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3. Inheritance and Variation

-CREATED BY-

PROF. DESHMUKH A. B.

ASST. TEACHER

Agasti Arts, Commerce and Dadasaheb Rupwate

Science Junior College, Akole

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CONTENT AT A GLANCE

3.1 Chromosomes and Mechanism of inheritance.

Reasons for Mendel’s Success

3.2 Genetic Terminology
3.3 Mendel’s Laws of Inheritance

1. Law of Dominance
2. Law of segregation (Law of purity of gametes)
3. Law of Independent Assortment

3.4 Back Cross and Test Cross

a. Back cross
b. Test cross

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3.5 Deviations from Mendel’s findings

a. Incomplete dominance
b. Co-dominance
c. Multiple alleles
d. Pleiotropy

3.6 Chromosomal Theory of Inheritance
3.7 Chromosomes : Function; Number of chromosomes; Structure of chromosome; Sex Chromosomes
3.8 Linkage and Crossing Over

Linkage -- I. Complete linkage; II. Incomplete linkage; Linkage Groups
Sex-linkage -- a. Complete sex linkage; b. Incomplete sex linkage
Crossing Over
Morgan’s Experiments showing linkage and crossing over

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3.9 Autosomal Inheritance

a. Widow’s peak
b. Phenylketonuria (PKU)

3.10 Sex Linked Inheritance

a. X-linked (sex linked) genes --- Colour blindness; Haemophilia (Bleeder’s disease)
b. Y-linked (Holandric) genes

3.11 Sex Determination

a. Sex Determination in human beings
b. Sex Determination in birds
c. Sex Determination in honey-bees

3.12 Genetic Disorders

Thalassemia; Down’s Syndrome (21st trisomy); Turner’s Syndrome; Klinefelter’s syndrome (XXY males)

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3.1 Chromosomes and Mechanism of inheritance:

Heredity / Inheritance: -

transmission of genetic information from generation to generation.

The mechanism of inheritance

successfully investigated before

chromosomes had been observed or
genes were known.

Gregor Mendel first gave the accurate explanation for the mechanism of inheritance by using hybridization technique.

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He studied seven traits in garden pea

individually one at a time or
in combination of two or three character at a time.

He processed the data mathematically and statistically.

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He postulated the principles of heredity which then became the fundamental laws of heredity, as proposed by Correns (1900).
He visualized that,

the traits as such are not inherited physically
but by ‘something’ present inside the gametic cell.
He assume that this something is responsible for expression of trait / character.
To this ‘something’, he coined term factors.
He proposed that factors are particulate in nature.
These factors occur in pairs in the parents and segregate from each other during gamete formation without blending/ mixing.
The Mendelian factors are now termed as genes.

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Reasons for Mendel’s Success :

His experiments were carefully planned and involved large sample.
He carefully recorded the number of plants of each type and expressed his results as ratios.
In the pea plant, contrasting characters can be easily recognized.
The seven different characters in pea plant were controlled by a single factor each.
The factors are located on separate chromosomes.

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These factors are transmitted from generation to generation.
He introduced the concepts of dominance and recessiveness.

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Before learning about Mendel’s experiments let us get acquainted with genetic terms and symbols.

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3.2 Genetic Terminology :

Character : It is a specific feature of an organism.

e.g., height of stem.

Trait : An inherited character and its detectable variant.

e.g., Tall or dwarf.

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Stem Height

Tall

Dwarf

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Factor: (unit of heredity)

A particle present in the organism which is responsible for the inheritance and expression of a character.
It is passed from one generation to the next through gametes.
It determines a genetical (biological) character of an organism.

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Gene:
It is a particular segment of DNA which is responsible for the inheritance and expression of that character.

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DNA

Chromosome

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Alleles or Allelomorphs:

The two or more alternative forms of a given gene (factor) are called alleles of each other.
They occupy identical loci (positions) on homologous chromosomes.
Allele is a short form of Allelomorph.

