Sex determination in humansChildren receive onlyanXchromosomefromXaXYOXXmother but X or Y fromfather.XMale:Female ratio isXXXYXOa1:1.Caucasian1.06;African-American1.025;Korea1.15(1969census data)Fig.4.6b4-16
Fig. 4.6 b Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 4-16 Sex determination in humans Children receive only Children receive only an X chromosome from an X chromosome from mother but X or Y from mother but X or Y from father. father. Male: Female ratio is Male: Female ratio is 1:1. Caucasian 1.06; African Caucasian 1.06; AfricanAmerican 1.025; Korea American 1.025; Korea 1.15 (1969 census data) 1.15 (1969 census data)
Role of X and Y chromosomes in human sexdeterminationmaleXYHave YmaleXXYfemaleXNo YfemaleXXConclusion:Y chromosome determines maleness.4-17
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 4-17 Role of X and Y chromosomes in human sex determination Y chromosome determines maleness. XY male XXY male X female XX female Conclusion: Have Y No Y
SRYgene11ZygoteTDFInthe absenceofaThe testis-determiningX XYchromosome,nofactor(TDF)is producedTDF is producedby a gene on the YEmbryonicchromosome.gonadsThelack of TDFTDF induces the medulla ofCortexMedullaallowsthecortex ofthe embryonic gonads tothe embryonicdevelop into testes.gonads to developDifferentiatedinto ovaries.gonadsThe testes producetestosterone,a hormone thatIn the absence ofTestesjOvaryinitiates development of maletestosterone,thesexual characteristics.embryo developsFemale sexualMale sexualfemale characteristics.characteristics.characteristics.4-18
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 4-18 X X In the absence of a Y chromosome, no TDF is produced The lack of TDF allows the cortex of the embryonic gonads to develop into ovaries. In the absence of testosterone, the embryo develops female characteristics. Female sexual characteristics. Male sexual characteristics. X Y The testis-determining factor (TDF) is produced by a gene on the Y chromosome. SRY gene ↓ TDF TDF induces the medulla of the embryonic gonads to develop into testes. The testes produce testosterone, a hormone that initiates development of male sexual characteristics. Differentiated gonads Zygote Embryonic gonads Ovary Testes Cortex Medulla
People carrySyndromeChromosomesFrequencyat birthabnormalSexchromosomes,femalesnumber of sexxo,monosomic1/5000Turnerxxx,trisomicchromosomes1/700Xxxx,tetrasomicare not rareXxxxx,pentasomicSexchromosomes,malesNormalXYY,trisomic1/10,000XXYY,tetrasomicXXXY,tetrasomicKlinefelter1/500XxxxY,pentasomicXxxxxY,hexasomicAbout0.4%ofallbabiesbornhaveadetectablechromosomalabnormalitythatgeneratesadetrimentalphenotype.4-19
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 4-19 Chromosomes Syndrome Frequency at birth People carry abnormal number of sex chromosomes are not rare
Variation between species in how chromosomesdetermine an individual's sexMalesOrganismChromosomes FemalesXXXYXX-XYMammals,DrosophilaXXXOXX-XOMoths,C.elegansZWZZZZ-ZWBirds,ButterfliesTable 4.24-20
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 4-20 Variation between species in how chromosomes Variation between species in how chromosomes determine an individual determine an individual’s sex _ Chromosomes Females Males Organism _ XX-XY XX XY Mammals, Drosophila XX-XO XX XO Moths, C.elegans ZZ-ZW ZW ZZ Birds, Butterflies _ Table 4.2