There are forms of PD that are influenced by genetic defects that appear to run in families. Most often these are ‘early-onset’ forms of PD cases where the disease begins earlier in life than usual. Heritable forms of PD account, however, for less than 10% of all cases of PD.
While genetics very likely plays a role in all forms of PD, genetic problems typically merely increase risk -- they do not inevitably cause the onset of the disease. If you have a relative with the disease, it does not mean that you will inevitably get the disease, too. It merely slightly increases your risk of getting it. Conversely if you have the disease, it does not mean that any of your children or grandchildren will get the disease. It merely indicates their risk for getting the disease is slightly higher than families without PD.
Stewart A Factor, DO and William J Weiner, MD. (eds) Parkinson’s Disease: Diagnosis and Clinical Management: Second Edition Edited by 2008 Demos Medical Publishing.