1

Chapter14

MendelandtheGeneIdea

HistoricalViewsofHeredity• Whatgeneticprinciplesaccountforthepassingoftraits

• Fromparentstooffspring?

• Acquiredtraits

• Traitsgainedbyparentsintheirlivesarepassedtooffspring

• “blending” hypothesis

• Ideathatgeneticmaterialfromthetwoparentsblendstogether

• Likeblueandyellowpaintblendtomakegreen

• The“particulate” hypothesis

• Ideathatparentspassondiscreteheritableunits(genes)

• Mendel

• Documentedaparticulatemechanismthroughhisexperimentswithgardenpeas

Fig. 14-1

2

Mendel’sExperimental,QuantitativeApproach• Mendeldiscoveredthebasicprinciplesof

heredity

• Bybreedinggardenpeasincarefullyplannedexperiments

• Advantagesofpeaplantsforgeneticstudy:

• Manyvarietieswithdistinctheritablecharactersortraits

• Matingofplantscanbecontrolled

• Eachpeaplant

• Hassperm-producingstamensandegg-producingcarpels

• Cross-pollination

• Canbeachievedbydustingoneplantwithpollenfromanother

Fig. 14-2TECHNIQUE

RESULTS

Parentalgeneration(P) Stamens

Carpel

1

2

3

4

Firstfilialgener-ationoffspring(F1)

5

• Mendeltrackedonlyeither-orcharacters

• Usedvarietiesthatweretrue-breeding

• Produceoffspringofsame

varietywhenself-pollinated

• Matedtwocontrasting,true-breedingvarieties

• hybridization

• P(parent)generation

• F1 generation- HybridoffspringofthePgeneration

• WhenF1 individualsself-pollinate,theF2generationisproduced

Mendel’sExperimental,QuantitativeApproach

3

TheLawofSegregation• Mendel

• Crossedcontrasting,true-breedingwhiteandpurplefloweredpeaplants

• AllF1 hybridswerepurple

• CrossedF1 hybrids

• ManyF2 plantshadpurpleflowers

• Butsomehadwhite

• Discoveredratioof~3:1

• Purpletowhiteflowers

• InF2 generation

EXPERIMENT

P Generation(true-breeding

parents) Purpleflowers

Whiteflowers

´

F1 Generation(hybrids)

All plants hadpurple flowers

F2 Generation

705 purple-floweredplants

224 white-floweredplants

• Mendel

• RealizedonlypurpleflowerfactoraffectingflowercolorinF1 hybrids

• Calledthepurpleflowercoloradominanttrait

• Andthewhiteflowercolorarecessivetrait

• Observedthesamepatternofinheritance

• Insixotherpeaplantcharacters

• Eachrepresentedbytwotraits

• WhatMendelcalleda“heritablefactor”iswhatwenowcallagene

Mendel’sExperimental,QuantitativeApproach

Mendel’sModel• Mendeldevelopedahypothesistoexplainthe3:1inheritancepatternhe

observedinF2 offspring

• Fourconceptscanberelatedtowhatwenowknowaboutgenesandchromosomes

• 1st: Alternativeversionsofgenesaccountforvariationsininheritedcharacters

• ie - purpleorwhite

• Alternativeversionsofagenearenowcalledalleles

• Eachgeneresidesataspecificlocusonaspecificchromosome

Allele for purple flowers

Homologouspair ofchromosomes

Locus for flower-color gene

Allele for white flowers

4

• 2nd:Foreachcharacteranorganisminherits twoalleles

• Onefromeachparent

• Didnotknowtheroleofchromosomes

• Twoallelesatalocusonachromosomemaybeidentical

• True-breedingplants

• Alternatively,thetwoallelesatalocusmaydiffer

• Hybrids

Mendel’sModel

Allele for purple flowers

Homologouspair ofchromosomes

Locus for flower-color gene

Allele for white flowers

• 3rd: Ifthetwoallelesatalocusdiffer

• Thenonedeterminestheorganism’sappearance

• Dominantallele

• Andtheotherhasnonoticeableeffectonappearance

• Recessiveallele

• Intheflower-colorexample,theF1 plantshadpurpleflowersbecausethealleleforthattraitisdominant

