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The chromosomes in heredity
Author: W. S. Sutton
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"THECHROMOSOMESINHEREDITY.WALTERS.SUTTON.Inarecentannouncementofsomeresultsofacriticalstudyofthechromosomesinthevariouscell-generationsofBrac/zystola'theauthorbrieflycalledattentiontoapossiblerelationbetween thephenomenatheredescribedandcertainconclusionsfirstdrawnfromobservationsonplanthydridsbyGregorMendelin21865,andrecentlyconfirmedbyaIlurliberofableinvestigators.FurtherattentionhasalreadybeeticalledtothetheoreticalaspectsofthesubjectinabriefcommunicationbyProfessorE.B.Wilson.3Thepresentpaperisdevotedtoamoredetailed discussionoftheseaspects,thespeculativecharacterofwhichmaybejustifiedbytheattempttoindicatecertainlinesofworkcalculatedtotestthevalidityoftheconclusionsdrawn.Thegeneralconceptionshereadvancedwereevolvedpurelyfromcytologicaldata,beforetheauthorhadknowledgeoftheMendehanprinciples,andarenowpresentedasthecontributionofa cytologistwhocanmakenopretensionstocompletefamiliaritywiththeresultsofexperimentalstudiesonheredity.Aswillappearhereafter,theycompletelysatisfytheconditionsintypicalMendeliancases,anditseemsthatmanyoftheknowndeviationsfromtheMendeliantypemaybeexplainedbyeasilyconceivablevariationsfromthenormalchromosomicprocesses. Ithaslongbeenadmittedthatwemustlooktotheorganizationofthegerm-cellsfortheultimatedeterminationofhereditaryphenomena.Mendelfullyappreciatedthisfactandeveninstitutedspecialexperimentstodeterminethenatureofthatorganization.Fromthemhedrewthebrilliantconclusionthat,while, 1Sutton,WalterS.,�??� OntheMorphologyoftheChromosomeGroupinBrachystolamagna,�? BI0L.BULL.,IV.,1,1902.2Mendel,GregorJohann,�??� VersuchetiherPfianzen-Hybriden,�? Ve'-h.naturf.Vers.iuRi/inn1V.,andinOsterwald'sKiassikeri/eresak/enWissenshaft.Eng@lishtranslationinJoan:.Rot'.II'rt.Soc.,XXVI.,1901.LaterreprintedwithmodificationsandcorrectionsinBateson's�??� Mendel'sPrinciplesofHeredity,�? Cam bridge,5902,p.40.Wilson,E.B.,�??� Mendel'sPrinciplesofHeredityandtheMaturationoftheGerm-Cells,�? Science,XVI.,456.231 232 \VALTERS.SUTTON.intheorganism,maternalandpaternalpotentialitiesarepresentinthefieldofeachcharacter,tilegerm-cellsinrespecttoeachcharacterarepure.Littlewasthenknownofthenatureofcelldivision,andMendelattemptednocomparisonsinthatdirection; buttothosewhoinrecentyearshaverevivedandextendedhisresultstheprobabilityofarelationbetweencell-organizationandcell-divisionhasrepeatedlyoccurred.Bateson'clearlystateshisimpressioninthisregardinthefollowingwords:�??� ItisimpossibletobepresentedwiththefactthatinMendehiancasesthecross-bredproducesonanaverageequalnumbersofgametes ofeachkind,thatistosay,asymmetricalresult,withoutsuspectingthatthisfactmustcorrespondwithsomesymmetricalfigureofdistributionofthegametesinthecelldivisionsbywhichtheyareproduced.�? Nearlyayearagoitbecameapparenttotheauthorthatthehighdegreeoforganizationinthechromosome-groupofthe germ-cellsasshowninBrachystolacouldscarcelybewithoutdefinitesignificanceininheritance,for,asshowninthepaper2alreadyreferredto,ithadappearedthat:I.Thechromosomegroupofthepresynapticgerm-cellsismadeupoftwoequivalentchromosome-series,andthatstronggroundexistsfortheconclusionthatoneoftheseispaternal andtheothermaternal.2.Theprocessofsynapsis(pseudo-reduction)consistsintheunioninpairsofthehomologousmembers(i.e.,thosethatcorrespondinsize)ofthetwoseries.33.Thefirstpost-synapticormaturationmitosisisequationalandhenceresultsinnochromosomicdifferentiation. 4.Thesecondpost-synapticdivisionisareducingdivision,resultingintheseparationofthechromosomeswhichhaveconjugatedinsynapsis,andtheirrelegationtodifferentgerm-cells. @.Thechromosomesretainamorphologicalindividualitythroughoutthevariouscell-divisions.1Bateson,\V.,�??� Mendel'sPrinciplesofHeredity,�? Cambridge,5902,p.30.2Sutton,\V.S.,bc.cit.�??� Theconclusionthatsynapsisinvolvesaunionofpaternalandmaternalchromo somesinpairswasfirstreachedbyMontgomeryin1901.Montgomery,T.H.,Jr.,�??� AStudyoftheChromosomesoftheGerm-CellsofMetazoa,�? Trans.Amer.Phil.Soc.,XX. THECHROMOSOMESINHEREDITY.233Itiswellknownthatintheeggsofmanyformsthematernalandpaternalchromosomegroupsremaindistinctlyindependentofeachotherforaconsiderablenumberofcleavage-mitoses,andwiththisfactinmindtheauthorwasatfirstinclinedtoconclude thatinthereducingdivisionsallthematernalchromosomesmustpasstoonepoleandallthepaternalonestotheother,andthatthegerm-cellsarethusdividedintotwocategorieswhichmightbedescribedasmaternalandpaternalrespectively.Butthisconception,whichisidenticalwiththatrecentlybroughtforwardbyCannon,'wassoonseentobeatvariancewithmanywell knownfactsofbreeding;thus:i.Ifthegerm-cellsofhybridsareofpuredescent,noamountofcross-breedingcouldaccomplishmorethantheconditionofafirst-cross.2.Ifanyanimalorplanthasbuttwocategoriesofgermcells,therecanbeonlyfourdifferentcombinationsintheoff springofasinglepair.3.Ifeithermaternalorpaternalchromosomesareentirelyexcludedfromeveryripegerm-cell,anindividualcannotreceivechromosomes(qualities)frommorethanoneancestorineachgenerationofeachoftheparentallinesofdescent,e.g.,couldnotinheritchromosomes(qualities)frombothpaternalorboth maternalgrandparents.Movedbytheseconsiderationsamorecarefulstudywasmadeofthewholedivision-process,includingthepositionsofthechromosomesinthenucleusbeforedivision,theoriginandformationofthespindle,therelativepositionsofthechromosomesandthedivergingcentrosomes,andthepointofattachmentof thespindlefiberstothechromosomes.Theresultsgavenoevidenceinfavorofparentalpurityofthegameticchromatinasawhole.Onthecontrary,manypointswerediscoveredwhichstronglyindicate2thatthepositionofthebivalentchromosomes�??