BasisofGeneticsGenetics-“origin”,adisciplineofbiology,isthescienceofheredityandvariationinlivingorganisms.Thefactthatlivingthingsinherittraitsfromtheirparentshasbeenusedsinceprehistorictimestoimprovecropplantsandanimalsthroughselectivebreeding.Themodernscienceofgenetics,whichseekstounderstandtheprocessofinheritance,onlybeganwiththeworkofGregorMendelinthemid-nineteenthcentury.Althoughhedidnotknowthephysicalbasisforheredity,Mendelobservedthatorganismsinherittraitsinadiscretemanner—thesebasicunitsofinheritancearenowcalledgenes.DNA,amoleculecomposedofachainoffourdifferenttypesofnucleotides—thesequenceofthesenucleotidesisthegeneticinformationorganismsinherit.Adenine(A),Cytosine(C),Guanine(G),Thymine(T).DNADoubleHelixHistonesChromatidsTelomereCentromereTelomereNuclearNucleosome1Blendinginheritance:theideathatindividualsinheritasmoothblendoftraitsfromtheirparents.2Inheritanceofacquiredcharacteristics:thebeliefthatindividualsinherittraitsstrengthenedbytheirparents3OthertheoriesincludedthePangenesisofCharlesDarwin(whichhadbothacquiredandinheritedaspects)andFrancisGalton'sreformulationofpangenesisasbothparticulateandinherited.HistoryMendelianandclassicalgeneticsMendelbeganstudyingplantbreadingbytryingtofindtheeffectsofcrossingdifferentstrainsofcommongardenpea(Modelorganism)Hecarriedouthisresearchwithmoreprecisionthanhadyetbeenused.Healsousedthenewscienceofstatisticstoanalysetheresultsofhisexperiments.Thisuseofmathematicstodescribebiologicalphenomenawasanewconcept.Mendeldidhispioneeringworkfrom1856to1865andhisresultswerepublishedinonepaper(reports)in1866.Withthelackofcommunicationatthosetimes,itisnotsurprisingthathisworkwentunnoticeduntil1900whenanothergroupifscientistsrepeatedtheexperimentsandthenfoundthatMendelhadalreadydevelopedtheideassome40yearsprevious.OutofMendel'sworkcametwo"Laws"ofinheritance:1)Mendelproposedthathereditywascontrolledbypairedfactorsthatsegregatedwhengametesformedandrejoinedatfertilisationand,2)Theprincipleofindependentassortmentindicatesthatthes...
