Gregor Mendel Mendelian Genetics. Beliefs about Heredity Fig. 1. De la propagation du genre humain, ou manuel indispensable pour ceux qui veulent avoir

Gregor MendelMendelian GeneticsMendelian Genetics

Beliefs about Heredity

Fig. 1. De la propagation du genre humain, ou manuel indispensable pour ceux qui veulent avoir de beaux enfants de l’un ou l’autre sexe (Paris, Year VII). Image courtesy of the Bibliothèque Interuniversitaire de Médecine, Paris.

HomunculusHow is “heredity passed on:

Spermist vs Ovists

Spermist conception of a

human sperm

Homunculus

Leeuwenhoek’s black male and white female rabbit experiments: spermist “proof”

Darwin

What he got right. What he got wrong.

• Acquired characteristics

Ex. Blind cave animals

• Sex. Repro.- gemmules from all over body are packed in sperm and egg

• Blended inheritance

• Likes produce likes• Change can be

permanent• There is no limit to

cumulative change

Mendel’s Three Principles

• Dominance

• Segregation

• Independent Assortment

The foundation of “classical” scienceThe foundation of “classical” science

(1822-1884)

Dominance

• Traits of both parents inherited, but one shows over the other

• Traits are not blended

Dominance Mechanism

• Two alleles are carried for each trait

• In true-breeding individuals, both alleles are the same.

• Hybrids, on the other hand, have one of each kind of allele.

• One trait is dominant, the other trait is recessive

Segregation

• Half the gametes (egg or sperm) will carry the traits of one parent and half the traits for the other parent

Pairs of alleles are separated (=segregated) during meiosisPairs of alleles are separated (=segregated) during meiosis

Two different parental characteristics will be inherited independently of one another during gamete formation.

Independent Assortment

Example: flower color and leaf shape

Mendel’s Mendel’s Genetics Genetics Experiments Experiments with Peaswith Peas

Pea Characters Pea Characters InvestigatedInvestigated

Genetic Loci & AllelesGenetic Loci & Alleles

allele for purple flowers

allele for white flowers

gene locus for flower color

Genetic Recombination at Genetic Recombination at FertilizationFertilization

Mendel’s Test for Evaluating the Mendel’s Test for Evaluating the Inheritance of Seed Shape in Inheritance of Seed Shape in

Pea PlantsPea Plants Recognized two different traits for the Recognized two different traits for the

seed shape character in pea plants: seed shape character in pea plants: round versus wrinkled seeds.round versus wrinkled seeds.

Established true-breeding varieties Established true-breeding varieties for each of these traits.for each of these traits.


Pea PlantsPea Plants

Crossed true-breeding round seed Crossed true-breeding round seed variety with the true breeding variety with the true breeding wrinkled seed variety (this represents wrinkled seed variety (this represents the Parental, or P, cross).the Parental, or P, cross).

Offspring (FOffspring (F11 generation) are called generation) are called “hybrids”.“hybrids”.

100% of these hybrids produced 100% of these hybrids produced round seeds. round seeds.

Crossed these FCrossed these F11 generation hybrids generation hybrids

among each other (individual among each other (individual crosses being selected at random).crosses being selected at random).



Results for their offspring (FResults for their offspring (F22

generation):generation):5474 (74.7%) were plants that 5474 (74.7%) were plants that

produced round seeds.produced round seeds.



Results for their offspring (FResults for their offspring (F22

generation):generation):1850 (25.3%) were plants that 1850 (25.3%) were plants that

produced wrinkled seeds.produced wrinkled seeds.



Results for their offspring (FResults for their offspring (F22 generation): generation): Ratio of round:wrinkled = approx. 3:1Ratio of round:wrinkled = approx. 3:1



Mendel’s Explanation for his Mendel’s Explanation for his ResultsResults

For every character (e.g., seed shape) For every character (e.g., seed shape) an individual possess two instruction an individual possess two instruction sets (alleles).sets (alleles).

One of these alleles was originally One of these alleles was originally derived from the individual’s mother, the derived from the individual’s mother, the other allele being originally derived from other allele being originally derived from the individual’s father.the individual’s father.


In true-breeding individuals, both In true-breeding individuals, both alleles are the same.alleles are the same.

Hybrids, on the other hand, have Hybrids, on the other hand, have one of each kind of allele.one of each kind of allele.


Mendel believed that only two alleles Mendel believed that only two alleles were possible for a given genetic were possible for a given genetic character, and that one of the alleles character, and that one of the alleles (the dominant one) masked the (the dominant one) masked the expression of the other (the expression of the other (the recessive one) in the hybrid.recessive one) in the hybrid.


When the hybrid formed gametes, When the hybrid formed gametes, only one of the two possible alleles only one of the two possible alleles would end up in a gamete.would end up in a gamete.

However, both alleles possessed an However, both alleles possessed an equal chance of appearing in a equal chance of appearing in a gamete.gamete.


Defining alleles:Defining alleles:

R = the round allele (dominant).R = the round allele (dominant).

r = the wrinkled allele (recessive).r = the wrinkled allele (recessive).


Defining genotypes & their phenotypes:Defining genotypes & their phenotypes:

RR genotype RR genotype (homozygous dominant) = (homozygous dominant) = round phenotype.round phenotype.

rr genotype rr genotype (homozygous recessive) = (homozygous recessive) = wrinkled phenotype.wrinkled phenotype.

