A sudden random change in the genetic material of a cell that potentially can cause it and all cells derived from it to differ in appearance or behaviour (i.e. in phenotype) from the normal type. An organism affected by a mutation (especially one with visible effects) is described as a mutant. Somatic mutations affect the nonreproductive cells and are therefore restricted to the tissues of a single organism. These can lead to diseases such as cancer. Germ-line mutations, which occur in the reproductive cells or their precursors, may be transmitted to the organism’s descendants and cause abnormal development or early embryonic death.

Mutations occur naturally at a low rate but this may be increased by radiation and by some chemicals (see mutagen). Most are point mutations, which consist of changes in the nucleotide sequence of the DNA, but some (the chromosome mutations) affect the appearance or the number of the chromosomes. An example of a chromosome mutation is that giving rise to Down’s syndrome.

According to the neutral theory of molecular evolution, the majority of point mutations in DNA are neither useful nor harmful and can spread throughout a population by genetic drift. Mutations that alter phenotypes are generally harmful, but a very small proportion may increase an organism’s fitness; these spread through the population over successive generations by natural selection. Mutation is therefore essential for evolution, being the ultimate source of genetic variation. See also copy number; molecular clock; single nucleotide polymorphism.

http://www.dnaftb.org/27/ First of two topics on mutation from the online resource DNA from the Beginning