Objectives*for*Exam*2*PCB*3063,*Summer*2012***Maternal*Inheritance**!***Know!what!the!term!maternal!inheritance!refers!to,!know!that!mitochondria!and!chloroplasts!contain!a!circular!DNA!molecule!that!codes!for!some!of!the!proteins!needed!for!respiration!and!photosynthesis.!Be!able!to!identify!maternal!inheritance!in!a!human!pedigree.!!!Linkage*and*Mapping*(Chapter*7)*!Q:!Important!Terms/concepts!to!know:!!LinkageJ*genes!that!are!part!of!the!same!chromosome!and!thus!canʼt!undergo!independent!assortment.!- Genes!on!the!same!chromosome!are!said!to!be!linked.!- Linkage!is!usually!less!than!100%!because!of!crossing!over!during!meiosis.!*Crossing*overJ*reciprocal!exchange!of!chromosome!segments!when!homologs!are!paired.!!!Parental*or*noncrossover*gametesJ*complete!linkage;!2!parental!gametes!are!formed!in!equal!proportions.!- noncrossover!(or!nonrecombinant!progeny)!are!most!frequent.!!Recombinant*or*crossover*gametesJ*when!exchange!occurs!between!nonsister!chromatids;!can!never!be!more!than!50%!of!the!total.!!!Single*crossover,*double*crossoverJ*when!a!single!crossover!occurs!between!two!nonsister!chromatids,!the!other!two!chromatids!of!the!tetrad!are!not!involved!in!this!exchange!are!enter!the!gamete!unchanged.!- When!a!double!crossover!occurs,!double!exchanges!of!genetic!material!have!happened.!Map*unitJ*is!equal!to!1%!recombination!between!2!genes(centimorgans).!!Genetic*map,*physical*map**- Genetic!maps!are!generated!by!using!the!recombination!process!as!we!have!been!learning!about.!- physical!maps!are!calculated!by!using!physical!distances!along!the!chromosome!as!the!number!of!base!pairs.** !!!!Q:!Essential!to!chapter!7!is!to!be!able!to!do!problems!involving!twoTpoint!and!threeTpoint!crosses!leading!to!construction!of!a!genetic!map.!As!part!of!this!you!should!understand!why!recombination!frequencies!can!never!exceed!50%!! !- Genes!on!the!same!chromosome!that!are!more!than!50!map!units!apart!undergo!recombination!so!frequently!that!they!assort!independently!T!they!are!inherited!as!if!they!were!on!different!chromosomes.!!- The!frequency!with!which!crossing!over!occurs!between!any!two!linked!genes!is!generally!proportional!to!the!distance!separating!the!respective!loci!along!the!chromosomes.!- As!the!distance!between!the!two!genes!increases,!the!proportion!of!recombinant!gametes!increases!and!that!of!the!parental!gametes!decreases.!- *The*farther*apart*the*better*the*chances*of*recombination**!Q:!You!should!know!the!relationship!between!genetic!maps!produced!by!studying!recombination!frequencies!and!physical!maps!of!chromosomes!that!are!produced!by!DNA!sequencing!(they!are!coT!linear!but!the!distances!between!genes!can!vary!because!some!areas!on!chromosomes!are!hot!spots!for!recombination!and!in!some!areas!(like!centromeres)!recombination!is!suppressed).!!!T!The!closer!2!loci!reside!along!the!axis!of!the!chromosome,!the!less!likely!any!single!crossover!event!will!occur!between!them!!!Quantitative*Genetics/Polygenic*Inheritance*(Chapter*22)**!Q:!Important!Terms/concepts!to!know:!!Polygenic*inheritanceJ*also!known!as!quantitative!inheritance,!refers!to!inheritance!of!a!phenotypic!characteristic!(trait)!that!is!attributable!to!two!or!more!genes,!or!the!interaction!with!the!environment,!or!both.!Unlike!monogenic!traits,!polygenic!traits!do!not!follow!patterns!of!Medelʼs!inheritance!(separated!traits).!Instead,!their!phenotypes!typically!vary!along!a!continuous!gradient!depicted!by!a!bell!curve.!!- Trait!is!determined!by!3+!gene!pairs!which!contribute!additively!to!the!phenotype!- each!pair!has!an!additive!and!a!nonTadditive!allele!!- phenotype!reflects!the!total!number!of!additive!alleles!Discontinuous*TraitsJ*individuals!can!be!grouped!into!distinct!phenotypes!!Continuous*traitsJ*individuals!display!a!continuous!range!of!phenotypes!!! ex:!height,!eye!color,!skin!color,!blood!pressure,!IQ,!behavioral!traits;!bellTshaped!distribution!!!!!Multifactorial*traitsT!polygenic!traits!that!are!influenced!by!the!environment.!Quantitative*traitsT!!Additive*and*nonadditive*allelesT!Complicated!of!simple!!Twin*studies:**ConcordanceT!When!a!pair!of!twins!display!the!same!phenotype!!MonozygoticT!twins!should!be!100%!concordant!!Dizygotic*twinsT!less!than!100%!concordant!!***You!should!understand!the!genetic!basis!for!traits!that!display!continuous!variation!(polygenic!inheritance!involving!additive!or!nonadditive!alleles)!and!have!a!general!understanding!of!the!types!of!traits!that!have!this!genetic!basis.!!!***You!should!be!able!to!do!simple!problems!that!require!determining!the!number!of!genes!that!control!a!polygenic!trait!and!the!amount!each!allele!contributes!to!the!phenotype,!similar!to!those!in!homework!2.!!!!Q:!You!should!also!be!able!to!analyze!human!twinTstudy!data!and!identify!traits!that!are!determined!primarily!by!the!environment,!primarily!by!genes,!or!by!a!mixture!of!both!genes!and!environment.!!!- Many!polygenic!traits!are!also!influenced!by!the!environment!(multifactorial!traits)**- Polygenic!traits!may!be!more!complicated!that!simple!additive!and!nonTadditive!alleles!*- When!a!pair!of!twins!display!the!same!phenotypeTTConcordant!*- If!the!trait!is!controlled!completely!by!genes,!monozygotic!twins!should!be!100%!concordant!and!Dizygotic!twins!should!be!<100%!concordant!*-
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