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In addition to the normal karyotype, wild populations of many animal, plant, and fungi species contain B chromosomes (also known as supernumerary or accessory chromosomes). By definition, these chromosomes are not essential for the life of a species, and are lacking in some (usually most) of the individuals. Thus a population would consist of individuals with 0, 1, 2, 3 (etc.) supernumeraries. Most B chromosomes are mainly or entirely heterochromatic (and so would be largely non-coding), but some, such as the B chromosomes of maize, contain sizeable euchromatic segments. In general it seems unlikely that supernumeraries would persist in a species unless there was some positive adaptive advantage, which in a few cases has been identified. For instance, the British grasshopper ''Myrmeleotettix maculatus'' has two structural types of B chromosomes: metacentrics and submetacentrics. The supernumeraries, which have a satellite DNA, occur in warm, dry environments, and are scarce or absent in humid, cooler localities. B chromosomes are not to be confused with marker chromosomes or additional copies of normal chromosomes as they occur in Trisomies. ==Origin== The evolutionary origin of supernumerary chromosomes is obscure, but presumably they must have been derived from heterochromatic segments of normal chromosomes in the remote past. In general "we may regard supernumeraries as a very special category of genetic polymorphism which, because of manifold types of accumulation mechanisms, does not obey the ordinary Mendelian laws of inheritance." (White 1973 p173) Next generation sequencing has shown that the B chromosomes from rye are amalgamations of the rye A chromosomes. Similarly, B chromosomes of the cichlid fish ''Astatotilapia latifasciata'' also have been shown to arise from rearrangements of normal A chromosomes. 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「B chromosome」の詳細全文を読む スポンサード リンク
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