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Intro toIntro toMendelelian Mendelelian GeneticsGenetics

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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 Mendel

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Gregor Johann MendelGregor Johann Mendel

Austrian monkAustrian monkBetween 1856 and 1863, Mendel Between 1856 and 1863, Mendel cultivated and tested some 28,000 cultivated and tested some 28,000 pea plantspea plantsHe found that the plants' He found that the plants' offspring retained offspring retained traitstraits of the of the parents.parents.Called theCalled the ““Father of Genetics"Father of Genetics"

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

Particulate InheritanceParticulate Inheritance

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Genetic TerminologyGenetic Terminology

TraitTrait - any characteristic - any characteristic that can be passed from that can be passed from parent to offspring parent to offspring HeredityHeredity - passing of - passing of traits from parent to traits from parent to offspring offspring GeneticsGenetics - study of - study of heredity heredity

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Types of Genetic Types of Genetic CrossesCrossesMonohybrid cross Monohybrid cross - - cross cross

involving a single traitinvolving a single traite.g. flower color e.g. flower color Dihybrid crossDihybrid cross - - cross cross involving two traits involving two traits e.g. flower color & plant e.g. flower color & plant heightheight

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Punnett SquarePunnett Square

Used to help Used to help solve solve genetics genetics problemsproblems

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Designer Designer ““GenesGenes””

AllelesAlleles - - two forms of a two forms of a gene gene (dominant & recessive)(dominant & recessive) DominantDominant - - stronger of two genes stronger of two genes expressed in the hybrid; expressed in the hybrid; represented byrepresented by aa capital letter (R)capital letter (R) RecessiveRecessive - - gene that shows up gene that shows up less often in a cross; represented less often in a cross; represented by aby a lowercase letter (r)lowercase letter (r)

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GenotypeGenotype - - gene gene combination for a traitcombination for a trait (e.g. RR, Rr, rr) (e.g. RR, Rr, rr) PhenotypePhenotype - - the physical the physical feature resulting from a feature resulting from a genotypegenotype (e.g. red, white) (e.g. red, white)

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Genotype & Phenotype in Genotype & Phenotype in FlowersFlowers

Genotype of alleles:Genotype of alleles:RR = red flower= red flowerrr = yellow flower= yellow flower

All genes occur in pairs, so All genes occur in pairs, so 22 allelesalleles affect a characteristic affect a characteristic

Possible combinations are:Possible combinations are:

GenotypesGenotypes RRRR RRrr rrrr

PhenotypesPhenotypesRED RED RED RED YELLOWYELLOW

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GenotypesGenotypesHomozygousHomozygous genotype - gene genotype - gene

combination involving 2 combination involving 2 dominant or 2 recessive genes dominant or 2 recessive genes (e.g. RR or rr);(e.g. RR or rr); also called also called pure pure  HeterozygousHeterozygous genotype - gene genotype - gene combination of one dominant & combination of one dominant & one recessive allele    (one recessive allele    (e.g. Rr);e.g. Rr); also calledalso called hybridhybrid

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Genes and Environment Genes and Environment Determine CharacteristicsDetermine Characteristics

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MendelMendel’’s Pea s Pea Plant Plant

ExperimentsExperiments

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Why peas,Why peas,Pisum Pisum sativumsativum??Can be grown in a Can be grown in a

small areasmall area Produce Produce lots of lots of offspring offspring Produce Produce purepure plants when allowed plants when allowed to to self-pollinateself-pollinate several generations several generations Can be Can be artificially artificially cross-pollinatedcross-pollinated

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Reproduction in Flowering Reproduction in Flowering PlantsPlants

Pollen contains Pollen contains spermspermProduced by the Produced by the stamenstamen

OvaryOvary contains eggs contains eggsFound inside the flowerFound inside the flower

Pollen carries sperm to Pollen carries sperm to the eggs for the eggs for fertilizationfertilization

Self-fertilizationSelf-fertilization can can occur in the same occur in the same flowerflower

Cross-fertilizationCross-fertilization can can occur between occur between flowersflowers

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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 (y)(y)Pod ShapePod Shape --- Smooth --- Smooth (S)(S) or wrinkled or wrinkled (s)(s)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 (p)(p)

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MendelMendel’’s Experimental s Experimental ResultsResults

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Generation Generation ““GapGap””Parental PParental P11 GenerationGeneration = the parental = the parental

generation in a breeding experimentgeneration in a breeding experiment..

