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P RINCI P LES OF INHER I T ANC E AND V ARI A TION

P RINCI P LES OF INHER I T ANC E AND V ARI A TION. P R INCIPL E S OF I N H ERI T AN C E A ND V A R I A T I ON. Genetics: deals with the inheritance, as well as the variation of characters from parents to offsprings.

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P RINCI P LES OF INHER I T ANC E AND V ARI A TION

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  1. PRINCIPLESOFINHERITANCEAND VARIATION

  2. PRINCIPLESOFINHERITANCEAND VARIATION • Genetics:dealswiththeinheritance,aswellasthevariationofcharactersfromparentstooffsprings. • Inheritance:istheprocessbywhichcharactersarepassedonfromparenttoprogeny. • Variation:isthedegreebywhichprogenydifferfromtheir • parents.

  3. MENDEL’SLAWSOF INHERITANCE: • GregorMendel.Conductedhybridizationexperimentsongardenpeasforsevenyears(1856– 1863)andproposedlawsofinheritance. • Mendelconductedartificialpollination/crosspollinationexperimentsusingseveraltrue-breedingpealines. • Atruebreedinglineisonethat,havingundergone • continuousself-pollinationforseveralgenerations. • Mendelselected14true-breeding peas’ plant varieties, as pair’s which were similar except for one character with contrastingtraits.

  4. INHERITANCEOFONEGENE:

  5. Mendel’sproposition: •  • Mendelproposedthatsomethingwasbeingstablypasseddown,unchanged,fromparenttooffspringthroughthegametes,oversuccessivegenerations.Hecalledthesethingsas‘factors’. • Nowadaywecallthemasgenes. • Geneisthereforearetheunitsofinheritance. • Geneswhichcodesofapairofcontrastingtraitsareknownasalleles,i.e.theyareslightlydifferentformsofthesamegene.

  6. Alphabetsused: • CapitallettersusedforthetraitexpressedattheF1stage. • Smallalphabetfortheothertrait. • ‘T’ isusedforTalland‘t’ is used for dwarf. • ‘T’ and‘t’ are alleles of each other. • HenceinplantsthepairofallelesforheightwouldbeTT.Tt.ortt. • Inatruebreedingtallordwarfpeavarietytheallelicpairofgenes forheightareidenticalorhomozygous,TTandttrespectively. • TTandttarecalledthegenotype. • Ttplantisheterozygousforgenescontrollingonecharacter(height). • Descriptivetermstallanddwarfarethephenotype.

  7. Testcross: • WhenF1hybridiscrossedbackwiththerecessiveparent,itisknownastestcross. • Itisusedtoknowthegenotypeofthegivenplant/animal.

  8. LawofDominance: • Charactersarecontrolledbydiscreteunitscalledfactors. • Factorsoccurinpairs. • Inadissimilarpairoffactorsonememberofthepairdominates(dominant)theother(recessive).

  9. LawofSegregation: • TheallelesdonotshowanyblendingandthatboththecharactersarerecoveredassuchintheF2generationthoughoneoftheseisnotseenattheF1stage. • Theparentscontaintwoallelesduringgameteformation;thefactorsorallelesofapairsegregateorseparatefromeachothersuchthatagametereceivesonlyoneofthetwofactors. • Homozygousparentproducesallgametesthataresimilari.econtainsametypeofallele. • Heterozygous parents’ produces two kinds of gametes each • havingoneallelewithequalproportion.

  10. Incompletedominance: • Whenacrossbetweentwopurebreedisdoneforonecontrastingcharacter,theF1hybridphenotypedosenotresembleeitherofthetwoparentsandwasinbetweenthetwo,calledincompletedominance. • Inheritanceofflowercolorinthedog • flower(snapdragonorAntirrhinumsp.)isagood • exampleofincompletedominance. • F2generationphenotypicratiois1:2:1insteadof3:1asMendelianmonohybridcross. • GenotypicratioofF2generationis1:2:1.

