Mendelian genetics

MendelelianMendelelian
GeneticsGenetics
copyright cmassengale 1

2
Gregor MendelGregor Mendel
(1822-1884)(1822-1884)
ResponsibleResponsible
for the Lawsfor the Laws
governinggoverning
Inheritance ofInheritance of
TraitsTraits

Gregor Johann MendelGregor Johann Mendel
Austrian monkAustrian monk
Studied theStudied the inheritanceinheritance of traits inof traits in pea plantspea plants
His paper in 1866 –Natural History Society ofHis paper in 1866 –Natural History Society of
BrunnBrunn
Developed theDeveloped the laws of inheritancelaws of inheritance
Mendel's work was not recognized until 1900Mendel's work was not recognized until 1900
Correns, De Varies and Teshermark-Correns, De Varies and Teshermark-
rediscovered Mendel work and appreciatedrediscovered Mendel work and appreciated
3

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"

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

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

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

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

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)

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)

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

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

Mendel’s Pea PlantMendel’s Pea Plant
ExperimentsExperiments

Why peas,Why peas, Pisum sativumPisum sativum??
Can be grown in aCan be grown in a small area, Short growthsmall area, Short growth
periodperiod
Inheritance of one character at a timeInheritance of one character at a time
ProduceProduce lots of offspringlots of offspring
ProduceProduce purepure plants when allowed toplants when allowed to self-self-
pollinatepollinate several generationsseveral generations
Can beCan be artificially cross-pollinatedartificially cross-pollinated
Maintained statistical record – derived ratioMaintained statistical record – derived ratio

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

Mendel’s ExperimentalMendel’s Experimental
MethodsMethods
•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 withCovered each flower with
a cloth baga cloth bag
•He traced traits through theHe traced traits through the
several generationsseveral generations
•

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

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)
• 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))

Mendel’s Experimental ResultsMendel’s Experimental Results

• Did the observed ratio match theDid the observed ratio match the
theoretical ratio?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 difference is due toThe difference is due to statistical errorstatistical error
TheThe larger the samplelarger the sample the more nearlythe more nearly
the results approximate to thethe results approximate to the
theoretical ratiotheoretical ratio

Generation “Gap”Generation “Gap”
• Parental PParental P11 GenerationGeneration = the parental generation= the parental generation
in a breeding experimentin a 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= the second-generation offspring
in a breeding experiment.in a breeding experiment.
(2nd filial generation)(2nd filial generation)
– From breeding individuals from the FFrom breeding individuals from the F11
generationgeneration

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

MonohybridMonohybrid
CrossesCrosses

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

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

FF11 Monohybrid CrossMonohybrid Cross
• Cross:Cross: RoundRound seedsseeds xx RoundRound seedsseeds
• RrRr xx RrRr
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

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

Back Cross and Test CrossBack Cross and Test Cross
• Mendel then crossed aMendel then crossed a purepure & a& a hybridhybrid
from hisfrom his F1F1generationgeneration
• This is known as anThis is known as an F1 or test crossF1 or test cross
• There areThere are twotwo possible testcrosses:possible testcrosses:
Homozygous dominant x HybridHomozygous dominant x Hybrid
Homozygous recessive x HybridHomozygous recessive x Hybrid

F1 Monohybrid Cross (1F1 Monohybrid Cross (1stst
))
• Cross:Cross: RoundRound seedsseeds xx RoundRound seedsseeds
• RRRR xx RrRr
R
R
rR
RR
RrRR
Rr
Genotype:Genotype: RR, RrRR, Rr
PhenotypePhenotype: RoundRound
GenotypicGenotypic
Ratio:Ratio: 1:11:1
PhenotypicPhenotypic

F1 Monohybrid Cross (2nd)F1 Monohybrid Cross (2nd)
• Cross:Cross: WrinkledWrinkled seedsseeds xx RoundRound seedsseeds
• rrrr xx RrRr
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

F1 Monohybrid Cross ResultsF1 Monohybrid Cross Results
 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

Mendel’s LawsMendel’s Laws

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,, oneone
from the mother and the other from thefrom the mother and the other from the
fatherfather
• True-breeding individuals areTrue-breeding individuals are
homozygous ( both alleles)homozygous ( both alleles) are the sameare the same

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)

Law of DominanceLaw of Dominance

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""recombined"
at fertilizationat fertilization, producing the genotype, producing the genotype
for the traits of the offspringfor the traits of the offspring.

