mendelian genetics1

8/11/2019 Mendelian Genetics1

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MendelelianGenetics

co ri ht cmassen ale


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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

occur between flowersco ri ht cmassen ale


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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

theoretical ratio co ri ht cmassen ale


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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

generationco ri ht cmassen ale


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Following the Generations

Cross 2PurePlants

TT x tt

Resultsin all

HybridsTt

Cross 2 Hybridsget3 Tall & 1 Short

TT, Tt, ttco ri ht cmassen ale


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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)


f


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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


f d d


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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


Dih b id C


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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


Dih b id C


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Dihybrid Cross

Round/Yellow: 9

Round/green: 3wrinkled/Yellow: 3wrinkled/green: 1

9:3:3:1


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


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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Rr

Rr

Rr

Rr

R

R

r

All Rr =pink(heterozygous pink)

produces theF1generation

r




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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


h C d i P bl


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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


S x link d T it


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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


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


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


Solution:


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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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mendelian genetics1

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