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Chapter7

MendelandtheGeneIdeaBIOL100

HistoricalViewsofHeredity•  Whatgene.cprinciplesaccountforthepassingoftraits

•  Fromparentstooffspring?

•  Acquiredtraits

•  Traitsgainedbyparentsintheirlivesarepassedtooffspring

•  “blending”hypothesis

•  Ideathatgene.cmaterialfromthetwoparentsblendstogether

•  Likeblueandyellowpaintblendtomakegreen

•  The“par.culate”hypothesis

•  Ideathatparentspassondiscreteheritableunits(genes)

•  Mendel

•  Documentedapar.culatemechanismthroughhisexperimentswithgardenpeas

Fig. 14-1

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Mendel’sExperimental,QuanCtaCveApproach•  Mendeldiscoveredthebasicprinciplesof

heredity

•  Bybreedinggardenpeasincarefullyplannedexperiments

•  Advantagesofpeaplantsforgene.cstudy:

•  Manyvarie.eswithdis.nctheritablecharactersortraits

•  Ma.ngofplantscanbecontrolled

•  Eachpeaplant

•  Hassperm-producingstamensandegg-producingcarpels

•  Cross-pollina.on

•  Canbeachievedbydus.ngoneplantwithpollenfromanother

Fig. 14-2

TECHNIQUE

RESULTS

Parental generation (P) Stamens

Carpel

1

2

3

4

First filial gener- ation offspring (F1)

5

•  Mendeltrackedonlyeither-orcharacters

•  Usedvarie.esthatweretrue-breeding

•  Produceoffspringofsamevarietywhenself-pollinated

•  Matedtwocontras.ng,true-breedingvarie.es

•  hybridizaCon

•  P(parent)generaCon

•  F1generaCon-HybridoffspringofthePgenera.on

•  WhenF1individualsself-pollinate,theF2generaConisproduced

Mendel’sExperimental,QuanCtaCveApproach

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TheLawofSegregaCon•  Mendel

•  Crossedcontras.ng,true-breedingwhiteandpurplefloweredpeaplants

•  AllF1hybridswerepurple

•  CrossedF1hybrids

•  ManyF2plantshadpurpleflowers

•  Butsomehadwhite

•  Discoveredra.oof~3:1

•  Purpletowhiteflowers

•  InF2genera.on

EXPERIMENT

P Generation (true-breeding parents)

Purple flowers

White flowers

×

F1 Generation (hybrids)

All plants had purple flowers

F2 Generation

705 purple-flowered plants

224 white-flowered plants

•  Mendel

•  Realizedonlypurpleflowerfactoraffec.ngflowercolorinF1hybrids

•  Calledthepurpleflowercoloradominanttrait

•  Andthewhiteflowercolorarecessivetrait

•  ObservedthesamepaYernofinheritance

•  Insixotherpeaplantcharacters

•  Eachrepresentedbytwotraits

•  WhatMendelcalleda“heritablefactor”iswhatwenowcallagene

Mendel’sExperimental,QuanCtaCveApproach

Mendel’sModel•  Mendeldevelopedahypothesistoexplainthe3:1inheritancepaYernhe

observedinF2offspring

•  Fourconceptscanberelatedtowhatwenowknowaboutgenesandchromosomes

•  1st:Alterna.veversionsofgenesaccountforvaria.onsininheritedcharacters

•  ie-purpleorwhite

•  Alterna.veversionsofagenearenowcalledalleles

•  Eachgeneresidesataspecificlocusonaspecificchromosome

Allele for purple flowers

Homologous pair of chromosomes

Locus for flower-color gene

Allele for white flowers

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•  2nd:Foreachcharacteranorganisminheritstwoalleles

•  Onefromeachparent

•  Didnotknowtheroleofchromosomes

•  Twoallelesatalocusonachromosomemaybeiden.cal

•  True-breedingplants

•  Alterna.vely,thetwoallelesatalocusmaydiffer

•  Hybrids

Mendel’sModel

Allele for purple flowers

Homologous pair of chromosomes

Locus for flower-color gene

Allele for white flowers

•  3rd:Ifthetwoallelesatalocusdiffer

•  Thenonedeterminestheorganism’sappearance

•  Dominantallele

•  Andtheotherhasnono.ceableeffectonappearance

•  Recessiveallele

•  Intheflower-colorexample,theF1plantshadpurpleflowersbecausethealleleforthattraitisdominant