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Dominant:
It is an allele that expresses its trait even in heterozygous condition only.
Alternatively, the allele that expresses in F₁ is called dominant.
It is an allele of a pair that masks the expression of other allele in F₁ generation.

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Parents:

Phenotype:

First Filial

Generation (F1):

Tall

X

Dwarf

Dominant

Tall

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Recessive:
The allele which is not expressed in presence of an alternative allele.
It expresses only in the presence of another identical allele.
It is an allele that does not express in F₁ hybrid.

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Parents:

Phenotype:

First Filial

Generation (F1):

Tall

X

Dwarf

Recessive

Tall

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Phenotype:
The external appearance of an individual for any trait is called phenotype for that trait.
It is observable and is determined by different combinations of alleles.
e.g., In pea, for the height of stem (plant) tall and dwarf are the two phenotypes (Tall is determined by TT or Tt and dwarf by tt).

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Genotype:
Genetic constitution or genetic make up of an organism with respect to a particular trait.
It is representation of the genetic constitution of an individual with respect to a single character or a set of characters.

e.g., pea tall plants can have genotype TT or Tt and dwarf has tt.

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Tall

Dwarf

TT / Tt

tt

Genotype:

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Homozygous (pure):
An individual possessing identical alleles for a particular trait, is called homozygous or pure for that trait.
Homozygous breeds true to the trait and produces only one type of gametes.

e.g., tall with TT and dwarf with tt.

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Tall

Dwarf

TT

tt

Genotype:

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Heterozygous:
An individual possessing contrasting alleles for a particular trait, is called heterozygous.
Heterozygous does not breed true for that trait and produces two types of gametes

e.g., F1 generation hybrids (Tt).

Heterozygous individual is also called hybrid.

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Parents:

Phenotype:

First Filial

Generation (F1):

Tall

X

Dwarf

All

Tall

Genotype:

TT

tt

Tt

Heterozygous

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Pure line:
An individual or a group of individuals which is homozygous or true breeding for one or more traits, constitutes pure line i.e., plant which breeds true for a particular character.
It is a descendent of a single homozygous parent produced after self fertilization.

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Tall

TT

All Tall

TT

Parent Plant

F1 Generation

All Tall

F2 Generation

1

Plant

20 Plants

350 Plants

Selfing

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Monohybrid:
It is heterozygous for one trait.
It is produced from a cross between two pure parents differing in single pair of contrasting characters.

e.g., Hybrid tall produced in a cross between pure tall and pure dwarf parents.

It is a heterozygote for a single pair of alleles.

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Phenotype:

First Filial

Generation (F1):

Tall

X

Dwarf

All

Tall

Genotype:

TT

tt

Tt

Monohybrid

Parents:

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F₁ generation:
It refers to the first filial generation.
It consists of all offsprings produced from a parental cross.
Alternatively, it is first generation from a given mating between pure parents having contrasting characters.

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Phenotype:

First Filial

Generation (F1):

Tall

X

Dwarf

All

Tall

Genotype:

TT

tt

Parents:

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F₂ generation:
The second generation (progeny) produced by selfing (inbreeding) of F1 generation offsprings is called second filial generation.

e.g., Progeny produced from a cross between two F₁ individuals (e.g., Tt × Tt).

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Phenotype:

First Filial

Generation (F1):

Tall

X

Dwarf

All Tall (Tt)

Genotype:

TT

tt

Parents:

Tall

Tt

Tall

Tt

X

TT

Tt

tt

Second Filial

Generation (F2):

Selfing:

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Punnett square:

(checkerboard)

It is a probability table.
It represents different permutations and combination of fertilization between gametes of the opposite mating types.
In short, it is a diagrammatic representation of a particular cross to predict the progeny of a cross.

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Homologous Chromosomes:
The morphologically, genetically and structurally essentially identical chromosomes present in a diploid cell, are called homologous chromosomes.
Such chromosomes synapse during meiosis.

