B.sc. agri i pog unit 3 mendelian genetics

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Mendelian Genetics
Subject: Principles of
Genetics Unit 3
copyright cmassengale

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Gregor MendelGregor Mendel
(1822-1884)(1822-1884)
ResponsibleResponsible
for the Lawsfor the Laws
governinggoverning
Inheritance ofInheritance of
TraitsTraits

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Gregor Johann MendelGregor Johann Mendel
Austrian monkAustrian monk
Studied theStudied the
inheritanceinheritance ofof
traits intraits in pea plantspea plants
Developed theDeveloped the
laws of inheritancelaws of inheritance
Mendel's workMendel's work
was not recognizedwas not recognized
until the turn ofuntil the turn of
thethe 20th century20th century

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Gregor Johann MendelGregor Johann Mendel
BetweenBetween 18561856
and 1863,and 1863, MendelMendel
cultivated andcultivated and
tested sometested some 28,00028,000
pea plantspea plants
He found thatHe found that
the plants'the plants'
offspring retainedoffspring retained
traits of thetraits of the
parentsparents
Called theCalled the
“Father of“Father of

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Mendel stated thatMendel stated that
physical traits arephysical traits are
inherited asinherited as “particles”“particles”
Mendel did notMendel did not
know that theknow that the
“particles” were“particles” were
actuallyactually ChromosomesChromosomes
& DNA& DNA
Particulate InheritanceParticulate Inheritance

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Genetic TerminologyGenetic Terminology
TraitTrait - any characteristic- any characteristic
that can be passed from parentthat can be passed from parent
to offspringto offspring
HeredityHeredity - passing of traits- passing of traits
from parent to offspringfrom parent to offspring
GeneticsGenetics - study of heredity- study of heredity

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Types of Genetic CrossesTypes of Genetic Crosses
Monohybrid crossMonohybrid cross -- crosscross
involving a single traitinvolving a single trait
e.g. flower colore.g. flower color
Dihybrid crossDihybrid cross -- crosscross
involving two traitsinvolving two traits
e.g. flower color & plant heighte.g. flower color & plant height

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Punnett SquarePunnett Square
Used to helpUsed to help
solve geneticssolve genetics
problemsproblems
2

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DesignerDesigner “Genes”“Genes”
AllelesAlleles -- two forms of atwo forms of a genegene
(dominant & recessive)(dominant & recessive)
DominantDominant -- stronger of two genesstronger of two genes
expressed in the hybrid;expressed in the hybrid;
represented byrepresented by aa capital letter (R)capital letter (R)
RecessiveRecessive -- gene that shows upgene that shows up
less often in a cross; representedless often in a cross; represented
by aby a lowercase letter (r)lowercase letter (r)

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More TerminologyMore Terminology
GenotypeGenotype -- gene combinationgene combination
for a traitfor a trait (e.g. RR, Rr, rr)(e.g. RR, Rr, rr)
PhenotypePhenotype -- the physicalthe physical
feature resulting from afeature resulting from a
genotypegenotype (e.g. red, white)(e.g. red, white)

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Genotype & Phenotype in FlowersGenotype & Phenotype in Flowers
Genotype of alleles:Genotype of alleles:
RR = red flower= red flower
rr = yellow flower= yellow flower
All genes occur in pairs, soAll genes occur in pairs, so 22
allelesalleles affect a characteristicaffect a characteristic
Possible combinations are:Possible combinations are:
GenotypesGenotypes RRRR RRrr rrrr
PhenotypesPhenotypes RED REDRED RED YELLOWYELLOW

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GenotypesGenotypes
HomozygousHomozygous genotype - genegenotype - gene
combination involving 2 dominantcombination involving 2 dominant
or 2 recessive genesor 2 recessive genes (e.g. RR or(e.g. RR or
rr);rr); also calledalso called pure pure
HeterozygousHeterozygous genotype - genegenotype - gene
combination of one dominant &combination of one dominant &
one recessive allele (one recessive allele (e.g. Rr);e.g. Rr);
also calledalso called hybridhybrid

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Why peas,Why peas, Pisum sativumPisum sativum??
Can be grown inCan be grown in
aa small areasmall area
ProduceProduce lots oflots of
offspringoffspring
ProduceProduce purepure
plants when allowedplants when allowed
toto self-pollinateself-pollinate
several generationsseveral generations
Can beCan be
artificially cross-artificially cross-
pollinatedpollinated

