12th Class Biology Principle Of Inheritance And Variation 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 :

• Reshuffling of genes/chromosomes by chance separation of chromosomes
• Crossing over
• Chance combination of chromosomes during meiosis and fertilization.

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.

(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 :

(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.