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Homologous Chromosomes

Non-Homologous Chromosomes

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Back cross :
It is a cross of F₁ progeny with any of the parents

e.g., F₁ tall × pure tall; F₁ tall × pure dwarf i.e., Tt × TT/tt.

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Phenotype:

First Filial

Generation (F1):

Pure

Tall

X

Pure

Dwarf

All Tall (?)

Genotype:

Parents: (P)

Back Cross

F1 Hybrid

Any

Parent(P)

X

OR

Tall (?)

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Test cross:
It is a cross of F₁ progeny with homozygous recessive parent.

e.g., F₁ tall × pure dwarf i.e., Tt × tt.

It is used to test the homozygous/ heterozygous nature of hybrid.
It is a kind of back cross.

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Phenotype:

First Filial

Generation (F1):

Tall

X

Dwarf

All Tall (?)

Genotype:

TT

tt

Parents: (P)

Test Cross

F1 Hybrid

Recessive

Parent (P)

X

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Phenotypic ratio:
It is the ratio of the offsprings produced in F₂ and subsequent generation with respect to their physical appearance

e.g., 3Tall : 1 dwarf, is F₂ ‘Phenotypic ratio’ in monohybrid cross.

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Parents (P):

Phenotype:

Genotype:

Gametes:

F1

Generation:

Selfing of F1:

Genotype:

Gametes:

F2

Generation:

Tall

X

Dwarf

TT (♀)

tt(♂)

T

t

Tt

Tall

Tt

T

t

T

t

T

t

T

t

TT

Tt

tt

Phenotypic

Ratio:

Tall

Dwarf

:

=

3:1

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Genotypic ratio:
It is the ratio of the offsprings produced in the F₂ and subsequent generation with respect to their genetic make up.

e.g., 1 TT : 2Tt : 1 tt, is F₂ genotypic ratio in monohybrid cross.

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Genotypic

Ratio:

=

1:2:1

TT : Tt : tt

Parents (P):

Phenotype:

Genotype:

Gametes:

F1

Generation:

Selfing of F1:

Genotype:

Gametes:

F2

Generation:

Tall

X

Dwarf

TT (♀)

tt(♂)

T

t

Tt

Tall

Tt

T

t

T

t

T

t

T

t

TT

Tt

tt

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Monohybrid Cross

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Always keep in Mind –

Mendel selected Garden Pea (Pisum sativum) for his hybridization experiments.
He performed hybridization experiments to study inheritance pattern.
He selected 7 different traits with their detectable variants.
Initially, he considered only one trait at a time and focused on the inheritance pattern of the selected trait.

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Mendel followed three steps of Hybridization –

A. Selection of parents and obtaining pure lines:

Selection of Parents:

To perform the first experiment, he focused on the character Height of the stem.
He selected Tall pea plant as female parent and Dwarf pea plant as male parent .

Obtaining pure lines by selfing up to 3 generations.

As pea plant is naturally self pollinated,
Mendel allowed self pollination in selected parents up to three generations to verify their purity for selected trait.

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Tall Plant

Dwarf Plant

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Tall

All Tall

TT

Parent Plant

F1 Generation

All Tall

F2 Generation

1

Plant

20 Plants

350 Plants

Selfing

Obtaining pure lines by selfing up to 3 generations

If this not happen, he selected another plant and follow the same procedure.

Collected & sowed seeds

Selfing

Collected & sowed seeds

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B. Artificial Cross of the selected parents to raise F₁ generation:

Performed artificial cross
Collected and open the pods
Collected seeds and sowed them in soil to raise F₁ generation
Observed the F₁generation
Note down the results

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C. Selfing of F₁ hybrids to raise F₂ generation:

Allowed selfing in F₁hybrids
Collected seeds and sowed them in soil to raise F₂ generation
Observed the F₂ generation
Note down the results

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Monohybrid cross :

A cross between parents differing in only one heritable trait is called monohybrid cross.
e.g., Cross of pure tall and pure dwarf plants.