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Reproduction in Flowering PlantsReproduction in Flowering Plants
Pollen contains spermPollen contains sperm
Produced by theProduced by the
stamenstamen
Ovary contains eggsOvary contains eggs
Found inside theFound inside the
flowerflower
Pollen carries sperm to thePollen carries sperm to the
eggs for fertilizationeggs for fertilization
Self-fertilizationSelf-fertilization cancan
occur in the same floweroccur in the same flower
Cross-fertilizationCross-fertilization cancan
occur between flowersoccur between flowers

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Mendel’s ExperimentalMendel’s Experimental
MethodsMethods
MendelMendel hand-pollinatedhand-pollinated
flowers using aflowers using a paintbrushpaintbrush
He couldHe could snip thesnip the
stamensstamens to preventto prevent
self-pollinationself-pollination
Covered each flowerCovered each flower
with a cloth bagwith a cloth bag
He traced traits throughHe traced traits through
thethe several generationsseveral generations

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How Mendel BeganHow Mendel Began
MendelMendel
producedproduced
purepure
strains bystrains by
allowing theallowing the
plants toplants to
self-self-
pollinatepollinate
for severalfor several
generationsgenerations

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Eight Pea Plant TraitsEight Pea Plant Traits
Seed shapeSeed shape --- Round--- Round (R)(R) or Wrinkledor Wrinkled (r)(r)
Seed ColorSeed Color ---- Yellow---- Yellow (Y)(Y) or Greenor Green ((yy))
Pod ShapePod Shape --- Smooth--- Smooth (S)(S) or wrinkledor wrinkled ((ss))
Pod ColorPod Color --- Green--- Green (G)(G) or Yellowor Yellow (g)(g)
Seed Coat ColorSeed Coat Color ---Gray---Gray (G)(G) or Whiteor White (g)(g)
Flower positionFlower position---Axial---Axial (A)(A) or Terminalor Terminal (a)(a)
Plant HeightPlant Height --- Tall--- Tall (T)(T) or Shortor Short (t)(t)
Flower colorFlower color ------ PurplePurple (P)(P) or whiteor white ((pp))

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Mendel’s Experimental ResultsMendel’s Experimental Results

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Did the observed ratio matchDid the observed ratio match
the theoretical ratio?the theoretical ratio?
The theoretical or expected ratio ofThe theoretical or expected ratio of
plants producing round or wrinkled seedsplants producing round or wrinkled seeds
isis 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 toThe discrepancy is due to statisticalstatistical
errorerror
TheThe larger the samplelarger the sample the more nearlythe more nearly
the results approximate to thethe results approximate to the
theoretical ratiotheoretical ratio

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Generation “Gap”Generation “Gap”
Parental PParental P11 GenerationGeneration = the parental= the parental
generation in a breeding experimentgeneration in a breeding experiment..
FF11 generationgeneration = the first-generation= the first-generation
offspring in a breeding experiment.offspring in a breeding experiment. (1st(1st
filial generation)filial generation)
From breeding individuals from the PFrom breeding individuals from the P11
generationgeneration
FF22 generationgeneration = the second-generation= the second-generation
offspring in a breeding experiment.offspring in a breeding experiment.
(2nd filial generation)(2nd filial generation)
From breeding individuals from the FFrom breeding individuals from the F11
generationgeneration copyright cmassengale

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Following the GenerationsFollowing the Generations
Cross 2Cross 2
PurePure
PlantsPlants
TT x ttTT x tt
ResultsResults
in allin all
HybridsHybrids
TtTt
Cross 2 HybridsCross 2 Hybrids
getget
3 Tall & 1 Short3 Tall & 1 Short
TT, Tt, ttTT, Tt, tt

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MonohybridMonohybrid
CrossesCrosses

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Trait: Seed ShapeTrait: Seed Shape
Alleles:Alleles: RR – Round– Round rr – Wrinkled– Wrinkled
Cross:Cross: RoundRound seedsseeds xx WrinkledWrinkled seedsseeds
RRRR xx rrrr
PP11 Monohybrid CrossMonohybrid Cross
R
R
rr
Rr
RrRr
Rr
Genotype:Genotype: RrRr
PhenotypePhenotype: RoundRound
GenotypicGenotypic
Ratio:Ratio: All alikeAll alike
PhenotypicPhenotypic

