Pre-Mendelian Theories of Heredity: An Overview of Early Genetic Concepts, Slides of Plant physiology

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Pre-Mendelian ideas about Heredity – Vapour and fluid

theory, Magnetic power theory, Preformation theory,

Lamarck’s theory, Darwin’s theory, Germplasm theory and

Mutation theory.

Lecture – 11

Introduction

The history of genetics - started with the work of Gregor

Johann Mendel ( 1822 - 1884 ), known as the father of genetics.

Pea plants , published in 1866 , - Mendelian law of

Inheritance.

The year 1900 marked the “Rediscovery of Mendel” by Hugo

de Vries, Carl Correns and Erich von Tschermak and by this

rediscovery of Mendel the science of genetics got into

limelight.

1. Vapour and fluid theory

2. Magnetic power theory

3. Preformation theory

4. Theory of epigenesis

5. The theory of acquired characters (Lamark’s theory)

6. Theory of natural selection (Darwin’s theory)

7. Theory of pangenes

8. Germplasm theory

9. Mutation theory

10. Particulate theory

11. Theory of Spontaneous generation 4

Pre-Mendelian ideas about Heredity Some of the important concepts about the heridity in the Pre-mendelian period includes,

• Early Greek philosophers speculated that the hereditary information's of parents existed in the form of vapours of fluids. Pythagoras ( 500 B.C.) : Greek Philosophers A moist “vapour” descended from the brain, nerves and other body organs of the male, is responsible for the development of an embryo in the uterus of the female. According to him, the male transmitted all the characters of the embryo and the female does not. Empeddes : Greek philosopher Thought that each parent produces a fluid known as “semen” which arises directly from various parts of the body and is responsible for the production of an embryo in the uterus of the female.

1. Vapour and Fluid theory

5

In the 17 th century W. Harvey ( 1578 - 1657 ), after performing certain experiments on deer proposed the theory called magnetic power theory. He suggested that as iron by friction with a magnet possesses the magnetic properties, so that the uterus by the friction of male organ acquires some magnetic power to conceive an embryo.

2. Magnetic power theory

7

3. Preformation theory

Dutch Biologist Swammerdam and Bonnet (1720-1793)

A miniature human called humanculus was already present (preformed) in the egg and sperm. In other word, a miniature human was preformed in the gametes. Development of zygote resulted only in the growth of miniature human. ✓ This theory was rejected Swammerdam, an ovist, believed that the animal hemisphere of the frog’s egg contained a tiny frog. 8

5. Theory of Acquired characters Proposed by J.B. Lamarck ( 1744 - 1829 ), a French biologist. States that - A new character once acquired by individual shall pass on to its progeny. This theory was disproved by Weismann. He cut the tail of mice for successive generations and always got the baby mice with tail. Thus, this theory was rejected. 10

6. Theory of Natural Selection

In 1858 , Charles Darwin ( 1809 - 1882 ) and Wallace independently proposed this theory. He believed evolution is due to natural selection According to this theory, many more individuals of each species are born than can possibly survive and consequently there is always a struggle for existence. If hereditary differences occur within the wild species of plants, natural selection allowed only the fittest individuals of the species to survive & eliminating others. This is known as survival of fittest due to natural selection. Published Origin of Species ( 1859 ) and proposed the ‘Hypothesis of Pangenesis’ ( 1868 ).

8. Germplasm Theory

August Weismann ( 1834 - 1914 ) - a German zoologist. A reduction on chromosome number took place during the formation of egg & the sperm & that the original number was restored when the egg & sperm are fused. According to “ Germplasm Theory of Heredity ”, the body tissues are of two types viz., germplasm and somatoplasm. The heriditary particles called as ids ( genes ) situated in an idants (chromosomes) constituted the germplasm. The germplasm are independent of the body. Castle & Phillips proved the independence of the germplasm from the somatoplasm in Guinea pigs. 13

The Body of an individual

Somatoplasm Germplasm Do not contribute to sexual reproduction The somatoplasm includes all other body tissues which are not related to sexual reproduction. Basis of sexual reproduction The germplasm refers to the reproductive tissues or cells that produces the gametes. Contd…

• Thus transmission of characters from one generation to other takes place through germplasm.
• Any change in the germplasm will lead to change in the next generation.
• This theory was accepted in broad sense. 14

10. Particulate theory

• Proposed - Maupertius ( 1698 - 1759 )
• Both the parents produce semen, which contains many tiny particles. During capulation, the semen of both parents united and formed the embryo.
• Each organs of the embryo was supposed to be formed by the fusion of two particles produced by both the parents. 16

11. Theory of Spontaneous generation

• The Greek biologist Aristotle, was of the opinion that not only plants but also animals like flies, mosquitoes and snails originated from decaying organic matter.
• Many biologist believed that some primitive organisms originated from non-living matter such as decaying organic matter. 17

Pre-Mendelian ideas about Heredity

1. Vapour : Pythagoras

2. Fluid theory : Empeddes

3. Magnetic power theory : W. Harvey

4. Preformation theory : Swammerdam & Bonnet (humanculus)

5. Theory of epigenesis : Wolff (universally accepted)

6. The theory of acquired characters : Lamark’s

7. Theory of natural selection : Darwin’s “Origin of Species”

8. Theory of pangenes : Charles Darwin

9. Germplasm theory : August Weismann

10. Mutation theory : De Vries (Term “Mutation”)

11. Particulate theory : Maupertius

12. Theory of Spontaneous generation : Aristotle 19