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Gamete Formation

Meiosis

Diploid Mother Cell

Haploid Gametes

Possess 2 factors for each character

Each gamete possess only one factor for each character

TT

T

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Tall (♀)

Dwarf (♂)

Parents

X

All Tall

F1 Generation

Selfing in F1 Plants

F1 Tall

787 grew Tall 75%

277 grew Dwarf 25%

F2 Generation

Phenotypic Ratio

Tall : Dwarf = 3:1

Collected & Sowed 1064 Seeds

Collected & sowed seeds

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Conclusions:

Characters are inherited separately as discrete particles or units.

He coined the term factor or determiner for the unit of heredity.

Each factor exists in two contrasting /alternative forms.

e.g., factor for height of the stem have two forms, one for tallness and the other for dwarfness.

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One of the two forms is Dominant, and the other is recessive.
Only dominant expresses in F1 generation.

He used appropriate alphabets,

capital letter for dominant allele

e.g., Tall – ( T ), Round ( R ), etc.

small letter for recessive allele.

e.g., Dwarf – ( t ), Wrinkled ( r ), etc.

In an organism, inheritance of each character is controlled by a pair of factors.

One of the factor is contributed by male parent and the other female parent.
Now we know that sexually reproducing higher organisms are diploid (2n).

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From the reappearance of recessive trait in F2 generation he concluded that

In the F1 generation factors do not mix with each other but they just remain together.
During gamete formation these factors separate, or segregate and each gamete receives only one factor from each pair of factors.
Now we know that gametes are haploid (n).

The organism that receives, recessive factor from both the parents expresses the recessive trait.

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Parents (P):

Phenotype:

Genotype:

Gametes:

F1

Generation:

Selfing of F1:

Genotype:

Gametes:

Tall (♀)

X

Dwarf (♂)

TT

tt

T

t

Tt

Tall

Tall (♀/ ♂)

Tt

T

t

F2 Generation:

T

t

T

t

TT

Tt

tt

Phenotypic

Ratio:

Tall

Dwarf

:

=

3:1

Genotypic

Ratio:

=

1:2:1

TT : Tt : tt

Mendel was expecting Intermediate expression OR 50% Tall and 50% Dwarf in F1 generation.

To find out where the dwarfness has gone? He allowed selfing in F1 plants.

Tall

Dwarf

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Parents (P):

Phenotype:

Genotype:

Gametes:

F1

Generation:

Selfing of F1:

Genotype:

Gametes:

Tall (♂)

X

Dwarf (♀)

TT

tt

T

t

Tt

All Tall (Hybrid)

Tall (♀/ ♂)

Tt

T

t

F2 Generation:

T

t

T

t

TT

Tt

tt

Phenotypic

Ratio:

Tall

Dwarf

:

=

3:1

Genotypic

Ratio:

=

1:2:1

TT : Tt : tt

Reciprocal Cross:

Tall

Dwarf

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In all monohybrid cross experiments Mendel observed consistent pattern of results.

i.e., he got approximate 3:1 monohybrid ratio for all characters.

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SR. NO.	CROSS	F1	F2	RATIO
1	Tall X Dwarf	Tall	787 Tall, 277 Dwarf	2.84 : 1
2	Yellow X Green Seeds	Yellow Seeds	6022 Yellow, 2001 Green	3.01 : 1
3	Round X Wrinkled Seeds	Round Seeds	5474 Round, 1850 Wrinkled	2.96 : 1
4	Green X Yellow Pods	Green Pods	428 Green, 152 Yellow	2.82 : 1
5	Inflated X Constricted Pods	Inflated Pods	882 Inflated, 299 Constricted	2.95 : 1
6	Axial X Terminal Flowers	Axial Flowers	651 Axial, 207 Terminal	3.14 : 1
7	Purple X White Flowers	Purple Flowers	705 Purple, 244 White	3.15 : 1

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Always keep in Mind –

Mendel selected Garden Pea (Pisum sativum) for his hybridization experiments.
He performed hybridization experiments to study inheritance pattern.
He selected 7 different traits with their detectable variants.
Initially, he considered only one trait at a time and focused on the inheritance pattern of the selected trait.

