Genetics by Mendel Genetics by Mendel

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ONE GENE WITH TWO ALLELES CONTROLING TWO

CONTRASTING/ ALTERNATIVE FORMS OF A SPECIFIC TRAIT IS

CALLED MENDELELIAN GENETICS

IntroductionIntroductionGeneticsGenetics is the study of genes. is the study of genes.InheritanceInheritance is how traits, or is how traits, or

characteristics, are passed on from characteristics, are passed on from generation to generation.generation to generation.

Chromosomes are made up of genes, Chromosomes are made up of genes, which are made up of DNA.which are made up of DNA.

Genetic material (genes,chromosomes, Genetic material (genes,chromosomes, DNA) is found inside the nucleus of a cell.DNA) is found inside the nucleus of a cell.

Gregor MendelGregor Mendel is considered “The Father is considered “The Father of Genetics“of Genetics“

Mendel's work was not recognized untilMendel's work was not recognized until the turn of thethe turn of the 20th century 20th century

Mendelian GeneticsMendelian GeneticsDominantDominant traits-traits- traits that are expressed.traits that are expressed.RecessiveRecessive traits-traits- traits that are covered up.traits that are covered up.Alleles-Alleles- the different forms of a characteristic.the different forms of a characteristic.Punnett Squares-Punnett Squares- show how crosses are show how crosses are

made.made.Probability-Probability- the chances/ percentages that the chances/ percentages that

something will occur.something will occur.Genotype-Genotype- the types of genes (Alleles) the types of genes (Alleles)

present.present.Phenotype-Phenotype- what it looks like.what it looks like.Homozygous-Homozygous- two of the same alleles.two of the same alleles.Heterozygous-Heterozygous- two different alleles.two different alleles.

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

RecessiveRecessive - - gene that shows up less gene that shows up less often in a cross; represented by aoften in a cross; represented by a lower case letter (r)lower case letter (r)

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Gregor MendelGregor MendelAustrian Monk.Austrian Monk.Experimented with “pea plants”.Experimented with “pea plants”.Used pea plants because:Used pea plants because:

They were availableThey were availableThey reproduced quicklyThey reproduced quicklyThey showed obvious differences in the traitsThey showed obvious differences in the traits

Understood that there was something that Understood that there was something that carried traits from one generation to the carried traits from one generation to the next - “next - “FACTORFACTOR”.”.

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Gregor Gregor MendelMendel

(1822-1884)(1822-1884)Father of Father of geneticsgenetics

Responsible Responsible for the Laws for the Laws governing governing

Inheritance of Inheritance of TraitsTraits

Mendel cont……

In the mid-1800s, the rules underlying patterns of inheritance were uncovered in a series of experiments performed by an Austrian monk named Gregor Mendel.

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

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Mendel's Plant Breeding Experiments

Gregor Mendel was one of the first to apply an experimental approach to the question of inheritance.

For seven years, Mendel bred pea plants and recorded inheritance patterns in the offspring.

Particulate Hypothesis of Inheritance Parents pass on to their offspring separate and distinct factors (today called genes) that are responsible for inherited traits.

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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 Reproduction in Flowering PlantsPlants

Pollen contains spermPollen contains sperm• Produced by the stamenProduced by the stamen

Ovary contains eggsOvary contains eggs• Found inside the flowerFound inside the flower

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Pollen carries sperm to Pollen carries sperm to the eggs for fertilisationthe eggs for fertilisation

Self-fertilisationSelf-fertilisation can can occur in the same occur in the same flowerflower

Cross-fertilisationCross-fertilisation can can occur between flowersoccur between flowers

Mendel’s Experimental Mendel’s Experimental MethodsMethods

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

• He could He could snip the snip the stamensstamens to prevent to prevent self-pollinationself-pollination

• Covered each flower Covered each flower with a cloth bagwith a cloth 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

Mendel was fortunate he chose the Garden Pea

•Mendel probably chose to work with peas because they are available in many varieties.

•The use of peas also gave Mendel strict control over which plants mated.

•Fortunately, the pea traits are distinct and were clearly contrasting.

To test the particulate hypothesis, Mendel crossed true-breeding plants that had two distinct and contrasting traits—for example, purple or white flowers.

What is meant by “true breeding?”

Mendel cross-fertilized his plants by hand. Why is it important to control which plants would serve as the parents?

For each monohybrid cross, Mendel cross-fertilized true-breeding plants that were different in just one character—in this case, flower color. He then allowed the hybrids (the F1 generation) to self-fertilize.

Typical breeding experiment P generation (parental generation)

F1 generation (first filial generation, the word filial from the Latin word for "son") are the hybrid offspring.

Allowing these F1 hybrids to self-pollinate produces:

F2 generation (second filial generation).

It is the analysis of this that lead to an understanding of genetic crosses.

