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Mendel’s Mendel’s Breakthrough Breakthrough Patterns, Particles, Patterns, Particles, and Principles of and Principles of Heredity Heredity

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Mendel’s Breakthrough. Patterns, Particles, and Principles of Heredity. Outline of Mendelian Genetics. The historical puzzle of inheritance and how Mendel’s experimental approach helped solve it Mendel’s approach to genetic analysis including his experiments and related analytic tools - PowerPoint PPT Presentation

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Mendel’s Mendel’s BreakthroughBreakthrough

Patterns, Particles, and Patterns, Particles, and Principles of HeredityPrinciples of Heredity

Outline of Mendelian Outline of Mendelian GeneticsGenetics

The historical puzzle of inheritance The historical puzzle of inheritance and how Mendel’s experimental and how Mendel’s experimental approach helped solve itapproach helped solve it

Mendel’s approach to genetic Mendel’s approach to genetic analysis including his experiments analysis including his experiments and related analytic toolsand related analytic tools

A comprehensive example of A comprehensive example of Mendelian inheritance in humansMendelian inheritance in humans

Gregor Mendel (1822-Gregor Mendel (1822-1844)1844)

Fig. 2.2

Themes of Mendel’s workThemes of Mendel’s work

Variation is widespread in natureVariation is widespread in nature Observable variation is essential for Observable variation is essential for

following genesfollowing genes Variation is inherited according to Variation is inherited according to

genetic laws and not solely by genetic laws and not solely by chancechance

Mendel’s laws apply to Mendel’s laws apply to allall sexually sexually reproducing organisms.reproducing organisms.

The historical puzzle of The historical puzzle of inheritanceinheritance

Artificial selection has been an important Artificial selection has been an important practice since before recorded historypractice since before recorded history Domestication of animals Domestication of animals Selective breeding of plantsSelective breeding of plants

1919thth century – precise techniques for century – precise techniques for controlled matings in plants and animals controlled matings in plants and animals to produce desired traits in many of to produce desired traits in many of offspringoffspring

Breeders could not explain why traits Breeders could not explain why traits would sometimes disappear and then would sometimes disappear and then reappear in subsequent generations.reappear in subsequent generations.

State of genetics in State of genetics in early 1800’searly 1800’s

What is inherited?What is inherited?

How is it inherited?How is it inherited?

What is the role of chance What is the role of chance in heredity?in heredity?

Mendel’s workplaceMendel’s workplace

Fig. 2.5

Historical theories of Historical theories of inheritanceinheritance

One parent One parent contributes most contributes most features (e.g., features (e.g., homunculus, N. homunculus, N. Hartsoiker, 1694)Hartsoiker, 1694)

Blending inheritance Blending inheritance – parental traits – parental traits become mixed and become mixed and forever changed in forever changed in offspringoffspring

Fig.2.6

Keys to Mendel’s Keys to Mendel’s experimentsexperiments

The garden pea was an ideal organismThe garden pea was an ideal organism Vigorous growthVigorous growth Self fertilizationSelf fertilization Easy to cross fertilizeEasy to cross fertilize Produced large number of offspring each generationProduced large number of offspring each generation

Mendel analyzed traits with discrete alternative Mendel analyzed traits with discrete alternative formsforms purple vs. white flowerspurple vs. white flowers yellow vs. green peasyellow vs. green peas round vs. wrinkled seedsround vs. wrinkled seeds long vs. short stem lengthlong vs. short stem length

Mendel established pure breeding lines to Mendel established pure breeding lines to conduct his experimentsconduct his experiments

Monohybrid crosses reveal Monohybrid crosses reveal units of inheritance and units of inheritance and

Law of SegregationLaw of Segregation

Fig.2.9

Traits have dominant and Traits have dominant and recessive formsrecessive forms

Disappearance of traits in F1 Disappearance of traits in F1 generation and reappearance in the generation and reappearance in the F2 generation disproves the F2 generation disproves the hypothesis that traits blendhypothesis that traits blend

Trait must have two forms that can Trait must have two forms that can each breed trueeach breed true

One form must be hidden when plants One form must be hidden when plants with each trait are interbredwith each trait are interbred

Trait that appears in F1 is Trait that appears in F1 is dominantdominant Trait that is hidden in F1 is Trait that is hidden in F1 is recessiverecessive

Alternative forms of traits Alternative forms of traits are allelesare alleles

Each trait carries two copies of a Each trait carries two copies of a unit of inheritance, one inherited unit of inheritance, one inherited from the mother and the other from from the mother and the other from the fatherthe father

Alternative forms of traits are called Alternative forms of traits are called allelesalleles

Law of SegregationLaw of Segregation

Two alleles for Two alleles for each trait each trait separate separate (segregate) (segregate) during gamete during gamete formation, and formation, and then unite at then unite at random, one from random, one from each parent, at each parent, at fertilizationfertilization

Fig. 2.10

The Punnet SquareThe Punnet Square

Fig. 2.11

Rules of ProbabilityRules of Probability

Independent events - probability of two events occurring together

What is the probability that both A and B will occur?Solution = determine probability of each and multiply

them together.

