single nucleotide polymorphisms (SNPs) and compares the prevalence of the risk allele in those who are affected and unaffected (and can be related or unrelated, see panels B and C of Figure 2). SNPs are single base pair allelic variations that naturally occur across individuals. For instance, rs279871, is a SNP in intron 7 of the gamma-aminobutyric acid receptor A (GABRA2) gene. Dick and colleagues83 demonstrated that individuals with one or more copies of the A allele were more likely to report illicit drug dependence independent of the age of onset of drug dependence. However, an increase in risk for alcohol dependence, by genotype, was only noted in those with ages of onset of alcohol dependence in their mid-twenties. Thus, the association between this polymorphism and alcohol (but not drug) dependence appeared to be absent during adolescence. Further, in an adolescent sample, Corley and colleagues84 found several genes, including GABRA2, CNR1 and CHRNA2 to be associated with adolescent antisocial drug dependence.
