Genetics: Principles of Inheritance and Variation

In this article we will discuss about:- 1.History of reserches in genetics 2.Heredity 3.Variation 4.Gene theory 5.Theories of Blending Inheritance

• Genetics term was given by W. Bateson. W. Bateson is Father of Modern Genetics.
• Genetics = Collective study of heredity & Variations.
• Heredity = Transmission of genetic characters from parent to offsprings.
• Variation = individuals of same species have  some differences, these are called variation.

History of reserches in genetics

Muller - Proposed the term "Cytogenetics" (Cytology + Genetics) Father of Actinobiology Actinobiology - study the effect of radiation on living organisms.

Morgan – Father of Experimental genetics He experiment on Drosophila & proposed various concepts.

Gene theory - According to gene theory ; genes are linearly located on chromosome.

Linkage term, Theory of sex linkage, Crossing over term, Criss - cross inheritance, Linkage map on Drosophila given by Morgan.

Garrod = Father of human genetics & Biochemical genetics. Garrod discovered first human Metabolic genetic disorder which is calledalkaptonuria(black urine disease). In this disease enzyme homogentisic acid oxidase is deficient. Gave the concept 'One mutant gene one metabolic block'  

VARIATION

PRE-MENDELISM

To explain the like begets like (offsprings are similar to their parents) several theories were given. They are collectively known as Theories of Blending Inheritance. 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 offsprings 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 offsprings 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 :- According to this theory germinal [reproductive] cell contains "a miniature figure of a man"

According to Swammerdam  preformation of a miniature of man is found inside the egg is called Mankin.

Those scientists  who believed in the hypothesis of Swammerdam are known as Ovists. On contrary, according to Hartsoeker preformed miniature of man is present in sperm.

A miniature of man is present in sperm, he called - Homunculus Scientists, who believed in  Hart soeker view, are called "spermist."

4. Encasement theory :- This theory was proposed by Charles Bonnet.

According to his view, body of female is just like a chinese box. The all future progenies are packed in the body of female like chinese box.

5. Epigenesis theory :- This theory was proposed by K.Wolf.

According to his view, germinal cells possess an undifferentiate material. This material develops step by step [gradually] after the fertilization. Such type development is known 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 or Gemmules.

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 :- The view was proposed by A.Weisman (1886).

According to him living body of an individual possess two different types of fluid material - Somatoplasm and Germplasm.

Somatoplasm does not participate in the formation of germinal cells.  Therefore, variations are not transferred into the progeny. somatoplasm is mortal because eventually it dies.