)–  Homozygousrecessive(aa)–  Heterozygous(Aa)

CanTongueRoll(TT,Tt)orCan’t(f)

Dimples(DD,Dd)NoDimples(dd)

Right(RR,Rr)orLei(rr)handed?

Curly(HH,Hh)orStraight(hh)

HandClaspRightoverLei?

Allergies(AA,Aa)orNoAllergies(aa)

Normal(BB,Bb)ColorBlindness(bb)

Can you see the number inside

the circle?

If you cannot, you may be colorblind

Detached(EE,Ee)orafached(ee)Earlobes

Freckles(FF,Ff)NoFreckles(ff)

CleiChin(CC,Cc)orNoClei(cc)

Widow’sPeak(WW,Ww)orStraight(ww)

PunnefSquaresAPunnefsquareisusedtoorganizeallthepossiblecombina8onsofoffspringfrompar8cularparents.Theallelesforatrue-breeding,purple-floweredplantarewrifenasPP.Theallelesforatrue-breeding,white-floweredplantarewrifenaspp.

ThePunnefsquareforthiscrossisshownonthenextslide.Alloftheoffspringhavethesamegenotype:Pp.Thedominantallele,P,ineachgenotypeensuresthatalloftheoffspringwillbepurple-floweredplants.Therecessiveallele,p,maybepassedontothenextgenera8on.ThisPunnefsquareshowstheresultsofMendel’sfirstexperiments.

PunnefSquaresHereisthePunnefsquareforthiscross.Alloftheoffspringhavethesamegenotype:Pp.Thedominantallele,P,ineachgenotypeensuresthatalloftheoffspringwillbepurple-floweredplants.Therecessiveallele,p,maybepassedontothenextgenera8on.ThisPunnefsquareshowstheresultsofMendel’sfirstexperiments

PunnefSquaresInMendel’s2ndexperiment,heallowedthe1stgenera8onplantstoself-pollinate.Thepictureonthenextslideshowsaself-pollina8oncrossofaplantwiththegenotypePp.Whatarethepossiblegenotypesoftheoffspring?No8cethatonesquareshowsthegenotypePp,whileanothershowspP.Theseareexactlythesamegenotype.TheotherpossiblegenotypesoftheoffspringarePPandpp.Thecombina8onsPP,Pp,andpPhavethesamephenotype—purpleflowers.Thisisbecauseeachcontainsatleastonedominantallele(P).Onlyonecombina8on,pp,producesplantsthathavewhiteflowers.Thera8oofdominanttorecessiveis3:1,justasMendelcalculatedfromhisdata.

PunnefSquares

PunnefSquaresInaPunnefsquare,allthepossibleallelesfromoneparentarewrifenacrossthetop.Allthepossibleallelesfromtheotherparentarewrifendowntheleiside.ThecombinedallelesintheboxesofthePunnefsquarerepresentallthepossiblecombina8onsintheoffspring.Inagene+ccross,theallelethateachparentwillpassontoitsoffspringisbasedonprobability.

PunnefSquaresBelowishowtoconstructaPunnefsquare.Inthiscase,thePunnefsquareshowsacrossbetweentwohybridpeaplantswithroundseeds(Rr).Thealleleforroundseeds(R)isdominantoverthealleleforwrinkledseeds(r).Eachparentcanpasseitherofitsalleles,Rorr,toitsoffspring.TheboxesinthePunnefsquarerepresentthepossiblecombina8onsofallelesthattheoffspringcaninherit.

ProbabilityEachparenthastwoallelesforeachgene.Whentheseallelesaredifferent,asinPp,offspringareequallylikelytoreceiveeitherallele.Thinkofacointoss.Thereisa50%chanceyou’llgetheadsanda50%chanceyou’llgettails.Thechanceofreceivingonealleleoranotherisasrandomasacointoss.Themathema8calchancethatsomethingwillhappenisknownasprobability.Itishowlikelyaneventwilloccur,notnecessarilywhatwilloccur.Probabilityismostoienwrifenasafrac8onorpercentage.Ifyoutossacoin,theprobabilityoftossingtailsis1/2—youwillgettailshalfthe8me.Theresultofonecointossdoesnotaffecttheresultofthenexttoss.

Mathema8csofProbability

ProbabilityandGene8csWhenGregorMendelanalyzedtheresultsofhiscrossesinpeas,hecarefullycountedalltheoffspring.Over8me,herealizedthathecouldapplytheprinciplesofprobabilitytohiscrosses.Mendelwasthefirstscien8sttorecognizethattheprinciplesofprobabilitycanbeusedtopredicttheresultsofgene8ccrosses.Tohavewhiteflowers,apeaplantmustreceiveapallelefromeachparent.EachoffspringofaPpxPpcrosshasa50%chanceofreceivingeitherallelefromeitherparent.So,theprobabilityofinheri8ngtwopallelesis1/2x1/2,whichequals1/4,or25%.Traitsinpeaplantsareeasytopredictbecausethereareonlytwochoicesforeachtrait,suchaspurpleorwhiteflowersandroundorwrinkledseeds.

ProbabilityandGene8csYoucanuseaPunnefsquaretopredictprobabili8esasseenbelow.Acrossbetweenapurebredblackguineapigandapurebredwhiteguineapig.Thealleleforblackfurisdominantoverthealleleforwhitefur.No8cethatonlyoneallelecombina8onispossibleintheoffspring—Bb.Alloftheoffspringwillinheritthedominantalleleforblackfur.Becauseofthis,alloftheoffspringwillhaveblackfur.Thereisa100percentprobabilitythattheoffspringwillhaveblackfur.

IncompleteDominanceSinceMendel’sdiscoveries,researchershavefoundthatsome8mesonetraitisnotcompletelydominantoveranother.Thesetraitsdonotblendtogether,buteachallelehasitsowndegreeofinfluence.Thisisknownasincompletedominance.Acurly-hairedparentandastraight-hairedparenthavewavy-hairedchildrenbecauseofincompletedominance.Aclassicexampleofincompletedominanceisfoundinthesnapdragonflower.

