生物专业英语蒋悟生版本第四课

LessonFourFoundationsofGenetics——XX班XX1.EarlyTheoriesofinheritanceEarlyideasofinheritanceincludedHippocrates'theoryofpangenesisandAugustWeismann‘sgermplasmtheory.遗传学的早期理论包括希波克拉底的泛生论和奥古斯特·魏斯曼的种质理论。Basedonexperimentswithmice,Weismannproposedthathereditaryinformationingametestransmittedtraitstoprogeny.基于小鼠实验，魏斯曼提出配子中的遗传信息将性状传递给后代。Bothoftheseearlyviewsincorporatedtheblendingtheory:theyheldthatheritabletraitsofthetwoparentsblend,sothatthedistinctcharacteristicsofeacharelostinoffspring.这两种早期观点合起来形成融合理论：他们认为两个亲本的遗传性状混合，以至于每个亲本的鲜明特征在子代中丢失。2.GregorMendelandtheBirthofGeneticsGregorMendel,anAugustinianmonkinthemonasteryatBrunn,Austria,isknownasthefatherofgenetics.孟德尔，一名奥地利，布伦修道院的奥古斯丁修道士，是众所周知的遗传学之父。Havingbeenexposedtotheoriesoftheparticulatenatureofmatterwhileauniversitystudentandhavingabackgroundinmathematics,Mendelcarriedoutaseriesofcarefullyplannedexperimentsthatdemonstratedtheparticulatenatureofheredity.当他还是只有数学背景的大学生时就提出了物质的粒子属性，孟德尔进行了一系列周密安排的实验来证实遗传的颗粒性。HisrevolutionaryideaswereneitherunderstoodnoraccepteduntilmanyyearsafterMendeldied.直到孟德尔去世很多年以后，他的理论才被理解和接受。3.Mendel'sClassicExperimentsMendelstudiedgeneticsthroughplant-breedingexperimentswiththegardenpea,aplantspeciesthatisself-fertilizingandbreedstrue(eachoffspringisidenticaltotheparentinthetraitofinterest).孟德尔通过豌豆的植物育种实验研究遗传学，豌豆是自花授粉植物和纯品系。（每个子代在遗传性状上与亲本是完全相同的）Totesttheblendingtheory,hefocusedhisresearchonsevendistinctcharacters.为验证融合理论，他的研究主要集中在7个显著的特征上。Eachofthesecharacters,suchasseedcolorandplantheight,presentonlytwo,clear-cutpossibilities.这些性状中的每一个，例如，种子颜色，植株高度，只表现出两个明确的可能性。Healsorecordedthetypeandnumberofallprogenyproducedfromeachpairofparentpeaplants,andfollowedtheresultsofeachcrossfortwogenerations.他还记录了每一对亲本豌豆产生的所有个子代的类型和数量，并且记录每对亲本杂交产生子2代的结果。Foreachofthecharactershestudied,Mendelfoundthatonetraitwasdominantwhiletheotherwasrecessive.对于他研究的每对性状，孟德尔发现，一个性状是显性的，而另一个是隐性的。Inthesecondfilial(F2)generation,theratioofdominanttorecessivewas3:1.在子2代中，显性与隐性比为3∶1。Mendeldeducedthatthisresultwaspossibleonlyifeachindividualpossessesonlytwohereditaryunits,onefromeachparent.孟德尔推断，只有在每个个体仅拥有两个遗传单元，并且每个单元来自一个亲代时，这种结果才可能成立。TheunitsMendelhypothesizedaretodayknownasalleles,alternativeformsofgenes.孟德尔假设的遗传单元就是现在称作的等位基因，替代形式的基因。Genesarethebasicunitsofheredity.基因是遗传的基本单位。Anorganismthatinheritsidenticalallelesforatraitfromeachparentissaidtobehomozygousforthattrait;ifdifferentallelesforatraitareinherited,theorganismisheterozygousforthattrait.一个生物体从每个亲本获得相同的等位基因，称纯合子；如果获得不同的等位基因，称杂合子。