Class 12 Genetic Basis of Inheritance NEET AIIMS
Genetic Basis of Inheritance
Throughout history, genetics has had a profound effect on humankind. As knowledge in the discipline has grown, many different issues have arisen that have led to controversies at the interface between science and society. Because genetic processes are fundamental to the comprehension of life itself, the discipline of genetics is thought by many to sit at the centre of the field of biology. Genetic information directs cellular function, largely determines an organism’s external appearance, and serves as the link between generations in every species. Genetics unifies biology and serves as its “Core”. Thus, it is not surprising that genetics has a long, rich history.
The term genetics (Gk. genesis = descent, to generate,
or to become, or grow into) was introduced by W. Bateson (1905) for the branch of Biology dealing with heredity and variation. Heredity and variation are two important aspects of the life of the organism and organic evolution.
Heredity (L. hereditas = heirship or inheritance; Spencer, 1863) is the transmission of characters from parents to offspring. The occurrence of heritable or nonheritable differences among individuals of a population is called variation. The chief sources of variation among the characteristics of related organisms are genic differences or environmentally induced differences. The variation in sexually reproducing organisms can easily be observed.
l, In order to make a definite distinction between hereditary and environmental variations, Johannsen (1909), formulated the genotype-phenotype concept. According to him, the genotype of an individual represents the sum total of heredity. On the other hand, phenotype represents features, which are produced by the interaction between genotype and environment.
l A genotype can thus exhibit different phenotypes under different conditions. This is referred to as the individual’s norm of reaction to the environment. Therefore, similar genotypes may not have the same phenotype. Conversely, similar phenotypes do not necessarily mean the same genotype.
l As an interesting case, we may consider the example of Himalayan albino rabbits, which are characterized by black colour on feet, ears, nose and tail, the remaining body being white. If hairs from the white part were plucked and the rabbit was allowed to stay in a cold place, the developing hairs were found to be black rather than white. Such changes due to different environments are normally of an adaptive nature.
l When two genotypes produce the same phenotype due to different environments, one is called the phenocopy of the other, because they differ genotypically.
l Phenocopy (Goldschmidt,1935) is a nowhere dietary phenotypic modification, caused by the special environmental condition, that mimics a similar phenotype. For instance, in Drosophila melanogaster, normal body colour is brown and a heredity variant has a yellow colour. It was observed that brown and yellow flies reproduce sincerely, irrespective of changes in the environment. Cases are known where normal larvae (brown), when raised on food containing silver salts, develop into yellow flies. This is a phenocopy of yellow mutant but would give rise to brown flies in a normal environment.
Pre-Mendelian Ideas About Inheritance
l A number of viewpoints were put forward before Mendel to explain the transmission of characters from parents to offspring. They are often called theories of blending inheritance as they believed that characters of the parents blended or got mixed during their transmission to the offspring.
l Moist Vapour Theory: Pythagoras (580-500 B.C.) believed that each organ of the body produced moist vapours which formed the body parts of the embryo.
l Fluid Theory: Empedocles (504-433 B.C.), proposed that each body part produced a fluid. The fluid from different body parts of the two parents mixes up and is used in the formation of the embryo.
l Reproductive Blood Theory: Aristotle (384-322 B.C.) thought that the males produced highly purified reproductive blood containing the nutrients from all body parts. Females also produce reproductive blood but this is impure. The two reproductive types of blood coagulate in the body of the female and from the embryo. Due to the purity of reproductive blood, the contribution of characters by the male is more than the female.
l Performation Theory: The theory of preformation believes that the organism is already present in the sperm or egg in a miniature form called a homunculus. Fertilization is required to stimulate its growth. Sperms were observed for the first time by Leeuwenhoek, in 1672. The performative theory was advocated by Malpighi (1673). It was supported by Roux as late as 1888.
l Theory of Pangenesal: Maupertuis (1698-1957)
considered that heredity is controlled by minute particles which come from all parts of the body to the reproductive organs. The particles combine in the embryo and multiply there. Certain particles of one parent can dominate over those of others so that the offspring resembles one parent in certain characteristics and the second parent in others.
l Darwin (1868) modified the above theory in the light of cell theory and information available about embryology. He thought that every somatic cell and tissue of the body produces a tiny particle called gemmule or pangene which contains both the parental and the acquired characters. All the ‘gemmules or pangene’ of the body cells collect in the gametes and are passed on to the zygote offspring where they guide the growth of different parts of the embryo.
Objections to Blending Inheritance
l The trait of sex does not blend itself in unisexual organisms. Such an organism can either be male or female.
l Children of dark and fair coloured parents should be of intermediate colour if blending inheritance is true. This is not the case. The children are often of different colours, some fair coloured, some dark coloured and others of an intermediate colour.
l Many individuals show ancestral characters not found in their immediate parents. The phenomenon is called atavism (L. atavus – great grandfather, grandfather or forefather), reversion or throwback. For example, a short tail may be found in some babies. Some people are able to move pinna or external ear.
l Basis of Heredity: Mendel (1866) proposed that inheritance is controlled by paired germinal units or factors, now called genes. They are present in all cells of the body and are transferred to the next generation through gametes.
Factors or genes are thus the physical basis of heredity. Genes or factors are passed from one generation to the next or from one cell to its daughter cells in the form of chromosomes -the chromosomal basis of heredity. The genetic material present in chromosomes is DNA. Genes are segments of DNA called cistrons. Therefore, DNA is the chemical basis of heredity.
As early as 1760, a German Botanist Joseph Kolreuter, father of polygenic/quantitative inheritance, first crossed two species of tobacco and obtained the progeny. This process of crossing was called hybridisation and the progeny was termed as a hybrid. This work was followed by Knight (1799) and Goss (1822) in England, Gartner (1849) in Germany, Naudin (1862) in France and G.J. Mendel (1856) in Austria.
l Gregor Johann Mendel (1822-1864) was born on July 22, 1822, in Heinzendorf village, near the Austrian township of Brunn (now Burno in the Czech Republic) to a farmer’s family. In 1843 at the age of 21, he entered Augustinian monastery of St. Thomas in Brunn as a priest. There he earned the title, Gregor, in 1849. In 1846, he attended the courses of agriculture, pomiculture and viniculture at Philosophical Academy in Brunn. In 1849, he was appointed as a substitute teacher at Altbrunn Monastery in Austria. From 1851 to 1853, he studied mathematics and natural science. In 1856, he began hybridisation experiments on pea plants. In 1866, he became a founder member of the “Natural Science Society” of Brunn. In 1866, he published his paper “Experiments in Plant Hybridisation” in the Annual Proceedings of Natural Science Society.
Unfortunately, his remarkable piece of work on basic genetics remained unnoticed and unappreciated. Unrecognised and bitterly disappointed, Mendel died in 1884 of kidney failure.
Rediscovery of Mendel’s Work
(The Birth of Genetics) :
Mendel work was rediscovered in 1900 by three scientists :
l Hugo de Vries of Holland (working on evening primrose and maize)
Carl Correns of Germany (working on maize, peas and beans)
Erich von Tschermak of Austria (working in flowering plants, including peas).
In digging out old literature, the above three workers confirmed the works already done by Mendel and recognized his genius. As a mark of honour, he became famous as the “Father of Genetics”.
Correns raised the status of two Mendel’s generalisation to the level of laws of heredity -law of segregation and the law of independent assortment. Hugo de Vries also found out the paper of Mendel and got it published in ‘Flora’ in 1901.