Mutually exclusive events - probability of one or another eventoccurring.

What is the probability of A or B occurring?Solution = determine the probability of each and add

them together.

Probability and Mendel’s Probability and Mendel’s ResultsResults

Cross Yy xYy pea plants. Cross Yy xYy pea plants. Chance of Y sperm uniting with a Y eggChance of Y sperm uniting with a Y egg

½ chance of sperm with Y allele½ chance of sperm with Y allele ½ chance of egg with Y allele½ chance of egg with Y allele Chance of Y and Y uniting = ½ x ½ = ¼Chance of Y and Y uniting = ½ x ½ = ¼

Chance of Yy offpsringChance of Yy offpsring ½ chance of sperm with y allele and egg with Y ½ chance of sperm with y allele and egg with Y

alleleallele ½ chance of sperm with Y allele and egg with y ½ chance of sperm with Y allele and egg with y

alleleallele Chance of Yy – (½ x ½) + (½ x ½) = 2/4, or 1/2Chance of Yy – (½ x ½) + (½ x ½) = 2/4, or 1/2

Further crosses confirm Further crosses confirm predicted ratiospredicted ratios

Fig. 2.12

Genotypes and Genotypes and PhenotypesPhenotypes

Phenotype – observable Phenotype – observable characteristic of an organismcharacteristic of an organism

Genotype – pair of alleles present in Genotype – pair of alleles present in and individualand individual

Homozygous – two alleles of trait are Homozygous – two alleles of trait are the same (YY or yy)the same (YY or yy)

Heterozygous – two alleles of trait Heterozygous – two alleles of trait are different (Yy)are different (Yy)

Genotypes versus Genotypes versus phenotpyesphenotpyes

Yy Yy

1:2:1YY:Yy:yy

3:1yellow: green

Fig. 2.13

Test cross reveals unkown Test cross reveals unkown genotpyegenotpye

Fig. 2.14

Dihybrid crosses reveal the Dihybrid crosses reveal the law of independent law of independent

assortmentassortment A dihybrid is an individual that is A dihybrid is an individual that is

heterozygous at two genesheterozygous at two genes Mendel designed experiments to Mendel designed experiments to

determine if two genes segregate determine if two genes segregate independently of one another in independently of one another in dihybridsdihybrids

First constructed true breeding lines for First constructed true breeding lines for both traits, crossed them to produce both traits, crossed them to produce dihybrid offspring, and examined the F2 dihybrid offspring, and examined the F2 for parental or recombinant types (new for parental or recombinant types (new combinations not present in the combinations not present in the parents)parents)

Results of Mendels dihybrid Results of Mendels dihybrid crossescrosses

F2 generation contained both F2 generation contained both parental types and recombinant parental types and recombinant typestypes

Alleles of genes assort Alleles of genes assort independently, and can thus appear independently, and can thus appear in any combination in the offspringin any combination in the offspring

Dihybrid cross shows Dihybrid cross shows parental and recombinant parental and recombinant

typestypes

Fig. 2.15 top

Dihybrid cross produces a Dihybrid cross produces a predictable ratio of predictable ratio of

phenotypesphenotypes

Fig. 2.15 bottom

The law of independent The law of independent assortmentassortment

During gamete formation different pairs of During gamete formation different pairs of alleles segregate independently of each otheralleles segregate independently of each other

Fig. 2.16

Summary of Mendel's Summary of Mendel's workwork

Inheritance is particulate - not blendingInheritance is particulate - not blending There are two copies of each trait in a germ There are two copies of each trait in a germ

cellcell Gametes contain one copy of the traitGametes contain one copy of the trait Alleles (different forms of the trait) Alleles (different forms of the trait)

segregate randomlysegregate randomly Alleles are dominant or recessive - thus the Alleles are dominant or recessive - thus the

difference between genotype and difference between genotype and phenotypephenotype

Different traits assort independentlyDifferent traits assort independently

Laws of probability for Laws of probability for multiple genesmultiple genes

P RRYYTTSS X rryyttss

F1 RrYyTtSs X RrYyTtSs

F2 What is the ratio of different genotypes and phenotypes?

