B.Sc. Microb/Biotech II Cell biology and Genetics Unit 4 Mendelian Genetics

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Mendelian GeneticsSubject: Principles of Genetics Unit

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Gregor Gregor MendelMendel

(1822-1884)(1822-1884)Responsible Responsible for the Laws for the Laws governing governing

Inheritance Inheritance of Traitsof Traits


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Gregor Johann MendelGregor Johann MendelAustrian monkAustrian monkStudied the Studied the inheritanceinheritance of traits in of traits in pea plantspea plantsDeveloped the Developed the laws of laws of inheritanceinheritanceMendel's work was Mendel's work was not recognized until not recognized until the turn of thethe turn of the 20th 20th centurycentury


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Gregor Johann MendelGregor Johann MendelBetween Between 1856 and 1856 and 1863,1863, Mendel Mendel cultivated and tested cultivated and tested some some 28,000 pea plants28,000 pea plantsHe found that the He found that the plants' offspring plants' offspring retained retained traits of the traits of the parentsparentsCalled theCalled the “Father of “Father of Genetics"Genetics"


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Mendel stated that Mendel stated that physical traits are physical traits are inherited as inherited as “particles”“particles”Mendel did not know Mendel did not know that the “particles” were that the “particles” were actually actually Chromosomes & Chromosomes & DNADNA

Particulate InheritanceParticulate Inheritance


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Genetic TerminologyGenetic Terminology TraitTrait - any characteristic that can be - any characteristic that can be

passed from parent to offspring passed from parent to offspring HeredityHeredity - passing of traits from - passing of traits from

parent to offspring parent to offspring GeneticsGenetics - study of heredity - study of heredity


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Types of Genetic CrossesTypes of Genetic Crosses

Monohybrid cross Monohybrid cross - - cross involving a cross involving a single traitsingle traite.g. flower color e.g. flower color

Dihybrid crossDihybrid cross - - cross involving two cross involving two traits traits e.g. flower color & plant heighte.g. flower color & plant height


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Punnett SquarePunnett Square

Used to help solve Used to help solve genetics problemsgenetics problems


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Designer Designer “Genes”“Genes” AllelesAlleles - - two forms of a two forms of a gene gene (dominant & (dominant &

recessive)recessive) DominantDominant - - stronger of two genes stronger of two genes

expressed in the hybrid; represented byexpressed in the hybrid; represented by aa capital letter (R)capital letter (R)

RecessiveRecessive - - gene that shows up less often gene that shows up less often in a cross; represented by ain a cross; represented by a lowercase lowercase letter (r)letter (r)


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More TerminologyMore Terminology

GenotypeGenotype - - gene combination for a gene combination for a traittrait (e.g. RR, Rr, rr) (e.g. RR, Rr, rr)

PhenotypePhenotype - - the physical feature the physical feature resulting from a genotyperesulting from a genotype (e.g. red, (e.g. red, white) white)


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Genotype & Phenotype in FlowersGenotype & Phenotype in Flowers

Genotype of alleles:Genotype of alleles:RR = red flower= red flowerrr = yellow flower= yellow flowerAll genes occur in pairs, so All genes occur in pairs, so 22 allelesalleles affect a characteristic affect a characteristicPossible combinations are:Possible combinations are:GenotypesGenotypes RRRR RRrr rrrrPhenotypesPhenotypesRED RED RED RED YELLOWYELLOW


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GenotypesGenotypes HomozygousHomozygous genotype - gene genotype - gene

combination involving 2 dominant or 2 combination involving 2 dominant or 2 recessive genes recessive genes (e.g. RR or rr);(e.g. RR or rr); also also calledcalled pure pure

HeterozygousHeterozygous genotype - gene genotype - gene combination of one dominant & one combination of one dominant & one recessive allele (recessive allele (e.g. Rr);e.g. Rr); also called also called hybridhybrid


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Why peas,Why peas, Pisum sativumPisum sativum??Can be grown in a Can be grown in a small small areaarea Produce Produce lots of offspring lots of offspring Produce Produce purepure plants plants when allowed to when allowed to self-self-pollinatepollinate several several generations generations Can be Can be artificially cross-artificially cross-pollinatedpollinated


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Reproduction in Flowering PlantsReproduction in Flowering Plants

Pollen contains spermPollen contains spermProduced by the stamenProduced by the stamen

Ovary contains eggsOvary contains eggsFound inside the flowerFound inside the flower

Pollen carries sperm to Pollen carries sperm to the eggs for fertilizationthe eggs for fertilization

