This post first appeared on September 26, 2011

Another Layer of Complexity

So appaloosa patterns depend on a “master switch”, which, by itself, produces a pattern most equine artists would know as varnish roan. It is also responsible hoof striping, white sclera and mottled skin. When that master switch is paired with additional patterning genes, varnish roan is changed to the louder blanket and leopard patterns.

There is another factor besides the pattern genes that can change the appearance of a leopard complex horse. That’s because the leopard complex gene is incompletely dominant.

Most horsemen are familiar with incomplete dominance because of the cream gene. A chestnut horse that inherits one cream gene will be a palomino, while if that same horse inherited two he would be a cremello. Leopard complex works much the same way; horses that inherit two copies of the gene will look different from horses with one.

The most basic difference is seen in the spotting. Horses with one copy of leopard complex are spotted. Horses with spotted blankets and leopards are all carrying one copy of the leopard complex gene. Horses with two copies of the gene are (mostly) unspotted. These are the snowcap and fewspot horses.

So a single varnish roan gene (Lp), when paired with the leopard pattern (PATN1) might look something like this:

While that same pattern gene (PATN1) with two copies of varnish roan might look like this:

And horse with a blanket pattern (some combination of  “smaller effect” patterning genes) and one copy of the varnish gene (Lp) might look like this:

And that same combination and two copies of the varnish gene, might look like this:

There are other differences, too. Probably the most important of these to an artist is that while the hooves on the dark legs of a heterozygous appaloosa are striped, the hooves on the unmarked legs of a homozygous appaloosa will be predominantly shell colored. Remember the varnish roan from a few days ago?

His pale hooves suggest he’s probably homozygous for Leopard Complex. He doesn’t have a pattern gene, so there are no spots to remove, but his hooves are a clue.

Here is an elderly fewspot leopard with the same kind of coloring on his hooves.

They aren’t completely without stripes. Look at the difference on the varnish gelding’s marked foot, which is truly shell-colored, and the stripes on the dark feet. (This is easier to see in his picture than in the one of the fewspot.)

Appaloosa legs with regular white markings like socks and stockings will have ordinary shell hooves no matter whether the horse has one copy or two. Of course, marked feet on non-appaloosas vary in terms of how uniformly pale they are. But in general the homozygous appaloosa hooves look a little more striped than the typical marked foot, and a lot less striped (and a lot less pigmented) than heterozygous appaloosa hooves.

Here are the hooves on the fewspot:

Here is the hoof on a heterozygous (ie., spotted) appaloosa. Note that the legs in both pictures are dark, and in fact are even the same basic color. Both are genetically black horses that have the bronzing effect found in some appaloosas.

Here is the hoof of the same horse as pictured above, only on a leg with a white marking.

When horses have one of the major patterns like leopard or blanket, then the presence or absence of spots makes it pretty easy to tell whether the horse has one or two copies of leopard complex. It’s a bit harder with horses that don’t have an obvious pattern gene. Many varnish roans do not have spots, especially if the varnish pattern is very subdued. The hooves are often the best clue, though those are often hidden in pictures. Hoof coloring can also be hard to determine in pasture-kept horses. The hooves in this post are all on pasture horses that haven’t been recently trimmed, so what striping is there is a little more muted.

Here is the hoof from the post about the pony Thumper, showing the stripes on a recent trim.

A word of caution, though. There are things that can make a horse look like a fewspot or a snowcap, besides the presence of two Leopard Complex genes. When paired with the appaloosa patterns, sabino tends to whiten the pattern and reduce the size of the spots, which can give horses the appearance of being homozygous. These horses are sometimes called false snowcaps or false fewspots. When looking at something that might be a snowcap or fewspot, check for face and leg markings. If the horse has flashy markings, his spotless appearance might be due to that rather than having two appaloosa genes.