Mendelian Mendelian GeneticsGenetics

Section 10-2Section 10-2

Pages 277-282Pages 277-282

Gregor MendelGregor Mendel Austrian monk and plant breederAustrian monk and plant breeder He joined the monastery in 1843He joined the monastery in 1843 He carried out the first important He carried out the first important

studies of hereditystudies of heredity He was the first person to He was the first person to

succeed in predicting how succeed in predicting how traits are transferred from one traits are transferred from one generation to the next.generation to the next.

He published his findings in He published his findings in 1866.1866.

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Why peas?Why peas?Can be grown in a small Can be grown in a small areaarea Produce lots of Produce lots of offspringoffspring Produce pure plants Produce pure plants when allowed to self-when allowed to self-pollinate for several pollinate for several generations generations Can be artificially cross-Can be artificially cross-pollinated pollinated

Why Peas? Peas contain both gametes in the same flower Pollen contains sperm produced by the stamen Ovary contains eggs inside the flower Pollen carries sperm to the eggs for fertilizationPollen carries sperm to the eggs for fertilization

Self-fertilizationSelf-fertilization can occur in the same flower can occur in the same flowerCross-fertilizationCross-fertilization can occur between flowers can occur between flowers

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Genetic TermsGenetic Terms

Heredity – the passing on of Heredity – the passing on of characteristics from parents to characteristics from parents to offspringoffspring

Genetics – the branch of biology Genetics – the branch of biology that studies hereditythat studies heredity

Traits – characteristics that are Traits – characteristics that are inheritedinherited

PP11 generation – “parent” - the original generation – “parent” - the original

true-breeding parents true-breeding parents FF11 generation – “filial” (or son or generation – “filial” (or son or

daughter) offspring of the parent daughter) offspring of the parent plants plants

FF22 or “second filial generation” (or or “second filial generation” (or

granddaughter or grandson)granddaughter or grandson)

Mendel’s Experiments- Terms

Mendel’s ExperimentsFirst Generation (FFirst Generation (F11) )

Mendel grew seeds from a cross Mendel grew seeds from a cross between green-seed and yellow-seed between green-seed and yellow-seed plants. All of the offspring had yellow plants. All of the offspring had yellow seeds.seeds.

Second Generation (FSecond Generation (F22) )

Mendel allowed the plants in the 1Mendel allowed the plants in the 1stst generation to self pollinate. ¾ of the generation to self pollinate. ¾ of the plants had yellow seeds and ¼ had green.plants had yellow seeds and ¼ had green.

Ratio of 3:1 (3 yellow to 1 green)Ratio of 3:1 (3 yellow to 1 green)

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AllelesAllelesAlleles- alternative form of a single gene Alleles- alternative form of a single gene

passed from generation to generation.passed from generation to generation.

Organisms have 2 genes that control each trait. Organisms have 2 genes that control each trait. These genes are located on the These genes are located on the chromosomeschromosomes

Dominant trait – appears in the FDominant trait – appears in the F11 generation generation

Recessive trait – disappears in the FRecessive trait – disappears in the F11

generationgeneration

A plant could have:A plant could have:

2 alleles for tallness (TT)2 alleles for tallness (TT)

2 alleles for shortness (tt)2 alleles for shortness (tt)

1 allele for tallness and 1 for shortness (Tt) 1 allele for tallness and 1 for shortness (Tt)

The 2 alleles are located on different copies The 2 alleles are located on different copies of a chromosome – one copy inherited of a chromosome – one copy inherited from the female parent and one from the from the female parent and one from the male parentmale parent

Rule of dominanceRule of dominancePea plants that have at least 1 allele for Pea plants that have at least 1 allele for tallness (TT or Tt) are tall because the allele tallness (TT or Tt) are tall because the allele for tallness is dominant over the allele for for tallness is dominant over the allele for shortnessshortnessThe only way a plant can be short is if both The only way a plant can be short is if both height alleles are for a short plant (tt)height alleles are for a short plant (tt)

HomozygousHomozygous – an organisms 2 alleles for a – an organisms 2 alleles for a trait are the same (TT, tt)trait are the same (TT, tt)

HeterozygousHeterozygous - an organisms 2 alleles for a - an organisms 2 alleles for a trait are different (Tt)trait are different (Tt)

Phenotypes & GenotypesPhenotypes & Genotypes2 organisms can look alike 2 organisms can look alike but have different gene but have different gene combinationscombinations

PhenotypePhenotype – the way an – the way an organism looks and organism looks and behaves (tall or short)behaves (tall or short)

GenotypeGenotype – the gene – the gene combination an organism combination an organism contains (TT, Tt, tt)contains (TT, Tt, tt)

Law of SegregationLaw of Segregation

Law of Segregation- two alleles for each trait Law of Segregation- two alleles for each trait separate during meiosis. separate during meiosis.

