MendeleliaMendelelian Geneticsn Genetics

Gregor Gregor MendelMendel

(1822-1884)(1822-1884)Called theCalled the

““Father of Father of Genetics"Genetics"

Responsible for Responsible for the laws the laws

governing governing Inheritance of Inheritance of

TraitsTraits

Gregor Johann MendelGregor Johann Mendel

Austrian monkAustrian monk Studied the Studied the

inheritanceinheritance of of traits in traits in pea plantspea plants

Developed the Developed the laws of inheritancelaws of inheritance

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

Gregor Johann Gregor Johann MendelMendel

1856 and 1863 - 1856 and 1863 - Mendel cultivated Mendel cultivated and tested some and tested some 28,000 pea plants28,000 pea plants

He found that the He found that the plants' offspring plants' offspring retained traits of retained traits of the parentsthe parents

Site of Gregor Mendel’s experimental garden in the Czech Site of Gregor Mendel’s experimental garden in the Czech RepublicRepublic

Mendel used words Mendel used words likelike “particles”“particles” oror “units”“units” oror “factors”“factors” to to refer to inherited traits.refer to inherited traits.

Mendel did not know Mendel did not know that these were actuallythat these were actually Chromosomes & DNAChromosomes & DNA

Particulate InheritanceParticulate Inheritance

Mendel’s Pea Plant Mendel’s Pea Plant ExperimentsExperiments

Why peas,Why peas, Pisum Pisum sativumsativum??

Can be grown in a Can be grown in a small small areaarea

Reproduce sexuallyReproduce sexually

Produce Produce lots of lots of offspring offspring

Can beCan be self-pollinated self-pollinated

Can be Can be artificially artificially cross-cross-pollinatedpollinated

Reproduction in Reproduction in Flowering PlantsFlowering Plants

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

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

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

GametesGametes

• Sperm – male sex cell

• Egg – female sex cell

• Normal body cells have 2 copies of chromosomes

• Gametes only have 1 so that when put together, they have 2 for the new cell

Self-fertilizationSelf-fertilization can occur in the same flower -or- can occur in the same flower -or-

Cross-fertilizationCross-fertilization can occur can occur between flowersbetween flowers

*Mendel’s *Mendel’s Experimental Experimental

MethodsMethods• Mendel Mendel hand-hand-pollinatedpollinated flowers flowers using a using a paintbrushpaintbrush

• Snipped off Snipped off stamensstamens to prevent to prevent self-pollinationself-pollination

• He traced traits He traced traits through the through the several several generationsgenerations

*Eight Pea Plant Traits*Eight 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)

Mendel’s CrossesMendel’s Crosses

• His parental generation (P) were all true-breeding– All produce offspring identical to

themselves when self-fertilized

• When Mendel crossed two “pure-bred” plants, he made hybrids– Produced by parents of different

traits

Mendel’s ResultsMendel’s Results

• When Mendel crossed a plant with round seeds (R) and a plant with wrinkled seeds (r), all the plants produced had round (R) seeds!

• The first generation of plants produced is called the F1 Generation

F1 Generation

Parental Generation

Mendel’s ResultsMendel’s Results

• When Mendel crossed his F1 generation with itself, he got a surprise!

F1 Generation

Parental Generation

??

F2 Generation

Mendel’s Experimental Mendel’s Experimental ResultsResults

Mendel’s F2 ResultsMendel’s F2 Results

• Every time Mendel self-fertilized his F1 generation plants, he got the same results:

3:1He saw 3 plants (75%) with the “dominant” trait

And 1 (25%) with the “recessive” trait!