Self-fertilizationSelf-fertilization can can occur in the same occur in the same flowerflowerCross-fertilizationCross-fertilization can can occur between occur between flowersflowers


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Mendel’s Experimental MethodsMendel’s Experimental Methods

Mendel Mendel hand-pollinatedhand-pollinated flowers using a flowers using a paintbrushpaintbrush

He could He could snip the stamenssnip the stamens to prevent self-pollinationto prevent self-pollinationCovered each flower with a Covered each flower with a cloth bagcloth bag

He traced traits through the He traced traits through the several generationsseveral generations


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How Mendel BeganHow Mendel BeganMendel Mendel produced produced purepure strains by strains by allowing allowing the plants the plants to to self-self-pollinatepollinate for for several several generatiogenerationsns


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Eight Pea Plant TraitsEight Pea Plant Traits

Seed shapeSeed shape --- Round --- Round (R)(R) or Wrinkled or Wrinkled (r)(r)Seed ColorSeed Color ---- Yellow ---- Yellow (Y)(Y) or Green or Green ((yy))Pod ShapePod Shape --- Smooth --- Smooth (S)(S) or wrinkled or wrinkled ((ss))Pod ColorPod Color --- Green --- Green (G)(G) or Yellow or Yellow (g)(g)Seed Coat ColorSeed Coat Color ---Gray ---Gray (G)(G) or White or White (g)(g)Flower positionFlower position---Axial ---Axial (A)(A) or Terminal or Terminal (a)(a)Plant HeightPlant Height --- Tall --- Tall (T)(T) or Short or Short (t)(t)Flower color Flower color --- --- Purple Purple (P)(P) or white or white ((pp))



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Mendel’s Experimental Mendel’s Experimental ResultsResults


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Did the observed ratio match the theoretical ratio?Did the observed ratio match the theoretical ratio?

The theoretical or expected ratio of The theoretical or expected ratio of plants producing round or wrinkled plants producing round or wrinkled seeds is seeds is 3 round :1 wrinkled3 round :1 wrinkledMendel’s observed ratio was 2.96:1Mendel’s observed ratio was 2.96:1The discrepancy is due to The discrepancy is due to statistical statistical errorerrorThe The larger the samplelarger the sample the more the more nearly the results approximate to nearly the results approximate to the theoretical ratiothe theoretical ratio


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Generation “Gap”Generation “Gap”Parental PParental P11 Generation Generation = the parental generation in = the parental generation in

a breeding experimenta breeding experiment..FF11 generation generation = the first-generation offspring in a = the first-generation offspring in a

breeding experiment. breeding experiment. (1st filial generation)(1st filial generation)From breeding individuals from the PFrom breeding individuals from the P11

generationgenerationFF22 generation generation = the second-generation offspring in a = the second-generation offspring in a

breeding experiment. breeding experiment. (2nd filial generation)(2nd filial generation) From breeding individuals from the FFrom breeding individuals from the F11

generationgeneration


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Following the GenerationsFollowing the Generations

Cross 2 Cross 2 Pure Pure

PlantsPlantsTT x ttTT x tt

Results Results in all in all

HybridsHybridsTtTt

Cross 2 Cross 2 HybridsHybrids

getget3 Tall & 1 3 Tall & 1

ShortShortTT, Tt, ttTT, Tt, tt


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Monohybrid Monohybrid CrossesCrosses


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Trait: Seed ShapeTrait: Seed ShapeAlleles: Alleles: RR – Round – Round rr – Wrinkled – WrinkledCross: Cross: RoundRound seedsseeds xx Wrinkled Wrinkled seedsseeds

RRRR xx rr rr

PP11 Monohybrid Cross Monohybrid Cross

R

R

rr

Rr

RrRr

Rr

Genotype:Genotype: RrRrPhenotypePhenotype: RoundRoundGenotypicGenotypicRatio:Ratio: All alikeAll alikePhenotypicPhenotypicRatio:Ratio: All alike All alikecopyright cmassengale

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PP11 Monohybrid Cross Review Monohybrid Cross Review

Homozygous dominant x Homozygous Homozygous dominant x Homozygous recessiverecessive

OffspringOffspring allall HeterozygousHeterozygous (hybrids)(hybrids) Offspring calledOffspring called FF11 generation generation Genotypic & Phenotypic ratio isGenotypic & Phenotypic ratio is ALL ALIKEALL ALIKE


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Trait: Seed ShapeTrait: Seed ShapeAlleles: Alleles: RR – Round – Round rr – Wrinkled – WrinkledCross: Cross: RoundRound seeds seeds xx Round Round seedsseeds