A TT plant can only produce T gametesA TT plant can only produce T gametes A Tt plant can produce both T gametes and t A Tt plant can produce both T gametes and t

gametesgametes A tt plant can only produce t gametesA tt plant can only produce t gametes

The 2 alleles come together during The 2 alleles come together during fertilization.fertilization.

TT Tt

TT TT Tt Tt

TtTTTT Tt

T T

T T t

t

T

T

segregation

segregation

fertilization

fertilization

gametes

gametes

P

F1

F2

Segregation of Alleles

Mendel’s CrossesMendel’s Crosses Hybrid – offspring of parents that have Hybrid – offspring of parents that have

different traitsdifferent traits Mono – means oneMono – means one Di- means twoDi- means two Monohybrid crosses – the two parent Monohybrid crosses – the two parent

plants differ by a single trait (height) plants differ by a single trait (height) Dihybrid cross – parents differ by two Dihybrid cross – parents differ by two

traits (height and color)traits (height and color)

Monohybrid Cross (Seed color)

Genotype-

1 YY, 2 Yy, 1yy

or 1:2:1

Phenotype-

3 Yellow, 1 Green

or 3:1

Punnett SquaresPunnett Squares

• 1905, Reginald Punnett, an English 1905, Reginald Punnett, an English biologist, devised the shorthand way of biologist, devised the shorthand way of finding the expected proportions of finding the expected proportions of possible genotypes in the offspring of a possible genotypes in the offspring of a crosscross

• Used to predict the possible genotypes of Used to predict the possible genotypes of offspringoffspring

• In reality, you don’t get the exact ratio of In reality, you don’t get the exact ratio of results shown in the squareresults shown in the square

Punnett Square-Monohybrid CrossPunnett Square-Monohybrid Cross

Punnett SquarePunnett Square

T t

T TT Tt

t Tt tt

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• Trait: Seed ShapeTrait: Seed Shape• Alleles:Alleles: RR – Round– Round rr – Wrinkled– Wrinkled• Cross:Cross: RoundRound seeds x seeds x Round Round seedsseeds• RrRr xx Rr Rr

Monohybrid CrossMonohybrid 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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• Trait: Seed ShapeTrait: Seed Shape

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

• Cross:Cross: RoundRound seeds x seeds x Round Round seedsseeds

• RRRR xx Rr Rr

Monohybrid CrossMonohybrid Cross

R

R

rR

RR

RrRR

Rr

Genotype:Genotype: RR, RrRR, Rr

PhenotypePhenotype: RoundRound

GenotypicGenotypicRatio:Ratio: 1:11:1

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 seeds x seeds x Round Round seedsseeds• rrrr xx Rr Rr

Monohybrid CrossMonohybrid Cross

r

r

rR

Rr

rrRr

rr

Genotype:Genotype: Rr, rrRr, rr

PhenotypePhenotype: Round Round & Wrinkled& Wrinkled

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

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

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Law of Independent AssortmentLaw of Independent Assortment

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

This law can be illustrated using This law can be illustrated using dihybrid dihybrid crossescrosses..

Dihybrid Cross (Seed color and shape)

Phenotypes: 9 Round and Yellow 3 Wrinkled and Yellow 3 Round and Green 1 Wrinkled and Green or 9:3:3:1 ratio

Punnett Square-Dihybrid Cross

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Dihybrid CrossDihybrid Cross• Traits: Seed shape & Seed colorTraits: Seed shape & Seed color• Alleles: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 combinations

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Dihybrid CrossDihybrid CrossRYRY 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: 1

9:3:3:1 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

9:3:3:1

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Summary of Mendel’s lawsSummary of Mendel’s lawsLAWLAW 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