Mode of inheritance - DYT1 Dystonia
Your description is correct. Most genes do not show complete dominance as outlined in Mendelian genetics and inheritance is almost always more complicated than a simple 1 gene/2 allele system. The mutation you're studying is classified as autosomal dominant but even heterozygotes may not display the phenotype. The proportion of the population that has the mutation AND displays the mutant phenotype is called the penetrance and, for DYT1, has been estimated at around 30%. 

http://www.omim.org/entry/128100?search=Dyt1&amp;highlight=dyt1