AO-AD is also correlated with other features of drinking. For instance, earlier AO-AD is associated with more alcohol dependence symptoms,2 and this relationship may be influenced by shared genetic liability. Cloninger et al.7 also posited that Type II/early-onset alcoholism may represent a more heritable form of the disorder. While there have not been studies that examine the heritability of AO-AD, numerous studies have robustly documented the role of additive genetic influences on AD itself8, 9 and on both age at first drink10, 11, 12 and the speed of transitioning from first use to the development of alcohol problems.13, 14 In support of these findings, a recent genome-wide association study (GWAS) in extended pedigrees from the Collaborative Study of the Genetics of Alcoholism (COGA) found several single nucleotide polymorphisms (SNPs) that were significantly associated with AO-AD (P<5E−8).15 For rs2168784, the most significant SNP, 30% of those homozygous for the minor allele met criteria for alcohol dependence while only 19% of those homozygous for the major allele did. This SNP was also associated with AD diagnosis in the COGA dataset.
