Mendelian Genetics and Heredity
Genetics and Heredity
It is the branch of biological sciences which deals with the transmission of characteristics from one generation to the next and also the action of hereditary units called genes as they bring about the . characteristics which they control.
Modern genetics is concerned with the study of genes. The term genetics was coined and used for the first time by W. Bateson (1905). Genetics deals with the principles and mechanism of biological inheritance which includes both ‘heredity and variations’. ‘Heredity’ indicates ‘Like Begetes Like’ i.e., offsprings tend to resemble their parents. It is the transmission of characteristics, structural functional and behaviouristic, from the parents to the offspring or from one generation to another.
Variations, on the other hand, tell us about differences in characteristics, between one individual to the other. In case of sexually reproducing organisms, there is ample opportunity for genetic variations to arise. Two processes, occurring during meiosis and the fusion of gametes during fertilization provide the means of introducing unlimited genetic variations in the population. The basic principles of genetics were discovered by Gregor Johann Mendel. Mendel is therefore, called Father of Genetics.
Types of variations
Somatic or somatogenic variations:
These variations effect the somatic cells of an organism. They are also called modifications or acquired characters because they are acquired by an individual during its life time. They are caused by three factors:
Environment factors : The environmental factors such as light, temperature, nutrition, wind and water supply, etc., bring about changes in the plants as well as animals.
Use and disuse of organs : The continuous use of organs make them stronger and disuse make them weaker. For example, a player who exercises daily have a strong muscular body as compared to one who does not perform exercise.
– Conscious efforts: Modifications due to conscious efforts are observed only in those animals which have intelligence. Receiving education, slim bodies, boring of pinna, small feet and bonsai, etc., are some examples of conscious efforts.
Germinal of blastogenic variations:
They are inheritable variations formed mostly in germinal cells which are either already present in the ancestors or develop as new due to mutations. These are two types: continuous and discontinuous.
– Continuous variations : They are fluctuating variations which oscillate due to race, variety and species.
– Discontinuous variations: These are mutations which are sudden, unpredictable, inheritable, not connected by any intermediate stages. These variations are the source of all germinal variations.
Pre-Mendelian concepts of heredity:
Genetics is barely around a century old.The first marked pioneer and experimental work in this field was by Gregor Johann Mendel, but it was established as a distinct branch in 1900, when Mendel’s findings were rediscovered by deVries, Correns and Tschermak. Even before the foundation of this science, man had some vague idea of some general aspects of heredity.
Vapour and fluid theory
• Early Greek philosopher, Pythagoras (500 B.C.) proposed the theory that some moist vapours are given out from the brain, nerves and other parts of the body of male during coitus. On account of these vapours, the offsprings exhibit similarities with the male parent.
• Another Greek philosopher Empedocles suggested that both parents produce semen which arises directly from its various body parts. The semen from both parents mixes and produces a new individual.
• He suggested that the semen of male is highly purified blood and possesses life-giving power. The semen of females is their menstrual fluid which furnishes nourishing material to the developing offspring.
• Aristotle’s theory was accepted for about 2,000 years. The medical books of 17th century contain illustrations showing stages of coagulation of the embryo within the uterus from a mixture of semen of the parents.
An important stage in the discovery of the true physical basis of heredity was reached in seventeenth century following the invention of microscope. Anton Van Leeuwenhoek observed sperm and ova under microscope and also noted their fusion in fishes. With these observations, he concluded that sperms furnish the life and ova provide place for the nourishment and development of the embryo.
• Scientist Malpighi propounded the preformation theory. This theory holds that the sex cells (sperm and ova) had the miniature copy of adults and the development of embryo was actually only the enlargement of parts that were already present in the sperm or egg.
• Some of the scientists of this time such as Swa mmerda m supported this theory; Hartsoeker (1694) and Dalempotius (1699) even imagined that they could see a miniature human being homunculus.
• The preformation theory persisted upto eighteenth century until it was rejected by a German Scientist Wolff by providing detailed structure of ovum and sperm in chick.
• French biologist Maupertuis (1689 – 1759) proposed that the body of each parent gives rise to minute particles. In sexual reproduction, these particles from both the parents unite together to form the daughter individual.
. During latter part of eighteenth century, a German biologist Caspar Friedrich Wolff proposed that the germ cells contain define but undifferentiated substances which, after fertilization, become organized into various complex body organs which form the adult.
• This idea was referred to as epigenesis. The theory of epigenesis is similar to our present day gene concept of heredity.
. Charles Darwin propounded pangenesis theory. According to this theory every cell, tissue and organ of animal body produces many minute particles known as gemmules or pangenes.
• These gemmules are discharged in the blood stream and are deposited in the reproductive organs. The reproductives cells contain these pangenes and a child develops as a result of blending of the pangenes from two parents.
Weismann theory of germplasm
• August Weismann (1889) suggested the theory of continuity of germplasm.
• He referred to the reproductive cells as germplasm and to rest of the body as somatoplasm.
. The germplasm forms the bridge of life and passes from one generation to the next. In each generation it also produces the somatoplasm.
GREGOR JOHANN MENDEL’S work on Genetics and Heredity
• Gregor Johann Mendel proposed the theory of inheritance.
• From 1856 to 1864 (8 years); Mendel conducted breeding experiments on garden peas (Pisumsativum) in the garden of his monastery and was luckily the first to formulate clear-cut principles or laws of heredity.
• His paper “Experiments on Plant Hybridization”
was published in the fourth volume of “Annual Proceedings of Natural History Society of Brunn” in 1866.
• However, Mendel’s work remained unnoticed and unappreciated for some 34 years. This is because of the following:
– Limited circulation of the “Proceedings of Natural History Society of Brunn” in which it was published.
– Mendel’s conclusions about heredity were ahead of his time.
– He could not convince himself about his conclusions being universal since Mendel failed to reproduce the results on Hawk-weed (Hieracium) undertaken on the suggestion of Nageli.
– Lack of aggressiveness in his personality.
• It was in 1900 that three workers independently rediscovered the principles of heredity already worked out by Mendel. They were Hugo de Vries of Holland, Carl Correns of Germany and Erich von Tschermak of Austria.
• The process of identification of a particular gene responsible for a particular biological process is called genetics dissection. Mendel is regarded as a first genetic surgeon.
Reasons for Mendel’s success
(i) Mendel’s success, where other had failed can be attributed, at least in part, to his elegant model of experimental design and analysis. Mendel’s selection of garden pea was evidently not an accident, but the result of long insightful thought.
(ii) He chose garden pea that was easy to grow and to hybridize artificially. The pea plant is self fertilizing in nature, but it is easy to cross-breed experimentally. The plant reproduces well and grows to maturity in a single season.
(iii) Mendel then chose to follow seven visible features (unit characters), each represented by two contrasting forms or traits.
(iv) Mendel was fortunate in choosing a diploid plant because diploid organisms contain only two sets of chromosomes. If he had chosen a polyploid organism, an organism with more than two sets of chromosomes, he would not have obtained simple, understandable results.
(v) Through many generations of natural self- fertilization, garden peas had developed into pure line. A single alteration in a trait was therefore demonstrated by a visible difference between varieties.
(vi) He restricted his examination to one or very few pairs of contrasting traits in each experiment.
(vii) He also kept accurate quantitative records, a necessity in genetic experiments. From the analysis of his data, Mendel derived certain postulates that have become the principles of transmission human genetics.