Mendel’sModel

Allele for purple flowers

Homologous pair of chromosomes

Locus for flower-color gene

Allele for white flowers

•  4th:Nowknownasthelawof

segregaCon

•  Duringgameteforma.on,

twoallelesforaheritable

characterseparate

(segregate)andendupin

differentgametes

•  Thus,aneggoraspermgetsonlyone

ofthetwoalleles

•  Correspondstothedistribu.on

ofhomologouschromosomesto

differentgametesinmeiosis

Mendel’sModel

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•  segrega.onmodelaccountsforthe3:1ra.oinF2gensofhisnumerouscrosses

•  PunneVsquare

•  Forshowingallpossiblecombina.onsofspermandegg

•  diagramforpredic.ngtheresultsofagene.ccrossbetweenindividualsofknowngene.cmakeup

•  AcapitalleYerrepresentsadominantallele

•  P=purpleallele

•  LowercaseleYerrepresentsarecessiveallele

•  p=whiteallele

CalculaCngpossiblecombinaConsP Generation

Appearance: Genetic makeup:

Gametes:

Purple flowers White flowers PP

P

pp

p

F1 Generation

Gametes:

Genetic makeup: Appearance: Purple flowers

Pp

P p 1/2 1/2

F2 Generation Sperm

Eggs

P

P PP Pp

p

p Pp pp

3 1

Individual B: homozygous

dominant

Individual A: heterozygous

Individual C: homozygous

recessive

GeneCcVocabulary•  Homozygous

•  Anorganismwithtwoiden.calallelesforthegeneforacharacter

•  Homozygousdominant

•  Homozygousrecessive

•  Heterozygous

•  Anorganismthathastwodifferentallelesforageneforacharacter

•  Unlikehomozygotes

•  heterozygotesarenottrue-breeding

AlleleforblueeyesAlleleforbrowneyes

•  Traitsdonotalwaysrevealgene.ccomposi.on

•  Differenteffectsofdominantandrecessivealleles

•  Phenotype

•  physicalappearance

•  Genotype

•  gene.cmakeup

•  PPandPpplantshavethesamephenotype(purple)

•  butdifferentgenotypes

GeneCcVocabulary

Phenotype

Purple

Purple 3

Purple

Genotype

1 White

Ratio 3:1

(homozygous)

(homozygous)

(heterozygous)

(heterozygous)

PP

Pp

Pp

pp

Ratio 1:2:1

1

1

2

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TheTestcross•  Howcanwetellthegenotypeofan

individualwiththedominantphenotype?

•  Musthaveonedominantallele,

•  Butcouldbeeitherhomozygousdominantorheterozygous

•  Testcross:

•  Breedingthemysteryindividualwithahomozygousrecessiveindividual

•  Ifanyoffspringdisplaytherecessivephenotype,themysteryparentmustbeheterozygous

TECHNIQUE

RESULTS

Dominant phenotype, unknown genotype:

PP or Pp?

Predictions

Recessive phenotype, known genotype: pp

×

If PP If Pp or Sperm Sperm p p p p

P

P

P

p Eggs Eggs

Pp

Pp Pp

Pp

Pp Pp

pp pp

or All offspring purple 1/2 offspring purple and

1/2 offspring white

TheLawofIndependentAssortment•  Mendelderivedthelawof

segrega.on

•  byfollowingasingle

character

•  TheF1offspringproducedinthis

crossweremonohybrids

•  individualsthatare

heterozygousforone

character

•  Acrossbetweensuch

heterozygotesiscalleda

monohybridcross

•  Mendeliden.fiedhissecondlawofinheritancebyfollowingtwocharactersatthesame.me

•  Crossingtwotrue-breedingparentsdifferingintwocharacters

•  ProducesdihybridsintheF1genera.on

•  Heterozygousforbothcharacters

•  Adihybridcross

•  CandeterminewhethertwocharactersaretransmiYedtooffspringasapackageorindependently

TheLawofIndependentAssortmentEXPERIMENT

RESULTS

P Generation

F1 Generation

Predictions

Gametes

Hypothesis of dependent assortment

YYRR yyrr

YR yr

YyRr

×

Hypothesis of independent assortment

or Predicted offspring of F2 generation

Sperm

Sperm YR

YR

yr

yr

Yr

YR

yR

Yr

yR yr

YR YYRR

YYRR YyRr

YyRr

YyRr

YyRr

YyRr

YyRr

YYRr

YYRr

YyRR

YyRR

YYrr Yyrr

Yyrr

yyRR yyRr

yyRr yyrr

yyrr

Phenotypic ratio 3:1

Eggs Eggs

Phenotypic ratio 9:3:3:1

1/2 1/2

1/2

1/2

1/4

yr

1/4 1/4

1/4 1/4

1/4

1/4

1/4

1/4 3/4

9/16 3/16 3/16 1/16

Phenotypic ratio approximately 9:3:3:1 315 108 101 32

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•  Lawofindependentassortment