Eight Pea Plant Traits WITH Eight Pea Plant Traits WITH THEIR CONTRASTING ALLELESTHEIR CONTRASTING ALLELES

Seed shapeSeed shape --- Round --- Round (R)(R) or Wrinkled or Wrinkled (r)(r) Seed ColourSeed Colour ---- Yellow ---- Yellow (Y)(Y) or  Green or  Green ((yy))

Pod ShapePod Shape --- Smooth --- Smooth (S)(S) or wrinkled or wrinkled ((ss)) Pod ColourPod Colour ---  Green ---  Green (G)(G) or Yellow or Yellow (g)(g) Seed Coat ColourSeed Coat Colour ---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 colour Flower colour --- --- Purple Purple (P)(P) or white or white ((pp))

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Mendel studies seven characteristics in the garden pea

:

Statistics indicated a pattern.

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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” that the “particles” were actually were actually Chromosomes & DNAChromosomes & DNA

Particulate InheritanceParticulate Inheritance

Genetic TerminologyGenetic Terminology

TraitTrait - any characteristic that can - any characteristic that can be passed from parent to be passed from parent to offspring 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 cross involving a single traitinvolving a single traite.g. flower colour e.g. flower colour

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

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Punnett SquarePunnett Square

Used to help Used to help solve genetics solve genetics problemsproblems

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More TerminologyMore Terminology

GenotypeGenotype - - gene combination gene combination for a traitfor 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 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

GenotypesGenotypes

HomozygousHomozygous genotype - gene genotype - gene combination involving 2 dominant combination involving 2 dominant or 2 recessive genes or 2 recessive genes (e.g. RR or rr);(e.g. RR or rr); also calledalso 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 also calledcalled hybridhybrid

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Mendel’s Experimental Mendel’s Experimental ResultsResults

Did the observed ratio during the experiments match the Did the observed ratio during the experiments match the theoretical ratio?theoretical ratio?

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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 wrinkled

Mendel’s observed ratio was 2.96:1Mendel’s observed ratio was 2.96:1

The discrepancy is due to The discrepancy is due to statistical statistical errorerror

The The larger the samplelarger the sample the more the more nearly the results approximate to nearly the results approximate to the theoretical ratio (the theoretical ratio (see seed see seed colourcolour) )

GenerationsGenerations

Parental PParental P11 Generation Generation = the parental generation = the parental generation

in a breeding experimentin a 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

generationgeneration FF22 generation generation = the second-generation offspring = the second-generation offspring

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

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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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PP11 Monohybrid Cross Monohybrid Cross 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

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R

R

rr

Rr

RrRr

Rr

Genotype:Genotype: RrRr

PhenotypePhenotype: RoundRound

GenotypicGenotypicRatio:Ratio: All alikeAll alike

PhenotypicPhenotypicRatio:Ratio: All alike All alike

F1

PP11 Monohybrid SUMMARY Monohybrid SUMMARY

Homozygous dominant x Homozygous dominant x Homozygous recessiveHomozygous recessive

OffspringOffspring allall HeterozygousHeterozygous (hybrids)(hybrids) Offspring calledOffspring called FF11 generation generation

Genotypic & Phenotypic ratio isGenotypic & Phenotypic ratio is ALL ALL ALIKEALIKE

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FF11 Monohybrid Cross Monohybrid Cross Trait: Seed ShapeTrait: Seed Shape Alleles: Alleles: RR – Round – Round rr – Wrinkled – Wrinkled Cross: Cross: Heterozygous Heterozygous RoundRound seeds seeds xx Round Round

seedsseeds RrRr xx Rr Rr

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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

F2

FF11 Monohybrid SUMMARY Monohybrid SUMMARY

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?

……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 or test

crosscross There are There are twotwo possible test possible test

crosses:crosses:Homozygous dominant x HybridHomozygous dominant x HybridHomozygous recessive x HybridHomozygous recessive x Hybrid

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

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R

R

rR

RR

RrRR

Rr

Genotype:Genotype: RR, RR, RrRr

PhenotypePhenotype: RoundRound

GenotypicGenotypicRatio:Ratio: 1:11:1

PhenotypicPhenotypicRatio:Ratio: All alike All alike

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

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r

r

rR

Rr

rrRr

rr

Genotype:Genotype: Rr, rrRr, rr

PhenotypePhenotype: Round & Round & WrinkledWrinkled

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

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

FF22 Monohybrid SUMMARY Monohybrid SUMMARY

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 because the

offspring have offspring have SAMESAME genotype as genotype as parentsparents

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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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Genetic traits in humans can be tracked through family pedigrees

The inheritance of many human traits follows Mendel’s principles and the rules of probability

Figure 9.8A

Family pedigrees are used to determine patterns of inheritance and individual genotypes

Figure 9.8B

DdJoshuaLambert

DdAbigailLinnell

D_Abigail

Lambert

Female

DdElizabeth

Eddy

D_JohnEddy

? D_HepzibahDaggett

?

?

ddDdDdDdddDdDd

MaleDeaf

Hearing

ddJonathanLambert

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Breed the PBreed the P11 generation generation

tall (TT) x dwarf (tt) pea tall (TT) x dwarf (tt) pea plantsplants

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T

T

t t

Solution:Solution:

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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

Breed the FBreed the F11 generation generation

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

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T

t

T t

Solution:Solution:

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TT

Tt

Tt

tt

T

t

T tproduces theFF22 generation generation

1/4 (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