Rr genotype Rr genotype (heterozygous) = round (heterozygous) = round phenotypephenotype

Pea ShapesPea Shapes

RR Rr rr


Parental (P) CrossParental (P) Cross

Round X WrinkledRound X Wrinkled

RR rrRR rr

Gametes: R rGametes: R r


FF11 Generation Hybrids Generation Hybrids

100% Round100% Round

100% Rr100% Rr


FF11 Generation Cross Generation Cross

Round X RoundRound X Round

Rr RrRr Rr

Gametes: R r R rGametes: R r R r


FF11 Gametes: R r R r Gametes: R r R r

FF22 Generation Outcome Generation Outcome

Round WrinkledRound Wrinkled RR Rr Rr rrRR Rr Rr rrNote: 3:1 ratio of round:wrinkledNote: 3:1 ratio of round:wrinkled

Punnett Square Punnett Square DiagramDiagram

Rr

Rr

rR

r

R RR Rr

Rr rr

gametes

Mendel’s Law of Mendel’s Law of SegregationSegregation

In the formation of gametes, In the formation of gametes, two members of a gene pair two members of a gene pair (alleles) segregate into (alleles) segregate into different haploid gametes with different haploid gametes with equal probability.equal probability.

A TestcrossA Testcross

FF11 Hybrids (Rr) Hybrids (Rr)

xxFF22 Recessives (rr) Recessives (rr)

Punnett Square Diagram for Punnett Square Diagram for TestcrossTestcross

Rr

r

R Rr

rr

rr

rgametes

Rr x rr

Mendel’s Law of Mendel’s Law of Independent AssortmentIndependent Assortment

Whenever two or more pairs of Whenever two or more pairs of contrasting characters are brought contrasting characters are brought together in a hybrid, the alleles of the together in a hybrid, the alleles of the different pairs segregate different pairs segregate independently of one another during independently of one another during gamete formation.gamete formation.

Mendel’s Test Using the Seed Shape Mendel’s Test Using the Seed Shape Character with the Seed Color Character with the Seed Color

CharacterCharacter

Define Alleles and Associated Traits:Define Alleles and Associated Traits:

Seed Shape CharacterSeed Shape Character

R = round seed trait (dominant)R = round seed trait (dominant)

r = wrinkle seed trait (recessiver = wrinkle seed trait (recessive))


CharacterCharacter

Define Alleles and Associated Traits:Define Alleles and Associated Traits:

Seed Color CharacterSeed Color Character

Y = yellow seed trait (dominant)Y = yellow seed trait (dominant)

y = green seed trait (recessive)y = green seed trait (recessive)


CharacterCharacter Parental (P) cross: crossed true-Parental (P) cross: crossed true-

breeding plants that produce round-breeding plants that produce round-yellow (RRYY) seeds with true-yellow (RRYY) seeds with true-breeding plants that produce breeding plants that produce wrinkled-green seeds (rryy).wrinkled-green seeds (rryy).


CharacterCharacter

FF1 1 Generation: hybrids for both Generation: hybrids for both

characters (Rr & Yy; or RrYy) characters (Rr & Yy; or RrYy) were100% round-yellow.were100% round-yellow.

FF1 1 Generation Individuals (RrYy) Generation Individuals (RrYy)

crossed among each other.crossed among each other.


CharacterCharacter FF22 Generation Results: Generation Results:

315 (56.7%) round-yellow315 (56.7%) round-yellow 108 (19.4%) round-green108 (19.4%) round-green 101 (18.2%) wrinkled-yellow101 (18.2%) wrinkled-yellow 32 (5.8%) wrinkled-green32 (5.8%) wrinkled-green

No No Independent Independent AssortmentAssortment

With With Independent Independent AssortmentAssortment

Calculating Expected FrequenciesCalculating Expected Frequencies

Expect Expect 99//1616 of the F of the F22 generation generation offspring to be round-yellow.offspring to be round-yellow.

Therefore, of a total of 556 offspring Therefore, of a total of 556 offspring the expected number (frequency) of the expected number (frequency) of round-yellow offspring may be round-yellow offspring may be calculated as follows:calculated as follows:

Frequency = (Frequency = (99//1616)*556 = 312.75)*556 = 312.75

Round-YellowRound-Yellow


Expect Expect 33//1616 of the F of the F22 generation generation offspring to be round-green.offspring to be round-green.



Round-GreenRound-Green


Expect Expect 33//1616 of the F of the F22 generation generation offspring to be wrinkled-yellow.offspring to be wrinkled-yellow.



Wrinkled- YellowWrinkled- Yellow


Expect Expect 11//1616 of the F of the F22 generation generation offspring to be wrinkled-yellow.offspring to be wrinkled-yellow.



Wrinkled- GreenWrinkled- Green

Comparing Observed to Expected Comparing Observed to Expected ResultsResults

ObservedObservedRound-YellowRound-Yellow

315315Round-GreenRound-Green

108108Wrinkled-YellowWrinkled-Yellow

101101Wrinkled-GreenWrinkled-Green

3232

ExpectedExpectedRound-YellowRound-Yellow

312.75312.75Round-GreenRound-Green

104.25104.25Wrinkled-YellowWrinkled-Yellow

104.25104.25Wrinkled-GreenWrinkled-Green

34.7534.75

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Gregor Mendel Mendelian Genetics. Beliefs about Heredity Fig. 1. De la propagation du genre humain, ou manuel indispensable pour ceux qui veulent avoir