Variations are the individual differences in somatic or genetic level which due to the change in the genetic make up of the individual taking place during the recombination, segregation or during defective meiosis, or by induction through contamination. This changes in gene is due to which these variations appear is called mutation.

Thus mutation is defined as any sudden appearance of heritable variation in nature of any organism is known as mutation.

According to derives, mutations are sudden and drastic heritable changes not traceable or ascribable to segregation or recombination.

Mutation may be distinguished into genemutation, chromosomal mutation (Mutation in chromosome level due to change in chromosome structure) Ns geomantic mutation (mutation) due to changes in the basic chromosome number).

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Chromosomal Mutation:

Chromosomes are the structures with definite organisation of genes in linear order. Any alterations in the location or positions of gene or loss or addition of genes to the chromosome brings about alterations in gross chromosome structure called chromosomal mutation or chromosomal aberrations.

Chromosomal mutations take place at the depleting stage of meiosis at the time of crossing over due to breaking and reunion in abnormal manner.

There are four types of chromosomal mutations or aberrations.

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Deletions:

Number and position of genes on a chromosome is usually fixed. If a chromosome is broken in to several pieces and then healing or reunion of segments takes place, it may happen that two ends of the fragments unite together leaving one or more acentric parts free. Thus rejoined chromosome will be deficient of some of the genes.

Depending on the length of chromosome part lost, it is called either deficiency or deletion. If deficiency is large then it is called deletion. This deficiency may be interstitial or terminal.

It has both genetic and cytological effects i.e. lethal and pseudo dominance effects.

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Duplications (Additions):

Sometimes in the nucleus of a cell extra chromosomal parts may be found in addition to normal chromosomal complements.

Duplocatopms occur if a segment of the chromosome or a sequence of genes occur two or more times or repeatedly occur in a chromosome or chromosome pair is called duplicaton.

During meiotic pairing of the chromosome bearing the duplicated segments forms a loop to maximize the juxtaposition of homologous regions. In contrast to deficient loop the duplication loop is formed by the duplicated segments. Duplication cause more variability than the deficiency.

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Duplication may takes place due to an equal crossing over corresponding to deletion here new characters may be added but not lethal and has useful role in evolution of new species.

Translocation:

This is a type of inter chromosomal aberration which involves the shifting of a part of one chromosome to another non homologous chromosomes. If two non homologous chromosomes exchange their parts, which need not be of the same size there results a reciprocal translocation. The reciprocal translocation may be of following types.

Simple homozygous translocation:

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When a chromosome breaks in to two parts due to external and internal stress, open of the two segments of the broken chromosome may become attached to nearest chromosome which may not be its homolofue.

Heterozygotic translocation:

In heteozygotic translocation a considerable degree of meiotic irregularity occur. During meiosis an individual which is heterozyous irregularity occur. During meiosis, an individual which is heterozyopus for a reciprocal translocation must form a cross shaped configurantion in order to affect pairing of all homologous segments. This cross shaped configuration often opens out into a ring as dhiasma teriminalise. The meiotic product differs. Reciprocal translocation is like that of crossing over except that involves exchange between the segments of two non-homologous chromosomes.

Inversion:

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In this type of chromosomal aberrations the piece of chromosome which removed or delinked reinserted in reverse order, so that the sequence of genes alters with changes in the order of their arrangement but there is no loss genes.

Inversions may be percentric or paracentric inversions.

a. Pericentic inversion :

In this type the inversted segments include th kinetochores or centromeres. The kenetochore may move from middle to one end of chromosome, thus there is marked change in the appearance of chromosomes.

b. Paracentric inversion :

Here the inverted segment does not include the centromere or kinetochere. Centromeres lies outside the inverted segment.

Inversion can be evidenced by suppression of crossing over and possibly the appearance of mutation owing to position effect inversion does not involve no loss or gain of genes.

Role of chromosomal mutation in evolution:

Hugo de vries (1884-1935) propounded the mutation theory in 1901 to explain the origin of new species of living organism. He studied on prim-rose (Oevothera lamerc kiana) and marked sudden appearance of new characters which are inherited to next generation.

i. Oenothera lamarkiana ha 14 chromosomes but derives found some of the off springs have 15, 16, 20, 22, 24, 24, 28, 29 and 30 chromosomes. The new individuals different in flower size shape arrangement of buds and size of seeds.

ii. New elementary species appear suddenly and attain full constancy at once.

iii. The same new species are produced in large, number of individuals, this increase the chance of selection by nature.

iv. Mutability is something fundamentally different from fluctuating variability.

v. Mutations takes place in nearly all directions and may involve any character.

Example:

i. Inchlamydomonas, yeasta and in some fungi deficiency deletion mutations cause lithality in the individual help in selection.

ii. Duplication mutations in corn peas, etc have important impact on evolution of new species.

iii. Ring chromosome appear in datura pea wheat tradescantia as a result of translocation mutation which may cause evolution of semisterile individuals.

iv. Inversion mutations cause position effects which cause the alteration of biochemical reaction in the plant body.