subject forensic science - e-PG Pathshala

FORENSIC SCIENCE

PAPER No.12: Forensic Biology & Serology

MODULE No. 26: Mendelian Genetics

SUBJECT FORENSIC SCIENCE

Paper No. and Title PAPER No. 12: Forensic Biology & Serology

Module No. and Title MODULE No. 26: Mendelian Genetics

Module Tag FSC_P12_M26

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TABLE OF CONTENTS

1. Learning Outcomes

2. Mendelian Genetics

3.Mendel’s experimental approach

4. Seven traits studied by Mendel

5. Mendel’s data

6. Antagonistic traits

7. Mendel’s crossed pea plant

8. Conclusions of Mendel’s research

8.1. Law of Dominance

8.2. Law of Segregation

8.3 Law of Independent Assortment

9. Cross

10. Exceptions of Mendel’s laws

11. Summary

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1.Learning Outcomes

)After studying this module, you shall be able to know that:

1. Why Mendel chose pea plant for his research?

2. How Mendel carried out his research?

3. What are the conclusions drawn from Mendel‟s research?

4. What are the exceptions of Mendel laws?

1. to g

2. Mendelian Genetics

Gregor Johann Mendel (20 July,1822-6 January, 1884) was an Austrian monk

born in Sicilian village of Heinzendorf (Austria).

He was a scientist by occupation, because of his experiments in pea he formulated

the theory of heredity, and therefore he is often remembered as the „Father of

Genetics‟.

At first Mendel‟s work was rejected in the scientific community.

By 1900, Mendel‟s idea was recognized, many biologists realized the importance

of Mendel‟s work and thus it was rediscovered in 1902.

Mendel took 8 years (1856-1863) to perform experiment of genetics in pea plants.

He published his theory of Inheritance “Versuche uber Pflanzenhybriden”

(Experiment in plant hybridization) in 1865.

Figure 1: Gregor Mendel

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3. Mendel’s Experimental Approach

Mendel‟s research was with Pea(Pisum sativum) which began in 1856,in his own

monastery garden.

The success of Mendel‟s work was because of:

Selection of single character,

Keeping accurate record of each plant,

Mathematical analysis of data.

Mendel selected pea plant because of following reasons:

Easy to grow,

Easy to get in large no.,

Available in many variety,

Traits are easy to isolate,

Reproduce sexually,

Have male and female sex cells called gametes.

Figure 2: Reproductive structure of flower.nucleus

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4. Seven Traits Studied by Mendel

Basically seven traits were studied by Mendel in pea plant which are as follows:

Seed coat Colour/flower Colour: grey & purple

Seed Colour: yellow & green

Seed shape: Smooth & wrinkled

Pod Colour: Green & yellow

Pod shape: Inflated & pinched

Stem height: Tall & short

Flower position: Axial & Terminal

Figure: 4 Seven traits of pea plant

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5. Mendel’s Data

Figure: 5 Mendel’s data

The above figure shows the output of Mendel‟s research work.

Mendel crossed round and wrinkled seed, he obtained round seed in F1

generation, which means round trait is dominant over wrinkled trait.

The probability ratio obtained in F1 generation was 3:1 for round and wrinkled

seed.

In the same manner remaining traits of pea plant were crossed, one allele in each

pair was dominant over the other pair.

For all remaining six traits probability ratio obtained was 3:1.

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6.Antagonistic Traits

Figure: 6 Antagonistic traits

The above figure shows the antagonistic traits of pea plant which Mendel used for

his research are as follows:

Round seed shape is dominant whereas wrinkled seed shape is recessive,

Yellow seed colour is dominant whereas green seed colour is recessive,

Violet red flower colour is dominant whereas white flower colour is recessive,

Flower position axial is dominant whereas terminal flower colour is recessive,

Inflated pod shape is dominant whereas pinched pod shape is recessive,

Pod colour green is dominant whereas pod colour yellow is recessive,

Tall plant is dominant whereas short plant is recessive.

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7.Mendel’s crossed pea plant

Figure: 7 Mendel crossed pea plant

The above figure shows the diagrammatic representation of Mendel‟s experiment:

At first he removed the stamens from purple flower of pea plant,

He then transferred sperm bearing pollen from stamens of white flower to egg

bearing carpel of purple flower,

Then pollinated carpel matured into pod,

Then he planted seeds from pod,

He observed that in first generation all the offspring were of purple flowers.

8. Conclusions of Mendel’s Research

However, the three conclusions drawn from his research were:

Law of Dominance,

Law of Segregation, &

Law of Independent assortment.

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8.1. Law of Dominance

First of all we must understand the first law,”Law of Dominance”.