IncompleteDominanceBelowshowsacrossbetweenatrue-breedingredsnapdragon(R1R1)andatrue-breedingwhitesnapdragon(R2R2).Asyoucansee,allofthepossiblephenotypesfortheiroffspringarepinkbecausebothallelesofthegenehavesomedegreeofinfluence.

CodominanceForallofthetraitsinpeasthatMendelstudied,oneallelewasdominantwhiletheotherwasrecessive.Thisisnotalwaysthecase.Inaninheritancepaferncalledcodominance,theallelesareneitherdominantnorrecessive.Asaresult,bothallelesareexpressedintheoffspring.Codominantallelesarewrifenascapitalleferswithsuperscriptstoshowthatneitherisrecessive.

ImportanceofEnvironmentTheeffectsofgenesareoienalteredbytheenvironment—theorganism’ssurroundings.Manyofaperson’scharacteris+csaredeterminedbyaninterac+onbetweengenesandtheenvironment.Severalgenesdeterminehumanheight.However,environmentalsoinfluencespeople’sheights.People’sdietscanaffecttheirheight.Apoordietcanpreventapersonfromgrowingastallasmightbepossible.

Environmentalfactorscanalsoaffecthumanskills,suchasplayingamusicalinstrument.Forexample,physicaltraitssuchasmusclecoordina8onandagoodsenseofhearingwillhelpamusicianplaywell.Butthemusicianalsoneedsinstruc8ononhowtoplaytheinstrument.Musicalinstruc8onisanenvironmentalfactor.

MeiosisWherearegeneslocated?Howdogenespassinforma)on?Understandingreproduc)oncanprovidesomeanswers.Twokindsofreproduc8on:asexualandsexual.Asexualreproduc8onresultsinoffspringwithgenotypesthatareexactcopiesoftheirparent’sgenotype.Sexualreproduc8onproducesoffspringthatsharetraitswiththeirparentsbutarenotexactlylikeeitherparent.Infact,offspringthatsharethesametwoparentsvaryalotfromeachother,aswell.

Reproduc8onInasexualreproduc2on,onlyoneparentcellisneeded.Thestructuresinsidethecellarecopied,andthentheparentcelldivides,makingtwoexactcopies.Thistypeofcellreproduc8onisknownasmitosis.Mostofthecellsinyourbodyandmostsingle-celledorganismsreproduceinthisway.Insexualreproduc+on,twoparentcellsjointogethertoformoffspringthataredifferentfrombothparents.Theparentcellsarecalledsexcells.Sexcellsaredifferentfromordinarybodycells.Humanbodycellshave46,or23pairsof,chromosomes.

Reproduc8onChromosomesthatcarrythesamesetsofgenesarecalledhomologous(hohMAHLuhguhs)chromosomes.Imagineapairofshoes.Eachshoeislikeahomologouschromosome.Thepairrepresentsahomologouspairofchromosomes.Buthumansexcellsaredifferent.Theyhave23chromosomes—halftheusualnumber.Eachsexcellhasonlyoneofthechromosomesfromeachhomologouspair.Sexcellshaveonlyone“shoe.”Yourbodycellseachhave35,000genesthatcontrolatrait.

Mitosis

GenesandChromosomesIntheearly1900s,scien8stswereworkingtoiden8fythecellstructuresthatcarriedMendel’shereditaryfactors,orgenes.In1903,WalterSuRonobservedthatsexcellsingrasshoppershadhalfthenumberofchromosomesasthebodycells.Hewantedtounderstandhowsexcells(spermandegg)form.Hewouldfocusonthemovementofthechromosomesduringforma8onandhypothesizedthatchromosomeswerethekeytounderstandinghowoffspringhavetraitssimilartotheirparents.

GenesandChromosomesSufonno8cedthatgrasshoppersbodycellshave24chromosomesandwhenhelookedcloserhefoundthateachsexcellhadonly12chromosomes.Eachgrasshopperoffspringhadexactlythesamenumberofchromosomesinitsbodycellsaseachoftheparents.Hereasonedthatthechromosomesinbodycellsactuallyoccurredinpairs,withonechromosomeineachpaircomingfromthemaleandtheothercomingfromthefemale.Fromhisobserva8ons,Sufonconcludedthatgenesarelocatedonchromosomes.Heproposedthechromosometheoryofinheritance.Accordingtothechromosometheoryofinheritance,genesarecarriedfromparentstotheiroffspringonchromosomes.

Whenchromosomepairsseparate,sodotheallelescarriedonthechromosomes.Oneallelefromeachpairgoestoeachsexcell.Chromosomesaremadeupofmanygenesjoinedtogetherlikebeadsonastring.Eachchromosomecontainsalargenumberofgenes,eachgenecontrollingapar+culartrait.Eachchromosomepairhasthesamegenes.Thegenesarelinedupinthesameorderonbothchromosomes.However,theallelesforsomeofthegenesmightdifferfromeachother,makingtheorganismheterozygousforsometraits.Iftheallelesarethesame,theorganismishomozygousforthosetraits.

GenesandChromosomes

Organismsproducesexcellsduringmeiosis.Meiosisistheprocessbywhichthenumberofchromosomesisreducedbyhalftoformsexcells—spermandeggs.Duringmeiosis,thechromosomepairsseparateandaredistributedtotwodifferentcells.Theresul+ngsexcellshaveonlyhalfasmanychromosomesastheothercellsintheorganism.Whentheycombine,eachsexcellcontributeshalfthenumberofchromosomestoproduceoffspringwiththecorrectnumberofchromosomes.Beforemeiosisbegins,everychromosomeintheparentcelliscopied.Centromeresholdthetwochroma8dstogether.

