Chapter-9: MUTATION

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Chapter-9: MUTATION - Patuakhali Science and Technology

DR. MD. ARIFUL ALAM

Associate Professor

Department of Fisheries Biology and Genetics

Definition

Mutation is a sudden change in genotype which involving qualitative or quantitative alterations in the

genetic materials itself. Mutation was first discovered by Wright in 1791 in lamb which had short legs.

Later on mutation was reported by Hugo de Vries in 1900. Mutation refers to any change in the base sequence of DNA i.e. heritable change in the genome. The most common change is a substituion, addition,rearrangement, or a deletion of one or more bases. A mutation need not give rise to a mutant phenotype.

Mutagen:

A mutagen is a physical agent or a chemical reagent that causes mutation. For example, Nitrous acid

reacts with some DNA bases, changing their identity and hydrogen-bonding properties. So, it is a

mutagen.

Characteristics of mutation

¾ Nature of change:

Mutations are more or less permanent and heritable changes in the phenotype of an individual. Such changes occur due to alteration in number, kind or sequence of nucleotides of genetic material, i.e., DNA in most of the cases.

¾ Frequency:

Spontaneous mutations occur at a very low frequency. However, the mutation rate can be enhanced many fold by the use of physical and chemical mutagens. The frequency of mutation for a gene is calculated as follows:

Where,

M= number of individuals expressing mutation for a gene and

N= number of normal individuals in a population

¾ Mutation rate:

Mutation rate varies from gene to gene. Some genes exhibit high mutation rate then other. Such genes are known as mutable genes. In some genome, some genes enhance the natural mutation rate of other genes. Such genes are termed as mutator genes. In some case, some genes decrease the frequency of spontaneous mutations than other genes in same genome, which are referred to as antimutator genes.

¾ Direction of changes:

Mutations usually occur from dominant to recessive allele or wild type to mutant allele. However, reverse mutations are also known.

¾ Effects:

Mutations are generally harmful to the organism. In other words, most of the mutations have deleterious effects. Only about 0.1% of the induced mutations are useful in crop development. In majority of cases, mutant alleles have pleiotropic effects.

Frequency of mutation =

M M+N

DR. MD. ARIFUL ALAM

Associate Professor

Department of Fisheries Biology and Genetics

¾ Site of mutation:

An average gene contains 500 to 1000 mutational sites.

¾ Type of event:

Mutations are random events. They may occur in any gene (nuclear or cytoplasmic), in any cell (somatic or reproductive) and at any stage of development of an individual.

¾ Recurrence:

The same type of mutation may occur repeatedly or again and again in different individual of the same population. Thus mutations are recurrent nature.

Classification of mutations

There occur numerous kinds of mutations among living organisms. They have been classified variously

by different Geneticists, each of which adopted different criteria. A brief classification of mutation is

described in below:

1. Depending on the kinds of cell in which mutation occurs, it is classified into following two types:

A. Somatic mutation:

Mutation in somatic cells is called as somatic mutation. The somatic mutation is not passed along to the next generation by sexual means. To maintain this mutation to the next generation, the individual containing the mutation must be cloned.

B. Germinal mutation:

When a gene is altered in a germ cell, the mutation is termed as germinal mutation. As germ cell gives rise to gametes and the gametes will carry the mutation and it will be passed on to the next generation when the individual successfully mates.

2. According to the mode of origin mutation may be

A. Spontaneous mutation: The mutation that occurs spontaneously is termed as spontaneous mutation. Most of the mutation that originally studied occurs spontaneously. B. Induced mutations: When mutations occur by treating an organism with mutagenizing agent is termed as induced mutation.

3. According to the direction, mutations are classified into following two types:

A. Forward mutations: Any change from wild type allele B. Backward or Reverse mutations: Any change from mutant allele to wild type

4. According to the survival (effects), mutations are classified into following four types:

Lethal: A mutation which kills the individual that carries it Sub-lethal: When mortality is more than 50% of individuals that carry mutation Sub-vital: When morality is less than 50% of individuals that carry mutation

Vital: When all the mutant individual survives

5. According to the site: Based on sites, mutations are classified into following two types:

Nuclear mutations: A mutation in nuclear gene

Cytoplasmic mutations: A mutation in cytoplasmic gene

6. According to the characters: Based on characters, mutations are classified into following two types:

Morphological mutations: A mutation that alters morphological characters on an individual Biochemical mutations: A mutation that alters biochemical characters on an individualquotesdbs_dbs2.pdfusesText_2

[PDF] Chapter-9: MUTATION - Patuakhali Science and Technology