- Objective This study examined the influence of allelic variation in two dopamine genes, the dopamine receptor D4 (DRD4) gene and the dopamine transporter D1 (DAT1) gene, and paternal attention-deficit hyperactivity disorder (ADHD) symptomatology on the level of ADHD symptoms in 96 four and a half-year-old boys. Method DNA was collected by means of a buccal swab and genotyped for DRD4 and DAT1. Mothers completed the Dupaul ADHD checklist on their sons. ADHD symptomatology ratings for fathers were based on a summed father self-reported and spouse-reported symptoms (Conners Adult ADHD Rating Scale). Results There were main effects for DAT1 and father symptomatology for the child Total ADHD and Hyperactivity–Impulsivity scores. The main effects for DRD4 were limited to the child Hyperactivity–Impulsivity scores. Child Inattentive scores were influenced only by father symptomatology. Interaction effects between DAT1 and DRD4 and between DAT1 and the father ADHD risk group were found for child Hyperactivity–Impulsivity scores. Boys with the highest level of symptomatology were those with the 10/10 DAT1 genotype and the DRD4-7 genotype or fathers with high symptomatology. Conclusion The findings of this study indicate that the risk for ADHD, particularly hyperactivity–impulsivity, is exacerbated in the presence of dopamine risk genes and paternal ADHD symptomatology. This study adds to the growing literature on the efficacy of including multiple genetic and environmental risk factors in studies related to the development of psychopathology.