Despite substantial progress in identification, there are only a few known large pedigrees with PD. Furthermore, genetic linkage studies that use "identity-by-descent" mapping have been hampered because of the limited amount of DNA available from affected pedigree members, generally because of death, lack of consent, or geographic dispersion. Association or "identity-by-state" mapping is an alternate approach using groups of unrelated persons. Association studies measure differences in genetic variability between a group with the disease in question and a group of matched healthy persons. This method is most powerful in implicating genes for multigenic traits in homogeneous population isolates. Many past studies have been confounded by misconceived a priori notions about the causes of disease, by the candidate genes
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