Prof. Deshmukh A. B.

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Prof. Deshmukh A. B.

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Character	Dominant	Recessive
1. Stem Height	Tall ( T )	Dwarf ( t )
2. Seed colour (Cotyledon)	Yellow ( Y )	Green ( y )
3. Seed Shape	Round ( R )	Wrinkled ( r )
4. Pod Colour	Green ( G )	Yellow ( g )
5. Pod Shape	Inflated ( I )	Constricted ( i )
6. Flower Colour	Purple ( P ) or Coloured ( C )	White ( p ) or White ( c )
7. Flower Position	Axial ( A )	Terminal ( a )

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Let us take another example

If round seeded homozygous ( R ) plant is crossed with the wrinkled seeded ( r ) plant.

What will be the F1 generation?
What will be the Phenotypic and Genotypic ratio in F2 generation?

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Follow 3 steps:
Step 1 - Selection of Parents and obtaining pure lines

Step 2 - Artificial Cross of the selected parents to raise F₁ generation
Step 3 - Selfing of F₁ hybrids to raise F₂ generation

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Let us consider,
Round seeded (homozygous) pea plant is selected as female parent.

We know that Mendel found Round shape ( R ) of seed is a dominant trait over wrinkled shape ( r ) in sweet pea.

As it is homozygous, the genotype will be RR.

Wrinkled seeded plant is selected as male parent.

We know that recessive trait expresses only when 2 recessive alleles are together.

Therefore, the genotype of Wrinkled seeded plant will

be rr.

We then follow the procedure of cross pollination in selected parents.

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Prof. Deshmukh A. B.

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Parents (P):

Phenotype:

Genotype:

Gametes:

F1

Generation:

Selfing of F1:

Genotype:

Gametes:

Round Seeds (♀)

X

Wrinkled Seeds (♂)

RR

rr

R

r

Rr

Round Seeds

Round Seeds (♀)

Round Seeds (♂)

Rr

R

r

R

r

F2

Generation:

R

r

R

r

RR

Rr

rr

Phenotypic

Ratio:

Round Seeds

Wrinkled Seeds

:

=

3:1

Genotypic

Ratio:

=

1:2:1

RR : Rr : rr

(Selfing is allowing self pollination in same flowers on same plant…)

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In above cross, between homozygous plant bearing round seeds and a plant with wrinkled seeds, the following results will be obtained

All seeds obtained in F1 generation will be Round only.
In F2 generation, the character wrinkled seed shape reappears and
The phenotypic ratio obtained as Round : Wrinkled will be 3:1.
The genotypic ratio obtained as RR : Rr : rr will be 1:2:1.

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Solve following examples –

If a homozygous plant having green pods ( G )is crossed with a plant having yellow pods ( g ),

What will be the F1 generation?
What will be the Phenotypic and Genotypic ratio in F2 generation?

If a homozygous plant having Axial flowers ( A )is crossed with a plant having terminal flowers ( a ),

What will be the F1 generation?
What will be the Phenotypic and Genotypic ratio in F2 generation?

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Remember:

In an organism many characters are present.

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Prof. Deshmukh A. B.

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Pod Shape

Inflated Pod

Constricted Pod

Pod Colour

Yellow

Green

Seed Cotyledon Colour

Green

Yellow

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Each character is controlled by a pair of alleles.

In a diploid organism, one of the factors is contributed by male parent and the other by female parent.
e. g.

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Character

Stem Height

Dominant expression

Recessive expression

Homozygous Tall

Heterozygous Tall

Homozygous

(Dwarf)

TT

Tt

tt

Pair of Alleles

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Character

Seed Shape

Dominant expression

Recessive expression

Homozygous with Round Seeds

Heterozygous with Round Seeds

Homozygous

Wrinkled Seeds

RR

Rr

rr

Pair of Alleles

Let us see another Character:

Thus, each character is controlled by a pair of alleles.