� CannonsW.A.,�??� ACytologicalBasisfortheMendelianLaws,�? Bull.Torrey BotanicalClub,29,1902.tAbsoluteproofisimpossibleinapure-bredformonaccountoftheimpossibilityofdistinguishingbetweenmaternalandpaternalmembersofanysynapticpair.If,however,suchhybridsasthoseobtainedbyMoenkhaus(Moenkhaus,W.J.,�??� EarlyDevelopmentinCertainHybridSpecies,�? ReportofSecondMeetingofNaturalistsat Chicago,Science,XIII.,323),withfishescanberearedtosexualmaturityabsoluteproofofthispointmaybeexpected.ThisobserverwasableintheearlycellS 234 WALTERS.SUTTON.intheequatorialplateofthereducingdivisionispurelyamatterofchance�??� thatis,thatanychromosomepairmayliewithmaternalorpaternalchromatidindifferentlytowardeitherpoleirrespectiveofthepositionsofotherpairs�??� andhencethatalarge numberofdifferentcombinationsofmaternalandpaternalchromosomesarepossibleinthematuregerm-productsofanindividual.Toillustratethis,wemayconsideraformhavingeightchromosomesinthesomaticandpresynapticgerm-cellsandconsequentlyfourintheripegerm-products.Thegerm-cellseriesofthespeciesingeneralmaybedesignatedbythelettersA,13, C,D,andanycleavagenucleusmaybeconsideredascontainingchromosomesA,B,C,Dfromthefatheranda,b,c,d,fromthemother.SynapsisbeingtheunionofhomologueswouldresultintheformationofthebivalentchromosomesAa,Bb,Cc,Dd,whichwouldagainberesolvedintotheircomponentsbythereducingdivision.Eachoftheripegerm-cellsarisingfrom thereductiondivisionsmustreceiveonememberfromeachofthesytlapticpairs,buttherearesixteenpossiblecombinationsofmaternalandpaternalchromosomesthatwillformacompleteseries,towit:a,B,C,D;A,b,C,D;A,B,c,D;A,B,C,d;a,b,C,D;a,B,c,D;a,B,C,d;a,b,c,d;andtheirconjugatesA,b,c,d;a,B,c,ci;a,h,C,d;a,b,c,D;A,B,c,d;A, b,C,d;A,b,c,D;A,B,C,D.Henceinsteadoftwokindsofgametesanorganismwithfourchromosomesinitsreducedseriesmaygiveriseto16differentkinds;andtheoffspringoftwounrelatedindividualsmaypresenti6x16or256combinations,insteadofthefourtowhichitwouldbelimitedbyahypothesisofparentalpurityofgametes.Feworganisms,moreover,have sofewas8chromosomes,andsinceeachadditionalpairdoublesthenumberofpossiblecombinationsinthegerm-products'andquadruplesthatofthezygotesitisplainthatintheordinaryformhavingfrom24to36chromosomes,thepossibilitiesareimmense.Thetablebelowshowsthenumberofpossiblecom ofcertainfishhybridstodistinguishthematernalfromthepaternalchromosomesbydifferencesinform,andifthesamecanbedoneinmaturation-divisionsthequestionofthedistributionofchromosomesinreductionbecomesaverysimplematterobservation.�??� Thenumberofpossiblecombinationsin,thegerm.productsofasingleindividualofanyspeciesisrepresentedbythesimpleformula2�? inwhichnrepresentsthe numberofchromosomesinthereducedseries. THECHROMOSOMESINHEREDITY.235binationsinformshavingfrom2to36chromosomesinthepresynapticcells. ChromosomesCombinationsinCombinationsinGametes.Zygotes.SomaticSeries.ReducedSenes.2 I 2 44 2 4 i66 3 8 64 8 4 i6 25610 5 32 5,02412 0 64 4,09614 7 528 56,384i6 8 25665,5369 512262,144 20 10 1,0241,048,57622 11 2,0484,194,30424 1.24.09616,777,21626 13 8,1921)7,108,86428 14 56,384268,435,45630 15 32,7681,073,741,824 32 i6�?�� 65,5364,294,967,2963417131,07217,179,869,184iS 202,54468,71Q,476,736ThusifBardeleben'sestimateofsixteenchromosomesforman(thelowestestimatethathasbeenmade)becorrect,eachindividualiscapableofproducing256differentkindsofgerm productswithreferencetotheirchromosomecombinations,andthenumbersofcombinationspossibleintheoffspringofasinglepairis256X256or65,536;whileToxopneustes,with36chromosomes,hasapossibilityof262,144and68,719,476,736differentcombinationsinthegametesofasingleindividualandthezygotesofapairrespectively.Itisthispossibilityofsogreata numberofcombinationsofmaternalandpaternalchromosomesinthegameteswhichservestobringthechromosome-theoryintofinalrelationwiththeknownfactsofheredity;forMendelhimselffollowedouttheactualcombinationsoftwoandthreedistinctivecharactersandfoundthemtobeinheritedindependentlyofoneanotherandtopresentagreatvarietyofcombina tionsinthesecondgeneration.Theconstantsize-differencesobservedinthechromosomesofBracizystolaearlyledmetothesuspicion,which,however,astudyofspermatogenesisalonecouldnotconfirm,thattheindividualchromosomesofthereducedseriesplaydifferentrolesindevelopment.Theconfirmationofthissurmiseappearedlater 236WALTERS.SUTTON.intheresultsobtainedbyBoveri'inastudyoflarva@actuallylackingincertainchromosomesofthenormalseries,whichseemtoleavenoalternativetotheconclusionthatthechromosomesdifferqualitativelyandasindividualsrepresentdistinctpoten tialities.Acceptingthisconclusionweshouldbeabletofindanexactcorrespondencebetweenthebehaviorininheritanceofanychromosomeandthatofthecharactersassociatedwithitintheorganism.Inregardtothecharacters,Mendelfoundthat,ifahybridproducedbycrossingtwoindividualsdifferinginaparticular characterbeself-fertilized,theoffspring,inmostcases,conformtoaperfectlydefiniteruleasregardsthedifferentialcharacter.RepresentingthecharacterasseeninoneoftheoriginalparentsbytheletterAandthatoftheotherbya,thenalltheoffspringarisingbyself-fertilizationofthehybridarerepresentedfromthestandpointofthegivencharacterbytheformulaAA:2Aa:aa.