We found that, consistent with previous studies, and as expected, TS and OCD were heritable in our TS families11, 50, 51. TS had a somewhat lower heritability estimate than either OCD or ADHD; this is most likely an artifact caused by the decreased variation in the sample (i.e., the vast majority of sibs were TS-affected). TS and OCD had an extremely strong genetic correlation of over 90%, and no significant environmental correlation, suggesting that, in multiply-affected families at least, TS and OCD are likely to be due to the same underlying genetic factors. One possibility is that TS+OCD in offspring may represent a more severe expression of the TS phenotype, perhaps resulting from a greater genetic loading. Another is that OCD represents an alternative phenotypic expression of TS-susceptibility genes, a hypothesis that has been put forward in previous family studies52. Unfortunately we are unable in the current study to distinguish between these hypotheses, which could be best tested in multi-generational families.
