Here is the link to the Parti-Yorkie vs Biewer thread from 2008 that had a discussion of DNA differences between the two:
http://www.yorkietalk.com/forums/gen...vs-biewer.html
The DNA testing technology at that time was not as good as it is today, but the upshot of the DNA part of the discussion was that there is an AKC registered group of Parti-Yorkies that tests as purebred Yorkshire Terriers, whereas a group of Biewers tested in a group by itself and was different from Yorkshire Terriers, hence a discussion of considering Biewer Terriers as a separate breed from Yorkies.
There was also a discussion in that thread of the genes responsible for differences in coat color between Biewers and Parti-Yorkies. The consensus was that the color spotting pattern is different between the two groups (Biewers have a more consistent pattern) and therefore the genes responsible for their coloration are different. I could look into this further and see if there is any more current research to bear this out, but that thread was quite contentious at times, so I'm not sure I should re-open that can of worms.