(Huang et al., 2008). Developmental changes in ADHD symptoms were unrelated to changes in working memory maintenance regardless of DRD1 genotype. Furthermore, for DRD2, there were no significant associations between genotype, working memory manipulation and changes in ADHD symptoms. Finally, studying these associations developmentally was important, as no genetic effects were significant at baseline when the participants were 7–11 years old. Thus, it seems plausible that dopamine D1 receptor gene variants and the development of higher-order working memory skills interact to affect the trajectory of ADHD symptoms more so than the emergence of symptoms in early childhood (Halperin & Schulz, 2006). Conceptually, this suggests that DRD1 is a ‘modifier’ gene that influences clinical features of ADHD without altering susceptibility (Fanous & Kendler, 2005).
