Download Ok so we are going to focus on a set of chromosomes coming down

Oksowearegoingtofocusonasetofchromosomescomingdownhere.Thissetof
chromosomesiseventuallygoingtomakeitallthewaydowntoman,andasitcomesdown
herewe'llblowitup.We'regoingtofocusinparticularonthisonepair.Anordinarypairof
autosomesthatbecomethexandy,we'llcallitProtoXandProtoY.Nowweknowthatin
meiosisthingsfirstgetduplicated.Sowe'regoingtoneed4nstage,andthisisjustareminder
ofwhatwesawyesterdaythatthisordinarypairofautosomes,destinedtobecomeXandYcan
recombineandshufflegenes.Theycandoitateitherendlikeso.Thisisallreviewforyou,this
isallfamiliarstuff.Andjusttoremindyouthatrecombinationcanoccuranywherealongthe
lengthofthisordinarypairofautosomes.Nowjusttosimplifythroughouttherestofthe
animation,we'lljustshowoneversionofthexandthey.Hereisanancestralgene,whichhas
nowmutatedontheProtoY.Thisistheinitiatingeventofsexchromosomeevolution.We've
gotthistyrannicalmaledetermininggene.AndhereisRPS4,again,anancientancestor,which
survivestodayontheYchromosome.Nowhowisrecombinationsuppressedbetweenthex
andthey?WellitturnsouttheYunderwentaseriesofinversions,justliketheoneshown
here.We'llseeafewmore.Asaresultofalltheinversions,thexandtheynolongernicely
align.UGHiftheycan'tswapparts,what'sgoingtohappen?ThefruitsandveggiesontheYrot,
theultimate??formofrottingisjustplaingettinglost,ordeleted.SotheYisgettingshorter,
andnowrecombinationisrestrictedtotheseregions.Thesearestillnicelyaligned,nicely
pairedhereattheends.Alrightwe'regoingtozoombacktoourtimeline.We'renowgoingto
crossthepointwherewepartcompanywiththeduckbilledplatypus,andwe'regoinghave
someofthesamethemes.Wearegoingtohavesomeofthesamethemesreappear.We're
goingtohavethisYundergoaninversion,whichissortofaninternalrecombinationevent.It's
abilitytopairwiththeXisagain,reduced.Nowwhyisn'ttheXrotting?TheXisn'trotting
becausetwoX'scanstillrecombineinfemalemeiosis.SogenesontheXremainhealthy
becauseofongoingrecombinationonthefemaleside,whereastheYisrottingbecauseit'sonly
beingpassedthroughaclonallineofmales.Letthetaperoll.Ohsomorerotting,thisisjust...
justcoveryoureyesifyoucan'tstandthesightofit.Andit'sveryhardforsomeonewhoworks
ontheYchromosometoactuallyseethis.Thiscouldverywellbecontinuoustothisveryday.
Oksonowwe'regoingtopassthepointwherewepartcompanywiththekangaroo,andnow
somethinginterestingisgoingtohappen.Anotherautosomeisgoingtocontributeafresh
supplyofgenestoboththeXandtheY.Thiskindofrenewsthegame,startsthegamerolling
again,butlooktheYjustdidanotherinversionevenwithinsomeofthisnewlyacquired
autosomalDNA.Soguesswhat'sgoingtohappen?Morerottingfruitsandveggies.Thisisstill
happening.It'sprobablynothappeningintoomanyofthemenintheroomrightnow,butX-Y
recombinationisstillcontinuingjustoutattheends.Ithinkwe'regoingtobedesperatelyin
needofsomenewkindofthemehere.Fortunately,aswemovenowdownintoprimate
evolution,wellwe'renotquitethereyet,butwe'reshrinking.Reallyatoughtime...Thelast300
millionyearshavebeenquitetough,nowwe'regettingdownintoprimateevolution,andsome
goodstuffisgoingtohappentousguys.OK,nowthisotherautosomehasaspermatogenesis
gene,it'scalledDAZL.Thisautosomeiskindlygoingtocontributeacopyofthe
spermatogenesisgenetotheYwithoutaskingpermissionfromtheXchromosome.Let'sletthe
taperoll.Sowe'regoingtoseeacopy,asthetaperollswe'llseeacopyoftheDAZLgeneinsert
intotheY,whereit'snowcalledDAZ,andanamazingthinghappens.Thegenegetsduplicated
ontheYchromosome,andthenthosetwogetduplicatedagain,andweendupwith4copiesof
DAZ.TurnsoutthespermatogenesisgenesontheY,nowfortunatelynoneofthisredstuffgot
lost.SpermatogenesisgenesontheYinmanycasesarosethroughtheprocessofmigrationin
fromautosomes.Andsothereweare,we'vegotthesexdetermininggene,theSRY.Oneof
thesesurvivinghousekeepersaren’tjustforYspermatogenesisDAZ,thatgetsustothemodern
humanXandYchromosomes,ofcourseinthecontextofthegenome.