Oh my goodness ... You were misinformed that the parti colored yorkie is a result of a "genetic defect". The recessive genes for the parti color, have been in some of our dogs since the beginning of the breed, the gene is called the piebald gene.
The parti color is not a result of a defective, mutated or compromised gene as some genetically challenged/uneducated people have led you to believe, it's the result of a well known gene that produces white markings.
You need both mother and father to carry this spotting gene in order to produce the parti color, if only one parent carries the gene they will look traditional colored. If the carrier breeds to a non carrier, all of their pups will look traditional in color but 1/2 of the litter will carry the piebald gene, the other half did not receive a copy of the gene from their parti gene carrying parent. This can go on for many, many, many years and generations and will go undiscovered until the carrier offspring is bred to another parti gene carrier ... and than SURPRISE!
I am no Biewer expert ... but depending on which Biewer group you talk to, the Biewer is the same as a parti colored yorkie, in that they were produced from two traditional colored yorkshire terrier parents who both carried the parti gene. Unfortunately, this color was denied registration with their foreign kennel registry, so a club was started with their own registry, which allowed the parti colored biewers to be registered.