While I'm at it, there is another characteristic of Yorkshire Terrier coat color, and that is progressive greying (as opposed to dark black). Progressive greying is due to a dominant allele at the "G" locus. This gene explains why Yorkshire Terrier puppy coats start out black and progressively turn grey (steel blue or silver blue) over time. This gene has not been cloned yet (as far as I know!), so the underlying biochemical basis for it is not known, nor is there a DNA test for it. Here is a review article that describes a wide variety of dog coat color genes:
http://www.fleetfiretimbers.com/fft/...Review2007.pdf