�?? thatis,onefourthreceiveonlythecharacterofoneoftheoriginalpure-bredparents,onefourthonlythatoftheother;whileonehalfthenumberreceivethecharactersofbothoriginalparentsandhencepresenttheconditionofthehybridfromwhichtheysprang.Wehavenotheretoforepossessedgraphicformula@toexpressthecombinationsofchromosomesinsimilarbreedingexperi ments,butitisclearfromthedataalreadygiventhatsuchformula@maynowbeconstructed.ThereducedchromosomeseriesinBrachystolaismadeupofelevenmembers,notwoofwhichareexactlyofthesamesize.TheseIdistinguishedinmypreviouspaperbythelettersA,B,C,...K.Intheunreducedseriestherearetwenty-twoelements2whichcanbeseentomake uptwoserieslikethatofthematuregerm-cells,andhencemaybedesignatedasA,B,C..K+A,B,C..K.Synapsisresultsintheunionofhomologuesandtheproductionofasingleseriesofdouble-elementsthus:AA,BB,CC...KK;andthereducingdivisionaffectstheseparationofthesepairssothatonememberofeachpassestoeachoftheresultinggerm-products. 1Boveri,Th.,�??� UeberMehrpoligeMitosenalsMittelzurAnalysedesZellkerns,�? Verb.a'.Phj's.-Med.Ges.zuWiirzburg,N.F.,Bd.XXXV.,1902.ItappearsfromapersonalletterthatBoverihadnotedthecorrespondencebetweenchromosomicbehaviorasdeduciblefromhisexperimentsandtheresultsonplanthybrids�??asindicatedalsoinfootnoteI,1.c.,p.8i. 2 theaccessorychromosomewhichtakesnopartinsynapsis. THECHROMOSOMESINHEREDITY.237Thereisreasontobelievethatthedivision-productsofagivenchromosomeinBrachystolamaintainintheirrespectiveseriesthesamesizerelationasdidtheparentelement;andthis,takentogetherwiththeevidencethatthevariouschromosomesof theseriesrepresentdistinctivepotentialities,makeitprobablethatagivensize-relationischaracteristicofthephysicalbasisofadefinitesetofcharacters.Buteachchromosomeofanyreducedseriesinthespecieshasahomologueinanyotherseries,andfromtheaboveconsiderationitshouldfollowthatthesehomologuescoverthesamefieldindevelopment.Ifthisbethe casechromosomeAfromthefatheranditshomologue,chromosomea,fromthemotherinthepresynapticcellsoftheoffspringmayberegardedasthephysicalbasesoftheantagonisticunitcharactersAandaoffatherandmotherrespectively.Insynapsis,copulationofthehomologuesgivesrisetothebivalentchromosomeAa,whichasisindicatedabovewould,inthere ducingdivision,beseparatedintothecomponentsAanda.Thesewouldinallcasespasstodifferentgerm-productsandhenceinamonceciousformweshouldhavefoursortsofgametes,a@A9a9 whichwouldyieldfourcombinations,A@±A9=AA+a9=Aaa@A9=aA+a9=aa SincethesecondandthirdofthesearealiketheresultwouldbeexpressedbytheformtilaAA:2Aa:aawhichisthesameasthatgivenforanycharacterinaMendehiancase.Thusthephenoinenaofgerm-celldivisionandofheredityareseentohavethesameessentialfeatures,viz.,purityofunits(chromosomes,charactcrs)andtheindipendenttransmissionofthesame;whileasa corollary,itfollowsineachcasethateachofthetwoantagonisticunits(chromosomes,characters)iscontainedbyexactlyhalfthegametesproduced.Theobservationswhichdealwithcharactershavebeenmade 238 WALTERS.SUTTON.chieflyuponhybrids,whilethecytologicaldataaretheresultofstudyofapure-bredform;butthecorrelationofthetwoisjustifiedbytheobservationofCannonthatthematurationmitosesoffertilehybridsarenormal.Thisbeingthecaseitis necessarytoconclude,asCannonhasalreadypointedout,thatthecourseofvariationsinhybridseitherisaresultofnormalmaturationprocessesorisentirelyindependentofthenature01thosedivisions.Ifweconcludefromtheevidencealreadygiventhatthedoublebasisofhybridcharactersistobefoundinthepairsofhomologouschromosomesofthepresynapticgerm cells,thenwemustalsoconcludethatinpure-bredformslikewise,thepairedarrangementofthechromosomesindicatesadualbasisforeachcharacter.Inahypotheticalspeciesbreediiigabsolutelytrue,therefore,allthechromosomesorsubdivisionsofchromosomesrepresentinganygivencharacterwouldhavetobeexactlyalike,sincethecombinationofanytwo ofthemwouldproduceauniformresult.Asamatteroffact,however,specificcharactersarenotfoundtobeconstantquantitiesbutvarywithincertainlimits;andmanyofthevariationsareknowntobeinheritable.Henceitseemshighlyprobablethathomologouschromatin-entitiesarenotusuallyofstrictlyuniformconstitution,butpresentminorvariationscorrespondingto thevariousexpressionsofthecharactertheyrepresent.Inotherwords,itisprobablethatspecificdifferencesandindividualvariationsarealiketraceabletoacommonsource,whichisadifferenceintheconstitutionofhomologouschromatin-entities.Slightdifferencesinhomologueswouldmeancorresponding,slightvariationsinthecharacterconcerned�??