Mutation and Its Types

Variations may be defined as the process whereby closely related organisms come to differ amongst themselves. Variations are of utmost significance in organic evolution as without variation no change could occur. They may be small, gradual and continuous, but sometimes a sudden change in genetic constitution leads to large variations.




Mutation


Mutation is a phenomenon which results in alteration of DNA sequences and consequently results in changes in the genotype and the phenotype of an organism. Mutations in a broad sense include all those heritable changes which alter the phenotype of an individual. While working on evening primrose (Oenothera lamarckiana), Hugo de Vries (1901, 1903) used the term mutation to describe phenotypic changes which were heritable. In addition to recombination, mutation is another phenomenon that leads to variation in DNA.



Mutations may occur spontaneously in nature or they may be artificially induced. Spontaneous gene mutations are always occurring in nature. In about 10 million specimens of Drosophila melanogaster, a few hundred gene mutants were observed. These mutants produce true-breeding stable genotypes varying in characters like eye colour, wing type, etc. from the original wild type. The artificially induced mutations are similar to those produced spontaneously in nature due to the fact that the kinds of changes produced are alike to those created by spontaneous mutations.



Types of Mutations


Mutation is a heritable change in the structure of a gene or chromosome, or a change in chromosomes
number. Accordingly, mutations are of the three principal types (1) gene mutations, (2) chromosomal mutations or aberrations and (3) genomatic mutations or polyploidy.

1. Gene mutations. 

Hereditary characters are due to the effects of genes. Sometimes, a slight slip occurs in the replication of genes and this change in gene duplication is known as gene mutation. Thus gene mutations alter the information conveyed by a gene. The gene and the original gene are situated at the same fixed point on a particular chromosome. Since, a gene is located at a fixed point on the chromosome, a gene mutation is called point mutation.


2.Chromosomal mutations or aberrations.


The structural changes in chromosomes which appear phenotypically are known as chromosomal mutations or aberrations. These alterations do not involve changes in the number of chromosomes but result from changes in the number are sequence of genes on chromosomes. These changes were first analysed by H. J. Muller (1928) in Drosophila and by Barbara McClintock (1930) in Zea.


3. Genomatic mutation or polyploidy

They involve variations in chromosome number of a whole genome. These variations ( heteroploidy ) are mainly of two types :-
  1. Aneuploidy :- It is the presence of a chromosome number which is different than the multiple of basic chromosome number. This type of variation involves one or a few chromosomes but not the entire set.
  2. Euploidy :- Variations that involves entire sets of chromosomes are known as euploidy.

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