Development of Genetics as a Branch of Biology

In this article we will discuss about the Development of Genetics as a Branch of Biology:- 1. Subject-Matter of Genetics 2. History and Development of Genetical Science.  

Subject-Matter of Genetics:

All living things have a tendency to produce young ones. The offsprings of all the organisms (plants and animals) resemble their parents in several respects. Why is it so? It is a question related to the phenomenon of heredity.

Heredity literally means “tendency of like to beget like”, i.e., all living things tend to produce young ones like themselves, for example, cow can produce only a cow; an elephant can produce a little elephant but never a tiger. Similarly bean, gram and mango plants can produce only bean, gram and mango respectively.

Sometimes new types of plants and animals develop when unlike parents are crossed. Is it not heredity then? Yes, it is a pure and simple case related to heredity. Thus heredity makes new types of organisms come out in some mating between two unlike parents.

Very often the offsprings are unlike the parents, as for example, sometimes deaf children are born from the parents with normal hearing. This is also due to heredity.

The offspring may be similar to their parents but they are never exact repetitions. They differ not only from one another but from their parents also in many characters.

These differences are called variations which may be of two types:

1. Variations which are inherited from one generation to other, and

2. Variations which are non-heritable and appear in the organisms when they grow in the changed environmental conditions, such as temperature, food, moisture, light and so on.

The branch of biological science which deals with heredity and variations is termed as Genetics. The word ‘genetics’ (a Greek word meaning to generate) was proposed by William Bateson (1902). This branch of science seeks to understand the laws governing the transmission of hereditary potentialities from parents to offsprings.

History and Development of Genetical Science:

It is difficult to imagine what ideas prehistoric men might have had about the phenomenon of heredity. Hippocrates (400 B.C.), Aristotle (350 B.C.) and other Greek philosophers believed that reproductive material came from all parts of individual’s body and characters were directly transmitted to the progeny, and thus according to them, the inheritance of the characters was direct.

The ideas of the other hypotheses were baseless and fantastic. Those philosophers initiated fantastic stories to explain reproduction and sex determination, such as, banana plants were supposed to have resulted from a cross between acacia tree (Babool) and palm; the giraffe was thought to be a hybrid between camel and leopard, ostrich (Suturmurga) was supposed to have developed when camel mated with sparrow.

Many such stories were repeated before seventeenth century.

The science of heredity and variation found its way to development when details of sexual reproduction were discovered. The reproductive parts of the plants were reported for the first time by N. Grew in 1682 and sexual reproduction in plants was described for the first time by Camerarious in 1694. A.D. Leeuwenhoek observed sperms of several animals in 1677 and suggested their association with eggs.

Credit also goes to Camerarious for developing for the first time a plant hybrid from a cross between hop plants and hemp. Thomas Fairchild (1717) obtained an artificial hybrid by cross-pollinating light red flowered Sweet Williams with pink flowered related species of Sweet Williams and noted that the characters of both parents (mother and father) developed in the offsprings.

The hybrid was called “Fairchild Sweet Williams” or “Fairchild mule”. His experiments were designed carefully and the results were actual and not accidental. Later on many other workers performed similar artificial pollinations between different related species and found, more or less, similar results.

J. Swammerdam (1679) suggested that the development of an organism is a simple enlargement of a minute preformed individual called homunculus which could be present in the sperm or in ovum (Fig. 12.1). This is called “preformation theory”.

Ovists thought that homunculus was present in the ovum while the animaculists insisted that preformed homunculus existed in the sperms. K.F. Wolff (1738-1794), a German scientist rejected the theory of preformation and suggested that the gametes contained undifferentiated living substances capable of forming organized body after fertilization.

This idea was called the “theory of epigenesis”. According to this theory many tissues and organs absent in the gametes develop subsequently denovo due to mysterious vital forces.

Linnaeus (1707-1778) believed that new plant species arose by natural hybridization. Joseph Koelreuter put it to the test of experiment during the years from 1760 to 1770. He made 500 different crosses and maintained the records up to second filial generation.

He observed in his experiments that the hybrids between plant varieties appeared either intermediate or resembled one parent or the other. He also noticed uniformity of all the hybrids in first filial generation of all the crosses.

In the second filial generation, the progeny showed tendency to reversion to original parental characters (segregation).

These observations led Kohlreuter to conclude:

1. That both male and female parents contribute equally to the formation of their offsprings, and

2. That since the hybrids tend to revert sooner or later to parental species they cannot form new species. Carl Gaertner in the years 1830s and 1840s repeated and expanded Koelreuter’s experiments. In 1849, he published an account of his 10,000 breeding experiments together with copious references to the works of others in his book in German.

Such crosses were made by Thomas Andrew Knight in 1780s, John Goss and Alexander Seton in 1820s, French botanist Charles Naudin. Those workers noticed the same mode of appearance of characters in the hybrid as had already been observed by Koelreuter.