GenesandChromosomes

MeiosisSexcellsaremadeduringmeiosis(mieOHsis).Meiosisisacopyingprocessthatproducescellswithhalftheusualnumberofchromosomes.Eachsexcellreceivesone-halfofeachhomologouspair.Forexample,ahumaneggcellhas23chromosomes,andaspermcellhas23chromosomes.Thenewcellthatformswhenaneggcellandaspermcelljoinhas46chromosomes.Becausethegenesoftheparentsaresortedandrecombinedrandomlyintheoffspring,theoffspringisdifferentfromtheparents.Ifthesameparentshavemoreoffspring,thegeneswillbesortedagain,andtheseoffspringwillbedifferentfromeachotheraswellasfromtheparents.

WhatHappensDuringMeiosis:MeiosisPhase1•  A.Thechromosomepairslineupincenterofthecell.•  B.Thepairsseparateandmovetooppositeendsofthecell.•  C.Twocellsformwithhalfthenumberofchromosomes.Each

chromosomes8llhastwochroma8ds.MeiosisPhase2•  A.Thechromosomeswiththeirtwochroma8dsmovetothe

centerofthecell.•  B.Thecentromeressplit,andthechroma8dsseparate.Single

chroma8dsmovetooppositeendsofthecell.

WhatHappensDuringMeiosis:EndofMeiosis•  Foursexcellshavebeenproduced.Eachcellhasonlyhalfthe

normalnumberofchromosomesthattheparentcellhadatthebeginningofmeiosis.Eachcellhasonlyonechromosomefromeachoriginalpair.

MeiosisandMendel

Thepictureshowswhathappenstoapairofhomologouschromosomesduringmeiosisandfer8liza8on.Thecrossistrue-bredroundseedsandtrue-bredwrinkledseeds.

MeiosisandMendel

Eachfer8lizedegginthe1stgenera8onhadonedominantalleleandonerecessivealleleforseedshape.Onlyonegenotypewaspossiblebecauseallspermformedbythemaleparentduringmeiosishadthewrinkled-seedallele,andallofthefemaleparent’seggshadtheround-seedallele.Meiosisalsohelpedexplainotherinheritedcharacteris8cs.

MeiosisandPunnefSquaresInFigurebelow,youcanseehowthePunnefsquareaccountsforthesepara8onofallelesduringmeiosis.AsshownacrossthetopofthePunnefsquare,halfofthespermcellsfromthemaleparentwillreceivethechromosomewiththeTallele.Theotherhalfofthespermcellswillreceivethechromosomewiththetallele.Inthisexample,thesameistruefortheeggcellsfromthefemaleparent,asshowndowntheleisideofthePunnefsquare.Dependingonwhichspermcellcombineswithwhicheggcell,oneoftheallelecombina8onsshownintheBoxeswillresult.

HumanInheritanceInhumanstraitscanbecontrolledbyeither:

1.  Singlegeneswithtwoalleles2.  Singlegeneswithmul8plealleles3.  Mul8plegeneswithmul8plealleles.

HumanInheritanceSomehumantraitsarecontrolledbysinglegeneswithtwoalleles,andothersbysinglegeneswithmul+plealleles.S8llothertraitsarecontrolledbymanygenesthatacttogether.Heightandskincolorarebothexamplesofhumantraitscontrolledbymanygenes.Whenmorethanonegenecontrolsatrait,therearemanypossiblecombina8onsofgenesandalleles.Thereisanenormousvarietyofphenotypesforheight,forexample,andhumanskincolorrangesfromalmostwhitetonearlyblack,withmanyshadesinbetween.

HumanInheritanceManyhumantraitsarecontrolledbyasinglegenewithonedominantalleleandonerecessiveallele.Aswithtallandshortpeaplants,thesehumantraitshavetwodis8nctlydifferentphenotypes,orphysicalappearances.Forexample,thealleleforawidow’speak,whichisahairlinethatcomestoapointinthemiddleoftheforehead,isdominantoverthealleleforastraighthairline.

Somehumantraitsarecontrolledbyasinglegenethathasmorethantwoalleles.Suchageneissaidtohavemul+plealleles—threeormoreformsofagenethatcodeforasingletrait.Anexampleofahumantraitthatiscontrolledbyagenewithmul8pleallelesisbloodtype.Therearefourmainbloodtypes—A,B,AB,andO—controlledbythreealleles.

SingleGeneswithMul8pleAllelesThisoccurswhentherearemorethan2possibili8es.Ex:BloodtypewheretypeAandBarecodominantandtypeOisrecessive.

ApersonwithtypeAblood(heterozgyous)hasachildwithapersonwhoisTypeB(Heterozygous)bloodwhatarethepossiblebloodtypesofthechild?

Traitscontrolledbymul8plegenesCanhaveanenormousamountofvariety.Inthiscasemul8plegeneseachcontributeapieceofthefinaloutcome.Somehumantraitsshowalargenumberofphenotypesbecausethetraitsarecontrolledbymanygenes.Thegenesacttogetherasagrouptoproduceasingletrait.Atleastfourgenescontrolheightinhumans,sotherearemanypossiblecombina8onsofgenesandalleles.Skincolorisanotherhumantraitthatiscontrolledbymanygenes.

SexChromosomesSexchromosomescarrygenesthatdeterminesex.Oneofthepairofchromosomesthatdeterminethesexofanindividual.Inhumans,femaleshavetwoXchromosomes.ButhumanmaleshaveoneXchromosomeandoneYchromosome.

SexChromosomesDuringmeiosis,oneofeachofthechromosomepairsendsupinasexcell.FemaleshavetwoXchromosomesineachbodycell.Whenmeiosisproducestheeggcells,eachegggetsoneXchromosome.MaleshavebothanXchromosomeandaYchromosomeineachbodycell.MeiosisproducesspermwitheitheranXoraYchromosome.Aneggfer8lizedbyaspermwithanXchromosomewillproduceafemaleandifthespermcontainsaYchromosome,theoffspringwillbeamale.