Mendel’sModel

Allele for purple flowers

Homologouspair ofchromosomes

Locus for flower-color gene

Allele for white flowers

• 4th:Nowknownasthelawof

segregation

• Duringgameteformation,

twoallelesforaheritable

characterseparate

(segregate)andendupin

differentgametes

• Thus,aneggoraspermgetsonlyone

ofthetwoalleles

• Correspondstothedistribution

ofhomologouschromosomesto

differentgametesinmeiosis

Mendel’sModel

5

• segregationmodelaccountsforthe3:1ratioinF2 gensofhisnumerouscrosses

• Punnett square

• Forshowingallpossiblecombinationsof

spermandegg

• diagramforpredictingtheresultsofageneticcrossbetweenindividualsofknowngeneticmakeup

• Acapitalletterrepresentsadominantallele

• P=purpleallele

• Lowercaseletterrepresentsarecessiveallele

• p=whiteallele

CalculatingpossiblecombinationsP Generation

Appearance:Genetic makeup:

Gametes:

Purple flowersWhite flowersPP

P

pp

p

F1 Generation

Gametes:

Genetic makeup:Appearance: Purple flowers

Pp

P p1/2 1/2

F2 GenerationSperm

Eggs

P

PPP Pp

p

pPp pp

3 1

Individual B:homozygous

dominant

Individual A:heterozygous

Individual C:homozygous

recessive

GeneticVocabulary• Homozygous

• Anorganismwithtwoidenticalallelesforthegeneforacharacter

• Homozygousdominant

• Homozygousrecessive

• Heterozygous

• Anorganismthathastwodifferentallelesforagenefor

acharacter

• Unlikehomozygotes

• heterozygotesarenottrue-breeding

AlleleforblueeyesAlleleforbrowneyes

• Traitsdonotalwaysrevealgeneticcomposition

• Differenteffectsofdominantandrecessivealleles

• Phenotype

• physicalappearance

• Genotype

• geneticmakeup

• PPandPp plantshavethesame

phenotype(purple)

• butdifferentgenotypes

GeneticVocabulary

Phenotype

Purple

Purple3

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

individualwiththedominant

phenotype?

• Musthaveonedominantallele,

• Butcouldbeeitherhomozygous

dominantorheterozygous

• Testcross:

• Breedingthe mysteryindividual

withahomozygousrecessive

individual

• Ifany offspringdisplaytherecessive

phenotype,themysteryparentmust

beheterozygous

TECHNIQUE

RESULTS

Dominant phenotype,unknown genotype:

PP or Pp?

Predictions

Recessive phenotype,known genotype:

pp

´

If PP If PporSperm Sperm

p p p p

P

P

P

pEggs Eggs

Pp

Pp Pp

Pp

Pp Pp

pp pp

orAll offspring purple 1/2 offspring purple and

1/2 offspring white

TheLawofIndependentAssortment• Mendelderivedthelawof

segregation

• byfollowingasingle

character

• TheF1 offspringproducedinthis

crossweremonohybrids

• individualsthatare

heterozygousforone

character

• Acrossbetweensuch

heterozygotesiscalleda

monohybridcross

• Mendelidentifiedhissecondlaw

ofinheritancebyfollowingtwocharactersatthesametime

• Crossingtwotrue-breeding

parentsdifferingintwocharacters

• Producesdihybrids intheF1generation

• Heterozygousforbothcharacters

• Adihybrid cross

• Candeterminewhethertwo

charactersaretransmittedtooffspringasapackageor

independently

TheLawofIndependentAssortmentEXPERIMENT

RESULTS

P Generation

F1 Generation

Predictions

Gametes

Hypothesis ofdependentassortment

YYRR yyrr

YR yr

YyRr

´

Hypothesis ofindependentassortment

orPredictedoffspring ofF2 generation

Sperm

SpermYR

YR

yr

yr

Yr

YR

yR

Yr

yRyr

YRYYRR

YYRR YyRr

YyRr

YyRr

YyRr

YyRr

YyRr

YYRr

YYRr

YyRR

YyRR

YYrr Yyrr

Yyrr

yyRR yyRr

yyRr yyrr

yyrr

Phenotypic ratio 3:1

EggsEggs

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

1/41/43/4

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

Phenotypic ratio approximately 9:3:3:1315 108 101 32

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• Lawofindependentassortment

