I can't speak for the partis. However the Biewers are participating in rigid DNA testing. For complete information please read about it on my website or on the BTCA's website:
http://www.biewer-btca.com/
Also, due to the rigourous testing the Biewers have been accepted as a separate breed by the American Rare Breed Association (ARBA).
So the parti's and Biewers may look the same but genetically they are different.