"I, for one, have never known a gray mare to produce a Dun foal."

Happens a lot with highland ponies :) Because Dun and grey (with primitive markings) are the most common colours... As far as I undrestand it, grey is a dominant alelle, so if you get one grey gene, your horse is grey, it overwrites other recessive colour genes. I'm probably the worst person to explain this :p