•  Eachpairofallelessegregatesindependently

•  ofanyotherpairofallelesduringgameteforma.on

•  Appliesonlytogenesondifferent,nonhomologouschromosomes

•  Geneslocatedneareachotheronthesamechromosometendtobeinheritedtogether

TheLawofIndependentAssortment

ThelawsofprobabilitygovernMendelianinheritance

•  Mendel’slawsofsegrega.onandindependentassortment

•  Reflectedintherulesofprobability

•  Whentossingacoin

•  Outcomeofonetosshasnoimpactontheoutcomeofthenexttoss

•  Inthesameway

•  Allelesofonegenesegregateintogametesindependentlyofanothergene’salleles

Rr Rr × Segregation of

alleles into eggs

Sperm

R

R R R

R

R r r r

r

r

r 1/2

1/2

1/2

1/2

Segregation of alleles into sperm

Eggs 1/4 1/4

1/4 1/4

•  Ruleofmul.plica.onstates

•  Theprobabilitythattwoormoreindependenteventswilloccurtogether

•  Istheproductoftheirindividualprobabili.es

•  Pone(XY)=P(X)*P(Y)

•  ProbabilityinanF1monohybridcrosscanbedetermined

•  Usingthemul.plica.onrule

•  Segrega.oninaheterozygousplantislikeflippingacoin:

•  Eachgametehasachanceofcarryingthedominantalleleandachanceofcarryingtherecessiveallele

TheMulCplicaConandAddiConRules

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•  Ruleofaddi.on

•  Theprobabilitythatanyoneoftwoormoreexclusiveeventswilloccur

•  Iscalculatedbyaddingtogethertheirindividualprobabili.es

•  Pan(XY)=P(XY1)+P(XY2)+P(XY3)+P(XYn)…

•  Theruleofaddi.oncanbeusedtofigureouttheprobabilitythat

•  AnF2plantfromamonohybridcross

•  willbeheterozygousratherthanhomozygous

TheMulCplicaConandAddiConRules

Rr Rr × Segregation of

alleles into eggs

Sperm

R

R R R

R

R r r r

r

r

r 1/2

1/2

1/2

1/2

Segregation of alleles into sperm

Eggs 1/4 1/4

1/4 1/4

ComplexInheritancePaVerns•  Rela.onshipbetweengenotypeandphenotype

•  RarelyassimpleasinthepeaplantcharactersMendelstudied

•  Manyheritablecharactersarenotdeterminedbyonlyonegenewithtwoalleles

•  However,thebasicprinciplesofsegrega.onandindependentassortmentapply

•  EventomorecomplexpaYernsofinheritance

•  Inheritanceofcharactersbyasinglegene

•  MaydeviatefromsimpleMendelianpaYernsinthefollowingsitua.ons:

•  Whenallelesarenotcompletelydominantorrecessive

•  Whenagenehasmorethantwoalleles

•  Whenageneproducesmul.plephenotypes

DegreesofDominance•  Completedominance

•  Phenotypesoftheheterozygoteanddominanthomozygoteareiden.cal

•  Incompletedominance

•  Intermediatephenotype

•  Somewherebetweenthephenotypesofthetwoparentalvarie.es

•  Codominance

•  Twodominantallelesaffectthephenotype

•  Inseparate,dis.nguishableways

Red

P Generation

Gametes

White CRCR CWCW

CR CW

F1 Generation Pink CRCW

CR CW Gametes 1/2 1/2

F2 Generation

Sperm

Eggs

CR

CR

CW

CW

CRCR CRCW

CRCW CWCW

1/2 1/2

1/2

1/2

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•  Dominantalleles

•  Notnecessarilymorecommoninpopula.onsthanrecessivealleles

•  Especiallylethalones…

•  Forexample

•  Onebabyoutof400intheU.S.isbornwithextrafingersortoes

•  Polydactylyalleledominanttoalleleformorecommontraitoffivedigitsperappendage

•  therecessivealleleisfarmoreprevalent

•  Thanthepopula.on’sdominantallele

FrequencyofDominantAlleles

MulCpleAlleles

IA

IB

i

A

B

none (a) The three alleles for the ABO blood groups and their associated carbohydrates

