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Gregor Gregor MendelMendel

(1822-1884)(1822-1884)Responsible Responsible

for for discovering discovering

the the lawslaws governing governing

inheritance inheritance of traitsof traits

Austrian monkAustrian monkStudied the Studied the inheritanceinheritance of of traits in traits in pea plantspea plantsDeveloped the Developed the laws of inheritancelaws of inheritanceMendel's work Mendel's work was not recognized was not recognized until the turn of until the turn of thethe 20th century 20th century (1900’s)(1900’s)

Between Between 1856 1856 and 1863,and 1863, Mendel Mendel cultivated and cultivated and tested some tested some 28,000 pea plants28,000 pea plantsHe found that the He found that the plants' offspring plants' offspring retained retained traits of traits of the parentsthe parentsHe is called theHe is called the “Father of “Father of Genetics"Genetics"

copyright cmassengale

Mendel stated that Mendel stated that physical traits are physical traits are inherited as inherited as “particles”“particles”Mendel did not Mendel did not know that the know that the “particles” were “particles” were actually actually chromosomes & chromosomes & DNADNA

Chromosome TerminologyHomologous chromosomes: a matching pair of chromosomes that are inherited one from each parent

Sister chromatids are identical

Chromosome Terminology

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

HeredityHeredity - passing of traits - passing of traits from parent to offspring from parent to offspring

GeneticsGenetics - study of heredity - study of heredity

Monohybrid cross Monohybrid cross - - cross cross involving a single traitinvolving a single traite.g. flower color e.g. flower color

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

Used to help Used to help solve genetics solve genetics problemsproblems

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100%

0%

Homozygous DominantParent

Ho

mo

zyg

ou

s R

eces

sive

Par

ent

4 Heterozygous Offspring

AllelesAlleles – – possible forms of a possible forms of a gene (dominant & recessive)gene (dominant & recessive)

DominantDominant - - stronger of two stronger of two genes expressed in the hybrid; genes expressed in the hybrid; will mask a recessive trait; will mask a recessive trait; represented byrepresented by aa capital letter capital letter (R) (R)

RecessiveRecessive - - gene that shows up gene that shows up less often in a cross; hidden by a less often in a cross; hidden by a dominant trait; represented by adominant trait; represented by a lowercase letter (r) lowercase letter (r)

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

PhenotypePhenotype - - the physical the physical feature resulting from a feature resulting from a genotypegenotype

(example: red, white)(example: red, white)

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 RR RR RRrr rrrr

PhenotypesPhenotypesRED RED RED RED YELLOWYELLOW

Homozygous Homozygous genotype - gene genotype - gene combination involving 2 combination involving 2 dominant or 2 recessive genes dominant or 2 recessive genes (example: RR or rr); (example: RR or rr); also calledalso called pure pure 

Heterozygous Heterozygous genotype - gene genotype - gene combination of one dominant & combination of one dominant & one recessive allele one recessive allele (example: (example: Rr); Rr); also calledalso called hybridhybrid

Can be grown in a Can be grown in a small areasmall area Produce Produce lots of lots of offspring offspring Produce Produce purepure plants when allowed plants when allowed to to self-pollinateself-pollinate for for several generations several generations Can be Can be artificially artificially cross-pollinatedcross-pollinated

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

He could He could snip the snip the stamens stamens to prevent to prevent self-pollinationself-pollinationCovered each flower Covered each flower with a cloth bagwith a cloth bag

He traced traits through He traced traits through several generationsseveral generations

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

Flower color Flower color --- --- Purple Purple (P)(P) or white or white ((pp)) Flower positionFlower position----Axial --Axial (A)(A) or Terminal or Terminal

