What is Mutation?

Mutation can be defined as a permanent change in cell's DNA. The changes in the DNA are reflected by the change expression of the character. If these changes occur in somatic cell, their impact is limited to the affected organism's life. However, any change in the mutation in the germ line-cell may become hereditary.
Kinds of mutation: there are two kinds of mutation.
Chromosomal Aberration:
These are mega or big changes, which involve:
Presence of an extra chromosome.
Loss of chromosome from the genome.
Deletion i.e. removal of one or a few bases from DNA.
Insertion i.e. addition of one or few base in DNA.
Inversion i.e. inverted reunion of the middle segment after breakage of a chromosome at two points, resulting in a change in sequence of genes or nucleotides.
Such mega changes leads to Down's syndrome etc.
Point mutation: these are alteration of one or few base pairs in the coding sequence of the chromosome. Such mutations occur by:
Spontaneous pairing error during DNA replication.
Damage to the DNA caused by mutagens, usually radiations and chemicals. Point mutation are the cause of sickle cell anaemia, phenylalanine.
Disease caused by point mutations:
Sickle cell anaemia: it is due to a change of amino acid glutamic acid by valine at position from N terminal end in haemoglobin beta chain. It reduces the ability to carry oxygen.
Phenylketonuria: in this disease, phenylketonuria is not degraded because of defective enzyme phenylalanine hydroxylate. Phenylalanine consequently accumulates in the cells leading to mental retardation, as the brain fails to develop at the infant stage.