Heredity definition biology
Heredity definition biology
The term genetics was coined by Bateson (1960). Genetics is the study of principles and mechanism of heredity and variations. The resemblance amongst offspring is never 100% (except in monozygotic twins) due to reshuffling of chromosomes and their genes. Same genetic material traits are present in monoparental individuals formed through asexual reproduction or mitosis. Such individuals are called ramets while the whole group of similar individuals is called clone.
What is 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 offsprings.
(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.
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 blanded, could never be seperated again. Pattern of inheritance shown by atavism also speaks against blending theory. The traits of sex do not blend in unisexual organisms.
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 pea 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. Gregor 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.
To excel in your Biology exam must refer NCERT text book and solve the questions given in exercise of NCERT for reference you can use Physics Wallah NCERT solutions for class 11 Biology . Prepared by Physics Wallah experts .