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CBSEClass10Science
RevisionNotes
CHAPTER–09
HEREDITY&EVOLUTION
Genetics:BranchofsciencethatdealswithHeredityandvariation.
Heredity:Itmeansthetransmissionoffeatures/characters/traitsfromone
generationtothenextgeneration.
Variation:Thedifferencesamongtheindividualsofaspecies/populationarecalled
variations.Ittakesplaceduetoenvironmentchanges,crossingover,and
recombinationofgenesandmutation.
Genotype:Thecompletesetofgenesinanorganism’sgenomeiscalledgenotype.
Phenotype:Theobservablecharactersinanorganismmakethephenotype.
Phenotypeisamodifiedgenotypeandmanyofthephenotypescannotbeinherited.
Clonesarethoseorganismswhichareexactcopiesofeachother.
MendelandHisWorkonInheritance
GregorJohannMendel(1822&1884):Startedhisexperimentsonplantbreedingand
hybridisation.Heproposedthelawsofinheritanceinlivingorganisms.
LawofDominance,Lawofsegregation,LawofIndependentAssortment.
MendelwasknownasFatherofGenetics
PlantselectedbyMendel:Pisumsativum(gardenpea).Mendelusedanumberof
contrastingcharactersforgardenpea.
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Mendel'sExperiments:Mendelconductedaseriesofexperimentsinwhichhe
crossedthepollinatedplantstostudyonecharacter(atatime)
Crossbetweentwopeaplantswithonepairofcontrastingcharactersiscalleda
monohybridcross.
Crossbetweenatallandadraftplant(short).
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Phenotypicratio:3:1
Genotypicratio:1:2:1
Phenotype®Physicalappearance[TallorShort]
Genotype®Physicalappearance[Tallorshort]
ObservationsofMonohybridCross
1. AllF1progenyweretall(nomediumheightplant(halfwaycharacteristic)
2. F2progeny¼wereshort,3/4weretall
3. PhenotypicratioF2–3:1(3tall:1short)
GenotypicratioF2–1:2:1
Conclusions
1.TTandTtbotharetallplantswhilettisashortplant.
2.AsinglecopyofTisenoughtomaketheplanttall,whilebothcopieshavetobe‘t’forthe
planttobeshort.
3.Characters/Traitslike'T'arecalleddominanttrait(becauseitexpressitself)and‘t’are
recessivetrait(becauseitremainssuppressed)
Fromtheseobservation,Mendelputforwardtherulesofinheritance
LawofSegregation:Everyindividualpossessesapairofallelesforaparticulartrait.During
gameteformation,agametereceivesonlyonetraitfromthealleles.Aparticulartraitcanbe
dominantorrecessiveinaparticulargeneration.
DihybridCross:Acrossbetweentwoplantshavingtwopairsofcontrastingcharactersis
calleddihybridcross.
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PhenotypicRatio
Round,yellow:9
Round,green:3
Wrinkled,yellow:3
Wrinkled,green:1
Observations
1. WhenRRyywascrossedwithrrYYinF1generationallwereRrYyroundandyellow
seeds.
2. SelfpollinationofFplantsgaveparentalphenotypeandtwomixtures(recombinants
roundyellow&wrinkledgreen)seedsplantsintheratioof9:3:3:1
Conclusions
1.RoundandyellowseedsareDOMINANTcharacters
2.Occurrenceofnewphenotypiccombinationsshowthatgenesforroundandyellowseeds
areinheritedindependentlyofeachother.
LawofIndependentAssortment:Allelesofdifferentcharactersseparateindependentfrom
eachotherduringgameteformation.
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Intheaboveexample,allelesoftexturewereassortedindependentlyfromthoseofseed
colour.
SEXDETERMINATION
Determinationofsexofanoffspring.
FACTORS
SexChromosomes:Inhumanbeingsthereare23pairsofchromosome.Outofthese22
chromosomespairsarecalledautosomesandthelastpairofchromosomethathelpin
decidinggenderofthatindividualiscalledsexchromosome.
XX–female
XY–male
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Thisshowsthathalfthechildrenwillbeboysandhalfwillbegirls.Allchildrenwillinherit
anXchromosomefromtheirmotherregardlesswhethertheyareboysorgirls.Thussexof
childrenwillbedeterminedbywhattheyinheritfromtheirfather,andnotfromtheir
mother.
EVOLUTION
Evolutionisthesequenceofgradualchangeswhichtakesplaceintheprimitiveorganisms,
overmillionsofyears,inwhichnewspeciesareproduced.
