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Definition of genetics, heredity, inheritance, cytology, cytogenetics; Brief history of developments in genetics and cytogenetics
Genetics
Genetics is the biological science which deals with the mechanism of heredity and causes of variations in living beings.
The word genetics was derived from the Greek root ‘gen’ which means ‘to become’ or ‘to grow into’ and it was coined by Bateson in 1906.
The science of genetics deals with the principles that explain the similarities between parents and their progeny and the differences among individuals of a single species.
Children resemble their parents is due to Heredity and they also differ from their parents to some extent in because of variation.
Heredity may be defined as the potential of an individual to transmit its characters to the offspring.
Variation are the differences that the individual acquires due to the interaction with the environment.
The main aim of genetics is the study of Heredity and variation, while heredity tries to maintain uniformity, variation brings in plasticity.
Cytology (Greek words, Kytos = hollow vessel or cell; logous = to discourse) or cell biology is the biological science which deals with the study of structure, function, molecular organization, growth, reproduction and genetics of the cells.
Cytogenetics is a branch of genetics that correlates the structure, number and behavior of chromosomes with heredity and variation.
Spontaneous generation
An organism originates from a preexisting organism of similar kind.
Thus, organisms do not arise or originate from non-living matter.
But this was not universally accepted till the middle of 19th century.
Many biologists believed that some primitive organisms originated from non-living matter e.g. decaying organic matter. Such origin of living organisms from non-living materials is known as spontaneous generation.
Aristotle (384-322 B.C) the greatest biologists of early days, believe that not only plants but animals like fleas, mosquitoes and snails originated from decaying organic matter.
But later, Lois Pasteur proved that, organic matters decay due to the presence of microbes in them and that microbes do not originate spontaneous from the organic matter. (Pastern-Father of Microbiology).
Earlier concepts of heredity:
Earlier concepts of heredity made their beginning first with the discovery of sexuality in organisms, both plants and animals.
The invention of the microscope gave a great impetus to probe into the world of microorganisms and get a visual knowledge about the link between one generation and the other bringing about heredity transmission of characters.
Reproductive organs in plants were reported for the first time by Grew in 1682.
It was Cameratious however who for the first time described sexual reproduction in plants. In 1717 Fairchild produced a hybrid having characters of both the parents. The hybrid was called 'Fairchild's mule'
Preformation theory:
The discovery of sexuality revealed the physical link between one generation and another, thus providing a physical basis for the transmission of heredity from parents to progeny.
The homun calus theory of Swammerdam suggested that the development of an organism is a simple enlargement of a minute but completely preformed individual.
The preformed individual or homunculus could be present either in the sperm or in the ovum.
Subsequent studies, later clearly showed that the Preformation was wrong, but the entire Preformation theory was nothing but a figment of imagination.
Epigenesis theory:
Wolf (1738-1794) who opposed the Preformation concept, opined that, neither the egg nor the sperm had a preformed organism, but both of them have undifferentiated mass of living substance which developed into the organisms after fertilization.
According to this theory called epigenesis new organs and tissues are formed denovo (from the mass of living substances in the gametes) due to vital forces.
Pangenesis theory:
Hippocrates (400 BC) believed that reproductive material is produced from all parts of the body so that all the characters are represented in the progeny.
Charles Darwin (1809-1882) was also a Votary for this type of inheritance. He modified the view of Hippocrates slightly and called it the theory of progenies.
According to this all parts of the body produce invisible gemmules or pangenes which are carried into sex organs via blood stream.
There the pangenes are assembled into gametes. During fertilization, pangenes of mother and father combine to produce the character of the offspring.
Lamarckian theory :
J.B. Lamarck (1744-1829) of inheritance, often called inheritance of acquired characters opines that, any new character acquired during the life time of an individual inherited.
It will be seen from the above that Charles Darwin's pangene theory in nothing but a version of Lamarckian theory.
Germplasm theory of Weismann:
August Weinmann (1834-1914) disproved the pangene theory with his experiments on rats.
He cut the tails of rats and observed the progeny for 22 generations. In every generation the rat progeny continued to have tails in spite of the fact that the parents were tailless.
If the pangene theory were to the correct, the tailless parents could not have sent the tail pangenes to the gametes and consequently the progeny should have been tailless.
Weismann proposed his 'germplasm theory' which is essentially correct even today.
According to his theory, the body of an organism can be divided into two somatoplasm and germplasm. Somatoplasm constitutes the vegetative body of the organism, while the germplasm constitutes the reproductive part.
Somatoplasm develops newly in every individual and there is no continuity while the germplasm is continuous and links the generations.
With the information present in germplasm, every individual develops its own somatoplasm. Hence any change or variation occurring in somatoplasm cannot the transmitted to the next generation, while those of germplasm can be inherited.
Historical developments in Cytology, genetics and cytogenetics:
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