Both genetic factors and environmental factors are likely to play a role in the development of DS. Most cases of DS cases are sporadic, with only approximately 2-5% of patients having a familial pattern; families with large involvement are rarely reported. Both dominant forms and recessive forms of DS have been documented. In some families with dominant DS, the disease skips a generation (reduced penetrance) and ranges in severity (variable expressivity). Most familial cases are not associated with other anomalies.