mendelian genetics1
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MendelelianGenetics
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Gregor Mendel(1822-1884)
Responsiblefor the Laws
governing
Inheritance ofTraits
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Gregor Johann Mendel
Austrian monkStudied theinheritanceoftraits in pea plantsDeveloped the lawsof inheritanceMendel's work was
not recognized untilthe turn of the20th century
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Gregor Johann Mendel
Between 1856 and1863,Mendelcultivated andtested some 28,000
pea plantsHe found that theplants' offspringretained traits ofthe parentsCalled theFatherof Genetics"
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Site ofGregorMendels
experimentalgarden in theCzech
Republic
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Mendel stated thatphysical traits are
inherited as particlesMendel did not knowthat the particles
were actuallyChromosomes & DNA
Particulate Inheritance
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Genetic Terminology
Trait- any characteristic thatcan be passed from parent to
offspringHeredity- passing of traitsfrom parent to offspring
Genetics- study of heredity
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Types of Genetic Crosses
Monohybrid cross - crossinvolving a single traite.g. flower color
Dihybrid cross- cross involving
two traitse.g. flower color & plant height
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Punnett SquareUsed to help
solve geneticsproblems
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Designer Genes
Alleles- two forms of a gene(dominant & recessive)
Dominant- stronger of two genesexpressed in the hybrid;represented byacapital letter (R)
Recessive- gene that shows up lessoften in a cross; represented by alowercase letter (r)
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More Terminology
Genotype- gene combinationfor a trait(e.g. RR, Rr, rr)
Phenotype- the physicalfeature resulting from a
genotype(e.g. red, white)
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Genotype & Phenotype in Flowers
Genotype of alleles:R= red flowerr= yellow flower
All genes occur in pairs, so 2allelesaffect a characteristic
Possible combinations are:
Genotypes RR Rr rr
Phenotypes RED RED YELLOW
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Genotypes Homozygousgenotype - genecombination involving 2 dominantor 2 recessive genes (e.g. RR orrr);also calledpure
Heterozygousgenotype - gene
combination of one dominant &one recessive allele (e.g. Rr);also calledhybrid
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Genes and EnvironmentDetermine Characteristics
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Mendels Pea PlantExperiments
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Why peas,Pisum sativum?
Can be grown in asmall areaProduce lots of
offspringProduce pureplantswhen allowed to
self-pollinateseveral generationsCan be artificiallycross-pollinated
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Reproduction in Flowering Plants
Pollen contains spermProduced by thestamen
Ovary contains eggs
Found inside theflower
Pollen carries sperm to theeggs for fertilization
Self-fertilizationcanoccur in the same flower
Cross-fertilizationcan
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Mendels ExperimentalMethods
Mendel hand-pollinatedflowers using a paintbrush
He could snip thestamensto preventself-pollination
Covered each flower
with a cloth bagHe traced traits throughthe several generations
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How Mendel Began
Mendelproducedpurestrains byallowing theplants toself-
pollinatefor severalgenerations
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Eight Pea Plant Traits
Seed shape--- Round (R)or Wrinkled (r)Seed Color---- Yellow (Y)or Green ()
Pod Shape--- Smooth (S)or wrinkled (
Pod Color--- Green (G)or Yellow (g)Seed Coat Color---Gray (G)or White (g)Flower position---Axial (A)or Terminal (a)Plant Height--- Tall (T)or Short (t)
Flower color --- Purple (P)or white ( )co ri ht cmassen ale
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Mendels Experimental Results
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Did the observed ratio matchthe theoretical ratio?
The theoretical or expected ratio ofplants producing round or wrinkled seeds
is 3 round :1 wrinkledMendels observed ratio was 2.96:1
The discrepancy is due to statistical
errorThe larger the samplethe more nearlythe results approximate to the
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Generation GapParental P
1
Generation= the parentalgeneration in a breeding experiment.