Allele Carbohydrate

Genotype Red blood cell

appearance Phenotype

(blood group)

IAIA or IA i A

B IBIB or IB i

IAIB AB

ii O

(b) Blood group genotypes and phenotypes

•  Mostgenesexistinpopula.onsinmorethantwoallelicforms

•  ie-ABObloodgroup

•  Inhumansaredeterminedbythreeallelesfortheenzyme(I)thataYachesAorBcarbohydratestoredbloodcells

•  IA,IB,andI

•  EnzymeencodedbytheIAalleleaddstheAcarbohydrate,

•  EnzymeencodedbytheIBalleleaddstheBcarbohydrate

•  Enzymeencodedbytheialleleaddsneither

Pleiotropy•  Pleiotropy

•  Onegenewithmul.ple

phenotypiceffects

•  Forexample

•  Pleiotropicallelesareresponsibleforthe

mul.plesymptomsof

certainhereditary

diseases

•  Suchascys.cfibrosis,sickle-cell

disease,andPKU

Clumping of cells and clogging of

small blood vessels

Pneumonia and other infections

Accumulation of sickled cells in spleen

Pain and fever

Rheumatism

Heart failure

Damage to other organs

Brain damage

Spleen damage

Kidney failure

Anemia

Paralysis Impaired mental

function

Physical weakness

Breakdown of red blood cells

Individual homozygous for sickle-cell allele

Sickle cells

Sickle-cell (abnormal) hemoglobin

Abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped

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PolygenicInheritance•  QuanCtaCvecharacters

•  Thosethatvaryinthepopula.onalongacon.nuum

•  Quan.ta.vevaria.on

•  Usuallyindicatespolygenicinheritance

•  Anaddi.veeffectoftwoormoregenesonasinglephenotype

•  Skin/eyecolor,heightinhumans

(a) Continuous variation in human height (b) The bell curve

Beak depth (mm)

Num

ber o

f ind

ivid

uals beak depth

5 6 7 8 9 10 11 12 13 14 0 20 40 60 80

P generation

1 – 8

F1 generation

F2 generation

Frac

tion

of p

opul

atio

n

Skin color

Eggs

Sperm 1 – 8 1 – 8

1 – 8 1 – 8

1 – 8 1 – 8

1 – 8 1 – 8 1 – 8 1 – 8 1 – 8 1 – 8 1 – 8 1 – 8 1 – 8

aabbcc (very light)

AABBCC (very dark)

AaBbCc AaBbCc

1 –– 64 15 –– 64

6 –– 64 1 –– 64

15 –– 64 6 –– 64

20 –– 64

1 –– 64

15 –– 64

6 –– 64

20 –– 64

TheEnvironmentalImpactonPhenotype•  Phenotypeforacharactermay

dependonenvironmentaswellasgenotype

•  NormofreacCon

•  Phenotypicrangeofagenotypeinfluencedbytheenvironment

•  Forexample

•  Hydrangeaflowersofthesamegenotype

•  Rangefromblue-violettopink

•  Dependingonsoilacidity

PedigreeAnalysis•  Pedigree

•  Afamilytreethatdescribestheinterrela.onshipsofparentsandchildrenacrossgenera.ons

•  InheritancepaYernsofpar.culartraits

•  Canbetracedanddescribedusingpedigrees

Queen Victoria

Albert

Alice Louis

Alexandra Czar Nicholas II of Russia

Alexis

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Youshouldnowbeableto:

1.  Definethefollowingterms:truebreeding,hybridiza.on,monohybridcross,Pgenera.on,F1genera.on,F2genera.on

2.  Dis.nguishbetweenthefollowingpairsofterms:dominantandrecessive;heterozygousandhomozygous;genotypeandphenotype

3.  UseaPunneYsquaretopredicttheresultsofacrossandtostatethephenotypicandgenotypicra.osoftheF2genera.on

4.  Explainhowphenotypicexpressionintheheterozygotedifferswithcompletedominance,incompletedominance,andcodominance

5.  Defineandgiveexamplesofpleiotropyandepistasis

6.  Explainwhylethaldominantgenesaremuchrarerthanlethalrecessivegenes

7.  Explainhowcarrierrecogni.on,fetaltes.ng,andnewbornscreeningcanbeusedingene.cscreeningandcounseling