RrRr xx Rr Rr

FF11 Monohybrid Cross Monohybrid Cross

R

r

rR

RR

rrRr

Rr

Genotype:Genotype: RR, Rr, RR, Rr, rrrrPhenotypePhenotype: Round Round && wrinkled wrinkledG.Ratio:G.Ratio: 1:2:11:2:1P.Ratio:P.Ratio: 3:1 3:1copyright cmassengale

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FF11 Monohybrid Cross Review Monohybrid Cross Review

Heterozygous x heterozygousHeterozygous x heterozygous Offspring:Offspring:

25% Homozygous dominant25% Homozygous dominant RRRR50% Heterozygous50% Heterozygous RrRr25% Homozygous Recessive25% Homozygous Recessive rrrr

Offspring calledOffspring called FF22 generation generation Genotypic ratio isGenotypic ratio is 1:2:11:2:1 Phenotypic RatioPhenotypic Ratio is 3:1 is 3:1


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What Do the Peas Look Like?What Do the Peas Look Like?


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……And Now the Test CrossAnd Now the Test Cross

Mendel then crossed a Mendel then crossed a purepure & a & a hybridhybrid from his from his FF2 2 generationgeneration

This is known as an This is known as an FF22 or test cross or test cross

There are There are twotwo possible testcrosses: possible testcrosses:Homozygous dominant x HybridHomozygous dominant x HybridHomozygous recessive x HybridHomozygous recessive x Hybrid


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Trait: Seed ShapeTrait: Seed ShapeAlleles: Alleles: RR – Round – Round rr – Wrinkled – WrinkledCross: Cross: RoundRound seedsseeds xx Round Round seedsseeds

RRRR xx Rr Rr

FF22 Monohybrid Cross (1 Monohybrid Cross (1stst))

R

R

rR

RR

RrRR

Rr

Genotype:Genotype: RR, RR, RrRrPhenotypePhenotype: RoundRoundGenotypicGenotypicRatio:Ratio: 1:11:1PhenotypicPhenotypicRatio:Ratio: All alike All alike


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Trait: Seed ShapeTrait: Seed ShapeAlleles: Alleles: RR – Round – Round rr – Wrinkled – WrinkledCross: Cross: WrinkledWrinkled seedsseeds xx Round Round seedsseeds

rrrr xx Rr Rr

FF22 Monohybrid Cross (2nd) Monohybrid Cross (2nd)

r

r

rR

Rr

rrRr

rr

Genotype:Genotype: Rr, rrRr, rrPhenotypePhenotype: Round & Round & WrinkledWrinkledG. Ratio:G. Ratio: 1:11:1P.Ratio:P.Ratio: 1:1 1:1copyright cmassengale

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FF22 Monohybrid Cross Review Monohybrid Cross Review

Homozygous x heterozygous(hybrid)Homozygous x heterozygous(hybrid) Offspring:Offspring:

50% Homozygous 50% Homozygous RR or rrRR or rr50% Heterozygous50% Heterozygous RrRr

Phenotypic RatioPhenotypic Ratio is 1:1 is 1:1 Called Called Test CrossTest Cross because the offspring because the offspring

have have SAMESAME genotype as parents genotype as parents


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Practice Your CrossesPractice Your Crosses

Work the PWork the P11, F, F11, and both F, and both F22 Crosses for each of the other Crosses for each of the other

Seven Pea Plant TraitsSeven Pea Plant Traits


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Mendel’s LawsMendel’s Laws


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Results of Monohybrid CrossesResults of Monohybrid Crosses

InheritableInheritable factors or genesfactors or genes are are responsible for all heritable responsible for all heritable characteristics characteristics

PhenotypePhenotype is based on is based on GenotypeGenotype Each traitEach trait is based onis based on two genestwo genes, , one from one from

the mother and the other from the the mother and the other from the fatherfather

True-breeding individuals are True-breeding individuals are homozygous homozygous ( both alleles)( both alleles) are the same are the same


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Law of DominanceLaw of Dominance

In a cross of parents that are In a cross of parents that are pure for contrasting traitspure for contrasting traits, , only one form of the trait will only one form of the trait will appear in the next generation.appear in the next generation.All the offspring will be All the offspring will be heterozygous and express heterozygous and express only the only the dominant trait.dominant trait.RR x rr RR x rr yieldsyields all Rr (round all Rr (round seeds)seeds)


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Law of DominanceLaw of Dominance


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Law of SegregationLaw of Segregation

During the During the formation of gametesformation of gametes (eggs or (eggs or sperm), the sperm), the two allelestwo alleles responsible for a responsible for a trait trait separateseparate from each other. from each other.

