Introduction
The science dealing with the study of mechanism of heredity and the causes of variation in all living beings is called as Genetics. The word Genetics was derived from the Greek root word ‘gen’ which means to grow. The term Genetics was coined by Bateson in 1906 for the study of physiology of heredity and variations. Basically genetics is the study of two contradictory aspects of the nature i.e. Heredity and Variation.
Heredity: Because of this phenomenon of heredity, offspring’s of all living organisms resemble their parents in several aspects. Hereditary exactly gives the meaning “like produces like”, all living organisms have a tendency to produce offspring’s like themselves.
Hence, heredity may be defined as the transmission of characters from one generation to other/from parents to their offspring’s via gametes in sexual reproduction or via some asexual reproductive bodies in asexual reproduction. These transferable characters are called “hereditary characters”. Thus, heredity is the cause of similarities between the offspring’s, so that the individuals of the same parents resemble each other in most of the aspects.
Variation: Though offspring’s receive all the characters from their parents, they are not exact copies of their parents. Differences are found even between two offspring’s of same parents. The progeny differs not only among themselves but also with the parents. These differences are called variations. Thus, variations may be defined as the visible differences between the parents and the offspring’s or between the offspring’s of same parents.
Genetics explains the mechanism and the basis for both similarities and differences between related individuals. Genetics also tries to explain the phenomenon of evolution of cytodiffrentiation.
Historical aspects of Genetics
About 6000 years ago people kept records of the pedigrees of domestic animals like horses, dog and food plants like rice. Still, the science of genetics is very young compared to other biological sciences. The origin of genetics can be traced back to 19th century when Mendel started his work. Before the initiation of the work by Mendel, people had an unclear knowledge about genetics and causes of heredity. The ideas and theories which have developed from time to time to explain the phenomenon of inheritance can be grouped into the following headings:
Vapor and fluid theories
- Around 500 BC, Pythagoras proposed that every organ of the animal body gives out some type of vapor and these vapors unite to form a new individual.
- Around 400 BC, Hippocrates believed that reproductive material is handed over from all parts of the body of an individual, so that the characters are directly handed over to the offspring.
- Aristotle in 350 BC thought that semen of man has some stimulating effect and he considered it as highly purified blood. According to him, mother provides inert matter and father provides motion to the new life.
Preformation theories:
The theories which advanced the concept of presence of preformed embryo in the sex cells are known as Preformation theories. Preformationists have often been divided into two schools,
Ovists- Ovists give more importance to the Egg/Ova. They believed that homunculus is a small humanoid creature present in the ovum.
Animalculists or Spermatists- Animalculists give more importance to the sperm. They believe that homunculus is a small humanoid creature present in the Sperm.
- Leonard da Vinci and Robert de Graff proposed that male and female parents contribute equally to the heredity of the offspring.
- William Harvey speculated that all the animals arise from eggs and that semen only plays a vitalizing role.
- Malpighi concluded that development of any organism is simply the growth of the preformed part.
- Antonie van Leeuwenhoek experimented on the sperms of several animals and suggested their association with corresponding eggs.
Finally, K F Wolff disproved the Preformation theory by proposing that neither egg nor the sperm has a structure like homunculus. He said that gametes contained undifferentiated living substance capable of forming the organized body after fertilization. This idea formed the basis of epigenesis. Wolff believed that tissues and organs develop de novo due to unknown vital forces.
Particulate theories
- Pierre Louis Maupertuis proposed that in sexual reproduction, both the parents unite together to form a new individual. In this process, both the parents provide minute particles. He believed that when the particles of the male parent are dominant, then it gives rise to a male offspring. Similarly, when the particles of female parent are dominant, then it gives rise to of a female offspring. This concept is called biparental inheritance.
- Lamarck proposed the phenomenon of inheritance of acquired characters among the living organisms. But he could not give convincing evidences in support of his concept.
