I was actually wondering that same question as well. I've owned My TB for four years and have never saw him have dapples, until this summer.
That makes me wonder about the whole - horses get dapples because they are in excellent condition and are getting good nutrition, because my TB was such a hard keeper and I could never get him to not be underweight. Until this past winter he put on a lot of weight, and when he started to shed out I noticed the dapples as he is still holding on weight, has a shiny coat, and his energy level has sky rocketed.
Chad Barnes 6-16-85~7-22-13
Hero Act - Thoroughbred Gelding ~ Gunner - Quarter Horse Gelding ~ John Deere - Mini Gelding