F1generation= the first-generationoffspring in a breeding experiment. (1st
filial generation)From breeding individuals from the P1generation
F2generation= the second-generationoffspring in a breeding experiment.(2nd filial generation)From breeding individuals from the F1
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Following the Generations
Cross 2PurePlants
TT x tt
Resultsin all
HybridsTt
Cross 2 Hybridsget3 Tall & 1 Short
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Monohybrid
Crosses
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Trait: Seed Shape
Alleles: RRound rWrinkledCross: Roundseeds x Wrinkled seeds
RR x rr
P1Monohybrid Cross
R
R
rr
Rr
RrRr
Rr
Genotype:Rr
Phenotype:Round
GenotypicRatio: All alike
PhenotypicRatio:All alike
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P1Monohybrid Cross Review
Homozygous dominant x Homozygousrecessive
OffspringallHeterozygous(hybrids)
Offspring calledF1generation
Genotypic & Phenotypic ratio isALLALIKE
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Trait: Seed Shape
Alleles: RRound rWrinkledCross: Roundseeds x Round seeds
Rr x Rr
F1Monohybrid Cross
R
r
rR
RR
rrRr
Rr
Genotype:RR, Rr, rr
Phenotype:Round &wrinkled
G.Ratio: 1:2:1
P.Ratio: 3:1
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F1Monohybrid Cross Review
Heterozygous x heterozygous Offspring:25% Homozygous dominantRR50% HeterozygousRr25% Homozygous Recessiverr
Offspring calledF2generation
Genotypic ratio is1:2:1 Phenotypic Ratiois 3:1
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What Do the Peas Look Like?
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And Now the Test Cross
Mendel then crossed a pure& ahybridfrom his F2 generation
This is known as an F2or testcross
There are twopossible
testcrosses:Homozygous dominant x HybridHomozygous recessive x Hybrid
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Trait: Seed Shape
Alleles: RRound rWrinkledCross: Roundseeds x Round seeds
RR x Rr
F2Monohybrid Cross (1st)
R
R
rR
RR
RrRR
Rr
Genotype:RR, Rr
Phenotype:Round
GenotypicRatio: 1:1
PhenotypicRatio:All alike
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Trait: Seed Shape
Alleles: RRound rWrinkledCross: Wrinkledseedsx Round seeds
rr x Rr
F2Monohybrid Cross (2nd)
r
r
rR
Rr
rrRr
rr
Genotype:Rr, rr
Phenotype:Round &
WrinkledG. Ratio: 1:1
P.Ratio:1:1
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F2Monohybrid Cross Review
Homozygous x heterozygous(hybrid) Offspring:50% Homozygous RR or rr50% HeterozygousRr
Phenotypic Ratiois 1:1
Called Test Crossbecause theoffspring have SAMEgenotype asparents
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Practice Your Crosses
Work the P1, F1, and both
F2Crosses for each of theother Seven Pea Plant
Traits
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Mendels Laws
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Results of Monohybrid Crosses
Inheritablefactors or genesareresponsible for all heritablecharacteristics
Phenotypeis based on GenotypeEach traitis based ontwo genes,one from the mother and theother from the father
True-breeding individuals arehomozygous ( both alleles)are thesame
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Law of Dominance
In a cross of parents that arepure for contrasting traits, onlyone form of the trait will appear in
the next generation.All the offspring will beheterozygous and express only thedominant trait.
RR x rr yieldsall Rr (round seeds)
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Law of Dominance
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Law of Segregation
During the formation of gametes(eggs or sperm), the two allelesresponsible for a trait separatefrom each other.
Alleles for a trait are then
"recombined" at fertilization,producing the genotype for thetraits of the offspring.
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Applying the Law of Segregation
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Law of IndependentAssortment
Alleles for differenttraits aredistributed to sex cells (&
offspring) independently of oneanother.
This law can be illustrated usingdihybrid crosses.
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Dihybrid Cross
A breeding experiment that tracksthe inheritance of two traits.
Mendels Law of IndependentAssortment
a. Each pair of alleles segregates
independentlyduring gamete formationb. Formula: 2n(n = # of heterozygotes)
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Question:How many gametes will be produced
for the following allele arrangements?
