Mutation: Theory and Types | Genetics

In this article we will discuss about the theory and types of mutation.  

Theory of Mutation:

Mutation is random, minute and irreversible, measureable change in the gene of an organism or a cell, with the necessary implications that any phenotypic manifestation of the change may potentially or actually be shown by all descendant organisms or cells.

Mutation in somatic cells is of the same general quality as in germ cells. Somatic mutation is associated with the functional activities of a cell type while germ cell mutation is responsible for introducing variations in the offspring.

Causes of mutation are not definitely known but possibly atomic radiation, and different stresses, viz. chemicals, heat, etc. are mutagenic (mutation inducing).

Mutations are always random and may take place in any direction. They are mostly lethal or deleterious but some mutations are useful when organisms are placed in environments other than those in which they usually occur. Environmental changes disrupt the adaptedness of an organism and the harmony between the environment and the organism can be restored only through a genotypic reconstruction by mutation.

Mutation and selection occur simultaneously. Most frequently selection works against mutation and the rate of change, if the mutations are not lethal, will be very slow. But if a particular mutation, advantageous to the organism, and mutation rate is appreciable, the combined action of mutation and selection might well cause a rapid change.

Modern theories of evolution consider mutation to be the source of raw materials from which evolutionary changes may be constructed.

Types of Mutation:

Mutations are of two types:

(a) Spontaneous mutations. These occur in nature in animals or plants not subjected to any artificial stress.

(b) Induced mutations. These are produced when the organism is subjected to different mutagens, viz. X-rays, gama-rays, chemicals, etc.

Stages of occurrence of mutation:

Mutations may take place in different stages in the life of an organism and also at different locations.

(a) Gametic. It occurs prior to the maturation of gametes.

(b) Zygotic. It appears immediately after the fertilization of egg.

(c) Somatic. It occurs in somatic cells as a result of which the cells produced by the multiplication of the mutant cells become genotypically different from the non- mutant cells.

Mutational changes can be grouped into two major classes. The mutation proper is caused by chemical alteration of individual genes, and chromosomal aberrations leading to a grosser structural change involving destruction, multiplication or spatial arrangement of the genes in the chromosomes.

The chromosomal aberrations may be of the following types:

I. Numerical changes:

Affecting the number of chromosomes.

(a) Haploidy:

Higher organisms for the major part of their life are mostly diploid. Under experimental conditions some diploid organisms have produced haploid aberrants with a single set of chromosomes in the cells instead of the normal double sets.

(b) Polyploidy:

Normally diploid organisms in some case may produce forms having more than two sets of homologous chromosomes.

II. Structural changes:

Affecting the arrangement of the genes in the chromosomes.

A. Changes due to reduplication or loss of some genes:

(a) Duplication:

In addition to its normal location a section of the chromosomes may be present elsewhere. In the salivary gland cells of Drosophila certain sections of the normal chromosomes are represented two or more times in the haploid set.

(b) Deletion (deficiency):

A section of the chromosome containing one gene or a block of genes is lost resulting in a deficient chromosome.

B. Changes due to altered arrangement of genes:

(a) Translocation:

Two chromosomes may exchange parts giving rise to a new chromosome.

(b) Inversion:

Due to a rotation of 180° the arrangement of the genes in the chromosome may be altered.