Genetic TerminologyGenetic Terminology

TraitTrait - any characteristic that - any characteristic that can be passed from parent to can 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

Designer Designer “Genes”“Genes”

AllelesAlleles - - two forms of a two forms of a gene gene (dominant & recessive)(dominant & recessive)

DominantDominant - - stronger of two alleles; stronger of two alleles; represented byrepresented by aa capital letter (R)capital letter (R)

RecessiveRecessive – – weaker of the two weaker of the two allelesalleles; represented by a; represented by a lowercase lowercase letter (r)letter (r)

More TerminologyMore Terminology

GenotypeGenotype - - gene combination gene combination for a traitfor a trait (ex: RR, Rr, rr)(ex: RR, Rr, rr)

PhenotypePhenotype - - the physical the physical feature resulting from a feature resulting from a genotypegenotype (ex: red, white) (ex: red, white)

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 pairsAll genes occur in pairs, so , 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 combination involving 2 dominant or 2 recessive genes dominant or 2 recessive genes (ex: RR or rr)(ex: 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 (ex: Rr)(ex: Rr) also calledalso called hybridhybrid

Mendel’s PeasMendel’s Peas

• Mendel’s peas were all “true breeding”.

• What would the genotype of a true-breeding tall plant (T) plant look like?

• What would the genotype of a true-breeding short plant (t) look like?

Genes and Environment Genes and Environment Determine Determine

CharacteristicsCharacteristics

Generation “Gap”Generation “Gap”• Parental PParental P11 Generation Generation = the parental = the parental

generation generation

• FF11 generation generation = the first-generation offspring = the first-generation offspring (1st filial generation)(1st filial generation)

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

• FF22 generation generation = the second-generation = the second-generation offspring offspring (2nd filial generation)(2nd filial generation)

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

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

Types of Genetic CrossesTypes of Genetic Crosses

Monohybrid cross Monohybrid cross - - cross involving cross involving a single traita single trait

example: flower color example: flower color

Dihybrid crossDihybrid cross - - cross involving two cross involving two

traits traits example: flower color & plant example: flower color & plant

heightheight

Punnett SquarePunnett Square Used to Used to

help solve help solve genetics genetics problemsproblems

MonohybrMonohybrid id

CrossesCrosses

• 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 RrAll Rr

PhenotypicPhenotypicRatio:Ratio: All Round All Round

• 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

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

Mendel’s LawsMendel’s Laws

Results of Monohybrid Results of Monohybrid CrossesCrosses

• GENESGENES are responsible for all heritable are responsible for all heritable characteristics characteristics

• PhenotypePhenotype is based on is based on GenotypeGenotype • Each traitEach trait is based onis based on two allelestwo alleles, , one from one from

the mother and the other from the fatherthe mother and the other from the father • True-breeding individuals are True-breeding individuals are homozygous homozygous

( both alleles are the same)( both alleles are the same)• HeterozygousHeterozygous individuals have one of each individuals have one of each

alleleallele

Law of DominanceLaw of Dominance In a cross of parents that are In a cross of parents that are pure for contrasting traitspure for contrasting traits, , only one form of the trait will only one form of the trait will appear in the next generation.appear in the next generation.

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

RR x rr RR x rr yieldsyields all Rr (round all Rr (round seeds)seeds)

The Law of SegregationThe Law of Segregation

Law of SegregationLaw of Segregation

• During the During the formation of gametesformation of gametes (eggs or sperm), the (eggs or sperm), the two allelestwo alleles responsible for a trait responsible for a trait separateseparate from each other.from each other.

• Alleles for a trait are then Alleles for a trait are then "recombined" at fertilization"recombined" at fertilization, , producing the genotype for the producing the genotype for the traits of the offspringtraits of the offspring.

Law of Independent Law of Independent AssortmentAssortment

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

• This law can be illustrated This law can be illustrated using using dihybrid crossesdihybrid crosses..– A breeding experiment that tracks the A breeding experiment that tracks the

inheritance of two traitsinheritance of two traits..

Dihybrid Dihybrid CrossCross

Traits: Seed shape & Seed colorTraits: Seed shape & Seed colorAlleles: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

Dihybrid CrossDihybrid CrossRYRY RyRy rYrY ryry

RYRY

RyRy

rYrY

ryry

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

Dihybrid CrossDihybrid Cross

Round/Yellow: 9Round/green: 3wrinkled/Yellow: 3wrinkled/green: 1

9:3:3:1

Summary of Mendel’s Summary of Mendel’s lawslaws

LAWLAW PARENT PARENT CROSSCROSS OFFSPRINGOFFSPRING

DOMINANCEDOMINANCE TT x ttTT x tt tall x shorttall x short 100% Tt (tall)100% Tt (tall)

