The autosome-wide significant linkage peak on chromosome 4 was observed in EAs only. Notably, association of alcoholism with GABRA2, about 10.7 Mb from the linkage peak we identified on chromosome 4 for the current study, has been documented and replicated in both EAs and AAs [Edenberg et al., 2004; Lappalainen et al., 2005; Covault et al., 2008; Enoch et al., 2009; Enochet al., 2010; Ittiwut et al., 2011]. A significant association of alcohol dependence with a haplotype block that extends from the intergenic region between GABRA2 and GABRAG1 to GABRG1 intron 3 was observed in EAs but not in AAs [Covault et al., 2008]. Covault et al. [2008] also observed different comorbid MSD patterns in this study sample between AAs and EAs; for example, the percentage of the OD affecteds is much higher in EAs than in AAs (65.3% vs. 27.4%, Table I). This difference is likely due to the ascertainment scheme and not due to inherent differences between populations. Thus, the distinct comorbid MSD pattern or ascertainment differences in EAs and AAs might contribute to the linkage peak being identified only in EAs.
