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The Agriculture Guruji

Introduction:

Genetics is a biological science which deals with the principles of heredity and variation.

Heredity refers to the transmission of characters from parents to their offspring.

The foundation of this new branch of biology was laid by Mendel in 1866 when he discovered the basic principles of heredity.

Mendel’s findings came into light only in 1900 when similar results were obtained independently by three scientists, viz., de Vres, Carl Correns and Tschermak.

The term genetics was first used by Bateson in 1905.

The word genetics has been derived from the Greek word gene, which means to become or to grow.

Since characters are governed by genes, genetics is the study of structure, composition and function of genes.

Forward and Reverse Genetics:

Forward Genetics:

The area of genetics in which first the phenotype is known and the gene involved is identified is called reverse genetics.

1. First the phenotype is identifies.

2. Then the gene involved is identified.

3. It includes classical genetics.

4. It requires field facilities and simple laboratory.

5. It involves moderate technical skill.

6. The initial cost involved is low.

7. Identification of gene is easy.

Reverse Genetics:

The area of genetics in which first the gene (DNA sequence) is identified and then its effect is studied is called reverse genetics. It includes genomics.

1. First the gene (DNA sequence) is identified.

2. Then the phenotypic effect of the gene is studied.

3. It involves molecular techniques especially genomics.

4. It requires adequate laboratory and high technical skill.

5. The initial cost involved is higher.

6. Identification of gene sequence is easy and rapid.

Pre-Mendelian Concepts about Heredity:

i) Preformation theory:

Proposed by Swammerdam and Bonnet (1720 – 1793).

This theory states that a miniature human called humumculus was already present in the egg and sperm.

A miniature human was performed in the gametes. The development of zygote resulted only in the growth of miniature human who was already present in the egg and sperm.

However, this theory was soon given up because this could not be proved scientifically.

ii) Theory of Epigenesis:

Proposed by Wolff (1738 – 1794).

This theory states that egg or sperm cells do not contain miniature human.

In other words, egg or sperm cells are undifferentiated.

The differentiation into various organs or parts takes place only after fertilization from the zygote resulting into development of adult tissues and organs.

iii) Theory of Acquired Characters:

This concept was proposed by Lamarck (1744 – 1829).

This theory states that a new character once acquired by an individual shall pass on to its progeny. It means if a man develops a strong muscle by exercise all his children will have strong muscle.

On the other hand, if a person becomes weak all his children will be weak. This theory was disproved by Weismann.

He cut the tail of mice for 22 successive generations and always got the baby mice with tail.

iv) Theory of Pangenes:

Proposed by Charles Darwin (1809 – 1882).

This theory states that vey small, exact but invisible copies of each body organ and component (called gemmules) are transported by the blood stream to the sex organs.

These invisible copies of each body organ are called the gemmules. These gemmules are assembled in the gametes. After fertilization these gemmules move out to different parts of the body resulting in the development of respective organ. A defective gemmule will lead to the development of defective organ in an individual.

v) Germplasm theory:

Proposed by August Weismann (1889).

This theory states that body tissues are of two types, viz. germplasm and somatoplasm. The germplasm refers to the reproductive tissues or cells which produce gametes. The somatoplasm includes all other body tissues which are not related to sexual reproduction.

Transmission of characters from one generation to other takes place only through germplasm. Any change in the germplasm will lead to change in the next generation. This theory is accepted in a broad sense.