Applying the Law of SegregationApplying the Law of Segregation

Dihybrid CrossDihybrid Cross
• 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

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

Genotypic ratio

Incomplete DominanceIncomplete Dominance
andand
CodominanceCodominance

• 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

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

CodominanceCodominance
• Both alleles of a characters are equallyBoth alleles of a characters are equally
dominant and both of them express theirdominant and both of them express their
character in the F1 Generationcharacter in the F1 Generation
• None is MaskedNone is Masked
• Codominance is an allelic interactionCodominance is an allelic interaction
• Example:Example: blood typeblood type
• AB blood group is due to codominanceAB blood group is due to codominance
• AB group is controlled by the Gene LAB group is controlled by the Gene LAA
and Land LBB
•

• Both are equally dominant
• A produce A antigen , B produce B Antigen
• Example : 2 Coat colour in short horn cattleExample : 2 Coat colour in short horn cattle
• In short horn cattle there are two colour of hair ,In short horn cattle there are two colour of hair ,
red and whitered and white
• R colour is controlled by R gene and white color isR colour is controlled by R gene and white color is
controlled by rcontrolled by r
• When red and white is crossed , the F1 has roanWhen red and white is crossed , the F1 has roan
colour having both red and white hairscolour having both red and white hairs

Clones
• In biology, cloning is the process of producing similar
populations of geneticallyidentical individuals that
occurs in nature when organisms such
bacteria, insects, plants or animals reproduce asexua
lly. Cloning in biotechnology refers to processes used
to create copies of DNA fragments (molecular
cloning), cells (cell cloning), or organisms (organism
cloning).

phenocopy
• A phenocopy is a variation in phenotype (generally
referring to a single trait) which is caused by
environmental conditions (often, but not necessarily,
during the organism's development), such that the
organism's phenotype matches a phenotype which is
determined by genetic factors. It is not a type
of mutation, as it is non-hereditary.
• The term was coined by Richard Goldschmidt in
1935.
• He used it to refer to forms, produced by some
experimental procedure, whose appearance duplicatescopyright cmassengale 57

• Phenocopy can also be observed in Himalayan rabbits. When
raised in moderate temperatures, Himalayan rabbits are
white in colour with black tail, nose, and ears, making them
phenotypically distinguishable from genetically black rabbits.
However, when raised in cold temperatures, Himalayan
rabbits show black colouration of their coats, resembling the
genetically black rabbits. Hence this Himalayan rabbit is a
phenocopy of the genetically black rabbit.

Inbreeding
• Among living organism fundamentally following two system of
matting occur
• INBREEDING
• The process of mating of individual which are more closely
related than average of the population to which they belong is
called inbreeding
• For examples the self fertilization in plants is an extreme type of
inbreeding. While the marriage between brothers and sister or
first cousin is the another familiar example of inbreeding among
higher animals - Genetic effect of inbreeding
• the continuous inbreeding results, genetically in homozygosity.
It produce homozygoous stocks of dominant or recessive gene
and eliminate heterozygosity from the inbreed population.

Practical applications of inbreeding
• Homozygous – alleles will be able to express the
delerious phenotypic effect on the individual
• Homozygous dominant – express the beneficial
phenotypic effect
• To produce race horse, dogs, bulls and cattle
• Inbreeding depression
• Inbreeding increase the homozygosity is called
inbreeding depression

Heredity and Variation
• The study of inherited characteristic is called heredity
• Genes carry the inherited characteristics of a person
• Each characteristic of a person is controlled by a pair of
genes
• A gene for a certain characteristic may be a dominant
gene or a recessive gene