FF11 generationgeneration = the first-generation = the first-generation offspring in a breeding experiment. offspring in a breeding experiment. (1st (1st filial filial generation)generation)

From breeding individuals from the PFrom breeding individuals from the P11 generationgeneration

FF22 generationgeneration = the second-generation = the second-generation offspring in a breeding experiment. offspring in a breeding experiment. (2nd (2nd filial filial generation)generation)

From breeding individuals from the FFrom breeding individuals from the F11 generationgeneration

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Following the Following the GenerationsGenerations

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 Shape

Alleles: Alleles: RR – Round – Round rr – Wrinkled – Wrinkled

Cross: Cross: RoundRound seedsseeds xx Wrinkled Wrinkled seedsseeds

RRRR xx rr rr

PP11 Monohybrid Cross Monohybrid Cross

R

R

rr

Rr

RrRr

Rr

Genotype:Genotype: RrRr

PhenotypePhenotype: RoundRound

GenotypicGenotypicRatio:Ratio: All alikeAll alike

PhenotypicPhenotypicRatio:Ratio: All alike All alike

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PP11 Monohybrid Cross Monohybrid Cross ReviewReviewHomozygous dominant x Homozygous dominant x

Homozygous recessiveHomozygous recessiveOffspringOffspring allall HeterozygousHeterozygous (hybrids)(hybrids)Offspring calledOffspring called FF11 generation generation

Genotypic & Phenotypic ratio Genotypic & Phenotypic ratio isis ALL ALIKEALL ALIKE

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Trait: Seed ShapeTrait: Seed Shape

Alleles: Alleles: RR – Round – Round rr – Wrinkled – Wrinkled

Cross: 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, rrrr

PhenotypePhenotype: Round Round && wrinkled wrinkled

G.Ratio:G.Ratio: 1:2:11:2:1

P.Ratio:P.Ratio: 3:1 3:1

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FF11 Monohybrid Cross Monohybrid Cross ReviewReviewHeterozygous x heterozygousHeterozygous x heterozygousOffspring:Offspring:

25% Homozygous dominant25% Homozygous dominant RRRR50% Heterozygous50% Heterozygous RrRr25% Homozygous Recessive25% Homozygous Recessive rrrrOffspring calledOffspring called FF22 generation generationGenotypic ratio isGenotypic ratio is 1:2:11:2:1Phenotypic RatioPhenotypic Ratio is 3:1 is 3:1

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

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……And Now the Test And Now the Test CrossCross

Mendel then crossed a pure & a Mendel then crossed a pure & a hybrid from his Fhybrid from his F2 2 generationgeneration

This is known as an This is known as an FF22 or test crossor 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 Shape

Alleles: Alleles: RR – Round – Round rr – Wrinkled – Wrinkled

Cross: 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, RrRr

PhenotypePhenotype: RoundRound

GenotypicGenotypicRatio:Ratio: 2:22:2

PhenotypicPhenotypicRatio:Ratio: All alike All alike

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Trait: Seed ShapeTrait: Seed Shape

Alleles: Alleles: RR – Round – Round rr – Wrinkled – Wrinkled

Cross: 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, rr

PhenotypePhenotype: Round & Round & WrinkledWrinkled

G. Ratio:G. Ratio: 2:2

P.Ratio:P.Ratio: 2:2 2:2

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FF22 Monohybrid Cross Monohybrid Cross ReviewReviewHomozygous x Homozygous x

heterozygous(hybrid)heterozygous(hybrid)Offspring:Offspring:50% Homozygous RR or rr50% Homozygous RR or rr50% Heterozygous Rr50% Heterozygous RrPhenotypic Ratio is 1:1Phenotypic Ratio is 1:1Called Test Cross because the Called Test Cross because the offspring have offspring have SAMESAME genotype genotype as parentsas parents

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MendelMendel’’s Lawss Laws

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Results of Monohybrid Results of Monohybrid CrossesCrosses

Inheritable factors or Inheritable factors or genesgenes are are responsible for all heritable responsible for all heritable characteristics characteristics

Phenotype is based on Phenotype is based on GenotypeGenotype Each trait is based on two genes, Each trait is based on two genes,

one from the mother and the one from the mother and the other from the father other from the father

True-breeding individuals are True-breeding individuals are homozygous homozygous (both alleles are the (both alleles are the same)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 traits, pure for contrasting traits, only one form of the trait will only one form of the trait will appear in the next appear in the next generation.generation.

All the offspring will be All the offspring will be heterozygousheterozygous and express and express only the dominant trait.only the dominant trait.

RR x rr yields all Rr (round RR x rr yields 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 formation of During the formation of gametesgametes (eggs or sperm), (eggs or sperm), the the two alleles responsible for a two alleles responsible for a trait separate from each other.trait separate from each other.

Alleles for a trait are then Alleles for a trait are then "recombined" "recombined" at fertilization, at fertilization, 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 Applying the Law of SegregationSegregation

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Law of Independent Law of Independent AssortmentAssortment

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

This law can be illustrated This law can be illustrated using using dihybrid crossesdihybrid crosses..

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Dihybrid Dihybrid CrossCrossA breeding experiment that tracks A breeding experiment that tracks

the inheritance of the inheritance of two two traitstraits..

MendelMendel’’s s ““Law of Independent Law of Independent AssortmentAssortment””

a. Each pair of alleles a. Each pair of alleles segregatessegregates independently duringindependently during gamete gamete formationformation