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2R hypothesis
The 2R hypothesis or Ohno's hypothesis, first proposed by Susumu Ohno in 1970,〔Ohno S (1970). ''Evolution by Gene Duplication.'' London: Allen and Unwin, ISBN 0-04-575015-7.〕 is a hypothesis in genomics and molecular evolution suggesting that the genomes of the early vertebrate lineage underwent two complete genome duplications, and thus modern vertebrate genomes reflect paleopolyploidy. The name derives from the ''2 rounds'' of duplication originally hypothesized by Ohno, but refined in a 1994 version, and the term ''2R hypothesis'' was probably coined in 1999. Variations in the number and timings of genome duplications typically still are referred to as examples of the 2R hypothesis. The 2R hypothesis has been the subject of much research and controversy; however, with growing support from genome data, including the human genome, the balance of opinion has shifted strongly in favour of support for the hypothesis.
According to Hokamp ''et al.'' (2003),〔 the version of the genome duplication hypothesis from which 2R hypothesis takes its name appears in Holland ''et al.'' (1994) and the term was coined in Hughes (1999).
==Ohno's argument==
Ohno presented the first version of the 2R hypothesis as part of his larger argument for the general importance of gene duplication in evolution. Based on relative genome sizes and isozyme analysis, he suggested that ancestral fish or amphibians had undergone at least one and possibly more cases of "tetraploid evolution". He later added to this argument the evidence that most paralogous genes in vertebrates do not demonstrate genetic linkage. Ohno argued that linkage should be expected in the case of individual tandem duplications (in which a duplicate gene is added adjacent to the original gene on the same chromosome), but not in the case of chromosome duplications.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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