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Genetic variation is a fact that a biological system – individual and population – is different over space. It is the base of the genetic variability of different biological systems in space.〔Hedrick P. (2011): Genetics of populations. Jones & Bartlett Learning, ISBN 978-0-7637-5737-3.〕〔Rieger R. Michaelis A., Green M. M. (1976): Glossary of genetics and cytogenetics: Classical and molecular. Springer-Verlag, Heidelberg - New York, ISBN 3-540-07668-9; ISBN 0-387-07668-9.〕〔Griffiths, A. J. F. (1999). An Introduction to genetic analysis. W. H. Freeman, San Francisco, ISBN 0-7167-3520-2.〕〔Cavali-Sforza L. L., Bodmer W. F. (1999): The genetics of human populations. Dover Publications, Inc., Mineola, New York, ISBN 0-486-40693-8. 〕 Genetic variation is based on the variation in alleles of genes in a gene pool. It occurs both within and among populations, supported by individual carriers of the variant genes. Genetic variation is brought about by random mutation, which is a permanent change in the chemical structure of a gene.〔Mayr E. (1970): Populations, species, and evolution – An abridgment of Animal species and evolution. The Belknap Press of Harvard University Press, Cambridge, Massachusetts and London, England, ISBN 0-674-69013-3.〕〔Dobzhansky T. (1970): Genetics of the evolutionary process. Columbia, New York, ISBN 0-231-02837-7.〕 == Among individuals within a population == Genetic variation among individuals within a population can be identified at a variety of levels. It is possible to identify genetic variation from observations of phenotypic variation in either quantitative traits (traits that vary continuously and are coded for by many genes, e.g., leg length in dogs) or discrete traits (traits that fall into discrete categories and are coded for by one or a few genes (e.g., white, pink, red petal color in certain flowers)). Genetic variation can also be identified by examining variation at the level of enzymes using the process of protein electrophoresis. Polymorphic genes have more than one allele at each locus. Half of the genes that code for enzymes in insects and plants may be polymorphic, whereas polymorphisms are less common in vertebrates. Ultimately, genetic variation is caused by variation in the order of bases in the nucleotides in genes. New technology now allows scientists to directly sequence DNA which has identified even more genetic variation than was previously detected by protein electrophoresis. Examination of DNA has shown genetic variation in both coding regions and in the non-coding intron region of genes. Genetic variation will result in phenotypic variation if variation in the order of nucleotides in the DNA sequence results in a difference in the order of amino acids in proteins coded by that DNA sequence, and if the resultant differences in amino acid sequence influence the shape, and thus the function of the enzyme. == Between populations == Geographic variation in genes often occurs among populations living in different locations. Geographic variation may be due to differences in selective pressures or to genetic drift. 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「Genetic variation」の詳細全文を読む スポンサード リンク
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