We have always had one or more gray stallions over the last 50 years. We sold our last gray stud 2 years ago and he went to the UK.
We actually think there are two slightly different gray genes:
The one gets flea bitten as they gray out and never turns white; they just keep getting more speckles as they age. The flea bit spots will be reddish brown if they carry the red gene along with the gray gene, even if their base color was not red. Some of these horses turn white and then start getting more fleas bit spots as they get older. The last stallion that was like this was Classical Silver. He had no red gene so he only sired bays, browns and grays and sired 80% grays. ALL of his gray foals got flea bites unless they were out of gray mares that had turned plain white. Those had an 80% chance of getting the speckles.
The other gene turns lighter and lighter and sometimes never dapples if the base color is is not dark. They all end up white if they live long enough. The stallion we just sold was like this. He carried a red gene and was foaled grullo or dun before he turned gray. Few of his foals dappled out and none of them got the speckles if they were not out of gray mares. It was always fun to see what the foals would do out of daughters of the other gray stud that had the flea bitten and dappled foals. He sired a lot of duns and most of his gray foals were born dun and grayed out by the time they were 5 or 6, never dappled or got flea bites (unless the dam was a dappled gray or the foals were born dark) and the ones born dun kept the dorsal stripe until they were 10 or 12 like their sire did. We still have several of these faded out duns from the stud sold 2 years ago. They never dapple much but the ones foaled bay, brown or black (dark base color) dapple out beautifully.
Our gray Playgun son (sold about 10 years ago) carried a red gene and sired all colors. Playgun is white and our Playgun son also sired foals that turned plain white unless they were out of gray flea bitten mares. I still have one of his daughters (I think 12 or 13 years old) and she is almost white with no speckles coming at all. She was foaled bay so dappled out before turning snow white.
I also raised and trained Arabians back in the 60s and 70s and they also had many that turned snow white and others that got flea bitten. I think the numbers of snow white ones greatly outnumbered the flea bitten ones, but there were both. All of the ones that traced to Skowronek and Nabor* were snow white. We had some other Arabians, mostly Polish, that got flea bitten. Again, the ones that dappled out the most were born dark and not born red.
So, after so many years of breeding and owning several hundred gray horses, they all followed these these gray 'rules'. We have been waiting to see if they ever identify the two distinct gray 'patterns'. There has always been a parentage and thus genetic difference between the ones that turn snow white and the ones that get speckles.