It appears to be the case that one contributor to variability in 5HT and DA levels is gene polymorphisms. The action of the neurotransmitters is influenced, in part, by variation among individuals in at least four target genes: the serotonin transporter gene (5HTTLPR) and dopamine receptors genes DRD2, DRD3, and DRD4. The 5HTTLPR gene is triallelic. It has one short allele, s, and two long alleles, Lg and La (Beitchman et al., 2006). The s and the Lg alleles tend to be relatively low-expressing (they are present less often), and they are associated with lower brain levels of 5HT (Beitchman et al., 2006; Hu et al., 2006; Hu et al., 2005; Lakatos et al., 2003). The a1 allele on the D2 dopamine receptor gene is associated with a reduced number of DA receptors, and hence more brain DA (Noble, 2003). Similarly, the 7-repeat allele on the D4 receptor gene and the a1 and a2 alleles on the D3 receptor gene are all associated with higher levels of emotional reactivity and emotion-based rash action (Auerbach, Faroy, Ebstein, Kahana, & Levine, 2001; Lakatos et al., 2003; Munafó et al., 2003; Oniszczenko & Dragan, 2005).