  11. Co–dominance: • F1resembledeitherofthetwoparents(completedominance). • F1offspringwasin-betweenoftwoparents(incompletedominance). • F1generationresemblesbothparentssidebysideiscalled(co-dominance). • Bestexampleofco-dominanceistheABObloodgroupinginhuman. • ABObloodgroupiscontrolledbythegeneI. • TheplasmamembraneoftheRBChassugarpolymers(antigen)thatprotrudefromitssurfaceandthekindofsugariscontrolledbythegene-I. • ThegeneIhasthreeallelesIA,IBandi. • TheallelesIAandIBproduceaslightlydifferentformofsugarwhileallelei • doesn’t produce any sugar. • EachpersonpossessesanytwoofthethreeIgenealleles. • IAandIBarecompletelydominantoveri. • WhenIA,andIBpresenttogethertheybothexpresstheirowntypesofsugar;thisbecauseofco-dominance.HenceredbloodcellshavebothAandBtypesugars.

  12. MultipleAlleles: • ExampleofABObloodgroupingproducesagoodexampleofmultiplealleles. • Therearemorethantwoi.e.threeallele,governingthesamecharacter.

  13. Asinglegeneproductmayproducemorethanoneeffect: • Starchsynthesisinpeaseedsiscontrolledbyonegene. • IthastwoallelesBandb. • StarchissynthesizedeffectivelybyBBhomozygoteandtherefore, • largestarchgrainsareproduced. • The‘bb’ homozygoushaslessefficiencyhenceproducesmallergrains. • Aftermaturationoftheseeds,BBseedsareroundandthebbseedsarewrinkle. • Heterozygous(Bb)produceroundseedandsoBseemstobedominantallele,butthestarchgrainsproducedareofintermediatesize. • Ifstarchgrainsizeisconsideredasthephenotype,thenfromthisangletheallelesshowincompletedominance.

  14. INHERITANCEOFTWOGENES: • LawofindependentAssortment: • Whentwocharacters(dihybrid)arecombinedinahybrid,segregationofonepairoftraitsisindependentoftheotherpairoftraits.

  15. CHROMOSOMALTHEORYOFINHERITANCE: • Why Mendel’s theory was remained unrecognized? • Firstlycommunicationwasnoteasyinthosedaysandhiswork • couldnotbewidelypublicized. • Secondlyhis concept of genes (or factors, in Mendel’s word) as stableanddiscreteunitsthatcontrolledtheexpressionoftraitsand of the pair of alleles which did not’ blend’ with each other, wasnotacceptedbyhiscontemporariesasanexplanationfortheapparentlycontinuousvariationseeninnature. • ThirdlyMendel’s approach of using mathematics to explain biologicalphenomenawastotallynewandunacceptabletomanyofthebiologistsofhistime. • Finallyhecouldnotprovideanyphysicalprooffortheexistence • offactors.

  16. RediscoveryofMendel’sresult: • 1990threescientists(deVries,CorrensandvonTschermak)independently rediscovered Mendel’s result on theinheritanceofcharacter

  17. Chromosomaltheoryofinheritance: • ProposedbyWalterSuttonandTheodoreBoveryin1902. • Theyworkoutthechromosomemovementduringmeiosis. • Thebehaviorofchromosomeswasparalleltothebehaviorof • genes and used chromosome movement to explain Mendel’s laws. • SuttonunitedtheknowledgeofchromosomalsegregationwithMendelianprinciplesandcalleditthechromosomaltheoryofinheritance. • Chromosomeandgenesarepresentinpairsindiploidcells. • Homologouschromosomesseparateduringgameteformation(meiosis) • Fertilizationrestoresthechromosomenumbertodiploidcondition. • Thechromosomaltheoryofinheritanceclaimsthat,itisthe • chromosomesthatsegregateandassortindependently.

