Alzheimer's disease and Parkinson's disease.

N Engl J Med 2003;348:1356-64. Copyright © 2003 Massachusetts Medical Society. he incidence of many common diseases is increased among the relatives of affected patients, but the pattern of inheritance rarely follows Mendel’s laws. Instead, such common diseases are thought to result from a complex interaction among multiple predisposing genes and other factors, including environmental contributions and chance occurrences. Identifying the genetic contribution to such complex diseases is a major challenge for genomic medicine. However, as so clearly foreseen nearly 350 years ago by the English physiologist William Harvey, 1 finding the genetic basis for rarer, mendelian forms of a disease may illuminate the etiologic process and pathogenesis of the more common, complex forms. This is illustrated in the progress made in understanding Alzheimer’s disease and Parkinson’s disease through the investigation of the rare, clearly inherited forms of these diseases. The molecular basis of neurodegenerative disorders was reviewed in the Journal in 1999. 2