Sex-LinkedGenesGenesontheXandYchromosomesareoiencalledsex-linkedgenes,whichleadtosex-linkeddisorders.Traitscontrolledbysex-linkedgenesarecalledsex-linkedtraits.BecausemaleshaveonlyoneXchromosome,malesaremorelikelythanfemalestohaveasex-linkedtraitthatiscontrolledbyarecessiveallele.Oneexampleofasex-linkedtraitthatiscontrolledbyarecessivealleleisred-greencolorblindnessthatisontheXchromosomeMenaremostlikelytohavesex-linkeddisorders.Acarrierisapersonwhohasonerecessivealleleforatraitandonedominantallele.Althoughacarrierdoesnothavethetrait,thecarriercanpasstherecessivealleleontohisorheroffspring.Inthecaseofsex-linkedtraits,onlyfemalescanbecarriers.

Sex-LinkedGenes Agene+cdisorderisanabnormalcondi8onthatapersoninheritsthroughgenesorchromosomes.Somegene+cdisordersarecausedbymuta+onsintheDNAofgenes.Otherdisordersarecausedbychangesintheoverallstructureornumberofchromosomes.

Sex-LinkedDisorders

Peoplewhoarecolorblindordeficientcanhavetroubledis8nguishingbetweenshadesofredandgreen.

Sex-LinkedDisordersHemophilia(HEEmohFILeeuh)isanothersex-linkeddisordercontrolledbytherecessivealleleontheXchromosome.Hemophiliapreventsbloodfromclotng,andpeoplewithhemophiliableedforalong8meaiersmallcutsandneedtobecarefultoavoidsmallbumpsandbruises.Hemophiliacanbefatal.

Sex-LinkedDisordersCys8cFibrosis(SIS8kFIEbrohsis)

•  Causedbytworecessiveallelesononechromosome.TheresultofthreebasesremovedfromaDNAmolecule.

•  Cys8cFibrosiscausesthebodytoproduceabnormallythickmucusinthelungsandintes8nes.•  Thismakesithardforthemtobreath.Leadstochronic

problemswiththeirlungs.•  Lifeexpectancyof37years.

Sex-LinkedDisordersSickle-CellDisease

•  Sicklecelliscontrolledbyaco-dominanttraitwithanormalallele.Apersonwithtwosickle-cellalleleswillhavethedisease.Apersonwithonesicklecellallelecanproducenormalandabnormalhemoglobinandnothavethedisease.

Thesicklecellsthemselvescannotcarryasmuchoxygenandcanclogbloodvessels.Itaffectsthehemoglobin(proteininredbloodcellsthatcarriesoxygen).WhenO2islow,theredcellshaveanunusualshape.

Sex-LinkedDisordersSickle-CellDisease•  Leadstosickle-cellpaincrises,requirespainmanagement

plansandbloodtransfusions.•  Sideeffectofsickle-cellisimmunitytothediseaseofmalaria.

ChromosomalDisorderDownSyndrome

–  Causedbyapersonhaveanextracopyofchromosome21,whichmeansthattheyhavethreeofthatchromosomeinsteadofthenormalpair.

–  Causedbyafailureofthechromosomestoseparateduringmeiosis.

–  Effectsbraindevelopment,othersymptomscanvary.– Mostcommontohaveotherseverehealthdefectsalongwithdownsyndrome,toincludesomementaldeficits.Heartdefectsarecommonandcanbetreated.

–  Notreatmentforanyofthedisordersmen8oned.–  Symptomsaretreatedbyacombina8onofdrugsandcare,andcanresultinaproduc8venormallife.

PedigreesHowwouldyoutracetheoccurrenceofagene8cdisorderthroughseveralgenera8onsofafamily?Oneimportanttoolthatgene+cistsusetotracetheinheritanceoftraitsinhumansisapedigree.Apedigreeisachartor“familytree”thattrackswhichmembersofafamilyhaveapar8culartrait.Thetraitinapedigreecanbeanordinarytrait,suchasawidow’speak,oragene8cdisorder,suchascys8cfibrosis.Bymakingapedigree,acounselorcanoienpredictwhetherapersonisacarrierofahereditarydisease.

PedigreeKey•  Square=male•  Circle=female•  ½filledsquareorcircle=carrier•  Fullyfilledsquareorcircle=hastrait•  circleconnectedtosquarebyhorizontalline=cross•  Linesgoingdownrepresentchildren

PedigreesBelowshowsapedigreeforalbinism,acondi8oninwhichaperson’sskin,hair,andeyeslacknormalcoloring.

ManagingGene8cDisordersYearsago,doctorshadonlyPunnefsquaresandpedigreestohelpthempredictwhetherachildmighthaveagene8cdisorder.Today,doctorsusetoolssuchaskaryotypestohelpdiagnosegene+cdisorders.Peoplewithgene+cdisordersarehelpedthroughmedicalcare,educa+on,jobtraining,andothermethods.KaryotypesTodetectchromosomaldisorderssuchasDownsyndrome,adoctorexaminesthechromosomesfromaperson’scells.Thedoctorusesakaryotypetoexaminethechromosomes.Akaryotype(KAreeuhtyp)isapictureofallthechromosomesinacell.Thechromosomesinakaryotpearearrangedinpairs.Akaryotypecanrevealwhetherapersonhasthecorrectnumberofchromosomesinhisorhercells.IfyoudidtheDiscoverac8vity,yousawakaryotypefromagirlwithDownsyndrome.

Gene8cCounselingGene+cCounselingAcouplethathasafamilyhistoryofagene8cdisordermayturntoagene8ccounselorforadvice.Gene8ccounselorshelpcouplesunderstandtheirchancesofhavingachildwithapar8culargene8cdisorder.Gene8ccounselorsusetoolssuchaskaryotypes,pedigreecharts,andPunnefsquarestohelpthemintheirwork.

DealingwithGene8cDisordersPeoplewithgene8cdisordersfaceseriouschallenges,buthelpisavailable.Medicaltreatmentshelppeoplewithsomedisorders.Forexample,physicaltherapyhelpsremovemucusfromthelungsofpeoplewithcys8cfibrosis.Peoplewithsickle-celldiseasetakefolicacid,avitamin,tohelptheirbodiesmanufactureredbloodcells.Becauseofeduca8onandjobtraining,adultswithDownsyndromecanfindworkinhotels,banks,restaurants,andotherplacesofemployment.Fortunately,mostgene8cdisordersdonotpreventpeoplefromlivingac8ve,produc8velives.

