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1、ClinicalcytogeneticsDisordersoftheautosomesandthesexchromosomesOutlineFirstdiscussthemostcommonautosomaldisorders;SeconddiscusstheuniquebiologyofXandYchromosomesandsexchromosomeabnormalities.Thirddiscussdisordersofgonadaldevelopmentandsexualdifferentiation;AutosomaldisordersTrisomy21(Dow
2、nsyndrome)Trisomy18(EdwardsSyndrome)Trisomy13(PatauSyndrome)CommonphenotypeGrowthretardationMentalretardationMultiplecongenitalanomaliesDownsyndromeOrtrisomy21,isthemostcommonchromosomedisordersandisthemostcommongeneticcauseofmoderatementalretardation.About1in800isbornwithDownsyndrome.Wa
3、sfirstdescribedclinicallybyLangdonDownin1866.Downsyndromewasthefirstconditionstobefoundtohave47chromosomes,withanextrachromosome21.MotherageandincidenceofDownsyndromePhenotypeThechromosomesinDownsyndromeTrisomy21RobertsoniantranslocationMosaicDownsyndromeTrisomy21Inabout95%ofallpatientsR
4、esultingfrommeioticnondisjunction90%meioticerrorresponsibleforthetrisomyusuallyoccursduringmaternalmeiosis.10%occurinpaternalmeiosis.TheriskincreaseswithmaternalageDownSyndrome47,XY,+21Examples:(1)Down’sSyndrometrisomy211outof800childrenRobertsoniantranslocationInabout4%ofDownsyndromepat
5、ients,Thekaryotypeis46,XX(XY),rob(14;21),+21or46,XX(XY),rob(G;21),+21,Showsnorelationtomaternalage,buthasahighfrequencyriskinfamilieswhenaparentisacarrierofthetranslocation.ConsequencesofbalancedRobertsoniantranslocation21q21qTranslocationIsachromosomecomprisingtwochromosome21longarms.Or
6、iginateasanisochromosome.AcarrierisprobablytohaveonlychildrenwithDownsyndrome.Karyotypeof21q21qtranslocationMosaicDownsyndromeThekaryotypeis46,XX/47,XX,+21or46,XY/47,XY,+21Thephenotypemaybemilderthanthatoftypicaltrisomy.Partialtrisomy21Apatientinwhomonlyapartofthelongarmofchromosome21isp
7、resentintriplicate.Etiologyoftrisomy21The“olderegg”model:theoldertheoocyte,thegreaterthechancethatthechromosomeswillfailtodisjoincorrectly.RiskofDownsyndromeThepopulationincidenceinlivebirthisabout1in800,Theriskoftypicaltrisomy-21reaching1in25birthsintheoldestmaternalageg
