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Chapter14
MendelandtheGeneIdea
HistoricalViewsofHeredity• Whatgeneticprinciplesaccountforthepassingoftraits
• Fromparentstooffspring?
• Acquiredtraits
• Traitsgainedbyparentsintheirlivesarepassedtooffspring
• “blending” hypothesis
• Ideathatgeneticmaterialfromthetwoparentsblendstogether
• Likeblueandyellowpaintblendtomakegreen
• The“particulate” hypothesis
• Ideathatparentspassondiscreteheritableunits(genes)
• Mendel
• Documentedaparticulatemechanismthroughhisexperimentswithgardenpeas
Fig. 14-1
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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
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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
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• 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
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• 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