Pre-mendelian genetics:
Actual history of genetics started in 1990 with the rediscovery of mendel's work on plant hybridization. The following are some theories on genetics proposed in the pre-mendelian era:
Vapour theory:
The greek philosopher Pythagoras(600BC) speculated that a moist vapour liberated from the body of the male is responsible for the development of an embryo in the uterus of the female.
Fluid Theory:
The greek philosopher was the opinion that the offspring was produced by the mixing of semen of man and menstrual fluid of women during copulation. He regarded man's semen as a highly purified form of his blood. He considered the the women's menstrual fluid as female semen.
Performation theory:
It is an ancient theory giving an interesting explanation for the development of an organism. It was proposed by Malpighi according to this theory the gametes contain in them a mini perforated embryo.
Malpighi stated that the ovum contains a miniature adult animal called homunculus. During development the homunculus unfolds and increase in size.
Some other scientists like Leeuwenhock(1677) believed that the preformed embryo is inside the sperm. These scientists are called animalculists( animalcule is the old name of sperm).
In parthenogenesis, the egg develops into an animal without the assistance of spermatozoon. if the egg can develop without fertilization, the embryo cannot be preformed inside the spermatozoon. However, this theory not at all accepted.
Theory of Epigenesis:
This theory was proposed by Wolff in 1759. Wolff's explanations are based on the observation of the development of chick embryos. It is an antithesis of preformation theory. This theory states that there is no preformed embryo inside the egg. The egg is formed of only granules during development are arranged to form cellular layers called germinal layers. These layers grow thick in some parts, grow thin in some other parts and turn into pockets in other parts and at last become the body of an embryo.
Particulate theory:
Maupertius stated that the semen produced by the parents contained many tiny particles. During copulation, the semen of both parents united and the embryo is formed. Each organ of the embryo was formed by the fusion of two types of particles produced by the parents.
Pangenic Theory or theory of pangenesis:
This theory was proposed by Darwin. He assumed that each cell of the body of an organism produced a minute copy of its own called gemmules or pangenes. The gemmules were liberated into the blood and were deposited in the testes and ovaries. Thus the gonads were the store houses for the various types of gemmules. The gemmules were then given to the gamets. The young one formed from the gametes would be having all the gemmules characteristic of the parents.
Germplasm theory:
This theory was proposed by August Weismann. It explains both heredity and development of an animal. According to Weismann, the body of an organism is formed of two types of cells, namely somatic cells and germ cells. The main bulk of the body is formed of somatic cells. The somatic cells disappear with the death of animal. So any change affecting these cells is not heritable. The reproductive cells(gamets) are said to be germ cells.
The germ cells are carried to the descendants generation after generation. So any change affecting these cells is inherited. Each part of an animal is represented in the sex cells by a separate particle called determinant. It is located in the chromosome of the nucleus just as the modern genes. The sum total of determinants would represent the various parts of the adult organism. The complete set of determinants would be handed down from generation to generation and the transmission of determinants would account for the transmission of hereditary characters.