Noninduced Mutagenic Agents
Environmental agents can influence the mutation rate not only by increasing it, but also by decreasing it. For example, antioxidants, which are found commonly in fruits and vegetables, are thought by many to protect against mutagens that are generated by normal cellular respiration. In addition to protective agents, however, many plants also contain deleterious mutagens known as carcinogens. Many chemical mutagens exist both naturally in the environment and as a result of human activity. Benzo(a)pyrene, for example, is produced by any incomplete burning, whether of tobacco in a cigarette or of wood in forest fires.
Spontaneous (noninduced) mutations are very rare, and finding them is difficult because most are recessive. The recessive nature of most mutations means that they will not be evident in most of the individuals who inherit them, for they will be hidden by the presence of the dominant allele. The rarity of mutations means that many individuals must be examined to find a mutant, whether they are people, other organisms, or even cells in culture.