Family studies have repeatedly demonstrated that Gilles de la Tourette Syndrome (GTS) is highly familial. Establishing that there is familial aggregation does not “prove” that the disorder is influenced only by genetic factors, since family members also share common environmental factors. Nevertheless, results from these studies provide an important first step for determining whether genetic factors are important in the manifestation of the condition. Since first degree relatives share on average 50% of their genetic material, it is expected that the first degree relatives of an individual affected with a genetic disorder will have a greater chance of also being affected with that disorder compared to the general population [2]. Results from GTS family studies consistently show a 10 to 100 fold increase in the rates of GTS in first-degree relatives when compared to those rates in the general population [3–12] making it one of the most heritable childhood onset neuropsychiatric conditions [13]. Furthermore, chronic tics (CT) also occur significantly more frequently (reports range from 7 to 22%) among first degree relatives of GTS probands compared to relatives of control probands, suggesting that CT is a manifestation of the same underlying genetic susceptibility as GTS [3–12].