  18. Experimentalverificationofchromosomaltheory: • ExperimentalverificationofchromosomaltheoryofinheritancebyThomasHuntMorganandhiscolleagues. • Morganworkedwithtinyfruitflies,Drosophilamelanogaster. • WhyDrosophila? • Suitableforgeneticstudies. • Grownonsimplesyntheticmediuminthelaboratory. • Theycompletetheirlifecycleinabouttwoweeks. • Asinglematingcouldproducealargenumberofprogenyflies. • Cleardifferentiationofmaleandfemaleflies • Havemanytypesofhereditaryvariationsthatcanbeseenwithlowpowermicroscopes.

  19. LinkageandRecombination • Morganhybridizedyellowbodied,whiteeyedfemales • tobrown-bodied,redeyedmaleandintercrossedtheirF1 • progeny. • HeobservedthatthetwogenesdidnotsegregateindependentlyofeachotherandtheF2ratiodeviatedverysignificantlyfrom9:3:3:1ratio(expectedwhenthetwogenesareindependent). • Whentwogenesinadihybridcrossweresituatedonthesamechromosome,theproportionofparentalgenecombinationswasmuchhigherthanthenon-parentaltype. • Morganattributedthisduetothephysicalassociationorlinkageofthetwogenesandcoinedthetermlinkage.

  20. Linage:physicalassociationofgenesonachromosome. • Recombination:thegenerationofnon-parentalgene • combinations. • Morganfoundthatevenwhengenesweregroupedonthesamechromosome,somegeneswereverytightlylinked(showedverylowrecombination)whileotherswereloosely • linked(showedhigherrecombination). • Thegeneswhiteandyellowwereverytightlylinkedandshowed • 1.3percentrecombination. • Thegeneswhiteandminiaturewingshowed37.2percentrecombination,hencelooselylinked. • AlfredSturtevantusedthefrequencyofrecombinationbetweengenepairsonthesamechromosomeasameasureofthedistancebetweengenesand‘mapped’ theirpositiononthechromosome.

  21. POLYGENICINHERITANCE: • Humanhavenodistincttallorshortinsteadawholerangeofpossibleheights. • Suchtraitsaregenerallycontrolledbythreeormoregenesandarethuscalledpolygenictrait. • Besidestheinvolvementofmultiplegenespolygenicinheritancealsotakesintoaccounttheinfluenceofenvironment. • Humanskincolorisanotherclassicexampleofpolygenicinheritance. • Inapolygenictraitthephenotypereflectsthecontributionofeachallelei.e.theeffectofeachalleleisadditive. • AssumethatthreegenesA,B,Ccontroltheskincolourinhuman. • DominantformsA,B;ANDCresponsiblefordarkskincolourandtherecessiveformsa,b,cforlightcoloroftheskin. • Genotypewithdominantalleles(AABBCC)willhavedarkestskincolor. • Genotypewithrecessivealleles(aabbcc)willhavelightestskincolour. • Othercombinationsalwayswithintermediatecolour.

  22. PLEIOTROPY: • Asinglegenecanexhibitmultiplephenotypicexpression,suchgeneiscalledpleiotropicgene. • Themechanismofpleiotropyinmostcasesistheeffectofageneonmetabolicpathwayswhichcontributestowardsdifferentphenotypes. • Phenylketonuriaadiseaseinhumanisanexampleofpleiotropy. • Thisdiseaseiscausedduetomutationinthegenethatcodefor • theenzymephenylalaninehydroxylase. • Phenotypicexpressioncharacterizedby:- • Mentalretardation • Reductioninhairs. • Reductioninskinpigmentation.

  23. SEXDETERMINATION: • Henking(1891)tracedspecificnuclearstructureduringspermatogenesisofsomeinsects. • 50%ofthespermreceivedthesespecificstructures,whereas50%spermdidnotreceiveit. • HenkinggaveanametothisstructureastheX-body. • X-bodyofHenkingwaslateronnamedasX-chromosome.