(a)(a) Seed colorSeed color ---- Yellow ---- Yellow (Y)(Y) or  Green or  Green ((yy)) Seed shapeSeed shape --- Round --- Round (R)(R) or Wrinkled or Wrinkled (r)(r) Pod shapePod shape --- Smooth --- Smooth (S)(S) or wrinkled or wrinkled ((ss)) Pod colorPod color ---  Green ---  Green (G)(G) or Yellow or Yellow (g)(g) Stem length Stem length --- Tall --- Tall (T)(T) or Short or Short (t)(t)

Parental PParental P11 Generation Generation = the parental = the parental generation in a breeding experimentgeneration in a breeding experiment

FF11 generation generation = the first-generation = the first-generation offspring in a breeding experiment offspring in a breeding experiment From breeding individuals from the PFrom breeding individuals from the P11

generation with each othergeneration with each otherFF22 generation generation = the second-= the second-

generation offspring in a breeding generation offspring in a breeding experiment experiment From breeding individuals from the FFrom breeding individuals from the F11

generation with each othergeneration with each other

How Mendel ProceededHow Mendel ProceededMendel crossed his parental generation Mendel crossed his parental generation (P) (P) of a of a homozygous dominant trait (XX) with his parental homozygous dominant trait (XX) with his parental generation generation (P) (P) of a homozygous recessive trait (xx). of a homozygous recessive trait (xx). (He did not know about dominant and recessive traits, (He did not know about dominant and recessive traits, so he was lucky that the traits he chose were ones so he was lucky that the traits he chose were ones that were controlled by one gene!) The that were controlled by one gene!) The first generation first generation (F(F11) offspring ) offspring allall appeared with the dominant trait appeared with the dominant trait showing as a phenotype. All their genotypes were showing as a phenotype. All their genotypes were heterozygous (Xx). heterozygous (Xx).

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How Mendel ContinuedHow Mendel ContinuedMendel then crossed his first generation Mendel then crossed his first generation heterozygous heterozygous offspring (Foffspring (F11) ) with more of his with more of his first generation heterozygous first generation heterozygous offspring (Foffspring (F11)). . The The second generation (Fsecond generation (F22) ) had recessive had recessive traits show up again at a genotype ratio of traits show up again at a genotype ratio of 1:2:1, meaning 1 AA, 2 Aa and 1 aa.1:2:1, meaning 1 AA, 2 Aa and 1 aa.

Heterozygous Offspring

Het

ero

zyg

ou

s O

ffsp

rin

g

Ratios of 1:2:1 meaning 1AA:2Aa:1aa

P = Cross 2 Pure P = Cross 2 Pure PlantsPlants

AA x aaAA x aa

FF11 = Results in = Results in

4 Yellow Hybrids Aa4 Yellow Hybrids Aa

FF2 2 = Cross 2 Hybrids = Cross 2 Hybrids for for

3 Yellow and 1 Green3 Yellow and 1 Green1 AA, 2 Aa, 1 aa1 AA, 2 Aa, 1 aa

Mendel’s Experimental Mendel’s Experimental ResultsResults

Did the observed ratio match Did the observed ratio match the theoretical ratio?the theoretical ratio?

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 sample larger the sample the more nearly the more nearly the results approximate to the the results approximate to the theoretical ratiotheoretical ratio

Homozygous dominant x Homozygous dominant x Homozygous recessiveHomozygous recessive

Offspring Offspring allall Heterozygous Heterozygous (hybrids)(hybrids)

Offspring calledOffspring called FF11 generation generation Genotypic & Phenotypic ratio Genotypic & Phenotypic ratio isis ALL ALIKEALL ALIKE

Trait: Seed ShapeTrait: Seed ShapeAlleles: Alleles: R R – Round– Roundr r – Wrinkled– WrinkledCross: Cross: RoundRound seedsseeds xx WrinkledWrinkled

seedsseeds RRRR xx rrrr

R

R

rr

Rr

RrRr

RrGenotype:Genotype: RrRr

PhenotypePhenotype: RoundRound

Genotypic Ratio:Genotypic Ratio: All All alikealike

Phenotypic Ratio:Phenotypic Ratio: All All alikealike

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 3:1is 3:1

Trait: Seed ShapeTrait: Seed ShapeAlleles: Alleles: RR – Round – Roundrr – Wrinkled – WrinkledCross: Cross: RoundRound seeds seeds xx RoundRound seedsseeds

