The following information may help to address your question:
Among familial cases of Parkinson disease, how the condition runs in the family differs depending on which gene is altered. When a parent and child are affected, autosomal dominant inheritance must be ruled out. Autosomal dominant disorders tend to occur in every generation of an affected family. However, in Parkinson disease there are a number of genetic phenomenon that can influence how the gene alterations impact a given family.
As an example, mutations in the LRRK2 gene can cause autosomal dominant Parkinson disease. However, the age of onset of Parkinson's in people with LRRK2 gene alterations is highly variable (from 35-years to 78-years). Symptom and symptom severity can also vary, even between family members. Also, some people with LRRK2 gene alterations never develop Parkinson disease at all.
Alterations in LRRK2 is only one example of a cause of familial Parkinson disease. In addition to LRRK2, SNCA gene alterations can cause autosomal dominant Parkinson disease. Alterations in the genes PARK7 or PINK1 can cause an autosomal recessive form of Parkinson disease. Alterations in PARK2 can cause autosomal recessive or dominant disease. Furthermore, the inheritance pattern is unclear when an increased risk of Parkinson disease or parkinsonism is associated with alterations in the GBA, SNCAIP, and UCHL1 genes.
Clinical genetic testing is available for some, but not all gene alterations known to be associated with Parkinson disease. Also, there are likely additional gene alterations that can cause Parkinson disease that have not yet been identified. To learn more about how Parkinson disease may be running in your family and to learn more about your genetic testing options, we recommend that you speak with a genetic professional.