� acorrespondence whichisactuallyseenincasesofinbreeding,wherevariationiswellknowntobeminimizedandwhereobviouslyinthecaseofmanyofthechromosomepairsbothmembersmustbederivedfromthesamechromosomeofarecentcommonancestorandhencebepracticallyidentical.Inthevariousformsofparthenogenesiswemeettheclosest kindofinbreedingandabriefconsiderationofthevariabilitytobeexpectedineach,fromthestandpointofthechromosometheory,mayserveasaguidetosuchresearchaswilltestthe1Cannon,\V.A.,bc.cit. THECHROMO@OMESINHEREDITY.239validityofthelatter.Thesimplestform,ofwhichchemicalparthenogenesisinsea-urchinsisanexamble,isthatinwhichtheorganismhasonlyasinglechromosomeseries,toberepresentedbyA,B,C,D...N.Thusfarnorecognized@casesofthistype havebeenrearedtosexualmaturity,butitistobeexpectedthatnoreducingdivisionwillbefoundinthematurationofsuchforms,andthattheirparthenogeneticoffspringwillexactlyresembletheimmediateparent.Incasesofnaturalparthenogenesiswhichareaccompaniedbythereentranceofthesecondpolarbodyanditsfusionwiththe egg-nucleus(oritsfailuretoform)theremustbeadoublechromosomeseries;butwemaydistinguishtwoclassesaccordingasthereducingprocessisaccomplishedinthefirstorthesecondmaturationdivision.'Ifreductionisaccomplishedinthefirstdivision,onehalfthechromosomesoftheoögoniaarethrownoutandlostinthefirstpolarbody.Theseconddivision,being equational,wouldresultinapolarbodywhichwouldbetheexactduplicateoftheegg-nucleusasfaraschromosomesareconcernedandwhichaccordingly,byitsreentrancewouldaddnothingnewtotheegg-series.Theseriesafterfusionwould,therefore,berepresentedbythelettersA,B,C,D...N+A,B,C,D...N.Ifsuchatypeofparthenogenesisweretofollow sexualreproduction,thefirstgenerationofoffspringmightbeexpectedtodiffermateriallyfromtheparentbyreasonofthecastingout,inthefirstpolarbody,ofchromosomesrepresentingcertaindominantcharacters,andtheconsequentappearanceintheoffspringofthecorrespondingrecessives.Subsequentparthenogeneticgenerations,however,wouldineachcasebeendowed withachromosomeseriesexactlysimilartothatoftheimmediateparentandaccordinglymightbeexpectedtoshowthesamecharacters.Incasetheseconddivisionofaparthenogeneticeggwerethereducingdivision,thereentranceorsuppressionofthesecondpolarbodywouldaccomplishtherestorationoftheoögonial chromosome-series:Inthiscasethefirstparthenogeneticgen�??� Eithermustberegardedaspossibleincaseswherewehavenodefiniteknowl.edgesinceitisregularlydescribedasthesecondintheOrthoptera(McClung,SuttonandCopepoda(RUckert,HAcker)whileintheHemiptera-Heteropteraitisbelievedtobethefirst(Paulmier,Montgomery). 240 WALTERS.SUTTON.erationmightbeexpectedtoduplicatethecharactersoftheparent(ifenvironmentalconditionsremainedunchanged)andlittleornovariabilitywouldbeexpectedaslongasparthenogenesispersisted. Inrelationtotheseproblemsthereisgreatneedofasimultaneousstudyofthegerm-celldivisionsandthevariationofperiodicallyparthenogeneticforms.Wehaveseenreason,intheforegoingconsiderations,tobelievethatthereisadefiniterelationbetweenchromosomesandallelomorphs'orunitcharactersbutwehavenotbeforeinquired whetheranentirechromosomeoronlyapartofoneistoberegardedasthebasisofasingleallelomorph.Theanswermustunquestionablybeinfavorofthelatterpossibility,forotherwisethenumberofdistinctcharacterspossessedbyanindividualcouldnotexceedthenumberofchromosomesinthegerm-products;whichisundoubtedlycontrarytofact.Wemust,there fore,assumethatsomechromosomesatleastarerelatedtoanumberofdifferentallelomorphs.Ifthen,thechromosomespermanentlyretaintheirindividuality,itfollowsthatalltheallelomorphsrepresentedbyanyonechromosomemustbeinheritedtogether.Ontheotherhand,itisnotnecessarytoassumethatallmustbeapparentintheorganism,forherethe questionofdominanceentersanditisnotyetknownthatdominanceisafunctionofanentirechromosome.Itisconceivablethatthechromosomemaybedivisibleintosmallerentities(somewhatasWeismannassumes),whichrepresenttheallelomorphsandmaybedominantorrecessiveindependently.Inthiswaythesamechromosomemightatonetimerepresentbothdomi nantandrecessiveallelomorphs.Suchaconceptioninfinitelyincreasesthenumberofpossiblecombinationsofcharactersasactuallyseenintheindividualsandunfortunatelyatthesametimeincreasesthedifficultyofdeterminingwhatcharactersareinheritedtogether,sinceusuallyrecessivechromatinentities(allelomorphs?)constantlyassociatedin thesamechromosomewithusuallydominantoneswouldevadedetectionforgenerationsandthenbecomingdominantmightappearasreversionsinaveryconfusingmanner.IBateson'sterm.- THECHROMOSOMESINHEREDITY.241Intheirexperimentson.Tktatthiola,Bate3onandSaunders'mentiontwocasesofcorrelatedqualitieswhichmaybeexplainedbytheassociationoftheirphysicalbasesinthesamechromosome.�??� Incertaincombinationstherewasclosecorrelationbe tween(a)greencolorofseedandhoariness,(b)browncolorofseedandgrabrousness.Inothercombinationssuchcorrelationwasentirelywanting.