RrRr xx RrRr

R

r

rR

RR

rrRr

Rr

Genotype:Genotype: RR, Rr, rrRR, Rr, rr

PhenotypePhenotype: Round Round && wrinkled wrinkled

Genotype Ratio:Genotype Ratio: 1:2:11:2:1

Phenotype Ratio:Phenotype Ratio: 3:13:1

Mendel then crossed a Mendel then crossed a pure pure & 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 possible testcrosses:testcrosses:Homozygous dominant x HybridHomozygous dominant x HybridHomozygous recessive x HybridHomozygous recessive x Hybrid

Homozygous x Homozygous x heterozygous(hybrid)heterozygous(hybrid)

Offspring:Offspring:50% Homozygous 50% Homozygous RR or rrRR or rr50% Heterozygous50% Heterozygous RrRr

Phenotypic RatioPhenotypic Ratio is 1:1is 1:1 Called Called Test Cross Test Cross because the because the offspring have offspring have SAMESAME genotype genotype as parentsas parents

Trait: Seed ShapeTrait: Seed ShapeAlleles: Alleles: R R – Round– Roundr r – Wrinkled– WrinkledCross: Cross: RoundRound seedsseeds xx RoundRound seedsseeds

RRRR xx RrRr

R

R

rR

RR

RrRR

RrGenotype:Genotype: RR, RrRR, Rr

PhenotypePhenotype: RoundRound

Genotypic Ratio:Genotypic Ratio: 1:11:1

Phenotypic Ratio:Phenotypic Ratio: All All alikealike

Trait: Seed ShapeTrait: Seed ShapeAlleles: Alleles: RR – Round – Roundrr – Wrinkled – WrinkledCross: Cross: WrinkledWrinkled seedsseeds xx RoundRound seedsseeds

rrrr xx RrRr

r

r

rR

Rr

rrRr

rr

Genotype:Genotype: Rr, rrRr, rr

PhenotypePhenotype: Round & Round & WrinkledWrinkled

Genotype Ratio:Genotype Ratio: 1:11:1

Phenotype Ratio:Phenotype Ratio: 1:11:1

In a cross of parents that are In a cross of parents that are pure for contrasting traits (AA pure for contrasting traits (AA and aa)and aa), only one form of the , only one form of the trait will appear in the next trait will appear in the next generation.generation.

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

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

During the During the formation of formation of gametes gametes (eggs or sperm), the (eggs or sperm), the pair of alleles pair of alleles responsible for a responsible for a trait trait separateseparate from each other, from each other, so each gamete only receives so each gamete only receives one copy of a gene.one copy of a gene.

These alleles are then randomly These alleles are then randomly united at united at fertilizationfertilization, producing , producing the the genotypegenotype for the traits of the for the traits of the offspring.offspring.

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

This law can be illustrated This law can be illustrated using using dihybrid crossesdihybrid crosses..

A breeding experiment that tracks A breeding experiment that tracks the the inheritance of two traitsinheritance of two traits..

Mendel’s Mendel’s Law of Independent Law of Independent AssortmentAssortmentEach pair of alleles segregates Each pair of alleles segregates

independentlyindependently during gamete during gamete formationformation

Formula: 2Formula: 2nn (n = # of heterozygotes) (n = # of heterozygotes) for all gamete possibilitiesfor all gamete possibilities

Remember:Remember: 22nn (n = # of heterozygotes) (n = # of heterozygotes)