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After performing Monohybrid crosses, few questions were raised in his mind.
Does one pair of allele affect or influences the inheritance pattern of other pairs of alleles?

OR

Each pair is inherited independently as if in monohybrid cross?
To find out answer, Mendel performed dihybrid crosses and then trihybrid crosses.

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Dihybrid cross :

A cross between two pure (homozygous) parents differing in two heritable traits, is called dihybrid cross.

OR

A cross between two pure (homozygous)parents in which the inheritance pattern of two pairs of contrasting characters is considered simultaneously is called dihybrid cross.

e.g., cross of pure tall, round seeded plant with dwarf, wrinkled seeded plant.

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For his first dihybrid cross Mendel selected

A plant having Round seeds ( R )with Yellow cotyledons( Y ) as female parent

Genotype – RRYY

A plant having Wrinkled seeds ( r ) with Green cotyledons ( y ) as male parent

Genotype – rryy

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Mendel followed same three steps of Hybridization –

A. Selection of parents and obtaining pure lines

Selected Parents
Obtaining pure lines by selfing up to 3 generations.

B. Artificial Cross of the selected parents to raise F₁ generation

Performed artificial cross
Observed the F₁generation (seeds formed after fertilization)
Note down the results

C. Selfing of F₁ hybrids to raise F₂ generation

Sowed the seeds and raise F₁ plants
Allowed selfing in F₁hybrids
Observed the F₂ generation (seeds formed after fertilization)
Note down the results

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Mendel was expecting

75% Round Yellow and
25 % Wrinkled Green seeds.

But he was surprised.
He found four types of seeds in F₂ generation
Out of four,

Two were parental combinations

Round Yellow and Wrinkled Green

and

Two were new (recombinations)

Round Green and Wrinkled Yellow

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Parents (P):

Phenotype:

Genotype:

Gametes:

F1

Generation:

Selfing

of F1:

Genotype:

Gametes:

Round Yellow Seeds (♀)

X

Wrinkled Green Seeds (♂)

RRYY

rryy

RrYy

Round Yellow Seeds

RrYy

Round Yellow

Seeds (♀)

Round Yellow

Seeds (♂)

X

RrYy

RY, Ry, rY, ry

RY

ry

RrYy

R

r

Y

y

RY

ry

Ry

rY

Genotype of F1

Hybrid:

Segregation:

Gametes:

Let us see how alleles combine to form gametes

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Prof. Deshmukh A. B.

65

04-05-2024 06:50:03

F2 Generation:

Phenotypic Ratio:

RY

Ry

rY

ry

RY

Ry

rY

ry

RRYY

RRYy

RrYY

RrYy

RRYy

RrYY

Rryy

rrYy

rryy

RRyy

rrYY

Round Round Wrinkled Wrinkled

Yellow Green Yellow Green

:

9 : 3 : 3 : 1

Genotypic Ratio:

RRYY : RRYy : RrYY : RrYy : RRyy : Rryy : rrYY : rrYy : rryy

1 : 2 : 2 : 4 : 1 : 2 : 1 : 2 : 1

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How to write genotypic ratio:

Prof. Deshmukh A. B.

66

04-05-2024 06:50:03

Phenotypic Ratio:

Genotypes :

Round Yellow (9)

Round Green (3)

Wrinkled Yellow (3)

Wrinkled Green (1)

RRYY (1)

RRYy (2)

RrYY (2)

RrYy (4)

RRyy (1)

Rryy (2)

rrYY (1)

rrYy (2)

rryy (1)

Genotypic Ratio:

RRYY : RRYy : RrYY : RrYy : RRyy : Rryy : rrYY : rrYy : rryy

1 : 2 : 2 : 4 : 1 : 2 : 1 : 2 : 1

67 of 69

Prof. Deshmukh A. B.

67

04-05-2024 06:50:03

68 of 69

Prof. Deshmukh A. B.

68

04-05-2024 06:50:03

Thank You…..

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69

Prof. Deshmukh A. B.'s Biology Class