In genetics, the mutation rate is the chance of a mutation occurring in an organism or gene in each generation. See Luria-Delbruck experiment. The mutation frequency is the number of individuals in a population with a particular mutation. This is important in fields such as evolutionary biology and oncology.

If the mutation rate of a gene is assumed to be constant (clock like) the degree of difference between the same gene in two different species can be used to estimate how long ago two species diverged (see molecular clock). In fact, the mutation rate of an organism may change in response to environmental stress. For example UV light damages DNA, which may result in error prone attempts by the cell to perform DNA repair.

The human mutation rate is higher in the male germ line (sperm) than the female (egg cells), but is generally on the order of 10^-8 (1 in 100 million) per nucleotide per generationNachman, Michael W. & Crowell, Susan L. 2000. Estimate of the Mutation Rate per Nucleotide in Humans. Genetics 156, 297-304.. Specifically, mutation rate in eukaryotes is in general 10^-4 to 10^-6, and for bacteria and phages the rate is 10^-5 to 10^-7 per gene per generationPeter J. Russell, Fundamentals of Genetics, Addison Wesley, ISBN 0-321-04868-7, 412.

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