\citet*{29636375} discusses cases on the borders of TS, such as tic disorder beginning in adulthood and the question of functional (conversion) tics.
Etiology
Genetics
Tying genetics of TS to genetics of brain volumes \citep{Mufford_2018}
Polygenic risk scores from a GWAS validated in the Avon Longitudinal Study \citep{30424865}
Genetic overlap of OCD and ADHD subtypes with Tourette syndrome \citep{28651666}
Tic patients with 2 unaffected parents were more likely to have de novo DNA sequence variants, compared to tic patients with an affected parent \citep{30257206}. These potentially causative new mutations were more likely to affect genes involved in cell polarity, including the gene CELSR3.
A huge study of shared GWAS data from over a million people revealed that TS shared common variant genetic risk with ADHD, OCD, major depressive disorder and migraine with aura \citep{Anttila2018}.