I don't know if the mechanism by which the white spotting gene influences markings is via impaired migration of melanocytes during development or if it due to reduced numbers of melanocytes.  Either way, the net effect is decreased pigment on the dog, in patterns that are somewhat predictable based on the location of melanocytes embryologically.  


On Sep 4, 2012, at 5:18 PM, Rini wrote:

Thank you, Mary - I love reading stuff like this (and even understand some of it! Keep it coming).  :)  Yes, I picked the name up from the King Charles Spaniels, assuming it was used across the board. I've also heard them called Pythagorian Thumbprints but only from one breeder (Boston Terrier), so who knows. Google doesn't mention it. Did you find a different name for it?
So some of our dogs have slow melanocytes?  :)
  When there is a lot of white on the head, you may see little "islands" of pigmented fur around the ears and in the middle of the forehead corresponding to the 3 "original" pools of pigment producing cells... the melanocytes in those dogs don't migrate very far from their location of origin ;-).