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Atavism is the tendency to revert to ancestral type. In biology, an atavism is an evolutionary throwback, such as traits reappearing which had disappeared generations before. Atavisms can occur in several ways. One way is when genes for previously existing phenotypical features are preserved in DNA, and these become expressed through a mutation that either knock out the overriding genes for the new traits or make the old traits override the new one. A number of traits can vary as a result of shortening of the fetal development of a trait (neoteny) or by prolongation of the same. In such a case, a shift in the time a trait is allowed to develop before it is fixed can bring forth an ancestral phenotype. In the social sciences, atavism is a cultural tendency—for example, people in the modern era reverting to the ways of thinking and acting of a former time. The word ''atavism'' is derived from the Latin ''atavus''. An ''atavus'' is a great-great-great-grandfather or, more generally, an ancestor. ==Biology== Evolutionarily, traits that have disappeared phenotypically do not necessarily disappear from an organism's DNA. The gene sequence often remains, but is inactive. Such an unused gene may remain in the genome for many generations. As long as the gene remains intact, a fault in the genetic control suppressing the gene can lead to it being expressed again. Sometimes, the expression of dormant genes can be induced by artificial stimulation. Atavisms have been observed in humans as well. Babies have been born with a vestigial tail, called "coccygeal process", "coccygeal projection", and "caudal appendage".〔 Atavism can also be seen in humans who possess large teeth, like those of other primates. In addition, a case of "snake heart", the presence of "coronary circulation and myocardial architecture (resemble ) those of the reptilian heart", has also been reported in medical literature. Examples of observed atavisms in animals include: *Hind legs on whales〔 or snakes *Hind fins on dolphins〔 *Extra toes on horses, as in archaic horses *Re-emergence of sexual reproduction in the flowering plant ''Hieracium pilosella'' and the Crotoniidae family of mites. *Teeth in chickens〔Gould, S.J. "Hen's Teeth and Horses' Toes" In: Hen's Teeth and Horse's Toes, New York: W. W. Norton, ISBN 978-0-393-01716-8〕 *Supernumerary nipples on higher primates 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Atavism」の詳細全文を読む スポンサード リンク
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