mutation
n. a permanent change in the genetic material of an organism. It may consist of an alteration to the number or arrangement of chromosomes (a chromosomal mutation) or a change in the composition of DNA, generally affecting only one or a few bases in a particular gene (a point mutation). Mutations can occur spontaneously, but many are due to exposure to agents (mutagens) that significantly increase the rate of mutation; these include X-rays and other forms of radiation and certain chemicals. A mutation occurring in a body cell (i.e., a somatic mutation) cannot be inherited, whereas a mutation in a reproductive cell producing ova or spermatozoa (i.e., a germ-line mutation) can be transmitted to an individual’s offspring. Most mutations either have no discernible effect or have a deleterious effect; however, a tiny minority are beneficial and thus give an individual and his or her descendants a selective advantage.
Genetic testing of families or groups at high risk for single-gene disorders ascertains the presence or absence of mutations responsible for these disorders.