Alleles for a trait are then Alleles for a trait are then "recombined" at "recombined" at fertilizationfertilization, producing the genotype for , producing the genotype for the traits of the offspringthe traits of the offspring.


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Applying the Law of SegregationApplying the Law of Segregation


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Law of Independent AssortmentLaw of Independent Assortment

Alleles for Alleles for differentdifferent traits are distributed traits are distributed to sex cells (& offspring) to sex cells (& offspring) independently of one another.independently of one another.

This law can be illustrated using This law can be illustrated using dihybrid dihybrid crossescrosses..


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Dihybrid CrossDihybrid CrossA breeding experiment that tracks the A breeding experiment that tracks the

inheritance of two traitsinheritance of two traits..

Mendel’s Mendel’s “Law of Independent Assortment”“Law of Independent Assortment”a. Each pair of alleles segregates a. Each pair of alleles segregates independentlyindependently

during gamete formationduring gamete formationb. Formula: 2b. Formula: 2nn (n = # of heterozygotes) (n = # of heterozygotes)


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Question:Question:How many gametes will be produced for the How many gametes will be produced for the

following allele arrangements?following allele arrangements?

Remember:Remember: 22nn (n = # of heterozygotes) (n = # of heterozygotes)

1.1. RrYyRrYy

2.2. AaBbCCDdAaBbCCDd

3.3. MmNnOoPPQQRrssTtQqMmNnOoPPQQRrssTtQq


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Answer:Answer:1. RrYy: 21. RrYy: 2nn = 2 = 222 = 4 gametes = 4 gametes

RY Ry rY ryRY Ry rY ry

2. AaBbCCDd: 22. AaBbCCDd: 2nn = 2 = 233 = 8 gametes = 8 gametesABCD ABCd AbCD AbCdABCD ABCd AbCD AbCdaBCD aBCd abCD abCDaBCD aBCd abCD abCD

3. MmNnOoPPQQRrssTtQq: 23. MmNnOoPPQQRrssTtQq: 2nn = 2 = 266 = = 64 gametes64 gametes


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Dihybrid CrossDihybrid CrossTraits: Seed shape & Seed colorTraits: Seed shape & Seed colorAlleles:Alleles: R round

r wrinkled Y yellow y green

RrYy x RrYy

RY Ry rY ryRY Ry rY ry RY Ry rY ryRY Ry rY ry

All possible gamete combinationsAll possible gamete combinationscopyright cmassengale

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Dihybrid CrossDihybrid Cross

RYRY RyRy rYrY ryry

RYRY

RyRy

rYrY

ryry


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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: 19:3:3:1 phenotypic ratio

RYRY RyRy rYrY ryry

RYRY

RyRy

rYrY

ryry


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Round/Yellow: 9Round/green: 3wrinkled/Yellow: 3wrinkled/green: 1

9:3:3:1copyright cmassengale

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Test CrossTest CrossA mating between an individual of A mating between an individual of unknown unknown

genotype genotype and a and a homozygous recessivehomozygous recessive individual. individual.Example:Example: bbC__ bbC__ x x bbccbbcc

BB = brown eyesBB = brown eyesBb = brown eyesBb = brown eyesbb = blue eyesbb = blue eyes

CC = curly hairCC = curly hairCc = curly hairCc = curly haircc = straight haircc = straight hair

bCbC b___b___

bcbc


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Test CrossTest Cross

Possible results:Possible results:

bCbC b___b___

bcbc bbCc bbCc

C bCbC b___b___

bcbc bbCc bbccor

c


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Summary of Mendel’s lawsSummary of Mendel’s laws

LAWLAW PARENT PARENT CROSSCROSS OFFSPRINGOFFSPRING

DOMINANCEDOMINANCE TT x ttTT x tt tall x shorttall x short

100% Tt 100% Tt talltall

SEGREGATIONSEGREGATION Tt x TtTt x Tt tall x talltall x tall

75% tall 75% tall 25% short25% short

INDEPENDENT INDEPENDENT ASSORTMENTASSORTMENT

RrGg x RrGgRrGg x RrGg round & round & green x green x round & round & greengreen

9/16 round seeds & green 9/16 round seeds & green pods pods

3/16 round seeds & yellow 3/16 round seeds & yellow pods pods

3/16 wrinkled seeds & 3/16 wrinkled seeds & green pods green pods

1/16 wrinkled seeds & 1/16 wrinkled seeds & yellow podsyellow pods


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Incomplete DominanceIncomplete Dominanceandand

CodominanceCodominance


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Incomplete DominanceIncomplete Dominance

F1 hybrids F1 hybrids have an appearance somewhat in in betweenbetween the phenotypes phenotypes of the two parental varieties.

