Genetic studies, including twin studies,[6] have proven that the overwhelming majority of cases of Tourette syndrome are inherited, although the exact mode of inheritance is not yet known.[7] Tic disorders have long been thought to be inherited as an autosomal dominant gene, but recent research challenges the autosomal dominant hypothesis, and suggests an additive model involving multiple genes.[5][8] According to Roger Freeman, MD, "Genes that cause tics have not yet been identified; it's very unlikely there's just one. Tics are very familial, but not in a Mendelian pattern (dominant, recessive, etc.). You can't 'inherit' the committee decision to define a problem in a specific way, so TS itself can't be inherited.