A few of early views on heredity are as follows;• 1.Pythagoras -proposed ‘Moist vapour theory’ and
according to this, some moist vapour is given out from the
body parts of a male during coitus which induced the
formation of similar parts of embryo in the uterus of the
female.
• 2.Aristotle - proposed that the women’s menstrual fluid
forms the embryo while the semen of the male furnished
the life giving power and form to the embryo.
• 3.Epigenesis Theory: Wolf -proposed that the germ cells
contain some undifferentiated living substances capable of
forming the organized body after fertilization

• dominant gene Vs recessive gene
• A dominant gene is powerful and can mask the effect of
a less powerful or recessive gene in a pair
• A recessive gene is weak and cannot show its effect if it
is paired with a dominant gene
• Each pair of genes is represented by two letters, a
capital letter used for a dominant gene and a small letter
for a recessive gene

Variation
• The visible differences between the parents and the
offspring or between the offspring or between the
offspring of the same parents
• Types of Variations-The variations may be classified
into two types
• Hereditary Variations: The variations which arise as a
result of any change in the structure and function of
the gene and are inherited from one generation to
another are called hereditary variations

• Environmental Variations:
• Two individuals with the same genotype may become
different in phenotype when they come in contact with
different conditions of environment. Such differences
among organisms of similar heredity are known as
environmental variations.
• Based on the type of cells, variation is classified into two
types
• a) Somatic variation
• b) Germinal variation

• Somatic variation
• The variation which occurs in somatic cells is called
somatic variation. It is acquired by the organisms
during their own lifetime and are lost with death.
Hence, it is also called acquired variation.
• Germinal variation: The variation which affects the
germinal or reproductive cells is called germinal
variation
• Based on the degree of differences, variation is
classified into two types
• Continuous variation and Discontinuous variationcopyright cmassengale 69

Continuous variation Discontinuous variation
1.Small and indistinct
variations are called
continuous variations.
1.Large, distinct and sudden
variations are called
discontinuous variation.
2.These are non-heritable. 2.These are heritable.
3.They have no role in
evolution.
3.They provide raw materials
for evolution on which
selection is based.
4.They are most common
and occur in all organisms.
4.They are not common and
appear suddenly.

Heredity and Variations in Asexual Reproduction
• In sexual reproduction, the individuals are produced
from a single parent without the formation of gametes.
Hence, the offspring are genetically identical to the
parent plants. Only mitosis cell division is involved in this
method. Such a genetically identical population
produced from a single organism but asexul method is
called clone. Variations in such cases may arise due to
environmental factors such as temperature, light, water,
food conditions, etc and radiations

Significance of Variations:• 1 .Variations provide raw material for evolution.
• 2.Variations provide unique character to each organism.
• 3.It helps in adaptation of organisms to changing
environment.
• 4.It helps in the development of better varieties of
plants and animals.
• 5.It forms the basis of heredity.

Pure line
• A pure line is a population that originates through
inbreeding or self-fertilization, descendants always equal
to each other and to the progenitors
• Concept of Pure Line
• n the early 20th century, the Danish botanist Wilhelm
Johannsen defined pure line as a group of genetically
identical individuals, progenies of a single self-fertilized
individual
• Pure line selection
• Pure lines are obtained through repeated self-
fertilizations of heterozygous individuals or through a
sequence of inbreeding in a population initially
heterozygous

• Some advantages of pure lines selection
• 1. Easy and low cost crop improvement method;
• 2. Improved and uniforms varieties both in aspect as in
performance (higher-yielding, more disease-resistant,
etc.)
• 3. In a laboratory context, the use of a pure line allows
to detect any mutation or abnormality.
• Some disadvantages of pure lines selection
• 1. Weak adaptive capacity;
• 2. Very narrow genetic base;
• 3. Requires time and space

• Applications of the pure lines selection
• 1. Allows standardization of products of crops (colour,
size, shape, texture, etc.);
• 2. Allows to obtain different purified varieties; Can be
performed after hybridization for segregation of
different populations;
• 3. Pure line individuals can be used in crossing programs
for pedigree certification;
• 4. Pure lines are used in research in biology, medicine
and biochemistry to study mutations and other
biological mechanisms

Mendelian genetics

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