It is also possible that the observed relationship between TS and ADHD is partly mediated through OCD, as has been previously suggested26, 30, 31. For example, Geller and colleagues have suggested that, based on their studies of ADHD families and pediatric OCD families, ADHD+OCD exists as a distinct familial subtype rather than being alternative expressions of the same underlying genetic susceptibility30, 31. We found some evidence to support this hypothesis in our TS families as well. First, although a TS+ADHD phenotype (without comorbid OCD) does exist in our families, it is not a large proportion, comprising only 11.8% of the sample, whereas the proportion of individuals with TS+OCD+ADHD is much higher (34%)35. Second, we found strong genetic correlations between OCD and ADHD, and between TS and OCD+ADHD, although not between TS and ADHD. Third, we found that maternal ADHD was associated with offspring OCD, with an odds ratio of 1.8 (CI 1.1–3.1), in addition to being associated with offspring ADHD.