Remember: 2n (n = # of heterozygotes)
1. RrYy
2. AaBbCCDd
3. MmNnOoPPQQRrssTtQqco ri ht cmassen ale
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Answer:
1. RrYy: 2n
= 22
= 4 gametesRY Ry rY ry
2. AaBbCCDd: 2n= 23= 8 gametesABCD ABCd AbCD AbCd
aBCD aBCd abCD abCD
3. MmNnOoPPQQRrssTtQq: 2n= 26= 64gametes
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Dihybrid CrossTraits: Seed shape & Seed colorAlleles: R round
r wrinkledY yellow
y green
RrYy x RrYy
RY Ry rY ry RY Ry rY ry
All possible gamete combinationsco ri ht cmassen ale
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Dihybrid Cross
RY Ry rY ry
RY
Ry
rY
ry
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Dihybrid Cross
RRYY
RRYy
RrYY
RrYy
RRYy
RRyy
RrYy
Rryy
RrYY
RrYy
rrYY
rrYy
RrYy
Rryy
rrYy
rryy
Round/Yellow: 9
Round/green: 3
wrinkled/Yellow: 3
wrinkled/green: 1
9:3:3:1 phenotypic
ratio
RY Ry rY ry
RY
Ry
rY
ry
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Dihybrid Cross
Round/Yellow: 9
Round/green: 3wrinkled/Yellow: 3wrinkled/green: 1
9:3:3:1
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Test Cross
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Test CrossA mating between an individual of unknown
genotype and a homozygous recessiveindividual.Example: bbC__ x bbcc
BB = brown eyesBb = brown eyesbb = blue eyes
CC = curly hairCc = curly haircc = straight hair
bC b___
bc
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Summary of Mendels laws
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Summary of Mendel s laws
LAWPARENT
CROSSOFFSPRING
DOMINANCE TT x tttall x short
100% Tttall
SEGREGATIONTt x Tttall x tall
75% tall25% short
INDEPENDENTASSORTMENT
RrGg x RrGground & green
xround & green
9/16 round seeds & green
pods3/16 round seeds & yellow
pods3/16 wrinkled seeds & green
pods1/16 wrinkled seeds & yellow
pods
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Incomplete DominanceandCodominance
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I l t D i
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Incomplete Dominance
F1 hybrids have an appearance somewhatin betweenthe phenotypes of the twoparental varieties.
Example: snapdragons (flower)red (RR) x white (rr)
RR = red flowerrr = white flower
R
R
r r
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Incomplete Dominance
Rr
Rr
Rr
Rr
R
R
r
All Rr =pink(heterozygous pink)
produces theF1generation
r
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Incomplete Dominance
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Incomplete Dominance
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Codominance
Two allelesare expressed (multiplealleles) in heterozygous individuals.
Example: blood type
1. type A = IAIAor IAi
2. type B = IB
IB
or IB
i3. type AB = IAIB4. type O = ii
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Codominance Problem
Example:homozygous male Type B (IBIB)
xheterozygous female Type A (IAi)
IA
IB
IB
i
IAIB IBi
1/2 = IAIB
1/2 = IBiIB
IA i
IB
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Another Codominance Problem
Example: male Type O (ii)x
female type AB (IAIB)
IAi IBi
IAi IBi
1/2 = IAi
1/2 = IBi
i
IA IB
i
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Codominance
Question:If a boy has a blood type O and
his sister has blood typeAB, what are the genotypesand phenotypes of theirparents?
boy - type O (ii) X girl - typeAB (IAIB)
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Codominance
Answer:
IAIB
ii
Parents:
genotypes = IAiand IBiphenotypes = Aand B
IB
IA i
i
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Sex-linked Traits
Traits (genes) located on the sexchromosomes
Sex chromosomes are X and YXXgenotype for femalesXYgenotype for malesMany sex-linked traitscarried onXchromosome
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Sex-linked Traits
Sex Chromosomes
XX chromosome - female Xy chromosome - male
fruit fly
eye color
Example: Eye color in fruit flies
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Sex linked Trait Problem
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Sex-linked Trait ProblemExample: Eye color in fruit flies
(red-eyed male) x (white-eyed female)XRY x XrXr
Remember: the Y chromosome in males
does not carry traits.RR = red eyedRr = red eyedrr = white eyed
XY = maleXX = female
XR
Xr Xr
Y
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Sex-linked Trait Solution:
XR Xr
XrY
XR Xr
XrY
50% red eyedfemale
50% white eyedmale
XR
Xr Xr
Y
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Female Carriers
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Female Carriers
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Genetic PracticeProblems
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Breed the P1generation
tall (TT) x dwarf (tt) pea plants
T
T
t t
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Solution:
T
T
t t
Tt
Tt
Tt
Tt All Tt = tall(heterozygous tall)
produces theF1generation
tall (TT) vs. dwarf (tt) pea plants
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Breed the F1generation
tall (Tt) vs. tall (Tt) pea plants
T
t
T t
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Solution:
TT
Tt
Tt
tt
T
t
T tproduces the
F2generation1/4 (25%) = TT1/2 (50%) = Tt
1/4 (25%) = tt1:2:1 genotype3:1 phenotype
tall (Tt) x tall (Tt) pea plants
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