Selec8veBreedingGene8ctechniqueshaveenabledpeopletoproduceorganismswithdesirabletraits.Selec8vebreeding,cloning,andgene8cengineeringarethreemethodsfordevelopingorganismswithdesirabletraits. Theprocessofselec8ngorganismswithdesiredtraitstobeparentsofthenextgenera8oniscalledselec+vebreeding.Examplesincludefarmerssavingseedsfromthehealthiestplantsthatproducedthebestfood.Theywouldthenplantthoseseedsoverandovertodevelopplantsthatproducedbefercorps.Peoplehaveusedselec8vebreedingwithmanydifferentplantsandanimals.Twoselec8vebreedingtechniquesareinbreedingandhybridiza8on.

InbreedingThetechniqueofinbreedinginvolvescrossingtwoindividualsthathavesimilarcharacteris8cs.Forexample,supposeamaleandafemaleturkeyarebothplumpandgrowquickly.Theiroffspringwillprobablyalsohavethosedesirablequali8es.Inbredorganismshaveallelesthatareverysimilartothoseoftheirparents.Inbredorganismsaregene8callyverysimilar.Therefore,inbreedingincreasestheprobabilitythatorganismsmayinheritallelesthatleadtogene8cdisorders.Forexample,inheritedhipproblemsarecommoninmanybreedsofdogs.

Hybridiza8onInhybridiza8on(hybridihZAYshun),breederscrosstwogene8callydifferentindividuals.Thehybridorganismthatresultsisbredtohavethebesttraitsfrombothparents.Forexample,afarmermightcrosscornthatproducesmanykernelswithcornthatisresistanttodisease.Theresultmightbeahybridcornplantwithbothofthedesiredtraits.

CloningForsomeorganisms,atechniquecalledcloningcanbeusedtoproduceoffspringwithdesiredtraits.Acloneisanorganismthathasexactlythesamegenesastheorganismfromwhichitwasproduced.Itisn’thardtoclonesomekindsofplants,suchasanAfricanviolet.Justcutastemfromoneplant,andputthesteminsoil.Waterit,andsoonyouwillhaveawholenewplant.Thenewplantisgene8callyiden8caltotheplantfromwhichthestemwascut.Researchershavealsoclonedanimalssuchassheepandpigs.Themethodsforcloningtheseanimalsarecomplex.Theyinvolvetakingthenucleusofananimal’sbodycellandusingthatnucleustoproduceanewanimal.

Gene8cEngineeringGene8cistshavedevelopedanotherpowerfultechniqueforproducingorganismswithdesiredtraits.Scien8stscanmanipulateindividualgeneswithinorganisms.Thiskindofmanipula8oniscalledgene+cengineering.Inthisprocess,calledgene8cengineering,genesfromoneorganismaretransferredintotheDNAofanotherorganism.Gene8cengineeringcanproducemedicines,fabrics,andimprovefoodcrops.

Gene8cEngineeringinBacteriaOnetypeofgene8callyengineeredbacteriaproducesaproteincalledinsulin.Injec8onsofinsulinareneededbymanypeoplewithdiabetes.RecallthatbacteriahaveasingleDNAmoleculeinthecytoplasm.SomebacterialcellsalsocontainsmallcircularpiecesofDNAcalledplasmids.Onthenextslideyoucanseehowscien8stsinserttheDNAforahumangeneintotheplasmidofabacterium.Oncethegeneisinsertedintotheplasmid,thebacterialcellandallitsoffspringwillcontainthishumangene.Asaresult,thebacteriaproducetheproteinthatthehumangenecodesfor—inthiscase,insulin.Becausebacteriareproducequickly,largeamountsofinsulincanbeproducedinashort8me.

Gene8cEngineering

Gene8cEngineeringinOrganismsScien8stscanalsousegene8cengineeringtechniquestoinsertgenesintoanimals.Forexample,humangenescanbeinsertedintothecellsofcows.Thecowsthenproducethehumanproteinforwhichthegenecodesintheirmilk.Scien8stshaveusedthistechniquetoproducethebloodclotngproteinneededbypeoplewithhemophilia.Geneshavealsobeeninsertedintothecellsofplants,suchastomatoesandrice.Someofthegenesenabletheplantstosurviveincoldtemperaturesorinpoorsoil.Othergene8callyengineeredcropscanresistinsectpests.

Gene8cEngineering-GeneTherapySomedayitmaybepossibletousegene8cengineeringtocorrectsomegene8cdisordersinhumans.Thisprocess,calledgenetherapy,willinvolveinser8ngcopiesofagenedirectlyintoaperson’scells.Forexample,doctorsmaybeabletotreathemophiliabyreplacingthedefec8vealleleontheXchromosome.Theperson’sbloodwouldthenclotnormally.

ConcernsonGene8cEngineeringSomepeopleareconcernedaboutthelong-termeffectsofgene8cengineering.Forexample,somepeoplethinkthatgene8callyengineeredcropsmaynotbeen8relysafe.Peoplefearthatthesecropsmayharmtheenvironmentorcausehealthproblemsinhumans.Toaddresssuchconcerns,scien8stsaretryingtolearnmoreabouttheeffectsofgene8cengineering.

LearningAboutHumanGene8csRecentadvanceshaveenabledscien8ststolearnagreatdealabouthumangene8cs.TheHumanGenomeProjectandDNAfingerprin8ngaretwoapplica8onsofthisnewknowledge.

LearningAboutHumanGene8csTheHumanGenomeProjectImaginetryingtocrackacodethatis6billionleferslong.That’sexactlywhatscien8stsworkingontheHumanGenomeProjecthavebeendoing.AgenomeisalltheDNAinonecellofanorganism.ThemaingoaloftheHumanGenomeProjecthasbeentoiden+fytheDNAsequenceofeverygeneinthehumangenome.TheHumanGenomeProjecthascompleteda“firstdrai”ofthehumangenome.Thescien8stshavelearnedthattheDNAofhumanshasatleast30,000genes.Theaveragegenehasabout3,000bases.Scien8stswillsomedayknowtheDNAsequenceofeveryhumangene.

