Reminder--- here is primer on wiki about dopamine and its physiological effects in the body and brain:

https://en.wikipedia.org/wiki/Dopamine

And this is the Kegg pathway of tyrosine metabolism.

Each rectangular box shows a number, and those numbers are enzymes. These enzymes determine when and how much of the other biochemicals are to be made at any one time.

http://www.genome.jp/kegg/pathway/map/map00350.html

Only a tiny amount of dopamine is made in the brain. Dopamine is not a common neurotransmitter there, like histamine, glutamate, and others.
( http://neurotransporter.org/glutamate.html )

http://thebrain.mcgill.ca/flash/d/d_..._03_m_que.html

There is ongoing research today involving the role of glutamate in dopamine disorders (like schizophrenia). This is massively complex and maybe too much so for this forum, but you can Google "Glutamate and dopamine" to find the pathways for this.

The complexity of the chemistry for all the neurotransmitters, is far and above just providing the substrate for their manufacture by the brain. What you eat will be taken by the body as a whole to feed other reactions involving that substrate (which here is tyrosine--see the Kegg pathway link). Any genetic errors in transcripting those enzymes that are responsible for the various reactions, will impact the formation of the desired result.

The genetic research is just beginning. We on the PN forum are seeing that genetic errors, in DNA can lead to nerve damage and pain. The MTHFR mutation is showing up on our PN forum here for all the posters who have the testing done. It is estimated to be a common mutation (10-30%) involving moving methyl groups around in the body, to form the neurotransmitters and do other tissue repair. I believe Muireann, has posted about this here on this forum and if you search her name you will find her posts about it.

So while it is a good thing to consume foods with the tyrosine in them, I do not think that will guarantee you will get a result with that route exclusively to impact Parkinson's disease. There are just too many other factors to consider as well.