Transient tic disorders are more common in first degree (close) relatives of people who have tic disorders than in the general population. Chronic motor or vocal tic disorder and Tourette's Disorder appear to be genetically related.
TD runs in families. It is genetically transmitted; having the gene makes a person a "carrier," but doesn't necessarily mean he will have the symptoms of the disease. A parent of either sex who is a carrier or has TD has a 50% chance of passing the genetic vulnerability to a child of either sex. Female gene carriers have a 70% chance of expressing TD symptoms, whereas male gene carriers have a 90% chance, which explains the preponderance of males with a TD diagnosis.
Ten to 15% of people with TD do not acquire the disorder genetically. Non-genetic TD is thought to be associated with low birth weight, gestational complications, head trauma, carbon monoxide poisoning, chronic neuroleptic exposure and encephalitis lethargica.
In terms of brain chemistry, evidence suggests that the neurotransmitter dopamine is affected in TD, but norepinephrine and serotonin are also involved.