Family linkage studies examine genetically related individuals exposed to a similar familial and to some extent community environment; as such, they have been of great utility in genetic epidemiology and have proven effective in identifying genes of simple Mendelian diseases such as cystic fibrosis. Related individuals affected with the disease are recruited along with their unaffected family members and the inheritance pattern of the disease is examined via genomic markers. Differential transmission of alleles to the affected individuals indicates linkage of the marker with the phenotype measured. However, obtaining samples for family-based studies, particularly for psychiatric illnesses such as drug addiction, can be difficult given the associated stigma (40). Genotyping costs are also greater because of the higher density of markers required compared to studies with population-based samples, and the large size of chromosomal regions shared between family members makes it more difficult to isolate the signal associated with disease outcomes. Nevertheless, in comparison with GWA studies, which use population-based samples and are powered to detect common variants with modest effects, traditional family linkage studies may have greater power to
