A mutation is a change in the base sequence of DNA. Mutations are the only source of completely new alleles, so they are the original source of all genetic variation. Meiosis and fertilisation only shuffle alleles that already exist.There are two broad kinds. Gene mutations change the base sequence within a single gene. Chromosome mutations change the structure or number of whole chromosomes, and can affect many genes at once.Mutations happen spontaneously and at random, mostly as copying errors during DNA replication. The natural rate is low, but it is increased by mutagens. These include ionising radiation (X-rays, gamma rays), ultraviolet light, and some chemicals such as benzene and the tar in tobacco smoke.The effect of a mutation depends on where and when it happens:• A mutation in a body (somatic) cell affects only that individual and its descendant cells. It is not passed on to offspring, although it may lead to cancer.
• A mutation in a cell that produces gametes can be inherited, so it adds a new allele to the population.Most mutations are neutral (for example, in non-coding DNA, or they do not change the amino acid). Many harmful mutations reduce the chance of survival. A few are beneficial and give an advantage in a particular environment. These are the raw material on which natural selection acts, which is why mutation is so important in evolution.
• A mutation in a cell that produces gametes can be inherited, so it adds a new allele to the population.Most mutations are neutral (for example, in non-coding DNA, or they do not change the amino acid). Many harmful mutations reduce the chance of survival. A few are beneficial and give an advantage in a particular environment. These are the raw material on which natural selection acts, which is why mutation is so important in evolution.