Why families?

It is rare for Parkinson's to run in families. However, some gene variants have been strongly associated with Parkinson's particularly in people with a clear family history (with several other affected family members) or with a very early age at onset (when the condition developed). We all carry a large number of rare genetic variations, across 20,000 genes. The easiest way to pinpoint the variation relevant to Parkinson's is to compare family members with and without Parkinson's.