INTRODUCTION
The term genetics was first coined by W. Bateson in 1905, although beginning of the science of genetics was made in 1900 by rediscovery of Mendel’s work.
The word genetics was derived from the Greek root ’Gen’ which means ‘to become’ or ‘to grow into’. Genetics is often described as a biological science, which deals with the principles of heredity and variation. Heredity refers to the transmission of characters from parents to their offspring.
The differences among the individuals of a species for a character constitute the variation. The variations are mainly of two types namely (i)heritable (genetic), (ii) non-heritable (environmental).
Heredity variation refers to differences in inherited traits. These types of variations are found not only in progenies of parent, but also transmitted from generation to generation.
The foundation of this new branch of biology – genetics was laid by Mendel in 1866 when he discovered the basic principles of heredity. However, Mendel’s findings came into light only in 1900, when similar results were obtained independently by three scientists, viz., Hugo de Vries, Carl Correns and Von Tschermak. Thus, genetics was born in 1900
Pre – Mendelian ideas about heredity
In ancient times, there were speculations on the nature of heredity. Early
philosophers and workers had forwarded various ideas or theories to explain the phenomenon of inheritance. They are briefly presented below.
(i) Vapour and fluid theories:-
Early Greek Philosophers like Pythagoras (500 B.C) proposed the theory that a moist vapour descends from the brain, nerves and other body parts of the male during coitus and from this, similar parts are formed in the uterus of the female forming the embryo.
Empedocles proposed the theory that each parent produces ‘a semen’ which
arises directly from various parts of the body. In embryos the various parts are formed by this semen.
Aristotle thought that the semen of man has some “Vitalizing” effect and he
considered it as the highly purified blood. According to him the mother furnishes inert matter and the father gives the life-giving power, “dynamic” to the new life.
(ii) Preformation Theory:-
The theory was proposed by two Dutch biologists, Swammerdam and Bonnet (1720-1793). This theory states that a miniature human called ‘homunculus’ was already preformed in the egg and sperm.
The development of zygote resulted only in the growth of a miniature human who was already present in the egg and sperm. However, this theory was rejected because this could not be proved scientifically.
(iii) Theory of Epigenesis:-
This theory was proposed by Wolf (1738-1794), a German biologist and it
states that egg or sperm cells do not contain miniature human but that the gametes contained undifferentiated living substance capable of forming the organized body after fertilization. This proposition was called as epigenetic concept and remained universally accepted.
(iv) Theory of Acquired characters:-
This concept was proposed by Lamarck (1744-1829), a French biologist. This
theory states that a new character once acquired by an 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.
(V) Theory of Pangenes:-
This theory was proposed by Charles Darwin (1809 – 1882), an English
naturalist. According to him, each part of the animal body produces a minute copy of its own, called gemmule or pangene. The gemmules are collected in the reproductive organs.
The gemmulues were passed on to the gametes. The young one formed from the gametes would be having all the gemmules characteristics of the parents, and will represent a blending of the qualities of its two parents. Thus, theory of pangenesis is also known as the ‘theory of blending inheritance’.
(vi) Germplasm Theory:-
This theory was advocated by August Weismann (1834-1914), a German
biologist. According to this theory, organism’s body contains two types of cells namely somatic cells and reproductive cells. The somatic cells form the body and its various organ systems, while the reproductive cells form sperm and ova.
The somatic cells contain the ‘somatoplasm’ and reproductive cells contain the ‘germplasm’. The germplasm can form somatoplasm, but somatoplasm can not form germplasm. The cells of the somatoplasm become differentiated during the formation of the complex organs of the body while cells of the germ cells remain undifferentiated and retain their power to generate new life.
The germplasm thus goes on in continuous stream from generation to generation. Changes in the somatic cells (somatoplasm), which were caused by the environment, cannot influence the germplasm and hence acquired characters are not inherited.
II. Pre- Mendelian experiments
Knight (1779) conducted experiments on pea much before Mendel but failed to formulate the laws of inheritance because he could not use the mathematics to his results. He crossed pigmented variety with unpigmented variety and F1 was pigmented. When F1 was selfed, F2 showed pigmented and non-pigmented plants. Since he did not keep record on different types, he could not discover the mechanism of inheritance.