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 & yellow 9/16 round & yellow 3/16 round & green3/16 round & green

3/16 wrinkled & yellow3/16 wrinkled & yellow1/16 wrinkled & green1/16 wrinkled & green

Test CrossTest Cross• A mating between an individual of A mating between an individual of unknown unknown

genotype genotype and a and a homozygous recessivehomozygous recessive individual. individual.• Example:Example: bbC__ bbC__ x x bbccbbcc

• BB = brown eyesBB = brown eyes• Bb = brown eyesBb = brown eyes• bb = blue eyesbb = blue eyes

• CC = curly hairCC = curly hair• Cc = curly hairCc = curly hair• cc = straight haircc = straight hair

bCbC b___b___

bcbc

Test CrossTest Cross

•Possible results:Possible results:bCbC b___b___

bcbc bbCc bbCc

C bCbC b___b___

bcbc bbCc bbccor

c

Incomplete DominanceIncomplete Dominanceandand

CodominanceCodominance

Incomplete DominanceIncomplete Dominance• F1 hybridsF1 hybrids have an appearance somewhat in in

betweenbetween the phenotypes phenotypes of the two parental varieties.

• Example:Example: snapdragons (flower)snapdragons (flower)• red (RR) x white (rr)

RR = red flowerRR = red flowerrr = white flower

r

r

R R

Incomplete Incomplete DominanceDominance

RrRr

RrRr

RrRr

RrRr

rr

rr

RR RR

All Rr =All Rr = pink pink(heterozygous pink)(heterozygous pink)

FF11 generation generation

Incomplete DominanceIncomplete Dominance

CodominanceCodominance• BOTH allelesBOTH alleles are expressedare expressed (multiple (multiple

alleles) in alleles) in heterozygous individualsheterozygous individuals..• Example:Example: blood typeblood type

1.1. type Atype A = I= IAAIIAA or I or IAAii

2.2. type Btype B = I= IBBIIBB or I or IBBii

3.3. type ABtype AB= I= IAAIIBB

4.4. type Otype O = ii= ii

Codominance Codominance ProblemProblem• Example:homozygous male Type B (IBIB)

• x heterozygous female Type A (IAi)

IAIB IAIB

IBi IBi

1/2 = IAIB

1/2 = IBi

IA

IB IB

i

Another Codominance Another Codominance ProblemProblem

Example:Example: male Type O (ii) x female type AB (IAIB)

IAi IBi

IAi IBi

1/2 = IAi1/2 = IBi

i

IA IB

i

CodominanceCodominance• QuestionQuestion::

If a boy has a blood type O and his If a boy has a blood type O and his sister has blood type AB, sister has blood type AB, what are what are the genotypes and phenotypes of the genotypes and phenotypes of their parents?their parents?

boy - boy - type O (ii) type O (ii) X girl - X girl - type AB type AB (I(IAAIIBB))

CodominanceCodominance•Answer:Answer:

IAIB

ii

Parents:Parents:genotypesgenotypes = IAi and IBiphenotypesphenotypes = A and B

IB

IA i

i

Sex-linked TraitsSex-linked Traits• Traits (genes) located on Traits (genes) located on

the the sex chromosomessex chromosomes• Sex chromosomes are Sex chromosomes are X X

and Yand Y• XXXX genotype for females genotype for females• XYXY genotype for males genotype for males• Many Many sex-linked traitssex-linked traits

carried on carried on XX chromosome chromosome

Sex-linked TraitsSex-linked Traits

Sex ChromosomesSex Chromosomes

XX chromosome - female Xy chromosome - male

fruit flyeye color

Example: Example: Eye color in fruit Eye color in fruit fliesflies

Sex-linked Trait Sex-linked Trait ProblemProblem

Example: Eye color in fruit flies (red-eyed male) x (white-eyed female)

XRY x XrXr

Remember: the Y chromosome in males does not carry traits.

RR = red eyedRr = red eyedrr = white eyedXy = maleXX = female

XR

XR y

Xr

Sex-linked Trait Sex-linked Trait Solution:Solution:

XR Xr

XR Xr

Xr y

Xr y

50% red eyed female

50% white eyed male

Xr

XR y

Xr

Female CarriersFemale Carriers