Situation-I
Situation1:Greenbeetlesgotthesurvivaladvantageortheywerenaturallyselectedasthey
werenotvisibleingreenbushes.Thisnaturalselectionisexertedbycrowsresultingin
adaptationsinthebeetlestofitbetterintheirenvironment
Situation-II
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Situation2:Bluebeetlesdidnotgetsurvivalsadvantage.Elephantsuddenlycausedmajor
havocinbeetlepopulationotherwisetheirnumberwouldhavebeenconsiderablylarge.
Fromthiswecanconcludethataccidentscanchangethefrequencyofsomegenesevenif
theydonotgetsurvivaladvantage:Thisiscalledgeneticdriftanditleadstovariation.
SITUATION-III
Groupofredbeetles
Habitatofbeetles(bushes)
Sufferfromplantdisease
Averageweightofbeetles
decreasesduetopoornourishment
Numberofbeetleskeptonreducing
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Laterplantdiseasegetseliminated
Numberandaverageweightofthebeetles
increasesagain
Situation3:Nogeneticchangehasoccurredinthepopulationofbeetle.Thepopulationgets
affectedforashortdurationonlyduetoenvironmentalchanges.
ACQUIREDANDINHERITEDTRAITS
AcquiredTraits InheritedTraits
1.Thesearethetraitswhicharedeveloped
inanindividualduetospecialconditions
1.Thesearethetraitswhicharepassedfrom
onegenerationtothenext.
Theycannotbetransferredtotheprogeny 2.Theygettransferredtotheprogeny.
Theycannotdirectevolutioneg.Lowweight
ofstarvingbeetles
3.Theyarehelpfulinevolution.eg.Colourof
eyesandhair
SPECIATION
MicroEvolution:Itistheevolutionwhichisonasmallscale.e.g.changeinbodycolourof
beetles.
Theprocessbywhichnewspeciesdevelopfromtheexistingspeciesisknownasspeciation.
Speciation:itistheprocessofformationofnewspecies.
Species:Agroupofsimilarindividualswithinapopulationthatcaninterbreedandproduce
fertileoffspring.
Factorswhichleadtospeciation:Geographicalisolation,geneticdriftsandvariations.
Geneflow:Itisexchangeofgeneticmaterialbyinterbreedingbetweenpopulationsofsame
speciesorindividuals
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WAYSBYWHICHSPECIATIONTAKESPLACE
Speciationtakesplacewhenvariationiscombinedwithgeographicalisolation.
Geneflow:occursbetweenpopulationthatarepartlybutnotcompletelyseparated.
GENETICDRIFT
Itistherandomchangeinthefrequencyofalleles(genepair)inapopulationoversuccessive
generations.
*NaturalSelection:Theprocessbywhichnatureselectsandconsolidatethoseorganisms
whicharemoresuitablyadaptedandpossessesfavorablevariations
ResultsinAccumulationofdifferentvariationsin
Subpopulation and
Geneticdrift
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Geneticdrifttakesplacedueto:
(a)SeverechangesintheDNA
(b)Changeinnumberofchromosomes
EvolutionandClassification
EVOLUTION:theprocessbywhichdifferentkindsoflivingorganismarebelievedtohave
developedfromearlierformsduringthehistoryoftheearth
Bothevolutionandclassificationareinterlinked.
1.Classificationofspeciesisreflectionoftheirevolutionaryrelationship.
2.Themorecharacteristictwospecieshaveincommonthemorecloselytheyarerelated.
3.Themorecloselytheyarerelated,themorerecentlytheyhaveacommonancestor.
4.Similaritiesamongorganismsallowustogroupthemtogetherandtostudytheir
characteristic.
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TRACINGEVOLUTIONARYRELATIONSHIPS:JeanBaptisteLamarckgavethefirst
theoryofevolution.TheacceptedoneisTheOriginofSpeciesbyCharlesDarwin.
(EvidencesofEvolution)
I.HomologousOrgans:(Morphologicalandanatomicalevidences.Thesearetheorgansthat
havesamebasicstructuralplanandoriginbutdifferentfunctions.
Homologousorgansprovidesevidenceforevolutionbytellingusthattheyarederivedfrom
thesameancestor.
Example:
ForelimbofHorse (Running) Samebasicstructuralplan,
butdifferentfunctions
perform.
Windsofbat (flying)
Pawofacat (walk/scratch/attack)
II.AnalogousOrgans:Thesearetheorgansthathavedifferentoriginandstructuralplan
butsamefunctionexample:
Example:Analogousorgansprovidemechanismforevolution.