�? Suchresultsmaybeduetotheassociationinthesamechromosomesofthephysicalbasesofthetwocharacters.Whenclosecorrelationwasobserved,bothmaybesupposedtohavedominatedtheirhomologues;whencorrelation waswanting,onemayhavebeendominantandtheotherreces@sive.Inthenextparagraphtothatquotedisthestatement:�??� Therulethatplantswithflowerseitherpurpleorclaretarosefromgreenseedswasuniversal.�? Heremaybeacaseofconstantdominanceoftwoassociatedchromatin-entities.Dominanceisnotaconceptionwhichgrowsoutofpurely cytologicalconsideration.Cytologymerelyshowsusthepresenceinacelloftwochromosomes,eitherofwhichiscapableofproducingsomeexpressionofagivencharacter,anditislefttoexperimentineachcasetoshowwhattheeffectofthiscombinedactionwillbe.Theexperiment2hasshownthatanyoneofthethreetheoreticalpossibilitiesmayberealized,viz:(I)Oneorthe othermaydominateandobscureitshomologue.(2)Theresultmaybeacompromiseinwhichtheeffectofeachchromosomeistoberecognized.(3)Thecombinedactionofthetwomayresultinanentirelynewcastofcharacter.Incasesbelongingtothefirstcategory,thevisiblequality(allelomorph,chromatinentity)wasdescribedbyMendelasdominantandtheotheras recessive,andtheexperimentsofBatesonandSaundersandothers,aswellasthoseofMendel,haveshownthatinmanycasesadominantcharactertendstoremaindominantduringsuccessivegenerationsiftheenvironmentisnotmateriallychanged.Nevertheless,someexperimentscitedbyBateson2gotoshowthatdominancemaybevariableordefective.Furthermore,itis notonlyconceivable,buthighlyprobablethatinmost,ifnotall1BatesonandSaunders,ExperimentalStudiesinthePhysiologyofHeredity.ReportstotheEvolutionCommittee,I.London,1902,p.8i,paragraphsIiand12.2Cf.BatesonandSaunders,bc.cit. 242 WALTERS.SUTTON.cases,therearemanydifferentexpressionsofeachcharacter(i.e.,manydifferentallelomorphsassuggestedbyBateson3inregardtohumanstature),whichonvariouscombinationswouldnecessarilyexhibitrelativedominance.Theexperimentswithpeas showanalmostconstantdominanceofcertainalleloniorphs,suchasroundoverwrinkledinseeds,andofyellowovergreenincotyledons;butitisworthyofnotethathere,asinmostMendelianexperiments,onlytwoantagonisticcharactershavebeenused.Investigationsonvarieties,ingeneralsimilar,butexhibitingdifferentexpressionsofsomeparticularcharacter,willcer tainlyyieldinstructiveresults.Bateson'sobservationsoncrossesbetweensingle-,rose-andpea-combedfowls,representasimpleformofsuchacaseandmaybeexpectedoncompletiontoaddmuchtoourknowlegeofthenatureofdominance.InadditiontothemanyexamplesbroughtforwardbyBatesoninsupportoftheMendelianprinciplehecitesthreetypesof caseswhicharetoberegardedasnon-Mendelian.Theseare:i.Theordinaryblendedinheritanceofcontinuousvariation.2.Casesinwhichtheformresultingfromthefirstcrossbreedstrue.3.The�??� falsehybrids�? ofMillardet. i.BlendedInheritance.�??� IntreatingofthisclassBatesonclearlystatesthepossibilitythatthecasemaybeoneentirely�??� apartfromthosetowhichMendel'sprinciplesapply,�? butgoesontoshowhowitmaypossiblybebroughtintorelationwithtrueMendeliancases.Hesaysinpart:�??� Itmustberecognizedthatin,fo@example,thestatureofacivilizedraceofman,atypically continuouscharacter,theremustcertainlybeonanyhypothesismorethanonepairofpossibleallelomorphs.Theremaybemanysuchpairs,butwehavenocertaintythatthenumberofsuchpairsandconsequentlyofthedifferentkindsofgametesarealtogetherunlimited,eveninregardtostature.Iftherewereevensofewas,say,fourorfivepairsofpossibleallelomorphs,the varioushomo-andheterozygouscombinationsmight,onseriation,givesonearanapproachtoacontinuouscurvethatthepurityoftheelementswouldbeunsuspected,andtheirdetectionpracticallyimpossible.�? Thishypothesis,whichpresentsnodifficultiesfromthepointofviewofthechromosometheory,is THECHROMOSOMESINHEREDITY.243sufficientinthepresentstateofourknowledgetobringmanycasesofapparentlycontinuousvariationintodefiniterelationwithstrictlyMendeliancases;but,ontheotherhand,itseemsprobable,asalreadynoted(p.22!),thattheindividualvariationin manycharactersnowthoughttobestrictlyMendelianmayprovetobe'duetotheexistenceinthespeciesofmanyvariationsofwhatmayberegardedasthetypeallelomorphs,accompanyingsimilarvariationsofthehomologouschromatinentitiesrepresentingthosetypes. 2.FirstCrossesthatBreedTrite.�??� Itisobviousthatinthegerm-cellsoftrue-breedinghybrids'therecanbenoqualitativereduction.Inthenormalprocesssynapsismustbeaccountedforbytheassumptionofanaffinityexistingbetweenmaternalandpaternalhomologues,andconverselyreductionisthedisappearanceofthataffinityoritsneutralizationbysomegreater force.NowinHieraciumthecharactersofthehybridarefrequentlyintermediatebetweenthoseofthetwoparents,showingthatbothallelomorphs(orchromatin-entities)areatwork,butonselffertilizationthereisnoresolutionofa.llelomorphs(reductiondivision).Onthecontrary,allthegerm-cellsareequivalent,asshownbythefactthatallcombinationsproducesimilaroff springwhichinturnaresimilartotheparent.ThesuggestionmadebyBatesoninanotherconnection,that�??