I've been reading this thread with a great deal of interest. I'm hesitant to post as the main posters here seem to have done their homework but I do have a question. Isn't the word 'parti' the same as 'piebald'? If so, there are piebald colors in other breeds, and in those cases, it seems that the breed clubs don't accept them either due to potential health risks, mainly deafness. Is there something unique about the Yorkie that exempts it from these risks? I found fairly quickly a few sites about other breeds with piebald concerns, namely the Dachshund, Boston Terrier and Great Dane. Here's some links...
health issues....
WHITES: IS IT WORTH THE RISKS and, also
Health Issues I'm sure there are sites with piebald advocates too, but the piebald issue seems to be well known in the dog world and it seems to be well accepted that there are risks in breeding for that look. I have no expertise with this but am just an interested observer so I may be completely off base. They are lovely dogs but isn't it a little like playing with fire to breed for that look (color), regardless of the origin of the gene?