Here is Izzy and she is both fleabit and still has a few dapples, all at the age of 17. So she must have been a late "grayer." I'm sure she is heterozygous. He son is turning gray rather slowly too, or at least people act like he is. Some folks have even said to me "so he's a bay after all." And I'm like "nope, I still say he's a gray!"
I wish I could have seen her 3 yr old baby. The guy tells me he's a blue roan. But Izzy is a gray (base color black I think) and the sire is chestnut, so I know he's almost certainly a gray too. People really have trouble with roan vs. gray. But if you learn to look for white on the head, it becomes really easy to tell the difference. Grays usually have white hairs peeking from the center of the tail too.