Mutations

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 in a gene pool.Gene mutations include substitution, insertion and deletion of bases. Unless a multiple of three bases is involved, insertions and deletions cause a frame shift, altering every codon after the mutation. Larger chromosome mutations change the structure or number of chromosomes. Mutations happen spontaneously during DNA replication, and the rate is increased by mutagens such as ionising radiation, ultraviolet light and some chemicals, for example those in tobacco smoke.Only mutations in cells that produce gametes can be passed to offspring and so enter the gene pool. A mutation in a body cell is lost when the organism dies.The Hardy-Weinberg principle assumes that no mutation occurs. A new mutation adds a new allele to the gene pool and changes allele frequencies, so the population is no longer in equilibrium. Because mutation rates are very low (for a given gene, typically around one in 100 000 gametes or fewer), mutation on its own changes allele frequencies very slowly. Its importance is that it supplies the new alleles that other processes, such as natural selection and genetic drift, then act on.Most mutations are neutral or harmful. A few are beneficial in a particular environment, and these may increase in frequency over many generations.