LearningAboutHumanGene8csDNAFingerprin+ngDNAtechnologyusedintheHumanGenomeProjectcanalsoiden8fypeopleandshowwhetherpeoplearerelated.DNAfingerprin)ngiden8fiestheuniquepafernsinanindividual’sDNA.DNAsamplesarenowusedasevidenceincrimes.DNAfromaperson’scellsisbrokendownintosmallpieces,orfragments.SelectedfragmentsareusedtoproduceapaferncalledaDNAfingerprint.Exceptforiden8caltwins,notwopeoplehaveexactlythesameDNAfingerprint.

LearningAboutHumanGene8csDNAFingerprin+ngScien8stsarenowabletocreatesomethinglikeatwin,calledaclone.Acloneisaneworganismthathasanexactcopyofanotherorganism’sgenes.Clonesofseveraltypesoforganisms,includingsomemammals,havebeendevelopedbyscien8sts.However,thepossibilityofcloninghumansiss8llbeingdebatedamongbothscien8stsandpoli8cians..

DNAFormanyyears,thestructureofaDNAmoleculewasapuzzletoscien8sts.Inthe1950s,twoscien8stsdeducedthestructurewhileexperimen8ngwithchemicalmodels.TheylaterwonaNobelPrizeforhelpingsolvethispuzzle!

DNAInheritedcharacteris8csaredeterminedbygenes,andgenesarepassedfromonegenera8ontothenext.Genesarepartsofchromosomes,whicharestructuresinthenucleusofmostcells.ChromosomesaremadeofproteinandDNA.DNAstandsfordeoxyribonucleicacid(deeAHKSeeRIEbohnooKLEEikASid).DNAisthegene8cmaterial—thematerialthatdeterminesinheritedcharacteris8cs.Itstoresinforma8onneededforgrowthandreproduc8on.Storedinnucleusofallcells.CopiedinMitosis&Meiosis,passedtocells.ButwhatdoesDNAlooklike?

DNAScien8stsknewthatthematerialthatmakesupgenesmustbeabletodotwothings.First,itmustbeabletogiveinstruc8onsforbuildingandmaintainingcells.Second,itmustbeabletobecopiedeach8meacelldivides,sothateachcellcontainsiden8calgenes.Scien8ststhoughtthatthesethingscouldbedoneonlybycomplexmolecules,suchasproteins.TheyweresurprisedtolearnhowmuchtheDNAmoleculecoulddo.

DNAModelDNAismadeofsubunitscallednucleo8des.Nucleo+desconsistsofsugar-phosphatemolecules.Thesugarisknownasdeoxyriboseandarerungsoftheladderaremadeofnitrogenbases.Thefourbasesareadenine,thymine,guanine,andcytosine.Eachbasehasadifferentshape.Scien8stsoienrefertoabasebythefirstleferofthebase,A,T,G,andC.

TimelineofDNA-  1800-scien8stsfoundnucleicacidsinnucleus-  1950-foundwhatnucleicacidDNAwasmadeofbutdidn’t

understandthemeaning-  1952-RosalindFranklindiscoveredthespiralshapeofDNA

(likeatwistedladder)-  1953-JamesWatsonandFrancisCrickmadeamodelofDNA

TimelineofDNA1950-biochemistnamedErwinChargafffoundthattheamountofadenineinDNAalwaysequalstheamountofthymine.Andhefoundthattheamountofguaninealwaysequalstheamountofcytosine.HisfindingsareknownasChargaff’srules.Atthe8meofhisdiscovery,nooneknewtheimportanceofthesefindings.ButChargaff’sruleslaterhelpedscien8stsunderstandthestructureofDNA.

TimelineofDNA1952-SheusedaprocessknownasX-raydiffrac)ontomaketheseimages.Inthisprocess,X-raysareaimedattheDNAmolecule.WhenanX-rayhitsapartofthemolecule,theraybouncesoff.Thepafernmadebythebouncingraysiscapturedonfilm.Franklin’simagessuggestedthatDNAhasaspiralshape.

TimelineofDNA1953-twootherscien8stswerealsotryingtosolvethemysteryofDNA’sstructure.TheywereJamesWatsonandFrancisCrick.AierseeingFranklin’sX-rayimages,WatsonandCrickconcludedthatDNAmustlooklikealong,twistedladder.TheywerethenabletobuildamodelofDNAbyusingsimplematerialsfromtheirlaboratory.TheirmodelperfectlyfitwithbothChargaff’sandFranklin’sfindings.ThemodeleventuallyhelpedexplainhowDNAiscopiedandhowitfunc8onsinthecell.

DNA’sDoubleStructure

AstrandofDNAlookslikeatwistedladder.Thisshapeisknownasadoublehelix(DUBuhlHEELIKS).Thetwosidesoftheladderaremadeofalterna8ngsugarpartsandphosphateparts.Therungsoftheladderaremadeofapairofbases.Adenineononesideofarungalwayspairswiththymineontheotherside.Guaninealwayspairswithcytosine.

DNA’sDoubleStructureNo8cehowthedoublehelixstructurematchesChargaff’sobserva8ons.WhenChargaffseparatedthepartsofasampleofDNA,hefoundthatthematchingbaseswerealwayspresentinequalamounts.Tomodelhowthebasespair,WatsonandCricktriedtomatchChargaff’sobserva8ons.Theyalsousedinforma8onfromchemistsaboutthesizeandshapeofeachofthenucleo8des.Asitturnedout,thewidthoftheDNAladdermatchesthecombinedwidthofthematchingbases.Onlythecorrectpairsofbasesfitwithintheladder’swidth.

