Comorbidity of alcoholism and ADHD forms a distinct phenotype that shows an increased severity of the substance disorder. This phenotype contributes substantially to the so-called type 2 alcoholics according to Cloninger. In our sample, the functional relevant 5-HTT promoter and the 5-HT2c receptor Cys23Ser polymorphism do not contribute to the supposed common genetic predisposition of ADHD and alcohol dependence.

It is well established that genetic factors play a major role in the development of alcoholism in both sexes. Several twin studies demonstrated a nearly equally high magnitude of genetic influence for men and women. However, the genetic sources of vulnerability are supposed to only partially overlap in men and women. Therefore, we evaluated the gender-specific effects of two single nucleotide polymorphisms affecting dopaminergic neurotransmission (dopamine D2 receptor: -141C Ins/Del polymorphism; Dopamine D3 receptor: Bal I) in our large sample of primary alcoholics. Only a gender-specific analysis of subgroups with a putatively high genetic load, e.g., family-history-positive or presence of severe withdrawal complications, revealed significant differences in allele-/genotype-frequency. Our results demonstrate that a varying sex distribution in the samples investigated might contribute to the heterogeneous results reported in association studies for candidate genes in alcoholism and, therefore, should be taken into account in future studies.

In our sample, persistence of ADHD symptoms from childhood into adulthood and antisocial personality disorder contributes to vulnerability and morbidity of alcohol dependence with early onset and greater severity of disorder.