One study evaluated 43 pairs of same-sex twins, including 30 MZ pairs and 13 DZ pairs. MZ twin concordance was 77% for any tics, compared to 23% for DZ twins. For Tourette’s syndrome per se, the MZ twin concordance was 53%, compared to the DZ twin concordance of 8%. These differences are significant.
One adoption study found that among biological relatives of 38 Tourette’s probands, 8.3% had Tourette’s syndrome, 16.3% had chronic tics, and 9.5% had OCD. These risks are all significantly greater than the risks for the relatives of control subjects.
Although one study reported that the transmission of Tourette’s syndrome and chronic tics was consistent with a single-locus model, other studies have been unable to differentiate between a single-locus model and polygenic models.
A collaborative effort to use systematic genomic screening to find genes causing Tourette’s syndrome is expected that a reproducible linkage will be found, although large samples may be needed.