Gray (G) is a color modifying gene. It is also a dominant gene, which means one parent must be gray in order for the foal to be gray. Two non-grays cannot produce a gray. It gradually turns any color of horse, including any dilutes or pintos, white. The horse starts out its base color and then gradually turns white. One way to tell if a horse is going to gray when they are born is that they tend to be born their "adult" color rather than the tan or mousey color non gray foals are born. However not all foals born their adult color will gray and not all that are born tan/mousey wont gray, but I have found it to be pretty reliable. The study covering the gray gene in horses has just recently been released, in it they found a few things I personally found interesting.
1. Heterozygous grays are more likely to have fleabites.
2. Homozygous grays go gray faster.
3. Horses that are aa have a higher chance of getting melanomas.
4. Homozygous grays have a higher chance of getting melanomas.
5. Homozygous grays also have a higher chance of having vitiligo, or depigmentation of the skin.
This is Fuerst Gotthard from Rainbow Equus Meadows. He is homozygous for gray.
This is Lavita from Wild Turkey Farms. He is heterozygous for gray.