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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%
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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