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Genetic association is when one or more genotypes within a population co-occur with a phenotypic trait, more often than it would be expected by chance occurrence. Studies of genetic association aim to test whether single-locus alleles or genotype frequencies (or more generally, multilocus haplotype frequencies) differ between two groups of individuals (usually diseased subjects and healthy controls). Genetic association studies are based on the principle that genotypes can be compared "directly", i.e. with the sequences of the actual genomes. ==What is genetic association?== Genetic association can be between phenotypes, such as visible characteristics such as flower colour or height, between a phenotype and a genetic polymorphism, such as a single nucleotide polymorphism (SNP), or between two genetic polymorphisms. Association between genetic polymorphisms occurs when there is non-random association of their alleles as a result of their proximity on the same chromosome; this is known as genetic linkage. Much of genetic variation in the human genome is in the form of SNPs which is the result of point mutations that produce single base-pair differences (substitutions or deletions) among chromosome sequences. GWA studies have become an important tool for discovering susceptibility genes for complex diseases. Information, including genotype frequencies, linkage disequilibrium (LD), and recombination rates, across populations help researchers to conduct GWA analysis using millions of SNP markers.〔Fareed, M., Afzal, M (2013) "Single nucleotide polymorphism in genome-wide association of human population: A tool for broad spectrum service". ''Egyptian Journal of Medical Human Genetics'' 14: 123–134. http://dx.doi.org/10.1016/j.ejmhg.2012.08.001.〕 Linkage disequilibrium (LD) is a term used in the study of population genetics for the non-random association of alleles at two or more loci, not necessarily on the same chromosome. It is not the same as linkage, which is the phenomenon whereby two or more loci on a chromosome have reduced recombination between them because of their physical proximity to each other. LD describes a situation in which some combinations of alleles or genetic markers occur more or less frequently in a population than would be expected from a random formation of haplotypes from alleles based on their frequencies. Genetic association studies are performed to determine whether a genetic variant is associated with a disease or trait: if association is present, a particular allele, genotype or haplotype of a polymorphism or polymorphisms will be seen more often than expected by chance in an individual carrying the trait. Thus, a person carrying one or two copies of a high-risk variant is at increased risk of developing the associated disease or having the associated trait. 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 association」の詳細全文を読む スポンサード リンク
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