MakingcopiesofDNAThepairingofbasesallowsthecelltoreplicate,ormakecopiesof,DNA.Eachbasealwaysbondswithonlyoneotherbase.Thus,pairsofbasesarecomplementarytoeachother,andbothsidesofaDNAmoleculearecomplementary.Forexample,thesequenceCGACwillbondtothesequenceGCTG.ThisisdoneduringInterphasewhenchromosomescopy(theamountofDNAisdoubled).DNAunwindsandacopyofeachsideismadefromtheoppositeside.Nowyouhave2strandsofiden8calDNA.

MakingcopiesofDNADNAmoleculeissplitdownthemiddle,wherethebasesmeet.Thebasesoneachsideofthemoleculeareusedasapafernforanewstrand.Asthebasesontheoriginalmoleculeareexposed,complementarynucleo8desareaddedtoeachsideoftheladder.TwoDNAmoleculesareformed.HalfofeachofthemoleculesisoldDNA,andhalfisnewDNA,every8meitdivides.EachnewcellgetsacompletecopyofDNA.Thejobofunwinding,copying,andre-windingDNAisdonebyproteinswithinthecell.DNAIsusuallyfoundwithseveralkindsofproteins.Otherproteinshelpwithcarryingouttheinstruc8onswrifeninthecodeoftheDNA.

HowDNAWorksAlmosteverycellinyourbodycontainsabout2mofDNA.HowdoesalloftheDNAfitinacell?AndhowdoestheDNAholdacodethataffectsyourtraits?DNAisfoundinthecellsofallorganisms,includingbacteria,mosquitoes,andhumans.EachorganismhasauniquesetofDNA.ButDNAfunc8onsthesamewayinallorganisms.

UnravelingDNADNAisoienwoundaroundproteins,coiledintostrands,andthenbundledupevenmore.Inacellthatlacksanucleus,eachstrandofDNAformsalooseloopwithinthecell.Inacellthathasanucleus,thestrandsofDNAandproteinsarebundledintochromosomes.ThestructureofDNAallowsDNAtoholdinforma8on.Theorderofthebasesononesideofthemoleculeisacodethatcarriesinforma8on.Ageneconsistsofastringofnucleo8desthatgivethecellinforma8onabouthowtomakeaspecifictrait.ThereisanenormousamountofDNA,sotherecanbealargevarietyofgenes.

TheDNAConnec8onTheGene+cCodeThemainfunc8onofgenesistocontrolproduc8onofproteinsinanorganism’scells.Proteinshelptodeterminethesize,shape,color,andmanyothertraitsofanorganism.GenesandDNARecallthatchromosomesarecomposedmostlyofDNA.Belowyoucanseetherela8onshipbetweenchromosomesandDNA.No8cethataDNAmoleculeismadeupoffourdifferentnitrogenbases—adenine(A),thymine(T),guanine(G),andcytosine(C).ThesebasesformtherungsoftheDNA“ladder.”

TheDNAConnec8onAgeneisasec8onofaDNAmoleculethatcontainstheinforma8ontocodeforonespecificprotein.Ageneismadeupofaseriesofbasesinarow.Thebasesinagenearearrangedinaspecificorder—forexample,ATGACGTAC.Asinglegeneonachromosomemaycontainanywherefromseveralhundredtoamillionormoreofthesebases.Eachgeneislocatedataspecificplaceonachromosome.

TheDNAConnec8onOrderoftheBasesAgenecontainsthecodethatdeterminesthestructureofaprotein.Theorderofthenitrogenbasesalongageneformsagene+ccodethatspecifieswhattypeofproteinwillbeproduced.Rememberthatproteinsarelong-chainmoleculesmadeofindividualaminoacids.Inthegene8ccode,agroupofthreeDNAbasescodesforonespecificaminoacid.Forexample,thebasesequenceCGT(cytosineguanine-thymine)alwayscodesfortheaminoacidalanine.Theorderofthethree-basecodeunitsdeterminestheorderinwhichaminoacidsareputtogethertoformaprotein.

NitrogenBases4 types:

Adenine (A) Cytosine (C) Guanine (G) Thymine (T)

*The amount of adenine always equals the amount of

thymine. *The amount of guanine always equals the amount of

cytosine.

**A pairs with T **G pairs with C

HowCellsMakeProteinTheproduc8onofproteinsiscalledproteinsynthesis.Duringproteinsynthesis,thecellusesinforma+onfromageneonachromosometoproduceaspecificprotein.Proteinsynthesistakesplaceontheribosomesinthecytoplasmofacell.Asyouknow,thecytoplasmisoutsidethenucleus.Thechromosomes,however,arefoundinsidethenucleus.How,then,doestheinforma8onneededtoproduceproteinsgetoutofthenucleusandintothecytoplasm?TheRoleofRNA(ribonucleicacid-(RIEbohnooKLEEikASid).Beforeproteinsynthesiscantakeplace,a“messenger”mustfirstcarrythegene8ccodefromtheDNAinsidethenucleusintothecytoplasm.Thisgene8cmessengeriscalledribonucleicacid,orRNA.

HowCellsMakeProteinAlthoughRNAissimilartoDNA,thetwomoleculesdifferinsomeimportantways.UnlikeDNA,whichhastwostrands,RNAhasonlyonestrand.RNAalsocontainsadifferentsugarmoleculefromthesugarfoundinDNA.AnotherdifferencebetweenDNAandRNAisintheirnitrogenbases.LikeDNA,RNAcontainsadenine,guanine,andcytosine.However,insteadofthymine,RNAcontainsuracil(YOORuhsil).TypesofRNAThereareseveraltypesofRNAinvolvedinproteinsynthesis.MessengerRNAcopiesthecodedmessagefromtheDNAinthenucleus,andcarriesthemessagetotheribosomeinthecytoplasm.AnothertypeofRNA,calledtransferRNA,carriesaminoacidsthemtothegrowingprotein

TheDNAConnec8onTransla+ngtheCodeThefirststepisforaDNAmoleculeto“unzip”betweenitsbasepairs.ThenoneofthestrandsofDNAdirectstheproduc8onofastrandofmessengerRNA.ToformtheRNAstrand,RNAbasespairupwiththeDNAbases.TheprocessissimilartotheprocessinwhichDNAreplicates.Cytosinealwayspairswithguanine.However,uracil—notthymine—pairswithadenine.

