Monday, September 28, 2009

Variation and Kinds of Variation

Term variation means the differences in characteristics shown by organisms belonging to the same species. Organisms within the same species vary from one another variations may be

(1) Genetic Variations

(2) Environmental Variations

(1) GENETIC VARIATIONS

Variation between individuals are caused by genes. Most genes have different forms called alleles. In sexually reproducing organisms these alleles are reshuffled each time a new organism is produced. Reshuffling occurs during meiosis as a result of crossing over and independent assortment. The random fusion of gametes from two parent also produces new combinations of alleles in the offspring. Another source of genetic variation is mutation, crossing over, independent assortment and random fusion produce new combinations of alleles while mutations produce completely new alleles. Genetic variations are inheritable therefore when organisms produce they pass on some of their genes to their offspring genetic variations affect DNA of the organisms.

(2) ENVIRONMENTAL VARIATIONS

Some variations among individuals are not caused by their genes. Plants exhibit such variations. They may differ in size and colour of leaves because one may be growing in shade and nutrient deficient soil and other in light and nutrient rich soil. Such differences that arise during organisms life time are called environmental variations. These variations do not affect DNA of organisms, therefore are not inherited.

A study of phenotypic difference in any large population shows that two types of variation occur

(1) Discontinuous variation

(2) Continuous variation

(1) Discontinuous Variations: There are certain characteristics within a population which exhibit a limited form of variation. Variations in this case produce individuals showing differences with no intermediate forms. For example a person belongs to one of the four blood groups in ABO blood system, A, B, AB and O. There are no in-betweens and only a few clearly defined groups of individuals. Such variations are called discontinuous variations.

(2) Continuous Variations: The characteristics show complete gradation from one extreme to the other without any break. For example human skin or eye colour cannot be categorised into clearly defined colours. Continuous variations may be caused by genes or by environment of by both. Variation in leaf length is oxford ragwort plant is caused by environment entirely because all cells in the plant were produced by mitosis from a single zygote and so contain exactly the same genes.

Sources of variations

Genetic variation do not occur in asexually reproducing organisms as replication of DNA is perfect. Any apartment variation in these organisms may be due to influence of environment. There is opportunity for genetic variation to arise in sexually reproducing organisms.

The main causes of variation are:

(1) Gene Recombination

(2) Mutations

(1) Gene Recombination: Gene recombination originate as a result of crossing over during meiosis, random migration of chromosomes at the time of call division and random fusion of male and female gametes during fertilization.

(2) Mutation: The gene reshuffling do not generate major changes in the genotype, which are necessary to give rise to new species. These changes are produced by mutations.

A mutation is a change in the amount or the structure of DNA of an organisms. This produces change in the genotype which may be inherited by cells derived by mitosis or meiosis from the mutant cell. Mutation may result in the change in appearance of characteristic in a population. Mutations occurring in gamete cells are inherited, where as those occurring in somatic cells can only be inherited by daughter cells produced by mitosis. They are called somatic mutations.

A change in the amount of arrangement of DNA is known as chromosomal mutation or chromosomal aberration where as change in structure of DNA at a single locus is called point mutation or gene mutation. The term mutations is usually used to describe gene mutation.

Chemical Mutagens: Certain chemicals such as nitrous acid, hydroxylamine, dimethyl solfonate, methyl ethyl sulphate, acridive etc act as mutagens. In addition a variety of other chemical substances including mustard gas, Caffeine, formaldehyde, Colchinine, certain components of tocacco and increasing number of drugs, food preservatives and pesticides also cause mutations.

Induced Mutation: A mutation that is produced artificially during experimentation by using mutagens such as X-rays, ultra violet rays etc are called induced mutations.

Spontaneous mutation: The mutation which arises for no apparent reason and cause a genetic alteration is called spontaneous mutation. It is random and natural. It may occur at any site on a chromosome. It is found that majority of them produce just a slight effect and are harmful.

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