B.Sc. Microb/Biotech II Cell biology and Genetics Unit 4 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 of traits inof traits in
pea plantspea plants
Developed theDeveloped the laws oflaws of
inheritanceinheritance
Mendel's work wasMendel's work was
not recognized untilnot recognized until
the turn of thethe turn of the 20th20th
centurycentury

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Gregor Johann MendelGregor Johann Mendel
BetweenBetween 1856 and1856 and
1863,1863, MendelMendel
cultivated and testedcultivated and tested
somesome 28,000 pea plants28,000 pea plants
He found that theHe found that the
plants' offspringplants' offspring
retainedretained traits of thetraits of the
parentsparents
Called theCalled the “Father of“Father of
Genetics"Genetics"

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Mendel stated thatMendel stated that
physical traits arephysical traits are
inherited asinherited as “particles”“particles”
Mendel did not knowMendel did not know
that the “particles” werethat the “particles” were
actuallyactually Chromosomes &Chromosomes &
DNADNA
Particulate InheritanceParticulate Inheritance

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

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Types of Genetic CrossesTypes of Genetic Crosses
 Monohybrid crossMonohybrid cross -- cross involving across involving a
single traitsingle trait
e.g. flower colore.g. flower color
 Dihybrid crossDihybrid cross -- cross involving twocross involving two
traitstraits
e.g. flower color & plant heighte.g. flower color & plant height

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Punnett SquarePunnett Square
Used to help solveUsed to help solve
genetics problemsgenetics problems
2

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

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More TerminologyMore Terminology
 GenotypeGenotype -- gene combination for agene combination for a
traittrait (e.g. RR, Rr, rr)(e.g. RR, Rr, rr)
 PhenotypePhenotype -- the physical featurethe physical feature
resulting from a genotyperesulting from a genotype (e.g. red,(e.g. red,
white)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 dominant or 2combination involving 2 dominant or 2
recessive genesrecessive genes (e.g. RR or rr);(e.g. RR or rr); alsoalso
calledcalled purepure
 HeterozygousHeterozygous genotype - genegenotype - gene
combination of one dominant & onecombination of one dominant & one
recessive allele (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 in aCan be grown in a smallsmall
areaarea
ProduceProduce lots of offspringlots of offspring
ProduceProduce purepure plantsplants
when allowed towhen allowed to self-self-
pollinatepollinate severalseveral
generationsgenerations
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 the stamenProduced by the stamen
Ovary contains eggsOvary contains eggs
Found inside the flowerFound inside the flower
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 flowerscopyright cmassengale

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Mendel’s Experimental MethodsMendel’s Experimental Methods
MendelMendel hand-pollinatedhand-pollinated
flowers using aflowers using a paintbrushpaintbrush
He couldHe could snip the stamenssnip the stamens
to prevent self-pollinationto prevent self-pollination
Covered each flower with aCovered each flower with a
cloth bagcloth bag
He traced traits through theHe traced traits through the
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 match the theoretical ratio?Did the observed ratio match 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 copyright cmassengale

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

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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, ttcopyright cmassengale

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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 x HomozygousHomozygous dominant x Homozygous
recessiverecessive
 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 test crossor test cross
There areThere are twotwo possible testcrosses:possible 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 x heterozygous(hybrid)Homozygous x 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 the offspringbecause the offspring
havehave SAMESAME genotype as parentsgenotype as parents

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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 the otherCrosses for each of the other
Seven Pea Plant TraitsSeven Pea Plant Traits

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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 fromone from
the mother and the other from thethe mother and the other from the
fatherfather
True-breeding individuals areTrue-breeding individuals are homozygoushomozygous
( both alleles)( both alleles) are the sameare the same

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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(eggs or
sperm), thesperm), the two allelestwo alleles responsible for aresponsible for a
traittrait separateseparate from each other.from each other.
Alleles for a trait are thenAlleles for a trait are then "recombined" at"recombined" at
fertilizationfertilization, producing the genotype for, producing the genotype for
the traits of the offspringthe traits of the offspring.

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Applying the Law of SegregationApplying the Law of Segregation

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Law of Independent AssortmentLaw of Independent Assortment
Alleles forAlleles for differentdifferent traits are distributedtraits are distributed
to sex cells (& offspring)to sex cells (& offspring)
independently of one another.independently of one another.
This law can be illustrated usingThis law can be illustrated using dihybriddihybrid
crossescrosses..

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Dihybrid CrossDihybrid Cross
A breeding experiment that tracks theA breeding experiment that tracks the
inheritance of two traitsinheritance of two traits..
Mendel’sMendel’s “Law of Independent Assortment”“Law of Independent 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 produced for theHow many gametes will be produced for the
following allele arrangements?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 inin
betweenbetween 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 (multiple allelesmultiple alleles) 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 hishis
sister has blood type AB,sister has blood type AB, what arewhat are
the genotypes andthe genotypes and phenotypesphenotypes
of their parents?of their parents?
boy -boy - type O (ii)type O (ii) X girl -X girl - type AB (Itype AB (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

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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

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Genetic Practice ProblemsGenetic Practice Problems

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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

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

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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&detID=22
• Fig 4
http://www.ck12.org/user:dGVycnlyQHZhbGxleTI2Mi5vcmc./
section/Mendelian-Inheritance-%253A%253Aof%253A
%253A-
copyright cmassengale 72

Thank You
copyright cmassengale 73

B.Sc. Microb/Biotech II Cell biology and Genetics Unit 4 Mendelian Genetics

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