TheDNAConnec8onThemessengerRNAthenleavesthenucleusandentersthe

cytoplasm.Inthecytoplasm,messengerRNAafachestoaribosome.Ontheribosome,themessengerRNAprovidesthecodefortheproteinmoleculethatwillform.Duringproteinsynthesis,theribosomemovesalongthemessengerRNAstrand.

TheDNAConnec8onMoleculesoftransferRNAafachtothemessengerRNA.ThebasesonthetransferRNA“read”themessagebypairingupthree-lefercodestobasesonthemessengerRNA.Forexample,youcanseethatamoleculeoftransferRNAwiththebasesAAGpairswiththebasesUUConthemessengerRNA.ThemoleculesoftransferRNAcarryspecificaminoacids.Theaminoacidslinkinachain.Theorderoftheaminoacidsinthechainisdeterminedbyheorderofthethree-lefercodesonthemessengerRNA.

TheDNAConnec8onTheproteinmoleculegrowslongeraseachtransferRNAmoleculeputstheaminoaciditiscarryingalongthegrowingproteinchain.Onceanaminoacidisaddedtotheproteinchain,thetransferRNAisreleasedintothecytoplasmandcanpickupanotheraminoacid.EachtransferRNAmoleculealwayspicksupthesameaminoacid.

ChangesinGenesImaginethatyouhavebeeninvitedtorideonanewrollercoasteratthestatefair.Beforeyouclimbintothefrontcar,youaretoldthatsomeofthemetalpartsonthecoasterhavebeenreplacedbypartsmadeofadifferentsubstance.Wouldyous8llwanttoridethisrollercoaster?Perhapsastrongmetalwasusedasasubs8tute.Orperhapsamaterialthatisnotstrongenoughwasused.Imaginewhatwouldhappenifcardboardwereusedinsteadofmetal!

Muta8onsSubs8tutesintherollercoastercanaccidentallyhappeninDNA.Amistakecanoccurinonegeneofachromosome.InsteadofthebaseA,forexample,theDNAmoleculemighthavethebaseG.Changesinthenumber,type,ororderofbasesonapieceofDNAisknownasmuta+ons.Suchamistakeaboveisonetypeofmuta8onthatcanoccurinacell’shereditarymaterial.Recallthatamuta8onisanychangeinageneorchromosome.Muta8onscancauseacelltoproduceanincorrectproteinduringproteinsynthesis.Asaresult,theorganism’strait,orphenotype,maybedifferentfromwhatitnormallywouldhavebeen.Infact,thetermmuta8oncomesfromaLa8nwordthatmeans“change.”

Muta8onsIfabaseisleioutoftheDNA,thisisadele+on.Whenanextrabaseisaddedthisisainser+on.However,themostcommonchangethathappensiswhenthewrongbaseisused,whichisasubs+tu+on.(seeexamplesbelow)

Muta8onsIfamuta8onoccursinabodycell,suchasaskincell,themuta8onwillnotbepassedontotheorganism’soffspring.If,however,amuta8onoccursinasexcell,themuta8oncanbepassedontoanoffspringandaffecttheoffspring’sphenotype.TherearethreepossibleconsequencestochangesinDNA:animprovedtrait,nochange,oraharmfultrait.Fortunately,cellsmakesomeproteinsthatcandetecterrorsinDNA.Whenanerrorisfound,itisusuallyfixed.Butoccasionallytherepairsarenotaccurate,andthemistakesbecomepartofthegene8cmessage.Ifthemuta8onoccursinthesexcells,thechangedgenecanbepassedfromonegenera8ontothenext.

HowdoMuta8onsHappen?Somemuta8onsaretheresultofsmallchangesinanorganism’shereditarymaterial.TheyhappenregularlybecauseofrandomerrorswhenDNAiscopiedordamagedcausedbyabnormalthingsincells.Anyphysicalorchemicalagentthatcancauseamuta8oninDNAiscalledamutagen.Forexample,asinglebasemaybesubs8tutedforanother,oroneormorebasesmayberemovedfromasec8onofDNA.Thistypeofmuta8oncanoccurduringtheDNAreplica8onprocess.Othermuta8onsmayoccurwhenchromosomesdon’tseparatecorrectlyduringmeiosis.Whenthistypeofmuta8onoccurs,acellcanendupwithtoomanyortoofewchromosomes.Thecellcouldalsoendupwithextrasegmentsofchromosomes.

EffectsofMuta8onsBecausemuta8onscanintroducechangesinanorganism,theycanbeasourceofgene8cvariety.Somemuta8onsareharmfultoanorganism.Afewmuta8ons,however,arehelpful,andothersareneitherharmfulnorhelpful.Amuta8onisharmfultoanorganismifitreducestheorganism’schanceforsurvivalandreproduc8on.

EffectsofMuta8onsWhetheramuta8onisharmfulornotdependspartlyontheorganism’senvironment.Themuta8onthatledtotheproduc8onofawhitelemurwouldprobablybeharmfultoanorganisminthewild.Thelemur’swhitecolorwouldmakeitmorevisible,andthuseasierforpredatorstofind.However,awhitelemurinazoohasthesamechanceforsurvivalasabrownlemur.Inazoo,themuta8onneitherhelpsnorharmsthelemur.

Helpfulmuta8ons,ontheotherhand,improveanorganism’schancesforsurvivalandreproduc8on.An8bio8cresistanceinbacteriaisanexample.An8bio8csarechemicalsthatkillbacteria.Genemuta8onshaveenabledsomekindsofbacteriatobecomeresistanttocertainan8bio8cs—thatis,thean8bio8csdonotkillthebacteriathathavethemuta8ons.Themuta8onshaveimprovedthebacteria’sabilitytosurviveandreproduce.