Mutation:

A mutation is a physical or chemical change which changes the sequence of bases in DNA molecule. Anything which causes mutation is known as mutagen. A mutation consequences from ultraviolet light, ionisation radiations, viruses/chemicals. The alterations in sequence of bases in DNA are repaired by unique enzymes in the cell. There is a condition that if it is not, the protein generated has no biological activity and the cell dies.

These mutations frequently prove harmful and give rise to symptoms which cause diseases. Sickle-cell anaemia is one such instance. Such type of disease is passed on from one generation to the next generation.