This discussion always seems to bring up the same points with no real resolution. Parti breeders say that the parti is a Yorkie, and have dna results to prove it. Those adhering to the breed standard say, yes, but the color is a fault that shouldn't be bred for. Parti breeders will say something like it's denying the breed's history and make-up to not include the parti.
I don't have a strong opinion about this but have looked at what is written about it. It seems like this debate is as old as the breed. There really are things written to support both sides of the argument. Having an interest in history and genealogy, I've gotten used to connecting the dots when there is no conclusive evidence. It's not as good as cold, hard facts, but sometimes it just has to do.
My belief about the parti is that, yes, they have been around since the inception of the breed. I also believe that breed purists have discounted them since the beginning and have not considered them a true Yorkshire Terrier.
I base my belief on a few points. It may well be flawed but here goes.
1. Parti breeders point to the Maltese being bred into the Yorkshire Terrier since the beginning.
2. The breed that contributed the most to the Yorkshire Terrier is the Clydesdale Terrier.
3. The Clydesdale is the silky coated blue and tan version of the Skye Terrier.
4. In the early days, the broken haired Scotch Terrier class was a mish-mash of quite different dogs, which is a situation ripe for doodling around with.
There are some things written in the Skye Terrier's history about the Maltese being bred into them. This would support the parti position. That, taken with the amalgam of dogs being shown as broken haired Scotch Terriers, and you can see how there might have been white in the dogs very early on.
There have been some things posted by parti breeders here before of white Yorkies being mentioned (and discounted) as far back as 1904. It does seem that breed purists, even then, acknowledged their existence but discounted them.
I have found a reference dating back to 1876 where it is mentioned in a rebuttal about Maltese being used by Skye breeders to enhance the coat and win a ribbon. Here's the link...
The country - Google Books
So, it does seem like the Maltese was used from the beginning, at least by some breeders of the Skye. With the Clydesdale being a type of Skye, it's not much of a leap to conclude that the Maltese influence would have been present in some Clydesdales, thereby getting into the make-up of the Yorkshire Terrier.
That being said, it does seem like the standard bearers of the Yorkshire have always been adamant about it being a blue and tan dog. It was never the intent for there being any other color and that the blue and tan is a distinguishing feature of the Yorkshire Terrier. It seems that any other color was discounted form the beginning and that any dog exhibiting other colors was the result of a breeding not within the breed standard. As they were trying to distinguish a Yorkie from the other, very similar breeds, this (the color) would have been very important in setting the type and determining what was and wasn't a Yorkshire Terrier. Of course, once the genetic influence of the piebald gene was there in some specimens, it is easy to see how it carried forward.
After connecting the dots, my conclusion is that there have always been dogs with white in them, but they have always been discounted.The YTCA considers the parti Yorkie a fault, but I imagine the early breeders considered them not a Yorkie at all. They were the ones that were looking at all these dogs at that time in their class and they are the ones that determined what was and what was not a given breed.
Through that shake out, determined a lot by what was winning in the ring, several breeds were lost to history. The Yorkie was very similar to other breeds, but its features were determined to be the best, and those others breeds soon began to disappear. The piebald gene in the Yorkie, to me, is a carry over from all those years ago. It has always been in some lines, but not in the breed in general, and the Yorkshire Terrier purists would say that the Yorkie has always been a blue and tan dog and any other color should have been bred out long ago.
This is all my rambling opinion and could well be completely off base. I can see how dna tests would show the Parti to be a Yorkie due to the influence going so far back. I do think you have to look back to see what the breed's developers intended a Yorkie to be and then it is more clear that the piebald gene was never intended to be a part of the breed's make-up.
I do think the Parti is a beautiful dog and mean no offense to Parti lovers. I've had the opportunity to get to know some of you here and consider you my friends. I don't question your ethics or motives and wish you all the best. It's obvious that some of you take your breedings seriously and do have long-term goals, all commendable things.
It's just that the subject has come up so often that I began to look for answers and this is what I've found. It's kind of murky, but does paint somewhat of a picture.