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PP11 Monohybrid Cross ReviewMonohybrid Cross Review
Homozygous dominant xHomozygous dominant x
Homozygous recessiveHomozygous recessive
OffspringOffspring allall HeterozygousHeterozygous
(hybrids)(hybrids)
Offspring calledOffspring called FF11 generationgeneration
Genotypic & Phenotypic ratio isGenotypic & Phenotypic ratio is
ALL ALIKEALL ALIKE

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Cross:Cross: RoundRound seedsseeds xx RoundRound seedsseeds
RrRr xx RrRr
FF11 Monohybrid CrossMonohybrid Cross
R
r
rR
RR
rrRr
Rr
Genotype:Genotype: RR, Rr, rrRR, Rr, rr
PhenotypePhenotype: Round &Round &
wrinkledwrinkled
G.Ratio:G.Ratio: 1:2:11:2:1
P.Ratio:P.Ratio: 3:13:1

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FF11 Monohybrid Cross ReviewMonohybrid Cross Review
Heterozygous x heterozygousHeterozygous x heterozygous
Offspring:Offspring:
25% Homozygous dominant25% Homozygous dominant RRRR
50% Heterozygous50% Heterozygous RrRr
25% Homozygous Recessive25% Homozygous Recessive rrrr
Offspring calledOffspring called FF22 generationgeneration
Genotypic ratio isGenotypic ratio is 1:2:11:2:1
Phenotypic RatioPhenotypic Ratio is 3:1is 3:1

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

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……And Now the Test CrossAnd Now the Test Cross
Mendel then crossed aMendel then crossed a purepure & a& a
hybridhybrid from hisfrom his FF22 generationgeneration
This is known as anThis is known as an FF22 or testor test
crosscross
There areThere are twotwo possiblepossible
testcrosses:testcrosses:
Homozygous dominant x HybridHomozygous dominant x Hybrid
Homozygous recessive x HybridHomozygous recessive x Hybrid

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Cross:Cross: RoundRound seedsseeds xx RoundRound seedsseeds
RRRR xx RrRr
FF22 Monohybrid Cross (1Monohybrid Cross (1stst
))
R
R
rR
RR
RrRR
Rr
Genotype:Genotype: RR, RrRR, Rr
PhenotypePhenotype: RoundRound
GenotypicGenotypic
Ratio:Ratio: 1:11:1
PhenotypicPhenotypic

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Cross:Cross: WrinkledWrinkled seedsseeds xx RoundRound seedsseeds
rrrr xx RrRr
FF22 Monohybrid Cross (2nd)Monohybrid Cross (2nd)
r
r
rR
Rr
rrRr
rr
Genotype:Genotype: Rr, rrRr, rr
PhenotypePhenotype: Round &Round &
WrinkledWrinkled
G. Ratio:G. Ratio: 1:11:1
P.Ratio:P.Ratio: 1:11:1

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FF22 Monohybrid Cross ReviewMonohybrid Cross Review
Homozygous xHomozygous x
heterozygous(hybrid)heterozygous(hybrid)
Offspring:Offspring:
50% Homozygous50% Homozygous RR or rrRR or rr
50% Heterozygous50% Heterozygous RrRr
Phenotypic RatioPhenotypic Ratio is 1:1is 1:1
CalledCalled Test CrossTest Cross because thebecause the
offspring haveoffspring have SAMESAME genotype asgenotype as
parentsparents

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Practice Your CrossesPractice Your Crosses
Work the PWork the P11, F, F11, and both F, and both F22
Crosses for each of theCrosses for each of the
other Seven Pea Plantother Seven Pea Plant
TraitsTraits

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Mendel’s LawsMendel’s Laws

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Results of Monohybrid CrossesResults of Monohybrid Crosses
InheritableInheritable factors or genesfactors or genes areare
responsible for all heritableresponsible for all heritable
characteristicscharacteristics
PhenotypePhenotype is based onis based on GenotypeGenotype
Each traitEach trait is based onis based on two genestwo genes,,
one from the mother and theone from the mother and the
other from the fatherother from the father
True-breeding individuals areTrue-breeding individuals are
homozygous ( both alleles)homozygous ( both alleles) are theare the
samesame

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Law of DominanceLaw of Dominance
In a cross of parents that areIn a cross of parents that are
pure for contrasting traitspure for contrasting traits, only, only
one form of the trait will appear inone form of the trait will appear in
the next generation.the next generation.
All the offspring will beAll the offspring will be
heterozygous and express only theheterozygous and express only the
dominant trait.dominant trait.
RR x rrRR x rr yieldsyields all Rr (round seeds)all Rr (round seeds)

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Law of DominanceLaw of Dominance

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Law of SegregationLaw of Segregation
During theDuring the formation of gametesformation of gametes
(eggs or sperm), the(eggs or sperm), the two allelestwo alleles
responsible for a traitresponsible for a trait separateseparate
from each other.from each other.
Alleles for a trait are thenAlleles for a trait are then
"recombined" at fertilization"recombined" at fertilization,,
producing the genotype for theproducing the genotype for the
traits of the offspringtraits of the offspring.

