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Genetic association studies aim to detect association between one or more genetic polymorphisms and a trait, which might be some quantitative characteristic or a discrete attribute or disease. Association differs from linkage in that the same allele (or alleles) is associated with the trait in a similar manner across the whole population, while linkage allows different alleles to be associated with the trait in different families. However, genetic associations arise only because human populations share common ancestry and it has been argued that association studies are really just a special form of linkage study in which the extended family is the wider population. In linkage analysis, data from distantly related individuals are more powerful for detecting small effects than data from closely relate