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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.