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Applying the Law of SegregationApplying the Law of Segregation

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Law of IndependentLaw of Independent
AssortmentAssortment
Alleles forAlleles for differentdifferent traits aretraits are
distributed to sex cells (&distributed to sex cells (&
offspring) independently of oneoffspring) independently of one
another.another.
This law can be illustrated usingThis law can be illustrated using
dihybrid crossesdihybrid crosses..

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Dihybrid CrossDihybrid Cross
A breeding experiment that tracksA breeding experiment that tracks
thethe inheritance of two traitsinheritance of two traits..
Mendel’sMendel’s “Law of Independent“Law of Independent
Assortment”Assortment”
a. Each pair of alleles segregatesa. Each pair of alleles segregates
independentlyindependently during gamete formationduring gamete formation
b. Formula: 2b. Formula: 2nn
(n = # of heterozygotes)(n = # of heterozygotes)

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Question:Question:
How many gametes will be producedHow many gametes will be produced
for the following allele arrangements?for the following allele arrangements?
Remember:Remember: 22nn
(n = # of heterozygotes)(n = # of heterozygotes)
1.1. RrYyRrYy
2.2. AaBbCCDdAaBbCCDd
3.3. MmNnOoPPQQRrssTtQqMmNnOoPPQQRrssTtQq

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Answer:Answer:
1. RrYy: 21. RrYy: 2nn
= 2= 222
= 4 gametes= 4 gametes
RY Ry rY ryRY Ry rY ry
2. AaBbCCDd: 22. AaBbCCDd: 2nn
= 2= 233
= 8 gametes= 8 gametes
ABCD ABCd AbCD AbCdABCD ABCd AbCD AbCd
aBCD aBCd abCD abCDaBCD aBCd abCD abCD
3. MmNnOoPPQQRrssTtQq: 23. MmNnOoPPQQRrssTtQq: 2nn
= 2= 266
= 64= 64
gametesgametes

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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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RYRY RyRy rYrY ryry
RYRY
RyRy
rYrY
ryry

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

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Test CrossTest Cross
A mating between an individual ofA mating between an individual of unknownunknown
genotypegenotype and aand a homozygous recessivehomozygous recessive
individual.individual.
Example:Example: bbC__bbC__ xx 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

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Test CrossTest Cross
Possible results:Possible results:
bCbC b___b___
bcbc bbCc bbCc
C bCbC b___b___
bcbc bbCc bbccor
c

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Summary of Mendel’s lawsSummary of Mendel’s laws
LAWLAW
PARENTPARENT
CROSSCROSS
OFFSPRINGOFFSPRING
DOMINANCEDOMINANCE TT x ttTT x tt
tall x shorttall x short
100% Tt100% Tt
talltall
SEGREGATIONSEGREGATION
Tt x TtTt x Tt
tall x talltall x tall
75% tall75% tall
25% short25% short
INDEPENDENTINDEPENDENT
ASSORTMENTASSORTMENT
RrGg x RrGgRrGg x RrGg
round & greenround & green
xx
round & greenround & green
9/16 round seeds & green pods9/16 round seeds & green pods
3/16 round seeds & yellow3/16 round seeds & yellow
podspods
3/16 wrinkled seeds & green3/16 wrinkled seeds & green
podspods
1/16 wrinkled seeds & yellow1/16 wrinkled seeds & yellow
podspods

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Incomplete DominanceIncomplete Dominance
andand
CodominanceCodominance

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F1 hybridsF1 hybrids have an appearance somewhat
in betweenin between the phenotypesphenotypes of the two
parental varieties.
Example:Example: snapdragons (flower)snapdragons (flower)
red (RR) x white (rr)
RR = red flowerRR = red flower
rr = white flower
R
R
r r

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RrRr
RrRr
RrRr
RrRr
RR
RR
rr
All Rr =All Rr = pinkpink
(heterozygous pink)(heterozygous pink)
produces theproduces the
FF11 generationgeneration
r

