A horse of a different colour part 4 - coloured horses

Yes, it has been a long time, but here we are with part 4 of the horse genetics series, and here's where the fact that I'm an Englishman will be obvious.

We're going to discuss what over here are called 'coloured' horses - that is to say horses which have coats which are white with solid patches of another 'normal' coat colour. In the UK, black and white is called 'piebald', and brown/bay and white is 'skewbald' (bay and white - i.e. black mane/tail, brown patches on white - is sometimes called 'tricoloured'). Across the Atlantic, they're all called 'pinto'. However, this is a different pattern to Apaloosa horses (which are not unique to the US), and then there's the American Paint horse, which is another can of worms entirely.

The genetics of coloured horses is complex, and I'm not going to go into it in great depth. If you want a more detailed look, have a peek at the Wikipedia page on pintos. From a painting point of view, what really matters is the pattern of white - collectively, these genes are white-patterning genes.

Skewbald bay with classic
Tobiano markings.
Image by Jean-Pol GRANDMONT
CC-BY-2.5 

The main 'coloured' gene is Tobiano (To/to). It's a pretty normal dominant gene, so if your horse has one copy of it it will display coloured/pinto characteristics. The A/a and E/e genes (from article 1) will govern whether it's a black and white, brown and white or bay and white pinto/coloured. 'Classic' Tobiano markings are white legs, and vertical patches of white, with coloured chest, face and rump/withers.

Next up is Overo. This is somewhat complex, as there are several genes making it up, and we're in the realm of the American Paint breed, which means I need to tread carefully or I'll probably offend someone by getting it not 100% right or grossly over-simplifying.

Well, tough. 'Cause I'm going to over-simplify a bit. :D

Classic Overo pattern.
Image by www.horsevet.co.uk
CC-BY-SA-3.0

Overo patterning is different to Tobiano in that it's much more jagged and horizontal. It is a term used by the American Paint Horse Association to describe a number of 'coloured' horse patterns that aren't pinto/piebald/skewbald.

The most common genetic cause is the Frame gene (O/N). As you can guess from the fact that it's not uppercase/lowercase, its slightly different - it behaves like a dominant gene in that horses with one copy of O display the overo patterning in some form or other. Unfortunately, horses with two copies of O display Lethal White Syndrome - similar to the way Dominant White is lethal, except that the foal is born with a pure white coat and dies after about 72 hours due to an underdeveloped digestive system (and are usually put to sleep sooner).

[Edit: realised I'd missed an important one.]

Sabino patterning on a Clydesdale.
Note, this is not caused by SB1.
Public Domain photo.

Another relatively common (as these things go) patterning gene (or rather set of genes) is/are known as Sabino. They fall into the Overo category as far as the American Paint Horse Association is concerned, and the most common one, SB1/sb1, is another incomplete dominant gene, in that SB1/sb1 (i.e. one copy of the gene) has a different effect to SB1/SB1. Specifically, one copy produces irregular white patches on the face and extremities, and often 'belly spots'. Two copies, on the other hand, are yet another way of producing a white horse! Unlike both Dominant White and the Overo Lethal White, this isn't unpleasantly fatal, though.

Sabino-type patterning, however, doesn't always result from the SB1 gene - there are a number of similar patterned horses that don't test positive for the gene, so there's clearly more research to be done. It is a classic pattern in draft horses (Shire, Clydesdale) although whatever the gene is that causes it, it doesn't produce a white horse if the animal has two copies: it's also a pattern found in Arabian horses, again caused by yet another gene.

Appaloosa with one of
the many possible
patterns.
Wikipedia - CC-BY-SA-3.0

The Appaloosa is a whole different ball game - they display what's called 'leopard' spotting, a much more mottled pattern than either Tobiano or Overo. This is caused by a family of genes collectively giving rise to the 'Leopard complex'. The principal one is Lp/lp, which is an incomplete dominant gene, in that animals with one copy of Lp display different patterning to one with two copies of it. Generally, one copy of Lp produces more, larger spots than two.

I'm not going to try and describe all the possible spotting patterns here, but, as ever, Wikipedia will come to your rescue. It is worth noting that the Appaloosa patterns are not unique to the classic 'Indian pony' - there are records of horses with leopard spotting patterns from as far back as Ancient Greece.

As I said, by this article we're now in the realm of quite complex and not-perfectly-understood genetics - if you want to learn more, Wikipedia is the place to go. But otherwise, hopefully I've covered enough to allow you to get a paint-brush out and produce convincing coloured horses (as well as convincing-coloured horses!)

And I'll leave you with a question to see if you've been paying attention. Why do Romanies have black and white horses?