  24. Sex-determinationofgrasshopper: • Sex-determinationingrasshopperisXX-XOtype. • Alleggbears one‘X’chromosome along withautosomes. • Somesperms (50%)bear’sone‘X’chromosome and 50% do • not. • Egg fertilizedwith sperm (with‘X’chromosome) became • female(22+XX). • Egg fertilizedwith sperm (without‘X’chromosome) became • male(22+X0)

  25. Sexdeterminationininsectsandmammals (XX-XYtype): • Bothemaleandfemalehassamenumberofchromosomes. • FemalehaveautosomesandapairofXchromosomes.(AA+XX) • Malehaveautosomesandonelarge ‘X’ chromosome and • one very small ‘Y-chromosomes.(AA+XY) • Thisiscalledmaleheterogammetyandfemale • homogamety.

  26. Sexdeterminationinbirds: • FemalebirdshavetwodifferentsexchromosomesdesignatedasZandW. • MalebirdshavetwosimilarsexchromosomesandcalledZZ. • Suchtypeofsexdeterminationiscalledfemaleheterogammetyandmalehomogamety.

  27. SexdeterminationinHoneybee: • Sexdeterminationinhoneybeebasedonthenumberofsetsofchromosomesanindividualreceives. • Anoffspringformedfromthefertilizationofaspermandanegg • developedintoeitherqueen(female)orworker(female). • Anunfertilizedeggdevelopsasamale(drone),bymeansofparthenogenesis. • Themalehavehalfthenumberofchromosomethanthatof • female. • Thefemalearediploidhaving32chromosomesandmalesarehaploidi.e.having16numbersofchromosomes. • Thisiscalledhaplodiploidsexdeterminationsystem. • Maleproducespermsbymitosis,theydonnothavefatherandthuscannothavesons,buthavegrandsons.

  28. MUTATION: • MutationisaphenomenonwhichresultsinalterationofDNAsequencesandconsequentlyresultsinchangesinthegenotypeandphenotypeofanorganism. • Inadditiontorecombination,mutationisanotherphenomenonthatleadstovariationinDNA. • Loss(deletion)orgain(insertion/duplication)ofasegmentofDNAresultsinalterationinchromosomes. • Sincegenesarelocatedonthechromosome,alterationinchromosomesresultsinabnormalitiesoraberration. • Chromosomalaberrationsarecommonlyobservedincancerouscells. • MutationsalsoariseduetochangeinasinglebasepairofDNA.Thisisknownaspointmutation.E.g.sicklecellanemia. • DeletionandinsertionsofbasepairsofDNAcausesframeshiftmutations.

  29. GENETICDISORDERS: • PedigreeAnalysis: • Analysisoftraitsinseveralofgenerationsofafamilyiscalledthepedigreeanalysis. • Inthepedigreeanalysistheinheritanceofaparticulartraitisrepresentedinthefamilytreeovergenerations.

  30. AutosomalDominant • Affectedindividualshaveatleastoneaffectedparent • Thephenotypegenerallyappearseverygeneration • Twounaffectedparentsonlyhaveunaffected • offspring • Traitsarecontrolledbydominantgenes • Bothmalesandfemalesareequallyaffected • Traitsdonotskipgenerations • e.g.polydactyly,tonguerollingabilityetc

  31. Autosomalrecessive: • Unaffectedparentscanhaveaffectedoffspring • Traitscontrolledbyrecessivegenesand • Appearonlywhenhomozygous • Bothmaleandfemaleequallyaffected • Traitsmayskipgenerations • 3:1ratiobetweennormalandaffected. • Appearanceofaffectedchildrenfromnormalparents(heterozygous) • Allchildrenofaffectedparentsarealsoaffected. • e.g.-Albinism,sicklecellanaemiaetc.

  32. MendelianDisorder: • Geneticdisordersgroupedintotwocategories– • Mendeliandisorder • Chromosomaldisorder • Mendeliandisordersaremainlydeterminedbyalterationormutationinthesinglegene. • ObeytheprincipleofMendelianinheritanceduring • transmissionfromonegenerationtoother. • Canbeexpressedinpedigreeanalysis. • E.g.Haemophilia,colorblindness,Cysticfibrosis,Sicklecellanemia,Phenylketonuria,Thalasemiaetc.

