firstm if you have the gene, even if it is dominant there is only a 50% chance of it being passed on, then what if the other parents gene is more dominant? then it goes down to 40% and then add to the fact evolution… it is a recent mutation, carried on because survival of the fittest applies far less in our medically advanced culture. in the wild (darwins wild) these people would struggle to find a mate, and that is assuming they lived long enough, now they lead full and productive lives (not trying to insult, just answer your question). this all means that a mutation that might have grown out is still here but extremely rare as it is recent. modern science will hopefully help cure this condition as we are now unlocking the gene sequence and are beginning to understand such things (not me, but clever scientists of course P)

Just because it’s dominant, doesn’t mean it’s prevalent. There is just a low frequency of the Parkinson’s disease. The only difference is that there are no heterozygous people who don’t display the gene.

Also there may be so choice about reproduction if Parkinson’s runs in a family, thus limiting the frequency that way.