Genetics term was given by W. Bateson(Father of Modern Genetics).
- Genetics: Genetics is the study of principles and mechanism of heredity and variation.
Heredity: It is a process of transmission of traits from parents to their offspring’s either via asexual reproduction or sexual reproduction. These characteristics or traits are located on the chromosomes in the form of genes.
Variation: It is the degree by which progeny differ from their parents. Variation may be in terms of morphology, physiology, cytology and behavioristic traits of individual belonging to same species
Variation arise due to Reshuffling of gene/chromosomes, Crossing over or recombination, Mutation and effect of environment.
HISTORY OF RESEARCHERS IN GENETICS
Muller: Father of Actinobiology. Actinobiology is the study of the effects of radiation on living organisms.
Morgan: Father of Experimental genetics. He made experiments on Drosophila & proposed various concepts.
Gene theory: The Gene Theory is one of the basic principles of biology. The main concept of this theory is that traits are passed from parents to offspring through gene transmission. Genes are located on chromosomes and consist of DNA. They are passed from parent to offspring through reproduction.
To explain the like begets like (off springs are similar to their parents) several theories were given. They are collectively known as theories of Blending Inheritance
as they believed that characters of the parents blended or got mixed during their transmission to the offspring.
Some of them are as follows:1. Vapour fluid theory:
Greek philosopher Pythagoras
[500B.C.] proposed this theory. According to this theory, at the time of coitus of male and female, moist vapour secretes from the brain and due to this off springs are similar as their parents.
2. Semen theory:
This theory has given by Empedocles
. According to his view, the semen of male and female is mixed during coitus. Characters of parents appear into the off springs due to the mixture.
According to Aristotle
- a semen of male is considered as "highly purified blood"
which has power of life and it is nourished by semen of female.3. Preformation theory
: The theory of preformation believes that the organism is already present, that is preformed in the sperm or egg in a miniature form called homunculus
(Fig. 5.1). Fertilization is required to stimulate its growth. Sperms were observed for the first time by Leeuwenhoek, in 1672.
Preformation theory was given by Swammerdam (1679) and advocated by Malpighi (1673). It was believed by a number of workers of that period like Hartosoeker (1694) and Dalepatius (1694). It was supported by Roux as late as 1888 but discarded by Wolff who suggested that organs are formed step by step (theory of epigenesis). Fig: Homunculus - Spermist conception of a human sperm4. 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 chinese boxes. This was named as encasement theory.5. Epigenesis theory
: Wolff 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.6. Pangenesis theory
: The theory of pangenesis was described by C.Darwin
. This theory postulated that all part of a living body [tissues] synthesize "micro molecules."
These micro molecules are known as Pangene
Fig: C. Darwin
The male and female pangenes fuse together during the fertilization these are, further again distributed in the various organs of the body at the time of development.
7. Germplasm theory: This theory, advocated by August Weismann (1889), a German biologist, states that body tissues are of two types, viz., germplasm and somatoplasm. The germplasm refers to the reproductive tissues or cells which produce gametes.
The somatoplasm includes all other body tissues which are not related to sexual reproduction. Thus, transmission of characters from one generation to other takes place only through germplasm. Any change in the germplasm will lead to change in the next generation. This theory is accepted in a broad sense.