Genetic Variations
Category : NEET
Genetic Variations
Variations are differences found in morphological, physiological and cytological behaviouristic traits of individuals belonging to same species race and family. They appear in offspring or siblings due to: –
Types of variations
(1) Somatic variations: These variations influence the somatic or body cells. They appear after birth and are, also called acquired characters, modifications or acquired variations. Somatic variations are non-inheritable and usually disappear with the death of the individual. They are formed due to three reasons i.e. environmental factors, use and disuse of organs, and conscious efforts.
(i) Environmental factors: They have lesser effect on animals as compared to plants. Important environmental factors are as follows:
(a) Medium : Amphibious or emergent aquatic plants possess heterophylly, i.e. different types of submerged, floating and emerged leaves, e.g. Ranunculus aquatilis, Limnophila heterophylla and this meristic activities are due to change in depth and medium of water.
(b) Light: Partial shade causes elongation of internodes.
(c) Temperature: Plants of hot areas have extensive roots but smaller shoots. Human skin becomes darker with increase in environmental temperature.
(d) Nutrition: Honey bee larva feeding on royal jelly develops into queen while the ones obtaining ordinary nourishment (bee bread) grow into workers.
(e) Water: Water deficiency leads to several modifications in plants like succulent, spines, reduced leaves, thick bark, hair etc.
(ii) Use and disuse of organs: In higher animals and human beings, greater use of an organ leads to its better development as compared to other organs which are less used, e.g., stronger muscular body in a wrestler.
(iii) Conscious efforts: Acquired variations due to conscious efforts include education, training of pets boring of pinna, bonsai, etc.
(2) Germinal variations: They are inheritable variations formed mostly in germinal cells which are either already present in the ancestors or develop a new due to mutations. Germinal variations are of two types, continuous and discontinuous
(i) Continuous variations: They are fluctuating variations and also called recombinations because they are formed due to recombination of alleles as found in sexual reproduction. Darwin (1859) based his theory of evolution on continuous variations.
(ii) Discontinuous variations: They are mutations, which are ultimate source of organic variations. Discontinuous variations are caused by chromosomal aberrations, change in chromosome number and gene mutations. In pea seed coat colour changes gray to white is an example of spontaneous mutation.
Importance of variations
(1) Variations continue to pile up forming new species with time.
(2) They are essential in the struggle for existence.
(3) Adaptability is due to variations.
(4) Variations allow breeders to improve races of plants and animals.
(5) Discontinuous variations introduce new traits.
(6) Inbreeding between closely related organisms reduces variation.
Heredity
Heredity is the study of transmission of characters and variations from one generation to the next.
(1) Basis of heredity: Heredity involves the transfer of chromosomes from parents to offspring or one individual to another. Therefore, chromosome is the base of heredity. The physical basis of heredity are genes while chemical basis of heredity is DNA.
(2) Pre-Mendelian view points
(i) Vapour and fluid theory: Greek philosopher, Pythagoras proposed that some moist vapour is given out from the brain, nerves and all other parts of the body during coitus. On account of these vapours, the offspring exhibits similarities with the male parents.
(ii) Semen theory: Empedocles, suggested that both parents produce semen which arises directly from their various body parts. The semen from both the parents gets mixed and produces a new individual.
(iii) Preformation theory: Antony von Leeuwenhoek was the first to observe human sperms. This theory believes that one of the sex cells or gametes either sperm or egg, contained within itself the entire organism in perfect miniature form. Miniature form was called as 'homunculus'. The theory was supported by Malpighi, Hartosoeker and Roux.
(iv) Particulate theory: Maupertuis proposed that the body of each parent gives rise to minute particles. These particles unite together to form the daughter individual.
(v) Encasement theory: Charles Bonnet and his supporters presumed that every female contains within her body miniature prototypes of all the creatures which would descend from her, one generation within the other, somewhat like a series of chines boxes. This was named as encasement theory.
(vi) Theory of epigenesis: Wolgg proposed that the germ cells contain definite but undifferentiated substances, which after fertilization, become organised into various complex body organs that form the adult. This idea was referred to as epigenesis.
(vii) Pangenesis theory: This theory was proposed by Charles Darwin according to this theory every cell, tissue and organ of animal body produces minute invisible bodies, called gemmules or pangenes. They can produce offspring’s.
(viii) Weismann theory of germplasm: August Weismann (1889) suggested the theory of continuity of germplasm. He described reproductive cells as germplasm and rest of the body as somatoplasm. The germplasm forms the bridge of life between successive generations and is passed on from one generation to the next.
(3) Evidences against blending theory: Thus individual would represent the mixture of both the parents. The prevailing view of in pre-mendelian era was blending theory. The hereditary material was thought of as being analogous to a fluid. Under this concept, the progeny of a black and white animal would be uniformly grey. The further progeny from crossing the hybrids among themselves would be grey, for the black and white hereditary material, once blended, could never be separated again. Pattern of inheritance shown by atavism also speaks against blending theory. The traits of sex do not blend in unisexual organisms.
(4) Basic features of inheritance: In the middle of 18th century, Carolus Linnaeous a Swedish taxonomist and two German plant breeders Kolreuter and Gaertner performed artificial cross pollination in plants and obtained hybrid offspring. Kolreuter obtained experimental evidence that inherited traits tended to remain discrete, although his observations were similar to mendel but he was not able to interpret them correctly. Mendel's great contribution was to replace the blending theory with particulate theory. Few essential features of inheritance are: –
(i) Traits have two alternative forms.
(ii) Traits are represented in the individual by distinct particles which do not blend or change.
(iii) Traits may remain unexpected for one or more generations and reappear later unchanged.
(iv) Traits may remain together in one generation and separate in a later generation.
(v) One alternative of a trait may express more often then the other.
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