Mendel‟s first law states that one allele in pair dominates over other, the one

which get unexpressed is called recessive.

During his experiments, he mated true breeding parents called P Generation i.e.

Purple and white flowers.

The offspring of true breeding parents were all purple flower plant which is called

as hybrid or F1 generation.

Then he self-pollinated the F1 generation plant, he observed that in F2 generation

combination of purple and white flowers were produced in a ratio of 3(purple) :

1(white).

Figure: 8 Law of Dominance

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8.2. Law of Segregation

Next second law is “Law of segregation”.

Mendel considered monohybrid cross here i.e. only smooth and wrinkled seeds

were taken into consideration (one character).

It states that the two alleles for a characteristic separate during gamete formation

(egg & sperm).

Mendel‟s experiment showed that the factors (dominant or recessive) for the

characters in P generation during gametogenesis segregate from each other.

These factor then co-exist in F1 generation which later reappears in F2 generation

in the ratio of 3:1

Figure:-9 Law of Segregation

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Alleles Segregate Alleles Unite

Figure:-8 Allele segregate & unite.

The above diagram shows how in Mendel‟s second law alleles segregate and once

again unite.

The first figure shows that the alleles of yellow and green pea from pure breeding

plant grows into plant, during gamete formation it segregates from each other.

Whereas the second figure shows that during fertilization the segregate alleles

unite with each other which leads into the development of seed.

8.3. Law of Independent Assortment

Mendel considered dihybrid cross i.e. Smooth seed & yellow cotyledons; wrinkled seed

& green cotyledons.

Mendel‟s third law states that,

The alleles for different characteristics are distributed to reproductive cells

independently.

In this experiment round seed and yellow cotyledon was crossed with wrinkled

seed and green cotyledon.

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The hybrid seeds obtained during F1 generation were all smooth seed and yellow

cotyledon which means that smooth seed was dominant over wrinkled seed

whereas yellow cotyledon was dominant over green cotyledon.

Then hybrids of F1 generation were self-pollinated, where he obtained four

different combinations of F2 generations in the ratio of 9:3:3:1 i.e.

Smooth yellow (9),

Smooth green (3),

Wrinkled yellow (3),

Wrinkled green (1).

He concluded that alleles for different traits assort independently of one another.

Figure:-9 Law of Independent Assortment

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9. Cross

Out cross Test cross

F1 Phenotype Tall x Tall parents Tall x Dwarf

(pure)

F1 Genotype Tt T T Tt tt

Gametes T,t T,T T,t t,t

Back cross offspring TT Tt Tt tt

Phenotype Tall pure Tall Phenotype Tall Dwarf

Ratio 1 : 1 : 1 : 1

The cross made between hybrids of F1 generation and one of the parents is called

Back cross.

Back cross is of two types:

Out cross: a cross between F1 hybrids and dominant homozygous parents.

Test cross: a cross between hybrids of F1 generation and recessive homozygous

parents.

10. Exceptions of Mendel’s Law

Various exceptions that arose from Mendel laws were:

Incomplete dominance

Codominance

Multiple Alleles

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10.1 Incomplete Dominance

Figure: 10 Incomplete Dominance

In incomplete dominance an individual have an intermediate phenotypic effect.

Eg. In 4 O‟clock plant also called as Mirabilis Jalapa which is found both in red

and white Colour, when crossed the F1 generation produces pink Colour flower.

Moreover, in F2 generation, when self-pollination is done then 1 red, 2 pink & 1

white Colour flower is produced.

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10.2. Codominance

Figure:11 Blood shows codominance

10.3. Multiple Alleles

Figure: 12 Multiple Alleles for blood group

Three or more alleles of the same gene that code for the same trait are called as

multiple alleles.

Eg. In humans blood group, there are four types of blood group i.e. A, B, AB & O

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11. Summary

Gregor Mendal is known as the „Father of Genetics‟.

Mendel‟s research was with Pea (Pisum sativum).

Basically seven traits were studied by Mendel in pea plant.

However, the three conclusions drawn from his research were:

Law of Dominance,

Law of Segregation, &

Law of Independent assortment

Mendel‟s first law states that one allele in pair dominate over other, the one which

get unexpressed is called recessive.

Mendel‟s second law states that the factors (dominant or recessive) for the

characters in P generation during gametogenesis segregate from each other.

Mendel‟s third law states that, the alleles for different characteristics are

distributed to reproductive cells independently.

The cross made between hybrids of F1 generation and one of the parents is called

Back cross.

Exceptions of Mendel‟s laws are: Incomplete dominance,Codominance, Multiple

alleles.

In incomplete dominance an individual have an intermediate phenotypic effect.

Three or more alleles of the same gene that code for the same trait are called as

multiple alleles

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