� ifoneallelomorphwerealoneproducedbythemaleandtheotherbythefemaleweshouldhaveaspeciesconsistingonlyofheterozygotes,�? whichwouldcometrueaslongasbredtogether,atfirstsightseemslogicallyapplicabletothesecases.Forsuchanidea,however, wecanfindnocytologicaljustification,sinceifanyreductionoccursbothchromosomesoccurinbothmaleandfemalegermcellsinequalnumbers;andfurther,theevidenceisinfavorofagreatvarietyofcombinationsofmaternalandpaternalchromosomesinthegerm-cellssothattheexactchromosomegroupofahybridparentcouldhardlybeduplicatedexceptbyfusionof theverypairofcellsseparatedbythereducingdivision.Amoreplausibleexplanationfromthecytologicalstandpointisthattheunionofthechromosomesinsynapsisissofirmthatnoreductioncantakeplace,1.e.,thatineachcase,apaternalandCf.Mendel'sexperimentsonHieracium. 244WALTERS.SUTTON.amaternalchromosomefusepermanentlytoformanewchromosomewhichsubsequentlydividesonlyequationally.Theresultmustbegerm-cellswhichareidenticalwithoneanotherandwiththoseoftheparents,andhenceself-fertilizationwouldpro duceoffspringpracticallywithoutvariation.Ifthisexplanationbethecorrectonetheprocessisdistinctlypathologicalandhenceitisnotsurprisingthatsuchcases,asnotedbyBateson,shouldoftenpresent�??� aconsiderabledegreeofsterility.�? 3.The�??� FalseHybrids�? ofMillardet.�??� Millardet,deVriesandBatesonhavealldescribedexperimentsinwhichtheoffspring resultingfroniacrossbetweendissimilarindividualsshowedthecharacterofoneparentonly,thoseoftheotherparentbeingshownbyfurtherexperimenttobelostpermanently.TheobviouscytologicalexplanationofsuchaphenomenonishintedatbyBatesoninthewords�??� SuchphenomenamayperhapsberegardedasfulfillingtheconceptionofStrasburgerandBoveri, thatfertilizationmayconsistoftwodistinctoperations,thestimulustodevelopmentandtheunionofcharactersinthezygote.�? 1Dlvi�??� sionoftheeggwithoutfusionofthepronucleiinawell-knownphenomenonhavingbeenobservedineggstreatedwithchloral(Hertwigbrothers)orether(Wilson)andmaybesupposedtooccurundercertainunusualconditionsinnature.Intheex perimentsmentioned,however,bothpronucleicontinuetodivideseparately,whileforacytologicalexplanationoftheoccurrenceof�??� falsehybrids�? itisnecessarytoconceivenotonlythefailureofthenucleitocopulatebuttheentiredisappearanceofoneofthem.Suchacasewouldbecomparabletothatofchemicallyinducedparthenogenesisortothefertilizationofenucleateegg fragments,accordingasthenucleusremainingwasmaternalorpaternal.Speculationinthisconnection,however,isunprofitableexceptingsofarasitmayserveasaguidetoresearch.AcarefulstudyofthefertilizationofsuchcasesasMillardet'sstrawberries,deVries'sOenotheraandBateson'sMattiziolacrosseswillnodoubtbeproductiveofimmediateandpositiveresults. Mosaics.�??Afourthclassofnon-Mendeliancases,the�??� mosaics�? or�??� piebalds�? constituteagroupinrelationtowhich,asIbelieve,onlynegativeevidenceistobeexpected�??� BatesonandSaunders,bc.cit.,p.154. THECHROMOSOMESINHEREDIT\@.245fromdirectcytologicalstudy.Agoodexampleoftheclassisthe�??� mosaic�? fruitofDaturaobtainedbyBatesonandSaunders,which,althoughingeneralexhibitingthethornlessrecessivecondition,showedinexceptionalcasesathornypatch.Of thiscaseBatesonsays:�??� Unlessthisisanoriginalsportonthepartoftheindividual,suchaphenomenonmaybetakenasindicatingthatthegerm-cellsmayalsohavebeenmosaic.�? Imustconfessmyfailuretocomprehendjustwhatisheremeantbymosaicgerm-cells.Ihaveattemptedtoshowthatinallprobabilitythegerm-cellsarenormallyamosaicofmaternaland paternalchromosomes,butveryevidentlythisisnotBateson'smeaning.FromthestandpointofthechromosometheoryIwouldsuggestapossibleexplanationoftheconditionsasfollows:\Vehavealreadyassumedthatthesomaticchromosomegroup,havingasimilarnumberofmemberstothatofthecleavage nucleusandderivedfromitbyequationdivisions,ismadeupintilesamewayofpairsofhomologouschromosomes.Everysomaticcell,bythisconception,mustcontainadoublebasisinthefieldofeachcharacteritiscapableofexpressing.InstrictlyMendeliancasesoneoftilehomologuesisuniformlydominantthroughoutthepartsoftheorganisminwhichthecharacteris exhibited.Asal'readynoted,however,itisunlikelythatallthedescendantsofadominantchromatinentitywillbedominant.ThisisshownbytheexperimentofdeVrieswithsugarbeets,whicharenormallybiennialbutalwaysproduceasmallpercentageofannualplantsor�??� runners,�? whichlatterareregardedasrecessives.Thepercentageoftheserunnersmaybein creasedbyrearingtheplanisunderunfavorableconditionsandthisistakenasevidencethattherecessiveallelomorphsmaybecomedominantundersuchconditions.'Ifeachcellcontainsmaternalandpaternalpotentialitiesinregardtoeachcharacter,andifdominanceisnotacommonfunctionofoneofthese,thereisnothingtoshowwhyasaresultof somedisturbingfactoronebodyofchromatinmaynotbecalledintoactivityinonegroupofcellsanditshomologueinanother.ThiswouldproducejustthesortofamosaicwhichBatesonand1Cf.BatesonandSaunders,pp.135,236. 