B.Sc. Microb/Biotech II Cell biology and Genetics Unit 4 Mendelian Genetics
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Transcript of 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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Dihybrid CrossDihybrid 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: 19:3:3:1 phenotypic ratio
RYRY RyRy rYrY ryry
RYRY
RyRy
rYrY
ryry
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Dihybrid CrossDihybrid Cross
Round/Yellow: 9Round/green: 3wrinkled/Yellow: 3wrinkled/green: 1
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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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Incomplete DominanceIncomplete Dominance
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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Incomplete DominanceIncomplete Dominance
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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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CodominanceCodominance
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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CodominanceCodominance
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
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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-
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Thank You
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