Linkage and association designs are both used to map Mendelian and complex traits (Table 1). In the 1980s, it became feasible to use DNA markers to map genotype-phenotype correlations and begin to pin-point specific genomic loci that influence alcohol phenotypes. Genetic linkage studies determine inheritance of a trait or disorder (e.g., AUD) among family members in extensive pedigrees by evaluating if one or more genetic markers spaced across the 23 chromosomes segregate with the disorder. Linkage mapping is a locus-driven approach that can usually only roughly locate a broad chromosomal region, which may contain 10s of 100s of genes, that co-segregate with the disorder. Thus, further fine mapping with recombinant genetic markers in informative family members is required to get to the gene. One of the first studies of this kind, the Collaborative Study on the Genetics of Alcoholism (COGA) was formed in 1989 to map and characterize the genetic variants associated with alcoholism (Bierut et al., 2002). COGA is a family-based genetic study with nine research sites throughout the U.S. Nuclear and multigenerational families were ascertained through a proband