246WALTERS.SUTTON.SaundersfoundinDaturaorasTchermak'spiedyellowandgreenpeasobtainedbycrossingtheTelephonepeawithyellowvarieties.Corrensdescribestheconditionaspa?cilodynamousandhisconceptionofthecausesofthephenomenonasIunderstandit isparallelwiththatwhichIhaveoutlinedabove.ThelogicalpossibilitysuggestedbyBateson'thattherecessiveislandsinsuchcasesasthemosaicpeamaybeduetorecessiveallelomorphsinthepairedstatedoesnotaccordwiththetheoryofachromosomicbasisforthoseallelomorphs,sincethechromosomegroups,bothofcellsshowingtherecessivecharacterand ofneighboringcellsshowingthedominantone,arederived,sofarasweknow,bylongitudinalorequationdivisionfromthe-chromosomesofthesameoriginalcleavagenucleusandhencemustbealike.Theapplicationofthetheoryheresuggestedmaybeputtotestbyanexperimentinwhichhybridsofdissimilartrue-breed ingparentagearecrossedandathirdgenerationof�??� quarterbloods�? produced.Mosaicsoccurringinsuchanorganism,ifthistheorybecorrect,wouldshowonecharacterresemblingthatofoneofthematernalgrandparentsandoneresemblingthatofoneoftheoriginalpure-bredsofthepaternalside.Ifbothcharactersofthemosaicshouldbeclearlypaternalormaternal thetheoryasoutlinedisproveninadequate,sinceoneofeachpairofchromosomes,andhencethecorrespondingcharactergroup,isthrownoutbythereduction-divisionineachgeneration.Inconsideringthebehaviorofthetwochiomosomesformingthebasisofanygivencharacter,itwasnotedthatinsomecases theheterozygotecharacterresultingfromthecombinationsofdissimilarallelomorphsissometimestotallyunlikeeitherofthelatter.ThusMendelfoundthatincrossesbetweenpeasrespectivelyiand6feetinheighttheoffspringrangedfrom6to754feet.Indiscussingsimilarcases,Batesoncallsattentiontothelightwhichwouldbethrownonthephenomenonifweventured toassumethatthebasesofthetwoallelomorphsconcernedarechemicalcompounds;andhecomparesthebehavioroftheallelomorphstothereactionofsodiumandchlorineintheformation IBateonandSaunders,p.156. THECHROMOSOMESINHEREDITY.247ofsalt.Tileresultsofchemicalanalysisshowthatoneofthemostcharacteristicfeaturesofchromatinisalargepercentagecontentofhighlycomplexandvariablechemicalcompounds,thenucleo-proteids,andthereforeif,asassumedinthetheoryhere advanced,thechromosomesarethebasesofdefinitehereditarycharacters,thesuggestionofBatesonbecomesmorethanamerelyinterestingcomparison.Wehaveseenreasoninthecaseofthetrue-breedinghybridstosuspectthatthetransmissionbythehybridofheterozygotecharactersmaybeduetopermanantunionofthehomologous chromosomes.Fromthisitisbutashortsteptotheconclusionthatevenif,asisnormallythecase,thechromosomesdonotfusepermanently,theveryfactoftheirassociationinthesameliquidmediummayallowapossibilityofacertaindegreeofchemicalinteraction.Thismustnormallybeslight,sinceitseffectsdonotappeartobevisibleinasinglegeneration;buttheslightest ofvariationasaresultofrepeatednewassociation,eventhoughittendindiversedirections,mustintime,guidedbynaturalselection,resultinanappreciabledifferenceinadefinitedirectionbetweenachromosomeanditsdirectdescendantandhencebetweenthecharactersassociatedwiththem.Inthiswehaveasuggestionofapossiblecauseofindividualvariationinhomol ogouschromosomeswhichwehavealreadyseenreasontosuspect(pp.221and226).Finally,wemaybrieflyconsidercertainobservationswhichseematfirstsighttoprecludethegeneralapplicabilityoftheconclusionsherebroughtout.Ifitbeadmittedthatthephenomenonofcharacter-reductiondiscoveredbyMendelistheex pressionofchromosome-reduction,itfollowsthatformswhichvaryaccordingtoMendel'slawmustpresentareducingdivision.Butthevertebratesandfloweringplants�??theveryformsfromwhichmostoftheMendelianresultshavebeenobtained�??havebeenrepeatedlydescribedasnotexhibitingareducingdivision.Here,therefore,isadiscrepancyofwhichIventuretoindicate apossibleexplanationinthesuggestionfirstmadebyFick1andmorerecentlybyMontgomery.2Thisistotheeffectthatin1Fick,R.,�??� MittheilungueberEireifungbeiAmphibien,�? Supp.Anal.Anz.,XVI.2Montgomery,T.H.,Jr.,bc.cit. 248WALTERS.SUTTON.synapsisasitoccursinvertebratesandotherformspossessingloop-shapedchromosomes,theunionissidebysideinsteadofend-to-endtoasinArthropods.Invertebrates,twoparallellongitudinalsplits,theforerunnersofthetwofollowingdivisions, appearinthechromosomesoftheprimaryspermatocyteprophases.Bothbeinglongitudinal,theyhavebeendescribedasequationdivisions,butifitshallbefoundpossibletotraceonetotheoriginallineofunionofthetwospermatogonialchromosomessidebysideinsynapsis,thatdivisionmustbeconceivedasatruereduction.Anumberofobservationssupportingthisview willbebroughtforwardinmyforthcomingworkonBracizystola.Again,ifthenormalcourseofinheritancedependsupontheaccuratechromatin-divisionaccomplishedbymitosis,itwouldappearthattheinterjection,intoanypartofthegermcycle,ofthegrossprocessesofamitosiscouldresultonlyinaradicaldeviationfromthatnormalcourse.Suchanoccurrencehasactually beendescribedbyMeves,McGregorandothersintheprimaryspermatogoniaofamphibians.Inthesecases,however,itappearsthatfissionofthecell-bodydoesnotnecessarilyfollowamitoticdivisionofthenucleus.Iwouldsuggest,therefore,thepossibilitythattheprocessmaybeofnosignificanceininheritance,sincebythedisappearanceofthenuclearmembranesin preparationforthefirstmitoticdivision,theoriginalconditionisrestoged,andthechrotnosomesmayentertheequatorialplateasifnoamitoticprocesshadintervened.Thereisoneobservationinconnectionwiththeaccessorychromosomewhichdeservesmentioninanytreatmentofthechromosomesasagentsinheredity.Thiselementalwaysdi videslongitudinallyandhenceprobablyequationally.Itfailstodivideinthefirstmaturationmitosis,inwhichtheordinarychromosomesaredividedequationally,butpassesentiretooneoftheresultingcells.Inthesecondmaturationdivision,bywhichthereductionoftheordinarychromosomesiseffected,theaccessorydivideslongitudinally.2 IItisofinterestinconnectionwiththisquestionthatthereoccursregularlyineachofthespermatogonialgenerationsinBrachystolaaconditionofthenucleuswhichsuggestsamitosisbutwhichinrealityisnothingmorethantheenclosureofthedifferentchromosomesinpartiallyseparatedvesicles.Cf.Sutton,W.S.,�??