• Eachpairofallelessegregatesindependently

• ofanyotherpairofallelesduringgameteformation

• Appliesonlytogenesondifferent,nonhomologous chromosomes

• Geneslocatedneareachotheronthesamechromosometendtobeinheritedtogether

TheLawofIndependentAssortment

ThelawsofprobabilitygovernMendelian inheritance

• Mendel’slawsofsegregationandindependentassortment

• Reflectedintherulesofprobability

• Whentossingacoin

• Outcomeofonetosshasnoimpactontheoutcomeofthenexttoss

• Inthesameway

• Allelesofonegenesegregateintogametesindependentlyofanothergene’salleles

Rr Rr´Segregation of

alleles into eggs

Sperm

R

RR R

R

R rrr

r

r

r1/2

1/2

1/2

1/2

Segregation ofalleles into sperm

Eggs1/4 1/4

1/4 1/4

• Ruleofmultiplicationstates

• Theprobabilitythattwoormoreindependenteventswilloccurtogether

• Istheproductoftheirindividualprobabilities

• Pone(XY)=P(X)*P(Y)

• ProbabilityinanF1 monohybridcrosscanbedetermined

• Usingthemultiplicationrule

• Segregationinaheterozygousplantislikeflippingacoin:

• Eachgametehasachanceofcarryingthedominantalleleandachanceofcarryingtherecessiveallele

TheMultiplicationandAdditionRules

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• Ruleofaddition

• Theprobabilitythatanyoneoftwoormoreexclusiveeventswilloccur

• Iscalculatedbyaddingtogethertheirindividualprobabilities

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

• Theruleofadditioncanbeusedtofigureouttheprobabilitythat

• AnF2 plantfromamonohybridcross

• willbeheterozygousratherthanhomozygous

TheMultiplicationandAdditionRules

Rr Rr´Segregation of

alleles into eggs

Sperm

R

RR R

R

R rrr

r

r

r1/2

1/2

1/2

1/2

Segregation ofalleles into sperm

Eggs1/4 1/4

1/4 1/4

SolvingGeneticsProblemswiththeRulesofProbability• Predictingoutcomeofcrossesinvolvingmultiplecharacters(dihybrid ormore)

• Applythemultiplicationandadditionrules

• Equivalenttotwoormoreindependentmonohybridcrossesoccurringsimultaneously

• Calculatingthechancesforvariousgenotypes

• Consideredeachcharacterseparately

• Multiplyindividualprobabilitiestogether

ComplexInheritancePatterns• Relationshipbetweengenotypeandphenotype

• RarelyassimpleasinthepeaplantcharactersMendelstudied

• Manyheritablecharactersarenotdeterminedbyonlyonegenewithtwoalleles

• However,thebasicprinciplesofsegregationandindependentassortmentapply

• Eventomorecomplexpatternsofinheritance

• Inheritanceofcharactersbyasinglegene

• MaydeviatefromsimpleMendelian patternsinthefollowingsituations:

• Whenallelesarenotcompletelydominantorrecessive

• Whenagenehasmorethantwoalleles

• Whenageneproducesmultiplephenotypes

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DegreesofDominance• Completedominance

• Phenotypesoftheheterozygoteanddominanthomozygoteareidentical

• Incompletedominance

• Intermediatephenotype

• Somewherebetweenthephenotypesofthetwoparentalvarieties

• Codominance

• Twodominantallelesaffectthephenotype

• Inseparate,distinguishableways

Red

P Generation

Gametes

WhiteCRCR CWCW

CR CW

F1 GenerationPinkCRCW

CR CWGametes 1/2 1/2

F2 Generation

Sperm

Eggs

CR

CR

CW

CW

CRCR CRCW

CRCW CWCW

1/2 1/2

1/2

1/2

• Dominantalleles

• Notnecessarilymorecommoninpopulationsthanrecessivealleles

• Forexample

• Onebabyoutof400intheU.S.isbornwithextrafingersortoes

• Polydactylyalleledominanttoalleleformorecommontraitoffivedigitsperappendage

• Inthisexample,therecessivealleleisfarmoreprevalent

• Thanthepopulation’sdominantallele

FrequencyofDominantAlleles

MultipleAlleles

IA

IB

i

A

B

none(a) The three alleles for the ABO blood groups

and their associated carbohydrates

Allele Carbohydrate

GenotypeRed blood cell

appearancePhenotype

(blood group)