- Charles Darwin gave his famous theory of Pangenesis. It is completely based on particulate theory. According to this theory, each part of the animal body produces minute particles called as gemmules. These gemmules are initially collected in blood and then later concentrated in reproductive organs. When the animal reproduces, these gemmules are passed on to the new individual. The new individual has the gemmules combined from both the parents. This mechanism also explains the mechanism of acquired characters.
- Weismann postulated the theory of germplasm to explain heredity. According to germplasm theory the body of the organism has two types of cells namely somatic cells and reproductive cells. The somatic cells are contained in the somatoplasm. The somatic cells form the body organ while the reproductive cells contained in the germplasm, forms the gametes namely Sperms and eggs. As per Weismann the germplasm can form the somatoplasm but the somatoplasm cannot form the germplasm. Thus, changes in somatoplasm caused by environment cannot influence the germplasm. To prove this, Weismann experimented on the mice. He cut the tails of the mice for many generations, but still he always got a tailed mice offspring. This particulate theory formed the central core of modern understanding of genetics.
- Gregor Mendel is the founder of modern concept of particulate theory. He conducted experiments on Pea plants and concluded that inheritance is governed by certain factors which occur in the cells of the parents. But he failed to explain the exact process by which these factors are passed on.
Rediscovery of Mendel’s laws
With help of the landmark investigations in the fields of Cytology, Embryology and biochemistry, the science of Genetics has received a solid support. Three plant breeders Hugo de Vries, Karl Correns and Erich Tschermak rediscovered the Mendel’s Laws in 1900. Each of them reached similar conclusions before they knew of Mendel’s work.
- Bateson in his book “the principles of heredity” explained the terms Allelomorphs, homozygote, F1, F2 and Epistatic gene. He also published Mendel’s work in English and extended to animals as well. He coined the term Genetics in 1905.
- R C Punnett devised Punnett’s square for making theoretical gametic combinations.
- Walter Sutton in 1902, gave chromosome theory of heredity and postulated that Mendel’s hereditary factors were physically located on the chromosomes.
- T H Morgan in 1911, proposed the theory of linkage. He turned the chromosome theory of inheritance into the concept of genes being located in a linear fashion on each chromosome.
- Janssens in 1911 described the cytological basis of crossing over.
- Muller and Stadler independently discovered X-rays induced mutations.
- Bridges discovered the phenomenon of non-disjunction in Drosophila and also proposed Genic Balance mechanism of sex determination on Drosophila.
Prior to the discovery of chemical structure of the genetic material, the gene was an abstract indivisible unit of heredity. This period in history is referred to as classical or formal genetics. Then the era of molecular genetics followed the discovery of DNA structure when the fundamental unit of heredity was determined to be DNA nucleotide and the gene was found to consist of aggregate of nucleotides.
- Nirenberg and Khorana in 1968 discovered complicated DNA code known as Genetic code.
- Hedges and Jacob used the term Transposons for a DNA segment which could move from one molecule to another and carried resistance for antibiotic ampicillin in bacterial cells.
In late 1970’s the science of genetics entered a new era dominated by the use of Genetic Engineering. With the help of this technology it is possible to transfer the genes from mammals to bacteria, making the bacteria as tiny factories for producing proteins of choice. With the help of Genetic engineering it is now possible to produce various blood clotting factors, complement proteins and other substances for the improvement of Genetic deficiency diseases. Other prospective fields of genetic research are oncogenes helpful in cancer treatment, Antibody diversity helpful in immune-genetics, homeotic mutation and behavior.
- Who is the father of Genetics?
- Discuss various concepts included in vapour fluid theories, Preformation theories and Particulate theory.
- Write about the following: (a) Germplasm theory, (b) Pangenesis theory and (c) Preformation theory
- What is homunculus and what is it significance?
- Bring out various concepts included in Particulate theory.
- Discuss the rediscovery of Mendel’s laws?
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