Oh, Mark - that IS a heavy question
I can't claim to fully understand it, but did study genetics, so if you get totally befuzzled give me a shout and I'll see if I can work through it step-by-step
The site Allison has linked to is a good starting point, it has a lot less jargon that most
Like a lot of dog related issues, there isn't only one opinion - so you just think you've got it and another website suggests something different
The only way to know for sure is to wait until the whole canine genome is mapped - until then, there are a number of different genetic solutions that could explain the colourings we see in dogs