[Take 2 - Blogger's iPhone client very kindly lost the first take on this post for me on Sunday. Fortunately, it wasn't scheduled for posting till the day you're reading it, so all it required was me to remember what the heck I'd said. I'm sort of glad, since this version is better.]
Part the seventh. Now we've covered most of the saner genetic colour variations on a horse, it's time to consider some questions which are more related to wargames, specifically "what colour should the horses in my unit be?"
This has three answers, two of which cover 'colour' in the equine genetics sense, and one in the (non-equine) sense of paint (pots thereof). I think they're going to merit an article each to finish this off.
So, the first, short and glib answer to the question "what colour should the horses in my unit be?"
Yup. After all those interesting diversions into skewbald perlino dun roans, the predominant colour of your average, run of the mill cavalry horse is good old bay. Not 'brown'. Bay. There are umpty-froo hundred breeds of horse (check Wikipedia if you don't believe me) but when it comes down to it, they're all genetically horses - equus ferus caballus. The three hundred plus different 'breeds'? They're mostly our fault, with a bit of randomness and mutations among closed populations thrown in.
While researching for this article, I ran across a fascinating piece (rejoicing in the same overall title as this series, in fact) that links to proof that ancient horses were pretty much bay, full stop. It also mentions in passing that chestnut horses only started appearing about 9000 years ago. What this tends to suggest is that ancient horses were pretty much universally AA EE, and that a (and similarly, one would guess, e, G and most everything else) are simple random mutations that stuck. Sorry, you lovers of esoteric coat colours - your prize animal is a genetic fluke that managed to hang on!
There's a neat online tool that allows you to calculate what happens when two horses breed: it's pretty obvious that a pair of AA EE bays will always produce another, but equally if you breed Aa Ee with AA EE you will still always get a bay. It's not until you have two animals with e or a mating that you stand any chance of landing a black or a chestnut, and even then over half turn out bay.
And of course, here's where man comes in. Sooner or later, he or she decides, for example, that the black horse is somehow better, more sacred, whatever, and tries to breed from it. Not having the benefit of this series of articles, though, he's probably quite surprised when he breeds it with a bay, and only one in four of its issue turn out black. Eventually, though, he'll get a couple of blacks (EE aa or Ee aa) as a breeding pair, and hey presto, the majority of the progeny of a random pairing of those turn out to be black. But he'll be really surprised when he discovers that the small percentage that aren't are... chestnut??? Wait! What!?
Next along comes someone with a white stallion. By which, of course, I mean a grey horse."Wow," thinks our ancient horse farmer/herder/breeder. "Gotta get me one of those." So he or she pays for it to run with his herd for a bit, and, fingers crossed, it was genetically GG and presto, out pops a Gg foal. Which, to start with, won't look anything like its sire (recipe for disaster there if the white horse's owner can't explain that, although he will be 11 months away by then!) but will eventually turn into a grey horse. Sadly, its offspring will only have a 25% chance of being grey. If our horse breeder's lucky enough to get two foals out of the original deal, that goes up to 75% if he interbreeds them.
And so it goes on! But, still, because your original wildtype horses were AA EE, and both of those are dominant, bays are going to be very common unless your population has been chosen, bred or otherwise lucked into possessing a high frequency of other dominant genes. For an example of this, check out the Appaloosa - actively and quite strictly bred for LP - and as more interesting cases, the Icelandic horse, which as a breed has enough of the recessive a gene for chestnut to be the most common colour, and the Haflinger, which all trace their lineage back to a single aa stallion. (Of course, that doesn't mean there'd be no horses looking like a Haflinger before then!)
This leads me to the second answer to the question "what colour should the horses in my unit be?":
"Mostly bay, unless there's a historical or other reason why not."
Join us next time for a few examples of that answer.
 If I have achieved nothing else in the course of this series, I shall be happy if my readers understand what a bay horse looks like.
 My second success of this series of posts will be if you can explain that to the poor confused breeder.
 My third goal from this series will be achieved if all my readers understand what a grey horse is.
 Way back when, on vacation with my soon to be wife, she'd spent the week trying very hard not to be a vet, and had a fun time playing the dumb blonde (for someone who's done a 6 year veterinary medicine course, that's not typecasting!), and getting the tour company rep to help her with all manner of things. Last night of the holidays, there's a quiz (of the "run up, sit in chair, answer, win a prize or pay a forfeit" type), and the rep, who has her pegged as a real ditz, asks 'what's the gestation period of a horse?'. Cue my dearest, still in apparent full-on blonde mode: she rushes up, hair flying, plonks herself down, answers, a bit breathlessly, "Eleven months." Slight pause, then she, almost without thinking, drops into her professional 'of course, this is a rough estimate I'm giving you' tone. "Give or take a couple of weeks."
Rep looks at her. Looks at his answer card. Looks at her. Very surprised. "How did you know that?"
Herself, innocently, "I'm a vet."
I'm watching the rep's expression, and it's abundantly clear that he's in the process of going "She's a vet. That means she's intelligent. And I've been patronising her for a whole week..." and completely dumping a sizeable chunk of his worldview in the face of having made a bit of an ass of himself!