BY

PRASHANT VELLORE

BSc.BIOTECHNOLOGY

2011BSB817/11RSS75088

ASSIGNMENT ON PRE-MENDELIAN GENETICS

Ever since people began to wonder at the world around them they began to ask questions like….

i. How a horse always gives birth to a horse? ii. Why does a baby always resemble it’s parents or it’s grandparents? The first theories came down through the thoughts of the

early Greek philosophers also called the natural philosophers.

But before they explained these questions in rational terms they were already answered by myths, old enough.

WHAT PEOPLE WONDERED EARLIER

•About six thousand years ago people started keeping record of pedigrees of domestic animals such as horses and food plants like rice.

•The ideas or theories which have been forwarded from time to time to explain the phenomenon of inheritance can be categorized as….

THEORIESVAPOUR AND

FLUID THEORIESPREFORMATION THEORIES

PARTICULATE THEORIES

VAPOUR AND FLUID THEORIES

PYTHAGORAS(500BC) proposed that every

organ of animal body gives out some type of vapours . These vapours unite and form a new individual.

HIPPOCRATES(400BC)

believed that the reproductive material is handed over from all parts of the body of an individual , so that the characters are directly handed over to the progeny.

ARISTOTLE (350BC)

thought that the semen has some “vitalizing” effect and considered it as the highly purified blood. According to him the mother furnishes inert matter and the father gives motion to the new life.

PREFORMATION THEORIESLEONARDO DA VINCI

(1452-1519)

proposed a theory that the male and the female parents contribute equally to the heredity of the offspring.

WILLIAM HARVEY(1578-1657)

speculated that all animals arise from eggs and semen plays only the vitalizing role.

A.V. LEEUWENHOEK (1632-1723)

in 1677 he observed sperms of different animals like man, dog, and other mammals like frog, fish, etc. and named it as ‘animalcules’ and also suggested their association with eggs.

HOMUNCULUS

some scientists speculated that they saw a ‘little man’ or Homunculus in the sperms.

PREFORMATIONISTS HAVE OFTEN BEEN DIVIDED INTO TWO SCHOOLS

SPERMISTS/ANIMALCULISTS OVISTS

OVISTS

these group of scientists attached more importance to ova or egg, they thought that ‘homunculus’ was present in the ovum, and thus it was the women who carried the eggs containing boy and girl children and was responsible for the gender of the offspring.

SPERMISTS/ANIMALCULISTS

these group of scientists attached more importance to sperm, they thought that ‘homunculus’ or a complete organism was present in the sperm, and contended the only contribution of female was the womb where the ‘homunculus’ grew.

There was a theory which disapproved this theory.

It was given by K.F.Wolff and was called ‘THEORY OF EPIGENESIS’.

THEORY OF EPIGENESISK.F.WOLFF(1738-1794)

finally refuted the preformation theory by proposing that neither eggs nor sperm had the structure like ‘humunculus’ but gametes contained substances capable of forming the organized body after fertilization. This idea was the very core of THE THEORY OF EPIGENESIS.

This theory suggested that many new organs and tissues which were originally absent, develop subsequently.

However ,Wolff believed that these tissues and organs developed due to some mysterious vital forces.

French biologist MAUPERTUIS (1689-1759) proposed that the body of each parent gives rise to minute particles.

During sexual reproduction the particles of both individuals unite together to form a new individual.

He thought that in certain cases the particles of the male parent might dominate on those of the female parent and produce the male individual,while in the production the particles of female might be dominant on particles of the male.

Thus he proposed the concept of biparental inherentance by elementary particles.

PARTICULATE THEORIES

PARTICULATE THEORIES were given in different forms like

1) Inherentance of acquired characters,

2)Theory of Pangenesis,

3)Theory of Germplasm.

INHERITENCE OF ACQUIRED CHARACTERS

LAMARCK (1744-1829)

in 1844 proposed the phenomenon of “inherentance of acquired characters” among living organisms.

But he failed to provide convincing evidences in support of his concepts.

THEORY OF PANGENESISHIPPOCRATES

gave the central idea of the pangenesis theory.

CHARLES DARWIN In 1868 gave his famous theory of pangenesis which exclusively depends on the particulate theory. According to it each part of the animal body produces many minute particles known as gemmules ,which are first collected in the blood and then concentrated in the reproductive organs.When the animal reproduces into new individual ,these gemmules pass on to it and it has blending of both parents.

WEISMANN (1835-1934) and GALTON (1823-1911)

disapproved the Theory of Pangenesis.

WEISMANN in 1892 postulated “Theory of Germplasm” to explain

heredity.

THEORY OF GERMPLASM

WEISMANN (1835-1934)

according to this theory explained that the body of the organisms contain two types of cells namely ‘somatic cells’ & ‘reproductive cells’. The somatic cells make up the body and it’s various organs, while the reproductive cells form the sperm and ova.

The somatic cells contain the ‘somatoplasm’ and germinal or reproductive cells contain the ‘germplasm’.

According to Weismann the germplasm can form the somatoplasm but vice-versa does not take place.

The changes in the somatic cells due to environment cannot influence the reproductive cells.

Even after cutting the tail of mice for many generations he always got tailed mice.

Thus, by such experiments he rejected Lamarckism and Pangenesis Theory.

Though The particulate Theory faced many problems in it’s beginning but its basic concept has formed the central core of the modern understanding of Genetics.