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Nowwhatwe'regoingtodoiswatchavideothatconnectstwoofthismorning’sthemes:
meiosisandsexdetermination.Thequestionisbywhatmechanismdoesanembryocometo
carryaY-chromosomeorasecondX.TheanswerisfoundinmeiosisintheFather.Sointhis
videoofhumanmalemeiosisIwantyoutolookfor2things.Guesswhattheyare?Gene
swapping,theswappingpartsofpairedchromosomes,anddividingbytwo;halvingthenumber
ofchromosomespercell.Soyou'llrememberthathumancellshave23pairsofchromosomes,
buttoillustratetheprincipleswe'rejustgoingtoshowsixpairshere,sixpairsofchromosomes.
Andwe'regoingtolookindetailattwopairs,we'regoingtolookatthex-ypairhere,andapair
ofautosomes.Andyouseethatwehaveitarranged,sothatmom’schromosomes...Rightso
yougothalfyourchromosomesfrommomandhalffromdad,wehaveitarrangedsothat
Mom'schromosomesareontheleft,inred,andthechromosomesyougotfromDadarein
blue,ontheright.Andwhat'sthefirstthingthathappensinmeiosis?Theveryfirststepis,you
actuallydon'tdividebytwo,youmultiplybytwo.Youdoubleallthechromosomes,doubleall
thegenes.Soatthefirststepinmeiosisyougotoa4n,toa4nstage.That'swherewe're
beginning,let’srollthevideo.Oksowe'renowgoingtoblowupandfocusonthispairof
autosomesontheleft,andyouseetherethatrecombinationcanoccuratanypointalongthe
lengthofthisautosomalpair.Let'sseewhathappenswhenrecombinationoccursata
particularpoint.There'stheswapping,we'vejustswappedabunchofgenes,thesamecan
happenontheotherarmofthatpairofautosomes.Nowwe'regoingtoturnovertothesex
chromosomes,that'swheretheSRYgeneislocatedontheY.AnditturnsouttheXandtheY
canrecombineonly,theynormallyrecombineonlywithintheirends.Let'sseehowtheswap
occursindetail.Sowe'vejustswappedsomeparts,somestufffromMomandDaddownatthe
otherendslikeso.Soatthispointhowmanycopiesofeachautosomalgenedoesthiscell
have?Four,westillhavefour.WehavetwocopiesoftheXandtwooftheY,andfourofevery
autosome.Okandwhatwe'regoingtosee,whatarewegoingtodonow?We'vegottogo
fromfourcopiesdowntoonepercell.Sowe'reactuallygoingtohavetworoundsofdivision.
Andyou'regoingtoseetheXuphereisgoingtogototwocellsonthetop,theYisgoingtogo
totwocellsonthebottom,we'regoingtogetfourspermoutofthisonecell.Let’srollthe
video.Okherecomesthefirstdivision,thecellonthetophastwocopiesofeverygene,cellon
thebottomtoo.Nowcomestheseconddivision.Rememberwe'vegottheX'sinthetwocells
onthetoptheY'sonthetwocellsatthebottom.Andthesearegoingtogoontomatureinto
sperm.SowewillhaveXbearingspermonthetop,andYbearingspermonthebottom.And
nowthesespermhavetogoinsearchofeggs,let'srollthevideo.Nowthey'refiredup,inthe
experimentonthetoptheXbearingsperminpurplearegoingtowin,downbelowtheY
bearingsperm,weendupwithanXXfertilizedeggandanXYfertilizedegg,andaswesaid,
duringthefirstsixweeksofhumandevelopmentareanatomically,histologically
indistinguishableinmaleandfemale.Weprogressuptothe7-weekstagerighthereandit's
onlyatthispointthattheSRYgenefiresandleadstothebirthofamaleorfemale.Oksohow
doXXmalescarryingSRYarise?HowdoXYfemalesdeletedforSRYarise?Iwantyouto
rememberthatXXmalesarecarryingaterminalportionoftheYchromosome,theXYfemales
aremissingaterminalportionoftheYchromosome.We'regoingtoseehowthathappensina
rerunofpartofthevideowejustwatched.Let'srunthatvideonow.Oksohereagainwe'vegot
thesixchromosomesthatarerepresentingour23pairs.Nowwe'regoingtolookjustattheXY
pair.Andwe'regoingtowatchanaberrantrecombinationorgeneswapping.Sothereagainis
theSRYgeneontheYchromosomerecombinationwouldnormallyberestrictedoutatthevery
endsoftheXandtheY,butoccasionallyanaberrantlyplacedrecombinationevent-Look!That
oneistoofardown,andlookwhat'shappening,theSRYgeneisbeingpassedovertoanXchromosome,andlookhere'saYchromosomethathaslostSRYasaresultofthismisplaced
swappingevent.SoweendupwithanX-chromosomethatisSRY+andaY-chromosomethatis
SRY-ThatisexactlyhowXXmalesandXYfemalesinhumansaregenerated.