J.Kolreuter (1733-1806), a German botanist performed hybridization
experiments in tobacco and compared the hybrids with their parents. He
demonstrated that the hybrids may resemble one or the other parent or may be intermediate between them. He also showed that both the parents make equal contributions to the hybrids.
Gartner (1772-1850) and Naudin (1815-1899) done experiments similar to
Kolreuter and they observed the similar results. However they could not apply mathematics to their results.
Historical Developments in Genetics:-
Gregor Johann Mendel (1822-1884) was an Austrian botanist who laid the
foundation of the science of genetics. He worked with garden pea (Pisum sativum) and formulated two important laws of inheritance, viz., (i) law of segregation and (ii) law of independent assortment.
For this pioneer work he is called ‘the father of genetics’. He presented a paper entitled “Experiments in plant Hybridization” before Natural History Society of Brunn in February 1865 which was published in the proceedings of the society in 1866.
However, his results were neglected for 34years.
Mendel died in 1884 and his work came into being after 16 years of his death in 1900 when same results were independently discovered by de Vries, Correns and Tschermak.
Developments in genetics and cytogenetics:-
1906 : Yule advanced the idea of multiple factor hypothesis to explain the
inheritance of quantitative characters. According to this hypothesis,
quantitative characters are governed by several genes with small
effects and the effects are in additive nature.
1908 : Hardy and Weinberg formulated the “Hardy Weinberg law” relating
genotypic frequencies to gene frequencies in random mating
populations.
1909 : Johannsen introduced the word gene.
1909 : Garrod’s book Inborn Errors of Metabolism published.
1909 : Nilson – Ehle provided the genetic explanation for the inheritance of quantitative traits.
1910 : Morgan (Nobel Prize 1933) established the sex liked inheritance of white eyes in Drosophilla melanogaster.
1911 : Morgan postulated the chromosomal basis of linkage.
1927 : Muller (Nobel Prize 1946) demonstrated that x rays are mutagenic.
1928 : Griffth’s discovery of transformation in Diplococcus pneumonia.
1931 : Creighton and McClintock (maize) and Stern (Drosophila) papers
appeared demonstrating that genetic recombination is correlated with
the exchange of morphological markers on chromosomes.
1932 : Knoll and Ruksa produced first electron microscope.
1940 : Oliver’s demonstration of recombination within the lozenge functional unit in Drosphila.
1941 : Beadle and Tatum’s work (Nobel Prize 1958) on Neurospora was
published, establishing the one gene-one enzyme concept.
1944 : Avery, Macleod and McCartty demonstrated that the pneumococcal “transforming principle” is DNA.
1946 : Lederberg (Nobel Prize 1958) and Tatum’s discovery of conjugation in bacteria.
1946 : Present day Electron microscope became widely used in biology, the ultra structure of the cell was studied.
1950 : McClintock’s (Noble Prize 1983) first paper on “transposable elements” in maize.
1952 : Hershey (Nobel Prize 1969) and Chase demonstrated that the genetic material of bacteriophage T2 is DNA.
1952 : Zinder and Lederberg’s discovery of phage mediated transduction in bacteria.
1953 : Watson and Crick (Nobel Prize 1962) worked out the – double helix
structure of DNA using the X ray diffraction data of Wilkins (Nobel
Prize 1962) and the base composition data of Chargaff.
1955 : Benzer’s first paper on the fine structure of the phage T4 rII locus.
1978 : Discovery of splicing of adenovirus RNAs in three different laboratories.
1982 : Publication of the complete 48502 nucleotide pair sequence of phage lambda by Sanger and colleagues.
1983 : Cech and Altman (Nobel Prize 1989) established the existence of catalytic RNAs.
1988 : Watson’s acceptance of job as coordinator of the “human genome project”.
1989 : NIH Recombinant DNA Advisory Committee recommends approval of first “human gene transplant” experiment.
1990 : Tsui, Collins and colleagues clone the “cystic fibrosis gene’ the gene
whose mutant alleles account for the majority of the cases of this
dreaded disease that affects about one out of every 2000 children in
the United States.