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A mutagen is a substance which increases the frequency of mutation in a plant or animal population, which can lead to a variety of consequences. Some chemicals have mutagenic properties, and radiation such as ultraviolet light and x-rays is another common source of mutations. Because mutagens can lead to genetic mutations, some of them can contribute to the development of cancers, making these mutagens carcinogenic in addition to mutagenic.
There are a number of ways in which a mutagen can work within a living organism. Most attack the DNA, affecting the organism's genetic code. Some manage to insert themselves into the DNA directly, causing the animal to start reproducing the mutagen because it thinks that it belongs in the DNA. Others cause structural damage, leading to genetic errors which can become catastrophic as cells start replicating, and others manipulate the DNA, forcing it to produce something which is hazardous. Fetuses are particularly susceptible to mutagens because they are growing and developing so rapidly, which is why pregnant women are warned to be extra careful around radiation and chemicals.
People first began to understand how mutagens worked in the 1920s, when researchers in the process of exploring radiation noted a variety of mutations in organisms exposed to high levels of radiation. Over time, a mutagenic link between many forms of radiation and many chemicals was made, illustrating the need to observe precautions in research laboratories, and to test products carefully before releasing them to the general public.
As doctors learned to their dismay in the 1950s with thalidomide, mutagens are not always consistent or predictable. Even though many living organisms have very similar genetic codes, a mutagen may cause problems in one organism, but not in another. In the case of thalidomide, the medication caused birth defects in humans, but not in the animals it was tested on.
In addition to causing mutations in living organisms, as for example when exposure to a mutagen leads to the development of a cancerous tumor, mutagens can also cause birth defects. Furthermore, mutagen exposure may result in the transmission of a sort of genetic time bomb, a mutated gene or sequence which may become a problem in future generations. The mutagen might cause the development of a recessive trait which is brought out when two descendants of people exposed to the substance have children. The cause of the appearance of a birth defect may be difficult to pin down if the exposure occurred several generations ago, causing confusion for the parents and the supervising doctor.