This post first appeared on the studio blog on September 10, 2010
Appaloosa patterns have been on my mind lately. Some of that comes from watching the changes in my own mare’s pattern. After years of without much change, she has begun to roan more visibly.

She still has her rib stripes, though the background color is far closer to gray than chocolate these days. It never ceases to amaze me how very vertical the lines are. They do not follow the contour of the body (like the stripes on a zebra) or the direction of hair growth. Instead they look like someone drew them with a ruler.

Sprinkles did get more white hairs each year, but the process was so slow I thought she’d be quite old before she looked really different. Then last fall I noticed she was getting a few white dots on the back of her ears. I have tried a few times to photograph them, but getting her head to point away from me when I am holding something as interesting as a camera is hard to do!

This summer she started getting the same white dots on her legs. They are more numerous on her hind legs than her front, and far more to the inside than the outside. At the same time she is getting darker dots there, too, though they are much harder to catch since they are only visible in the right light. (The faded parts of her coat are somewhat iridescent.) The spots are quite muted and soft in outline, much like the Tetrarch spots some grey horses get.

You can also see that she has a completely shell hoof on the leg in the first image. Appaloosas have stripes on their hooves when they have solid legs, but when there are white markings they have shell hooves just like any other horse. That is, unless they are homozygous for the “master switch” for the appaloosa patterns. Those horses have shell hooves (or nearly so) no matter what color the leg is.

Which is why I find one of Sprinkle’s buddies so interesting. Jag is a black blanket appaloosa that I suspect also carries the splash gene. He is certainly not homozygous [for Leopard Complex] because his blanket is spotted; homozygous blanket appaloosas end up as snowcaps.

These are the two sides of his blanket pattern. He certainly has spots. He also has the neatest white patches that run all along his spine up to his withers. One of these days I’ll remember to get a shot of that, too.

Edit: I have included those pictures here, along with a shot from underneath his belly. What was interesting about his pattern was that larger irregular white patches followed his dorsal line up to his withers, but the smaller white patches on his belly flanked (and did not cross) the ventral line. 

So he is heterozygous and black. Yet his hooves are almost shell colored, they are so minimally striped.

I only got a shot of his two hind feet, but the front look much the same. They are faintly striped, and that one hind has a dark patch, but they are predominantly shell. Sprinkles, and most of the other genetically black appaloosas I have encountered, have had predominantly dark hooves on their solid legs. (I should mention that Jag has no white on his feet at all.)

I have wondered if this is just a normal variation of expression, or if it is related to his carrying the splash gene, or some other combination of factors. That is what makes appaloosas (and sabinos, for that matter) so very interesting to me. The appearance of the pattern depends on the interaction of many different genes rather than a single one, so it is a puzzle to determine which traits can occur in conjunction and which ones cannot.