Tuesday 5 April 2011

A horse of a different colour...

A brief diversion, brought on by a discussion over the weekend and at the club on Monday on the subject of painting cavalry.There is slightly more to the subject than slapping on some brown paint and hoping. My wife's a veterinary surgeon, and she would probably disown me for incorrectly painted horses!

A long time ago (somewhere about 1997) I used to play on one of the variants of an online MUD game that allowed players to code various things. This particular one was heavy on roleplaying as opposed to killing monsters, and one of the staff had coded up some rideable horses. These latter I showed to Anne, my wife, who was somewhat unimpressed at the way the code picked colours, so I set about learning more about horse coat colours and fixing the code.

So. Herewith part 1 of a course on horse colours. The easy bit. But first...

Basic genetics. It's all pretty simple, really. Most of the genes we're going to look at work much the same way, so let's consider a gene. In essence, it's a genetic switch between two characteristics, which we'll label as upper and lower case letters, say G and g. For a given gene, you have two copies - each of your parents gives you one chosen at random from the two they possess, so you potentially can either be GG, Gg (in one of two ways) or gg. [Yes, yes, spare me the gee-gee jokes!]

The important thing is that, in most of the cases we are considering, G is dominant - it swamps g - which means that if you have GG OR Gg, you will display the G characteristic, and ONLY if you have gg will you display the g characteristic.

With me so far? Good. So, how does this apply to horses?

There are TWO really important genes in horse colours. E or e, and A or a. Let's start with E.

  • E means the horse can produce black pigment in its coat - it has the potential to be a black horse.
  • e means it can't, it produces red/brown - it's a chestnut or sorrel.
Simples! So in the absence of any other genes, EE and Ee horses have some black in their coats, ee are chestnut. (And note that an EE horse can never produce a chestnut foal, because it must ALWAYS give a copy of E to its offspring, which will dominate whatever the other parent gives.) So, given random distribution of E and e, ONLY 25% of horses should be chestnut/sorrel.

And the rest? Well, that's where the other important gene comes in - A or a (it's called the Agouti gene).
Picture by extreme_eventer04
  • A means any black pigment in the horse's coat is restricted to its 'points' - ears, mane, tail, legs etc. i.e. if the horse has an it's a bay - red/brown body, black mane and tail. Recent research suggests that a horse with AA or Aa and EE is statistically likely to be a darker bay than one with Ee.
  • a means any black pigment will be everywhere: so IF the horse has an E it will be black
Again, A is dominant. So 75% of the remaining horses in our random selection will be bay (AA or two ways of getting Aa). The rest will be black.

So: ignoring the more complex colours and other genetic fun (I really went to town on the MUD code!), assuming a random gene pool, out of 16 horses, 4 will be chestnut, three will be black and the rest will be bays. 

(Aside: when I next paint some cavalry I'll add pictures of painted examples of each colour to these posts.)

Next time? White, both as markings and as grey/white horses.


  1. Thank you for the info!

    I'm ashamed to say that even though I learnt about Mendelian genetics in school I never got down to understanding coat colour inheritence.

    I've always wondered what proportion of my horses to paint in what colour, and your post has answered that for me.

  2. Thanks for this Mike - fascinating. Need to get myself some ink washes.


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