You will find a lot of controversial threads on here about the partis not being yorkies and so on. But the AKC DNA'd 42 litter of puppies before they were satisfied that they are purebred yorkies.
No one knows where the parti gene came from or why it is in some lines and not in other lines. There is a lot of information on the parti color and the history of the yorkie at
http://www.colorfulyorkie.com/CYTC_Parti_Color.htm