WingsofbatElongatedfingerswithskin
folds Differentbasicstructure,but
performsimilarfunctioni.e.,
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Wingsofbird Featherycoveringalongthe
arm
flight.
III.Fossils:(Paleontologicalevidences)
Theremainsandrelicsofdeadorganismsofthepast.
FOSSILSAREPRESERVEDTRACESOFLIVINGORGANISMS
FossilArchaeopteryxpossessfeaturesofreptilesaswellasbirds.Thissuggeststhatbirds
haveevolvedfromreptiles.
ExamplesofFossils
AMMONITE-Fossil-invertebrate
TRILOBITE-Fossil-invertebrate
KNIGHTIA-Fossil-fish
RAJASAURUS-Fossildinosaurskull
AGEOFTHEFOSSILS
EvolutionbyStages
Evolutiontakesplaceinstagesiebitbybitovergenerations.
I.FitnessAdvantage
EvolutionofEyes:Evolutionofcomplexorgansisnotsuddenitoccursduetominorchanges
inDNA,howevertakesplacebitbybitovergenerations.
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Insectshavecompoundeyes
Humanshavebinoculareyes
II.FunctionalAdvantage
EvolutionsofFeathers
Feathers{/tex}\to{/tex}provideinsulationincoldweatherbutlatertheymightbecome
usefulforflight.
Example:Dinosaurshadfeathers,butcouldnotflyusingfeathers.Birdsseemtohavelater
adaptedthefeatherstoflight.
EvolutionbyArtificialSelection:Humanshavebeenapowerfulagentinmodifyingwild
speciestosuittheirownrequirementthroughoutagesbyusingartificialselection.e.g.(i)
Fromwildcabbagemanyvarietieslikebroccoli,cauliflower,redcabbage,kale,cabbageand
kohlrabiwereobtainedbyartificialselection.(ii)Wheat(manyvarietiesobtaineddueto
artificialselection).
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MolecularPhylogeny
ItisbasedontheideathatchangesinDNAduringreproductionarethebasicevents
inevolution
Organismswhicharemoredistantlyrelatedwillaccumulategreaterdifferencesin
theirDNA
ToolstoStudyHumanEvolutionaryRelationship
Excavating
Timedating
Fossils
Determining
DNASequences
Althoughthereisgreatdiversityofhumanformsallovertheworldgetallhumansare
asinglespecies
GENETICFOOTPRINTSOFHUMANS
Hundreds/thousandofyearsago
Theydidnotgoinasingleline
Theywentforwardandbackward
MovedinandoutofAfrica
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Sometimescamebacktomixwitheachother.
Whatyouhavelearnt
Variationsarisingduringtheprocessofreproductioncanbeinherited.
Thesevariationsmayleadtoincreasedsurvivaloftheindividuals.
Sexuallyreproducingindividualshavetwocopiesofgenesforthesametrait.Ifthe
copiesarenotidentical,thetraitthatgetsexpressediscalledthedominanttraitand
theotheriscalledtherecessivetrait.
Traitsinoneindividualmaybeinheritedseparately,givingrisetonewcombinations
oftraitsintheoffspringofsexualreproduction.
Sexisdeterminedbydifferentfactorsinvariousspecies.Inhumanbeings,thesexof
thechilddependsonwhetherthepaternalchromosomeisX(forgirls)orY(forboys).
Variationsinthespeciesmayconfersurvivaladvantagesormerelycontributetothe
geneticdrift.
Changesinthenon-reproductivetissuescausedbyenvironmentalfactorsarenot
inheritable.
Speciationmaytakeplacewhenvariationiscombinedwithgeographicalisolation.
Evolutionaryrelationshipsaretracedintheclassificationoforganisms.
Tracingcommonancestorsbackintimeleadsustotheideathatatsomepointoftime,
non-livingmaterialmusthavegivenrisetolife.
Evolutioncanbeworkedoutbythestudyofnotjustlivingspecies,butalsofossils.
Complexorgansmayhaveevolvedbecauseofthesurvivaladvantageofeventhe
intermediatestages.
Organsorfeaturesmaybeadaptedtonewfunctionsduringthecourseofevolution.
Forexample,feathersarethoughttohavebeeninitiallyevolvedforwarmthandlater
adaptedforflight.
Evolutioncannotbesaidto‘progress’from‘lower’formsto‘higher’forms.Rather,
evolutionseemstohavegivenrisetomorecomplexbodydesignsevenwhilethe
simplerbodydesignscontinuetoflourish.
Studyoftheevolutionofhumanbeingsindicatesthatallofusbelongtoasingle
speciesthatevolvedinAfricaandspreadacrosstheworldinstages.