57

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Two allelesTwo alleles are expressed (are expressed (multiplemultiple
allelesalleles) in) in heterozygous individualsheterozygous individuals..
Example:Example: blood typeblood type
1.1. type Atype A = I= IAA
IIAA
or Ior IAA
ii
2.2. type Btype B = I= IBB
IIBB
or Ior IBB
ii
3.3. type ABtype AB = I= IAA
IIBB
4.4. type Otype O = ii= ii

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Codominance ProblemCodominance Problem
Example: homozygous male Type B (IB
IB
)
x
heterozygous female Type A (IA
i)
IA
IB
IB
i
IA
IB
IB
i
1/2 = IA
IB
1/2 = IB
i
IB
IA
i
IB

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Another Codominance ProblemAnother Codominance Problem
• Example:Example: male Type O (ii)
x
female type AB (IA
IB
)
IA
i IB
i
IA
i IB
i
1/2 = IA
i
1/2 = IB
i
i
IA
IB
i

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QuestionQuestion::
If a boy has a blood type O andIf a boy has a blood type O and
his sister has blood typehis sister has blood type
AB,AB, what are the genotypeswhat are the genotypes
andand phenotypes of theirphenotypes of their
parents?parents?
boy -boy - type O (ii)type O (ii) X girl -X girl - typetype
AB (IAB (IAA
IIBB
))

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Answer:Answer:
IA
IB
ii
Parents:Parents:
genotypesgenotypes = IA
i and IB
i
phenotypesphenotypes = A and B
IB
IA
i
i

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Sex-linked TraitsSex-linked Traits
Traits (genes) located on theTraits (genes) located on the sexsex
chromosomeschromosomes
Sex chromosomes areSex chromosomes are X and YX and Y
XXXX genotype for femalesgenotype for females
XYXY genotype for malesgenotype for males
ManyMany sex-linked traitssex-linked traits carried oncarried on
XX chromosomechromosome

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Sex-linked TraitsSex-linked Traits
Sex ChromosomesSex Chromosomes
XX chromosome - female Xy chromosome - male
fruit fly
eye color
Example:Example: Eye color in fruit fliesEye color in fruit flies

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Sex-linked Trait ProblemSex-linked Trait Problem
Example: Eye color in fruit flies
(red-eyed male) x (white-eyed female)
XR
Y x Xr
Xr
Remember: the Y chromosome in males
does not carry traits.
RR = red eyed
Rr = red eyed
rr = white eyed
XY = male
XX = female
XR
Xr
Xr
Y

66
Sex-linked Trait Solution:Sex-linked Trait Solution:
XR
Xr
Xr
Y
XR
Xr
Xr
Y
50% red eyed
female
50% white eyed
male
XR
Xr
Xr
Y

67
Genetic PracticeGenetic Practice
ProblemsProblems

68
Breed the PBreed the P11 generationgeneration
tall (TT) x dwarf (tt) pea plantstall (TT) x dwarf (tt) pea plants
T
T
t t

69
Solution:Solution:
T
T
t t
Tt
Tt
Tt
Tt All Tt = tall
(heterozygous tall)
produces the
tall (TT) vs. dwarf (tt) pea plantstall (TT) vs. dwarf (tt) pea plants

70
Breed the FBreed the F11 generationgeneration
tall (Tt) vs. tall (Tt) pea plantstall (Tt) vs. tall (Tt) pea plants
T
t
T t

71
Solution:Solution:
TT
Tt
Tt
tt
T
t
T t
produces the
1/4 (25%) = TT
1/2 (50%) = Tt
1/4 (25%) = tt
1:2:1 genotype1:2:1 genotype
3:1 phenotype3:1 phenotype
tall (Tt) x tall (Tt) pea plantstall (Tt) x tall (Tt) pea plants

•References
•Fig 1 www.gopixpic.com
•Principles of inheritance Mendal’s laws and and genetic
models by Springer
•Fig 2 https://askabiologist.asu.edu/punnett-squares
•Fig 3
http://www.exploringnature.org/db/detail.php?dbID=22&det
•Fig 4
http://www.ck12.org/user:dGVycnlyQHZhbGxleTI2Mi5vc
mc./section/Mendelian-Inheritance-%253A%253Aof
%253A%253A-
copyright cmassengale 72

B.sc. agri i pog unit 3 mendelian genetics

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B.sc. agri i pog unit 3 mendelian genetics