than Al/Al genotype. The authors propose that delay discounting and other aspects of reward-dependent impulsiveness may constitute a risk factor for the development of alcohol dependence, and that this could be partly mediated by the DRD2 gene (Limosin et al., 2003). Anghelescu et al. (2005) examined the association of the tryptophan hydroxylase (TPH) intron 7 A218C alleles with dimensions of the Temperament and Character Inventory (TCI; Cloninger & Syrakic, 1997) in alcohol-dependent individuals and age-matched controls. The TPH enzyme, which is implicated in serotonin biosynthesis, has been linked to alcoholism and impulsivity (Nielsen et al., 1998; Virkkunen, Godlman, Nielsen, & Linnoila, 1995). Although TCI scores were not related to the TPH genotypes in the complete sample the alcohol-dependent group included a higher proportion of individuals homozygous for certain TPH genotypes. Thus, although TPH polymorphisms may not explain variation in the expression of impulsivity in either alcohol dependent individuals or healthy controls, the difference in the TPH genotype distribution in alcohol dependent individuals suggests that there is a connection between serotonin function and alcoholism.