� TheSpermatogonialDivisionsinBrachytolaMagna,�? A'zns.Univ.Quart.,IX.,2.2ThechromosomexofProtenor,whichof,allchromosomesinnon-orthopteran THECHROMOSOMESINHEREDITY.249MyobservationsinregardtotheaccessorychromosomelendsupporttothehypothesisofMcClung'thatofthefourspermatozoaarisingfromasingleprimaryspermatocyte,thosetwowhichcontainthiselemententerintotheformationofmaleoff spring,whiletheothertwo,whichreceiveonlyordinarychromosomestakepartintheproductionoffemales.Ifthishypothesisbetrue,thenitisplainthatinthecharacterofsexthereductionoccursinthefirstmaturationmitosis,sinceitisthisdivisionwhichseparatescellscapableofproducingonlymalesfromthosecapableofproducingonlyfemales.Thusweareconfronted withtheprobabilitythatreductioninthefieldofonecharacteroccursinoneofthematurationdivisionsandthatofalltheremainingcharactersintheotherdivision.Thesignificanceofsuchanarrangement,thoughnoteasyofperception,isneverthelessgreat.Asregardstheirchromosomegroups,thetwocellsresultingfromeachreductionmitosis@reconjugatesand,there fore,oppositesfromthestandpointofanyindividualcharacter.ThusifweconsiderahypotheticalformhavingeightchromosomescomprisingthepaternalseriesA,B,C',Dandthematernalseriesa,6,c,d@oneofthecellsresultingfromthereductiondivisionmightcontaintheseriesA,6,c,D,inwhichcaseitssistercellwouldreceivetheconjugateseriesa,B,C,d.Itisplain thattheseconjugates,differingfromeachotherineverypossiblecharacter,representthemostwidelydifforentspermstheorganismcanproduce.Nowifreductioninthesex-determiningchromatinalsotookplaceinthisdivisionitisapparentthatthesetwodiametricallyoppositeserieswouldenterintoindividualsofdifferentsexes;butifthesex-reductionispreviouslyaccom plishedbytheasymmetricaldistributionoftheaccessoryinthefirstdivision,thenboththemembersofeachconjugatepairmusttakepartintheproductioneitherofmalesoroffemalesandthusformsmostcloselyresembletheaccessory,isalsodescribedbyMontgomery(1902) asdividinginthereducingdivision,andfailingtodivideintheequationdivision�??atactwhichisthemoreremarkablebecauseinProtenor,asinallHemiFtera.Heteropterathusfardescribed,reductionisaccomplishedinthefirstmaturationdivision.�??� McClung,C.H.,�??� TheAccessoryChromosome�??� SexDeterminant?�? Bioi..BULL.,III.,iand2,1902.�??� NotesontheAccessoryChromosome,�? Anal.Anz.,XX.,pp.220�??226. 250 WALTERS.SUTTON.allextremesofchromosomecombinationareprovidedforwithinthelimitsofeachsex.POSTSCRIPT.TheinterestingandimportantcommunicationofGuyer'on �??� HybridismandtheGerm-Cell�? isreceivedtoolateforconsiderationinthebodyofthispaper.Thisinvestigatoralsohasappliedconclusionsfromcytologicaldatatotheexplanationofcertainphenomenaofheredity,-@ndhiscomparativeobservationsonthespermatogenesisoffertileandinfertilehybridsareanimportantcontributiontothecytologicalstudyofthesubject.The conclusionsdrawnareofgreatinterestbut,Ithink,insomecases,opentocriticism.Inassumingthatthereisa�??� segregationofmaternalandpaternalchromosomesintoseparatecells,whichmaybeconsidered�??� pure'germ-cellscontainingqualitiesofonlyonespecies�? (p.19),herepeatstheerrorofCannonwhichhasalreadybeendealtwithintheearlypartofthispaper.No mentionismadeinthepaperofMendel'slawbutinconsideringtheinbredpigeonhybridsfromwhichhismaterialwasobtained,theauthorexpresseshisfamiliaritywithmanifestationsoftheMendelianprinciplebythestatementthat�??� inthethirdgenerationthereisgenerallyareturntotheoriginalcolorsofthegrandparents.�? Incaseswhichseemtoresembleonegrand parentinallparticularsitisclearthattheconceptionofpuregerm-cellsmaybestrictlyapplied,buttheauthorwasfamiliarwithcasesofinbredhybridswhichplainlyshowmixtures.Theseheisinclinedtoexplainintwowaysasfollows(I)�??� Unionoftwocellsrepresentingeachofthetwooriginalspecieswouldyieldanoffspringofthemixedtype.�? (2)�??� Besides throughthemixingjustindicated,variabilitymaybeduealsoinsomecasestothenotinfrequentinequalitiesinthedivisionofindividualchromosomes,throughwhichvaryingproportionsofthechromatinofeachspeciesmayappearincertainofthematuregerm-cells�? (p.20).Thefirstoftheseexplanationswouldaccordwiththeresultof Mendelianexperimentbutforthefactthatitiserroneouslyapplied(andwithoutcytologicalgrounds)toallthecharactersorchromosomesinsteadoftoindividuals.Asforthesecond�??� Guyer,M.F.,�??� HybridismandtheGerm.CeIl,�? BulletinoftheUniversityofCincinnati,No.22,1902.- "
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