The science of genetics is the study of two contradictory aspects of nature that is heredity and variation.
- The process of transmission of the characters from one generation to the next generation is called heredity or the inheritance it causes the similarities between individuals.
- Variation is the cause of differences between individuals. This is the reason that brothers and sisters who resemble each other are still unique individuals.
Heredity and variations play an important role in the formation of new species. The biological science which deals with the mechanism of heredity and causes of variations in living beings is known as genetics. The word genetics means ‘to grow’ and it was coined by Bateson in 1906 for the study of physiology of heredity and variation.
The ideas or theories which have been forwarded from time to time to explain phenomenon of inheritance can be categorized under following headings:
- Vapour and fluid theories
Greek philosophers such as Pythagoras proposed that every organ of animal body gives out some type of vapours. These vapours unit to form new individual.
Hippocrates believed that reproductive material is handed over from all parts of body of an individual, so that the characters are directly handed over to the progeny.
Aristotele thought that the semen of man has some effect and he considered it as the highly purified blood. Thus father gives the motion of the new life.
- Preformation Theories
A. V. Leeuwenhoek in 1677 observed sperm of several animals including man and also suggested their association with egg. Theory was given by Swammerdam and believes that one of the sex cells either sperm or egg Contained within it. The entire organism in perfect miniature form that was called as Homunculus. This theory was supported by the Malpighi, Hartsoeker and roux (and also Dalepatius).
- Theory of Pangenesis
In 1868 the well known naturalist Charles Darwin gave his famous theory of pangenesis. Central idea of theory was given by Hippocrates. According to this theory each part of animal body produces many minute particles known as gemmules. These gemmules first collected in the blood and later on concentrated in the reproductive organ. When reproduces into new individuals, these gemmules pass on to it and it has blending of both parents by this mechanism acquired characters would also be inherited.
- Theory of pangenesis was disapproved by Galton and Weismann in 1882-1892 postulated the theory of germplasm to explain heredity. According to This theory body of organism have two type’s cells somatic cells and reproductive cell.
The somatic cells of body form the various body organs, while reproductive cells form sperm and ova. Somatic cell contains somatoplasm and germ cells have germplasm. According to Weismann germplasm can form somatoplasm but somatoplasm cannot form germplasm. Thus the change in the structure of somatic cells that caused by the Environment cannot influence the reproductive cells or germplasm. This rejected the pengenesis theory. Pattern of the inheritance shown by Atavism also speak against blending theory or pengenesis theory. In such cases grand children may exhibit a feature of an earlier generation not seen in parents.