  33. Hemophilia: • Inthisdiseaseasingleproteinthatisapartofthecascadeofproteinsinvolvedintheclottingofbloodisaffected.Duetothisinanaffectedindividualasimplecutwillresultinnon-stopbleeding. • Sexlinkedrecessivedisease. • Thediseasestransmittedfromunaffectedcarrierfemaletosomeofthemaleprogeny. • Femalebecominghemophilicisextremelyrarebecausemotherofsuchafemaleatleastcarrierandthefathershouldbehemophilic. • Affectedtransmitsthediseaseonlytothesonnottothedaughter. • Daughtercanreceivethediseasefrombothmotherandfather.

  34. Sicklecellanaemia: • ThedefectiscausedduetosubstitutionofGlutamicacid(Glu)byValine(Val)atthesixthpositionofthebetaglobinchainofthehaemoglobinmolecule. • Substitutionofaminoacidtakesplaceduetothesinglebasesubstitutionatthesixth • codonofthebetaglobingenefromGAGtoGUG. • ThemutanthaemoglobinmoleculeundergoespolymerizationunderlowoxygentensioncausingthechangeintheshapeoftheRBCfrombiconcavedisctoelongatedsicklelikestructure. • Thisisanautosomeslinkedrecessivetrait. • Transmittedfromparentstotheoffspringwhenboththeparentsarecarrierforthegene • (heterozygous). • Thisdiseaseiscontrolledbysinglepairofallele,HbA,andHbS. • Therearethreepossiblegenotypes(HbAHbA,HbAHbS,andHbSHbS. • OnlyhomozygousindividualsforHbS(HbSHbS)showthediseasedphenotype. • Heterozygous(HbAHbS)individualsappearapparentlyunaffectedbuttheyarecarrierofthediseaseasthereis50percentprobabilityoftransmissionofthemutantgenetotheprogeny.

  35. Phenylketonuria: • Autosomalrecessivetrait. • Inbornerrorofmetabolism. • Theaffectedindividuallackoneenzymecalledphenylalaninehydroxylasethatconvertstheaminoacidphenylalaninetotyrosine. • Intheabsenceoftheenzymephenylalanineaccumulatedand convertedintophenylpyruvicacidandotherderivatives. • Accumulationoftheseresultsinmentalretardation. • Thesederivativesexcretedthroughkidney.

  36. Chromosomaldisorders: • Causedduetoabsenceorexcessorabnormalarrangementofoneormorechromosome. • Failureofsegregationofchromatidsduringcelldivisioncycleresultsinthegainorlossofchromosome(s), • calledAneuploidy. • Failureofcytokinesisaftertelophasestageofcelldivisionresultsinanincreaseinawholesetofchromosomeinanorganismandthisphenomenoniscalledpolyploidy. • Trisomy:additionalcopyofachromosomemaybeincludedinanindividual(2n+1). • Monosomy:anindividualmaylackoneofanyonepairof • chromosomes(2n-1)

  37. Downsyndrome: • Causedduetopresenceofanadditionalcopyofthechromosomenumber21(trisomyof21). • ThisdisorderwasfirstdescribedbyLangdonDown(1866). • Shortstaturewithsmallroundhead. • Furrowedtongue • Partiallyopenedmouth • Palmisbroadwithcharacteristicpalmcrease. • Physical,psychomotorandmentaldevelopmentisretarded.

  38. Klinefelter’ssyndrome: • CausedduetothepresenceofanadditionalcopyofX-chromosomeresultingintoakaryotypeof47,(44+XXY). • Overallmasculinedevelopment. • Alsodevelopfemininecharacter(developmentofbreasti.e.Gynaecomastia) • Individualsaresterile. • Turner’s syndrome: • CausedduetotheabsenceofoneoftheX-chromosomesi.e.45(44+X0). • Suchfemalesaresterileasovariesare • rudimentary. • Lackofothersecondarysexualcharacters.

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