Example:Example: snapdragons (flower)snapdragons (flower)red (RR) x white (rr)

RR = red flowerRR = red flowerrr = white flower

R

R

r r


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RrRr

RrRr

RrRr

RrRr

RR

RR

rr

All Rr =All Rr = pink pink(heterozygous pink)(heterozygous pink)

produces theproduces theFF11 generation generation

r


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CodominanceCodominanceTwo allelesTwo alleles are expressed ( are expressed (multiple allelesmultiple alleles) in ) in

heterozygous individualsheterozygous individuals..Example:Example: blood type blood type

1.1. type Atype A = I= IAAIIAA or I or IAAii2.2. type Btype B = I= IBBIIBB or I or IBBii3.3. type ABtype AB = I= IAAIIBB

4.4. type Otype O = ii= ii


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Codominance ProblemCodominance Problem

Example: homozygous male Type B (IBIB)x

heterozygous female Type A (IAi)

IAIB IBi

IAIB IBi

1/2 = IAIB

1/2 = IBi IB

IA i

IB


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Another Codominance ProblemAnother Codominance Problem

• Example:Example: male Type O (ii) x female type AB (IAIB)

IAi IBi

IAi IBi

1/2 = IAi1/2 = IBi

i

IA IB

i


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QuestionQuestion::If a boy has a blood type O and If a boy has a blood type O and his sister has blood type AB, his sister has blood type AB, what what are the genotypes and are the genotypes and phenotypes phenotypes of their parents?of their parents?

boy - boy - type O (ii) type O (ii) X girl - X girl - type AB (Itype AB (IAAIIBB))


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Answer:Answer:

IAIB

ii

Parents:Parents:genotypesgenotypes = IAi and IBiphenotypesphenotypes = A and B

IB

IA i

i


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Sex-linked TraitsSex-linked Traits

Traits (genes) located on the Traits (genes) located on the sex sex chromosomeschromosomes

Sex chromosomes are Sex chromosomes are X and YX and YXXXX genotype for females genotype for femalesXYXY genotype for males genotype for malesMany Many sex-linked traitssex-linked traits carried on carried on XX

chromosomechromosome


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Sex-linked TraitsSex-linked Traits

Sex ChromosomesSex Chromosomes

XX chromosome - female Xy chromosome - male

fruit flyeye color

Example: Example: Eye color in fruit Eye color in fruit fliesflies


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Sex-linked Trait ProblemSex-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 eyedXY = maleXX = female

XR

Xr Xr

Y


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Sex-linked Trait Solution:Sex-linked Trait Solution:

XR Xr

Xr Y

XR Xr

Xr Y

50% red eyed female

50% white eyed male

XR

Xr Xr

Y


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Genetic Practice ProblemsGenetic Practice Problems


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Breed the PBreed the P11 generation generation

tall (TT) x dwarf (tt) pea plantstall (TT) x dwarf (tt) pea plants

T

T

t t


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Solution:Solution:

T

T

t t

Tt

Tt

Tt

Tt All Tt = tall(heterozygous tall)

produces theFF11 generation generation

tall (TT) vs. dwarf (tt) pea tall (TT) vs. dwarf (tt) pea plantsplants


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Breed the FBreed the F11 generation generation

tall (Tt) vs. tall (Tt) pea plantstall (Tt) vs. tall (Tt) pea plants

T

t

T t


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Solution:Solution:

TT

Tt

Tt

tt

T

t

T tproduces theFF22 generation generation1/4 (25%) = TT1/2 (50%) = Tt1/4 (25%) = tt1:2:1 genotype1:2:1 genotype 3:1 phenotype3:1 phenotype

tall (Tt) x tall (Tt) pea tall (Tt) x tall (Tt) pea plantsplants


References• Fig 1 www.gopixpic.com• Principles of inheritance Mendal’s laws and and genetic

models by Springer• Fig 2 https://askabiologist.asu.edu/punnett-squares• Fig 3

http://www.exploringnature.org/db/detail.php?dbID=22&detID=2290

• Fig 4 http://www.ck12.org/user:dGVycnlyQHZhbGxleTI2Mi5vcmc./section/Mendelian-Inheritance-%253A%253Aof%253A%253A-


https://askabiologist.asu.edu/punnett-squares



Thank You


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B.Sc. Microb/Biotech II Cell biology and Genetics Unit 4 Mendelian Genetics