A careful perusal of early literatures reveals that numerous hybridization experiments were performed during 18th and 19th centuries in search of the laws governing the mechanism of heredity but the results did not lead to any appreciable hypothesis.

Greek thinkers of eighteenth century were of the view that inherited characters of the individuals were acquired through direct contact with environment.

The French scientist Jean Baptiste Lamarck (1744-1829) formulated the theory of inheritance of acquired characters (Lamarckian theory or Lamarckism). In the theory, he stated that only those organs of the body developed well which were in great use over long period of time and the organs which were in less or no use became reduced and finally disappeared.

The theory was criticized by several workers because it failed to demonstrate the mechanism experimentally.

Gregor Johann Mendel (1822-1884) began his experiments of cross breeding of different varieties of garden peas in 1856 with the intention to discover the relation between hybridization and history of evolution of organic forms. He got the results similar to those obtained by his predecessors.

The serious mistakes which Mendel’s predecessors committed were:

1. That they considered several characters at one time. Due to that they got confused and could not draw any conclusion from the results they obtained.

2. That they neither grew all the hybrids they got from various crosses nor they could maintain the record of number of progeny of each kind.

Mendel realised this and found a statistical law for the hybrids of second filial generation. This was the rewarding advancement which enabled him to detect the underlying mechanism of heredity. He read his paper in 1865 before an unappreciative audience of Natural History Society in Brunn, but unfortunately his work failed to attract their attention and remained unrecognised in the life time of Mendel.

Mendel seems to have found the details of all early experiments in crossing of varieties of edible peas which had already been carried out by Knight (1789), Goss (1820) and others. Mendel’s work was overlooked for 34 years. It was rediscovered by three independent workers De Vries, Correns and Tschermak in the year 1900. The classical works of these workers ensured worldwide acknowledgment of Mendel’s works.

Darwin (1868) modified Hippocrate’s classical concept and suggested that all the cells and tissues threw off very minute granules called “gemmules” or “pangenes” and these multiplied, circulated throughout the body of organism and were transmitted to the germ cells (gametes), which thus contained a multitude of minute granules from each individual part of body.

This is called “theory of pangenesis” (Fig. 12.2). Darwin regarded these particles as bearers of hereditary characters and thought that they were responsible for the appearance of ancestral characters in the offsprings.

The above idea of Darwin could attract little support by the time Mendel’s work was rediscovered. Much more knowledge accumulated about the chromosomes, and the biologists began to realize the important roles of chromosomes in explanation of Mendelian laws.

The theory of pangenesis was disproved by Galton (1823-1911) and A. Weismann (1835-1934). Weismann demonstrated that pangenesis could not be verified. He cut the tail of mice for twenty-two generations and got tailed mice in all the generations.

The experiment of mutilation may appear crude, but results definitely contradicted the theory of pangenesis and Lamarckism.

Weismann proposed the “theory of germplasm” according to which:

(i) the body of an organism contains two types of cells; somatic cells having somatoplasm and reproductive cells or germ cells with germplasm,

(ii) The somatoplasm cannot form germplasm but germplasm is meant for reproduction and can form somatoplasm as well and

(iii) Changes in the structure of somatoplasm caused by environment (acquired characters) cannot influence the reproductive cells or germplasm whereas the changes occurring in germplasm influence the progeny.

The field of modem genetics started progressing fast when Thomas Hunt Morgan (1910) put forward his “chromosome theory of heredity”. The theory states that

(i) The chromosomes are the bearers of hereditary characters,

(ii) The genetic determinants (genes) for many characters are arranged along the chromosomes in definite linear fashion, and

(iii) The gene is a factor for determining one or more hereditary characters in all organisms.

T. H. Morgan made investigations on the nature of genes and received Nobel Prize in 1934.

After Morgan, De Vries and many other popular biologists opened new chapters in the history of genetics. In recent years, biochemistry provided new dimensions to both genetics and evolution. H.J. Muller got Nobel Prize in 1946 for the discovery of induction of mutations by x-rays.

Biochemistry has contributed several very important ideas concerning heredity which is named “Biochemical Genetics”.

This branch of science is concerned with the chemical and physical nature of hereditary material and the mechanism which governs the action of genes. G. W. Beadle and J. Lederberg have made outstanding contributions to biochemical genetics and got Nobel Prize in 1958. A. Kornberg and Ochoa got Nobel Prize in 1959 for their work on the chemistry of DNA and RNA.

Watson, Crick and Wilkins got Nobel Prize in 1962 for the discovery of double helix model of DNA. Jacob, Monad and Leowulf discovered regulator genes and operator genes which regulate the activity of structural genes. Nurenberg, Hargovind Khorana and Holley worked independently on the genetic code and for their outstanding work they shared the Nobel Prize in 1968.