the last few years, advances in technology and, in particular, advances in molecular genetics have made it possible to search the human genome for disease causing genes. Thus far, mutant alleles have been described for such diseases as cystic fibrosis, Huntington's chorea, and certain cancers. In contrast to this success though, researchers have had more difficulty establishing the molecular basis of behavioral disorders. This has been in part due to the fact that psychiatric conditions do not typically display a clear mode of genetic transmission. The inheritance of alcoholism, for example, is characterized by incomplete penetrance and extremely variable clinical expression (10:346).
Two strategies which can be employed to identify the genetic locus for a given disease are genetic linkage approach and the population association approach. Genetic linkage occurs as a result of the tendency for nearby genes on the same chromosome to be cotransmitted from parent to offspring together. By establishing the frequency
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