We have previously examined the heritability of TS comorbid classes derived from latent class analysis (LCA), finding a highly comorbid (TS+OCD+ADHD) class that was highly heritable. The LCA approach, while useful for identifying groups of similar individuals (e.g., the TS+OCD+ADHD class), cannot be used for the identification of specific genetic and environmental relationships between the individual disorders of TS, OCD, and ADHD35. Thus, in this paper, we expand on those findings to examine the familiality of TS, ADHD and OCD independently in TS-affected families and explore the genetic and environmental relationships between them. Based on our previous findings and on the earlier family studies, we hypothesize that: 1) there will be high rates of parental TS, OCD, OCB and ADHD in the sample compared to the population rates, and that parental diagnoses will be associated with offspring OCD, OCB and ADHD; 2) there will be evidence of non-random mating for ADHD with TS and OCD, partly accounting for the increased prevalence of ADHD in TS-affected offspring; and 3) TS and OCB/OCD will share genetic factors, OCD and ADHD will share genetic factors, but TS and ADHD will share both genetic and environmental factors.
