I once owned a flea-bitten grey pony with the delightfully misleading name of ‘Diablo’. The rumour about this sweet, chubby gelding was that he was half Welsh Mountain Pony, half Standardbred, the product of an unsanctioned dalliance at a local agricultural show. This was completely plausible, since like most ponies he could get fat on the hint of a whiff of a single straw of hay, but wow could he trot; he regularly gave horses with at least two hands on him a fright as he floated gracefully past in our Pony Club trotting races.

Diablo’s grey colour was probably inherited from his dam the Welsh Mountain Pony, because this breed has a high proportion of greys (at least it does in the area where I grew up). He would have been born with a darker coat colour like bay, chestnut or black, and greyed with age, with the grey starting to appear with the loss of his foal coat, and white hairs becoming the majority by the time he was two or three years old (although the speed of this greying process does vary between horses and especially between breeds).

In horses, greying with age occurs when your horse inherits a version of the greying gene from a grey parent (it also happens if your horse inherits it from both parents, which can happen if they are both grey).

As I’ve said in the article about chestnut, black, and bay or brown, it’s important to remember three things when getting a grip on your horse’s genetics:

1. Your horse has two copies of each gene*, one inherited from either parent;
2. There are many different versions of each gene in the horse population, some of which can produce identical or nearly identical appearance and biological function, while other versions can produce a different coat colour or cause disease; and
3. Each physical characteristic of your horse, including coat colour, results from the combined effects of the two copies of each gene, AND the interactions between a large number of different genes.

The gene that governs greying with age versus not greying with age is called syntaxin-17 (STX17).

Now, why do we get glamorous grey dappling and the not-so-glamorous flea-bitten grey colouration? As far as I’ve heard (from actual reliable scientist-type people rather than random internet chat boards), no-one is completely sure. But the bad news is that dapples will inevitably fade away, while the dark flecks of a flea-bitten grey stay forever.

Horses that go grey with age have a greatly increased risk of developing melanomas, and can have spotty skin depigmentation too. An outstanding picture of this kind of depigmentation is at The Mare’s Tales blog.

For most people, their grey horse goes completely snow white and only then develops coloured flea-bitten grey flecking. This doesn’t necessarily happen all over the body at the same time: for example a horse’s head may be flea-bitten, most of its body white, and its legs still faintly dappled. It can also be very hard to tell when the dark flecking of a flea-bitten grey turns up in a horse that greys fast, because they only change coats twice a year, and we only see 6-monthly snap-shots of what’s happening to their coat.

If your horse is already almost white, Practical Horse Genetics can carry out genetic testing for the grey gene and the base coat colour genes.

Diablo was mostly flea-bitten when I got him, with some white parts and some darker grey remaining on his legs. His mane and tail were completely white, and I don’t recall that there were ever dark hairs in them … which brings me to the final story about him: someone STOLE HIS TAIL! Yes, his thick, snowy tail was cut off just below the tail bone to be a false tail for some loser whose horse didn’t have the luscious, snowy locks of my chubby trotting champion! You can no doubt imagine how frustrated and annoyed I was; I can only imagine what Diablo himself thought!

I only hope they bribed him to stand still with so many apples or sugar cubes that he thought it was worth the trade.

PS. I’ve heard that some people confuse white horses with grey horses, since some grey horses go completely white. From my experience, you could only make this mistake if you’d never seen a real white horse. It’s similar to the difference between a blonde and an albino – you might wonder if a very pale blonde person is albino, but when you see someone who really is albino there ain’t no wondering about it. Believe me, if you’re not thinking ‘Wow, a real white horse! That’s amazing!’, the horse is grey.

* Except for genes on the X and Y chromosomes in male horses, where there may be only one copy.