Genetics – The Science of heredity (Inheritance)
“Genetics” is the branch of biological science which deals with the mechanism of transmission of characters from parents to offsprings. The term Genetics was introduced by W. Bateson in 1906. The four major subdisciplines of genetics are
1. Transmission Genetics / Classical Genetics – Deals with the transmission of genes from parents to offsprings. The foundation of classical genetics came from the study of hereditary behaviour of seven genes by Gregor Mendel.
2. Molecular Genetics – Deals with the
structure and function of a gene at molecular level.
3. Population Genetics – Deals with heredity in groups of individuals for traits which is determined by a few genes.
4. Quantitative Genetics – Deals with heredity of traits in groups of individuals where the traits are governed by many genes simultaneously.
Heredity and variation
Genetics is often described as a science which deals with heredity and variation.
Heredity is the transmission of characters from parents to offsprings.
The organisms belonging to the
same natural population or species that shows a difference in the characteristics is called variation.
Variation is of two types
(i) Discontinuous variation and
(ii) Continuous variation
1. Discontinuous Variation:
Within a population there are some
characteristics which show a limited form of variation.
Example: Style length in Primula,
plant height of garden pea. In discontinuous variation, the characteristics are controlled by one or two major genes which may have two or more allelic forms. These variations are genetically determined by inheritance factors. Individuals produced by this variation show
differences without any intermediate form between them and there is no overlapping between the two phenotypes. The phenotypic expression is unaffected by environmental conditions. This is also called as qualitative inheritance.
2. Continuous Variation:
This variation may be due to the combining effects of environmental and genetic factors.In a population most of the characteristics exhibit a complete gradation, from one extreme to the other without any break. Inheritance of phenotype is determined by the combined
effects of many genes, (polygenes) and environmental factors. This is also known as quantitative inheritance. Example: Human
height and skin color.
Importance of variations
• Variations make some individuals better fitted in the struggle for existence.
• They help the individuals to adapt
themselves to the changing environment.
• It provides the genetic material for natural selection
• Variations allow breeders to improve better yield, quicker growth, increased resistance and lesser input.
• They constitute the raw materials for evolution.