Some people love it, others hate it; the Grey Gene is what causes all shades of grey horses, from dapple grey to white.
Grey is a simple dominant gene....kinda. This one is a bit tricky, but for the most part we can treat it as if it is a simple dominant gene. When a horse inherits grey, it will eventually lose pigment in its coat over time. A grey horse has to have at least one grey parent, but in some cases this gene can hide very well.
The homozygous recessive state of grey, is a horse that is "not grey". The recessive form has no effect on pigment. Now, here's the tricky part: Horses with one copy of the grey gene (Gg - and did you note the slang useage there?) tend to lose pigment in their manes and tails quickly, while horses who are homozygous dominant for grey (GG) tend to have dark manes and tails for longer, often into their 20s. Because of this, the arguement can be made that grey is really an incomplete dominant gene (3 distinct phenotypes for each combination of alleles: GG, Gg, and gg).
But, to make it easy, most people treat this gene as a simple dominant gene. You either have grey or you don't have grey. Both homozygous dominant horses (GG) and heterozygous horses (Gg) will lose pigment in their coat as they age. How fast depends upon a bunch of other factors, and to prevent headaches, I'll just say that breed, bloodlines, and white markings all play a role in determining how fast a horse will turn white.
Grey foals are often born with a trait called "Grey goggles". Sounds silly, but once you see them, they really do look like goggles on the foal. Take this appaloosa filly:
Various grey horses go through various shades of grey. Dapple grey is a favorite of many people, but it's not that common in stock horses. It's most often seen in draft and draft crosses, with a few select lines of Thoroughbreds dappling. Most horses simply fade out over time with each shedding. With the girls above (Oops and Phoenix) each spring we would feel as if we were currying away all their color, only to find a new and amazing look. Other common shades of grey are steel grey - a solid dark grey color across the body, rose grey - a pink cast to the coat from either a chestnut or bay colored base coat, and of course the end result is white.
Having an always color changing horse is pretty cool, but grey has a downside. Melanoma is known to be caused by the grey gene. This causes tumors across the horse's intestinal tract, and eventually you will find bumps under the horse's tail. Most people begin to see the tail bumps when the horse is in its 20s, and of course each horse is different. I once knew a 40 year old grey quarter horse who didn't have a tumor on him.
These tumors can result in blockages in the intestines though. The horse will begin to colic, because a tumor acts as an impaction, and no amount of medicine will cure it. Now, before you start to panic about your grey horse, realize that the tumors rarely affect the horse before its mid 20s, and on average, grey horses live 3 years shorter then non grey horses. The reason for knowing about the melanomas though, is to help make those medical decisions a bit easier. If your 25 year old grey horse keeps having minor colics, knowing that tumors are the likely cause makes the decision to have, or not to have surgery a whole lot clearer.
Some grey horses will show tumors in other places. Ash (20 years old) has a lump on her shoulder, and recently developed one on her girth area. They cause her no discomfort, but they do prevent me from riding her. The saddle rubs those lumps, and she becomes cranky. For me, this means that my baby has earned her retirement, and gets to live the life of grazing and babysitting foals. My mother's mare, Keeley (also 20), only has a few small bumps under her tail. So the effects of grey can vary widely.
Grey can hide on white patterned horses, such as Fewspots, or dominant whites. Because these horses are born white, there is no hair pigment to be lost over time. Horses with parti color coats, such as appaloosas, pintos and paints, will lose pigment where they have it, but will retain the black skin where they once had color. For these horses, once they have fully greyed out, you can still find their original markings when the horse is wet and you can see their skin.
Grey can be confused with other white patterns, such as rabicano, roaning, or appaloosa roaning. There are ways to tell, but pedigree and parentage is the most common. I believe a DNA test for grey is now available. The test was first developed by a private company who refused to license it to other labs, and sadly, I have not checked to see if it's publicly available.
Now... Next test.....Since every one loved the last so much. There are 2 possible correct answers!
What color is this horse: ee Aa CRcr Gg
A Note about Sugarbush Draft Horses
I see it over and over again, and no matter how many times it's said, it's still wrong. "Sugarbush Drafts are just an Appaloosa Draft Cross". Uh.... no. The Sugarbush Draft Horse was a breed created many years ago in Ohio. While the initial cross was made using Percherons to Appaloosas, in the many generations following, the breed has been solidified into a consistent type. Saying these horses are "just" a draft cross makes as much sense as saying that AQHA horses are "just" a Thoroughbred cross, American Cream Drafts are "just" a dilute Belgian, or that Morgans are "just" a grade.