Whenanorganismisheterozygousforatrait,theresultingphenotypeforthattraitexpressesonlythedominantallele.当生物是杂合子时，它的表型由显性等位基因决定。Thus,theorganism’sphenotype—itsphysicalappearanceandproperties-differsfromitsgenotype,whichmayincludebothadominantandarecessiveallele.因此，生物的表型，其外观和特征，不同于它的基因型，基因型可能既包括显性等位基因，又包括隐性等位基因。ApictorialrepresentationofallpossiblecombinationsofageneticcrossisknownasaPunnettsquare.一个遗传交叉的所有可能组合的图形描述称为旁纳特方格。TheresultsofMendel'sexperimentsondominantandrecessiveinheritanceledtoMendel'sfirstlaw:thelawofsegregation.孟德尔的显性和隐性遗传实验的结果遵循孟德尔第一定律：分离定律。Thislawstatesthatforagiventraitanorganisminheritsoneallelefromeachparent.该定律指出，对于一种特定的性状，生物只遗传每个亲本的一个等位基因。Togethertheseallelesformtheallelepair.这些等位基因一起组成等位基因对。Whengametesareformedduringmeiosis,thetwoallelesbecomeseparated(halvingofchromosomenumber).在减数分裂期形成配子时，两个等位基因分离。（染色体数目减半）TogainevidenceforhistheoryMendelperformedtestcrosses,matingplantsofunknowngenotypetoplantsthatwerehomozygousrecessiveforthetraitofinterest.为验证他的理论，孟德尔做了测交实验，即基因型未知的植物与纯合的隐性基因植物杂交。Theratioofdominantphenotypes(ifany)intheprogenymakesclearwhethertheunknowngenotypeisheterozygous,homozygousdominant,orhomozygousrecessive.显性表型（如果有的话）在子代中的比例可以明确测得未知基因型是否为杂合子、显性纯合子或隐性纯合子。4.Mendel'sIdeasandtheLawofindependentAssortmentMendelalsoperformeddihybridcrosses,whichenabledhimtoconsiderhowtwotraitsareinheritedrelativetooneanother.孟德尔还进行了双因子杂合子杂交，这使他考虑到两对相对性状是如何遗传的。Thisworkledtothelawofindependentassortment,whichstatesthattheallelesofgenesgoverningdifferentcharactersareinheritedindependently.这项工作产生了自由组合定律，表明控制不同性状的等位基因是独立遗传的。AnapparentexceptiontoMendel‘slawsisincompletedominance,aphenomenoninwhichoffspringofacrossexhibitaphenotypethatisintermediatebetweenthoseoftheparents.孟德尔定律的一个特例是不完全显性，即一个交叉的子代表现出一种介于亲本之间的表型的现象。However,incompletedominancereflectsthefactthatbothallelesforthetraitinquestionexertaneffectonthephenotype.然而，不完全显性反映了一个事实，两个等位基因对表型都有影响。Theallelesthemselvesremainseparate.这些等位基因本身会继续分离。Mendelpresentedhisideasin1866inascientificpaperpublishedbytheBrunnSocietyforNaturalHistory.1866年，孟德尔在自然史上发表了他的科学论文，陈诉了他的观点。Unfortunately,themeaningofhisresearchwasnotunderstoodbyotherscientistsoftheday.不幸的是，他的研究的意义不被当时其他科学家理解。Hisworkwasrediscoveredin1900byCarlCorrensandHugodeVries.在1900年，他的著作被卡尔•柯灵斯和德弗里斯重新发现。5.ChromosomesandMendelianGeneticsSoonafterMendel'sworkwasrediscovered,WalterSuttonandTheodorBoveriindependentlyproposedthatthehereditaryunitsmightbelocatedonchromosomes.孟德尔的著作被重新发现后不久，沃尔特•萨顿和西奥多•博韦里独自提出，遗传单位可能定位在染色体上。ExperimentstoprovethishypothesiswerecarriedoutbyThomasHuntMorganandhisstudentsatColum