The DRD2 Taql polymorphism and symptoms of attention deficit hyperactivitydisorder

The relationship of the DRD2 Taql-A1 allele to hyperactive/impulsive and in attentive symptoms of attention deficit hyperactivity disorder (ADHD) in ch ildren and adolescents was examined in a sample of clinic-referred children and their siblings, and control children and their siblings (n = 236). The contribution of genetic dominance and additivity to mean differences among the A2A2, A1A2, and A1A1 genotypes was estimated using structural equation modeling. The effect of genetic additivity was statistically significant f or both traits in an analysis of all children. The heritability from the DR D2 locus was estimated at 4.27% for hyperactive-impulsive symptoms and 2.12 % for inattentive symptoms. Children with the A2A2 genotype had the highest mean level of symptoms. To control for any possible effects of population stratification, this analysis was repeated with parental genotypes as contr ols. In this smaller sample, although the direction of the effect was the s ame as that in the whole sample, the genotypic differences failed to reach conventional significance levels and the effect sizes were smaller (h(2) = 1.62% and 0.79%, respectively). Furthermore, a genotype relative risk test of children who had questionnaire-based diagnoses of ADHD also failed to yi eld evidence of either association or linkage. Given that the Al allele was expected to be the high risk allele, and that results were non-significant in tests that controlled for population heterogeneity, we doubt that this DRD2 polymorphism influences symptoms of ADHD in childhood.