IAIA or IA i A

BIBIB or IB i

IAIB AB

ii O

(b) Blood group genotypes and phenotypes

• Mostgenesexistinpopulationsinmorethantwoallelicforms

• ie - ABObloodgroup

• Inhumansaredeterminedbythreeallelesfortheenzyme(I)thatattachesAorBcarbohydratestoredbloodcells

• IA,IB,andI

• Enzymeencodedbythe IA alleleaddstheAcarbohydrate,

• EnzymeencodedbytheIB alleleaddstheBcarbohydrate

• Enzymeencodedbythei alleleaddsneither

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Pleiotropy• Pleiotropy

• Onegenewithmultiple

phenotypiceffects

• Forexample

• Pleiotropicallelesare

responsibleforthe

multiplesymptomsof

certainhereditary

diseases

• Suchascystic

fibrosis,sickle-cell

disease,andPKU

Clumping of cellsand clogging of

small blood vessels

Pneumoniaand otherinfections

Accumulation ofsickled cells in spleen

Pain andfever

Rheumatism

Heartfailure

Damage toother organs

Braindamage

Spleendamage

Kidneyfailure

Anemia

ParalysisImpairedmental

function

Physicalweakness

Breakdown ofred blood cells

Individual homozygousfor sickle-cell allele

Sickle cells

Sickle-cell (abnormal) hemoglobin

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

Epistasis• Epistasis

• Ageneatonelocusaltersthephenotypicexpressionofageneatasecondlocus

• Inmiceandmanyothermammals,coatcolordependsontwogenes

• Onegenedeterminesthepigmentcolor

• WithallelesB forblackandbforbrown

• Anothergenedetermineswhetherthepigmentwillbedepositedinthehair

• WithallelesC forcolorandcfornocolor

BbCc BbCc

Sperm

EggsBC bC Bc bc

BC

bC

Bc

bc

BBCC

1/4 1/4 1/4 1/4

1/4

1/4

1/4

1/4

BbCC BBCc BbCc

BbCC bbCC BbCc bbCc

BBCc BbCc

BbCc bbCc

BBcc Bbcc

Bbcc bbcc

9 : 3 : 4

´

PolygenicInheritance• Quantitativecharacters

• Thosethatvaryinthepopulationalongacontinuum

• Quantitativevariation

• Usuallyindicatespolygenicinheritance

• Anadditiveeffectoftwoormoregenesonasinglephenotype

• 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 13 140

20

40

60

80

P generation

1–8

F1 generation

F2 generation

Frac

tion

of p

opul

atio

n

Skin color

Eggs

Sperm1–81–8

1–81–8

1–81–8

1–81–81–81–81–81–81–81–81–8

aabbcc(very light)

AABBCC(very dark)

AaBbCc AaBbCc

1––6415––64

6––641––64

15––646––64

20––64

1––64

15––64

6––64

20––64

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TheEnvironmentalImpactonPhenotype• Phenotypeforacharactermay

dependonenvironmentaswellasgenotype

• Normofreaction

• Phenotypicrangeofagenotypeinfluencedbytheenvironment

• Forexample

• Hydrangeaflowersofthesame

genotype

• Rangefromblue-violettopink

• Dependingonsoilacidity

PedigreeAnalysis• Pedigree

• Afamilytreethatdescribestheinterrelationshipsofparentsandchildrenacrossgenerations

• Inheritancepatternsofparticulartraits

• Canbetracedanddescribedusingpedigrees

QueenVictoria

Albert

Alice Louis

Alexandra CzarNicholas IIof Russia

Alexis

Youshouldnowbeableto:

1. Definethefollowingterms:truebreeding,hybridization,monohybridcross,Pgeneration,F1 generation,F2 generation

2. Distinguishbetweenthefollowingpairsofterms:dominantandrecessive;heterozygousandhomozygous;genotypeandphenotype

3. UseaPunnett squaretopredicttheresultsofacrossandtostatethephenotypicandgenotypicratiosoftheF2 generation

4. Explainhowphenotypicexpressionintheheterozygotedifferswithcompletedominance,incompletedominance,andcodominance

5. Defineandgiveexamplesofpleiotropy andepistasis

6. Explainwhylethaldominantgenesaremuchrarerthanlethalrecessivegenes

7. Explainhowcarrierrecognition,fetaltesting,andnewbornscreeningcanbeusedingeneticscreeningandcounseling