if this is for a level psychology, in studying Schizophrenia, all you have to know is that low levels of dopamine can cause parkinson's.
However when drugs are given to alleviate the symptoms of parkinsons (such as L. Dopa) they can cause schizophrenic symptoms, reinforcing the dopamine hypothesis of schizophrenia
any more in depth detail, i cant help you with, sorry
Low levels of dopamine in the Substantia Nigra part of your Basal Ganglia (in your brain) causes Parkinsons. Basically the cells which secrete dopamine die causing a decrease in the amount of the neurotransmitter within this part of your brain thus mucking up your motor functions and causing tremors (I.e. the characteristic "pill rolling" movement). Wikipedia it for more info.
Low levels of dopamine doesn't necessarily increase RISK of getting Parkinson's but can contribute to the disease symptoms. Parkinson's is characterised by a loss of dopamine secreting neurones which in turn can lead to motor dysfunctions. This can be caused by things such as oxidative stress in the mitochondria or a build up of the protein alpha-synuclein.
He's a an example because he developed the disease at a young age which raised questions about the idea that his fighting may have been a contributing factor, even though the disease is typically thought to be spontaneous and related to old age.