1.1. RrYyRrYy

2.2. AaBbCCDdAaBbCCDd

3.3. MmNnOoPPQQRrssTtQqMmNnOoPPQQRrssTtQq

1. 1. RrRr YyYy: 2: 2nn = 2 = 222 = 4 gametes = 4 gametes

RY Ry rY ryRY Ry rY ry

2. 2. AaAa BbBb CC CC DdDd: 2: 2nn = 2 = 233 = 8 gametes = 8 gametes

ABCD ABCd AbCD AbCdABCD ABCd AbCD AbCd

aBCD aBCd abCD abCD aBCD aBCd abCD abCD

3.3.MmMm NnNn OoOo PP QQ PP QQ RrRr ss ss TtTt QqQq: :

22nn = 2 = 266 = 64 gamete possibilities = 64 gamete possibilities

Traits: Seed shape & Seed colorTraits: Seed shape & Seed colorAlleles:Alleles: R round Y yellow r wrinkled 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

RYRY RyRy rYrY ryry

RYRY

RyRy

rYrY

ryry

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

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

9:3:3:1

A mating between an individual of A mating between an individual of unknown genotype unknown genotype and a and a homozygous homozygous recessiverecessive 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

Possible results:Possible results:

bCbC b___b___

bcbc bbCc bbCc

C bCbC b___b___

bcbc bbCc bbccor

c

LAWLAW PARENT PARENT CROSSCROSS OFFSPRINGOFFSPRING

DOMINANCEDOMINANCE(dominant trait masks (dominant trait masks

a recessive trait)a recessive trait)

TT x tt TT x tt tall x shorttall x short

100% Tt 100% Tt talltall

SEGREGATIONSEGREGATION(traits separate from (traits separate from

each other when each other when sorted)sorted)

Tt x Tt Tt x Tt tall x talltall x tall

75% tall 75% tall 25% short25% short

INDEPENDENT INDEPENDENT ASSORTMENTASSORTMENT

(different traits (different traits sort separately from sort separately from

each other)each other)

RrGg x RrGg RrGg 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

FF11 hybrids hybrids have an appearance somewhat in betweenin between 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

RrRr

RrRr

RrRr

RrRr

RR

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

produces theproduces theFF11 generation generation

R R R r

R r

r r

R r

R

r 1:2:11:2:1

produces theproduces theFF22 generation generation

Both allelesBoth alleles are expressed are expressed (multiple alleles) in (multiple alleles) in heterozygous heterozygous individualsindividuals..

Example: blood typeExample: blood type

1.1. type Atype A = I= IAAIIAA or I or IAAii2.2. type Btype B = I= IBBIIBB or I or IBBii3.3. type ABtype AB = I= IAAIIBB

4.4. type Otype O = ii= ii

Example: homozygous male Type B (IBIB)x

heterozygous female Type A (IAi)

IAIB IBi

IAIB IBi

1/2 = IAIB

1/2 = IBi

IB

IA i

IB

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

IAi IBi

IAi IBi

1/2 = IAi1/2 = IBi

i

IA IB

i

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))

Answer:Answer:

IAIB

ii

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

IB

IA i

i

Traits (genes) located on the Traits (genes) located on the sexsex chromosomes chromosomes

Sex chromosomes are Sex chromosomes are X and YX and YXXXX genotype for females genotype for femalesXYXY genotype for males genotype for malesMany Many sex-linked traits sex-linked traits carried carried

on on XX chromosome chromosomeMales will show recessive traits Males will show recessive traits

on their X chromosome without on their X chromosome without a matching gene on the Y a matching gene on the Y chromosome to mask it (ex: chromosome to mask it (ex: color blindness)color blindness)

Sex ChromosomesSex Chromosomes

XX chromosome - female Xy chromosome - male

fruit flyeye color

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

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 Xr

Y

XR Xr

Xr Y

XR Xr

Xr Y

50% red eyed female

50% white eyed male

XR

Xr Xr

Y

RR = red eyed Rr = red eyed rr = white eyed XY = male XX = female

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

T

T

t t

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

tall (Tt) vs. tall (Tt) pea tall (Tt) vs. tall (Tt) pea plantsplants

T

t

T t

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