gametes RYTS ryts

gametesRYTS

RyTS

rYTS

RYTs

RyTs

rYTs

RYtS

RytS

rYts

Ryts

RYts

rYTS

ryTs rYtS rYts ryts

RYTS

RyTS

rYTS

RYTs

RyTs

rYTs

RYtS

RytS

rYts

Ryts

RYts

rYTS

ryTs rYtS rYts ryts

Punnet Square method - 24 = 16 possible gamete combinations for each parent

Thus, a 16 16 Punnet Square with 256 genotypes

That’s one big Punnet Square!

Loci Assort Independently - So we can look at each locusindependently to get the answer.

F1 RrYyTtSs RrYyTtSs

What is the probability of obtaining the genotype RrYyTtss?

P RRYYTTSS rryyttss

Rr Rr

1RR:2Rr:1rr

2/4 Rr

Yy X Yy

1YY:2Yy:1yy

2/4 Yy

Tt Tt

1TT:2Tt:1tt

2/4 Tt

Ss Ss

1SS:2Ss:1ss

1/4 ss

Probability of obtaining individual with Rr and Yy and Tt and ss.

2/4 2/4 2/4 1/4 = 8/256 (or 1/32)

F1 RrYyTtSs RrYyTtSs

P RRYYTTSS rryyttss

What is the probability of obtaining a completely homozygousgenotype?

Genotype could be RRYYTTSS or rryyttss

Rr Rr

1RR:2Rr:1rr

1/4 RR1/4 rr

Yy Yy

1YY:2Yy:1yy

1/4 YY1/4 yy

Tt Tt

1TT:2Tt:1tt

1/4 TT1/4 tt

Ss Ss

1SS:2Ss:1ss

1/4 SS1/4 ss

(1/4 1/4 1/4 1/4) + (1/4 1/4 1/4 1/4) = 2/256

Rediscovery of MendelRediscovery of Mendel

Mendel’s work was unappreciated Mendel’s work was unappreciated and remained dormant for 34 yearsand remained dormant for 34 years

Even Darwin’s theories were viewed Even Darwin’s theories were viewed with skepticism in the late 1800’s with skepticism in the late 1800’s because he could not explain the because he could not explain the mode of inheritance of variationmode of inheritance of variation

In 1900, 16 years after Mendel died, In 1900, 16 years after Mendel died, four scientists rediscovered and four scientists rediscovered and acknowledged Mendel’s work, giving acknowledged Mendel’s work, giving birth to the science of geneticsbirth to the science of genetics

1900 - Carl Correns, Hugo 1900 - Carl Correns, Hugo deVries, and Erich von deVries, and Erich von

Tschermak rediscover and Tschermak rediscover and confirm Mendel’s lawsconfirm Mendel’s laws

Fig. 2.19

Mendelian inheritance in Mendelian inheritance in humanshumans

Most traits in humans are due to the Most traits in humans are due to the interaction of multiple genes and do not interaction of multiple genes and do not show a simple Mendelian pattern of show a simple Mendelian pattern of inheritance.inheritance.

A few traits represent single-genes. A few traits represent single-genes. Examples include sickle-cell anemia, Examples include sickle-cell anemia, cystic fibrosis, Tay-Sachs disease, and cystic fibrosis, Tay-Sachs disease, and Huntington’s disease (see Table 2.1 in Huntington’s disease (see Table 2.1 in text)text)

Because we can not do breeding Because we can not do breeding experiments on humans,.experiments on humans,.

In humans we must use In humans we must use pedigrees to study inheritancepedigrees to study inheritance

Pedigrees are an orderly diagram of a Pedigrees are an orderly diagram of a families relevant genetic features families relevant genetic features extending through multiple generationsextending through multiple generations

Pedigrees help us infer if a trait is from Pedigrees help us infer if a trait is from a single gene and if the trait is a single gene and if the trait is dominant or recessivedominant or recessive

Anatomy of a pedigreeAnatomy of a pedigree

Fig. 2.20

A vertical pattern of A vertical pattern of inheritance indicates a rare inheritance indicates a rare

dominant traitdominant trait

Hunitington’s disease: A rare dominant traitAssign the genotypes by working backward through the pedigree

Fig. 2.20

A horizontal pattern of A horizontal pattern of inheritance indicates a rare inheritance indicates a rare

recessive traitrecessive trait

Cystic fibrosis: a recessive conditionAssign the genotypes for each pedigree

Fig.2.21