The LP gene is the main gene needed for Appaloosa coat color. LP stands for Leopard comPlex. It causes a few things to happen on its own:
White Sclera. This is the white part around the eye. In most horses they have solid dark eyes, but appaloosas (small "a" refers to color pattern, big A refers to the breed) have a human like eye.
Then there's the striped hooves. Not all appaloosas have striped hooves. Some have solid amber colored hooves. Usually, but not always, the amber colored hooves are seen on horses that are homozygous for the leopard complex gene, or LPLP.
Striped hooves are not unique to appaloosas, but they are one of the many traits. Many horses will have striped hooves on white or partially white legs, while an appaloosa will have striped hooves on a dark leg. The second picture shows an amber hoof that's smudged with dirt. Sadly, I never took pictures of amber hooves, so had to go looking at what I could cut out of another picture! The amber colored hoof on a dark leg is indicative of homozygosity in this horse. Sadly, most of my homozygous horses have 4 white legs, which would have a "white" hoof on them anyway.
Then there's mottled skin. The same way that appaloosas have polka dotted hair patterns, they also get spotty skin. The amount of mottling in the skin often progresses with age, just as the horse's roaning does. This mottling is often seen around the eyes and lips, as well as the genitals. Basically places where there's less hair.
And then, there's the progressive roaning. Here's an example of a horse that roaned severely in just a couple of years:
Shades of Olympic (aka Shadow) as a yearling
As a 2 year old the next spring:
But the most important thing to know about the LP gene, is that it "lights up" the appaloosa patterns. LP is an incompletely dominant gene. This means that with 2 recessive copies (lplp) you get a solid horse, or non appaloosa horse (paints and pintos are lplp). With one dominant copy and one recessive (heterozygous or LPlp) you will get polka dots in the coat. This is the type of patterning most people think of when they think "appaloosa". And with 2 dominant copies you will get a horse with no or very few dots in their white pattern. These are called snowcaps when there is only a white area on the rump/torso, and fewspots or near fewspots where there's white on over 80% of the horse's body.
Lets see if this helps:
He is heterozygous for LP (LPlp) so has polka dots in his "dense white area".
And this is a snowcap:
This area is called the "dense white patterning" or a variation thereof (dense white pattern, dense pattern area, etc).
This filly is homozygous for LP, or LPLP. And yet both of these horses would be considered to have the same "pattern" but a different 'zygosity.
And of course, a horse with 2 recessive lp alleles would look like this:
Ok, I think that sentence made MY head hurt. Lets use pictures instead!
So, if you bred her to a horse with no pattern at birth... such as my lovely Scorch....
They could produce a foal that looks like this:
Formula One Farms. Yes, he's an Appaloosa not a Stonewall Sport Horse, but I'm hoping you get the idea.
So, those solid mares from loud pedigrees are rather important in keeping the color alive in the LP breeds. So often they are thought of as a "waste" or a junk horse, when in fact they have some of the most important genes out there.
If you keep breeding horses with "recognizable" appaloosa patterns back to each other, eventually the recessive lp gene will be lost. There are many examples of breeding out recessive alleles due to a desired appearance.
Cleveland Bay horses are always bay. It's extremely rare to find a horse that carries the recessive agouti allele "a". Friesians require a black coat color for registration. Before DNA testing, almost all of the stallions were homozygous for Extension of Black (EE). Today, stallions must be genetically proven to be homozygous in order to be approved for breeding. You will only find the recessive extension allele "e" in the mares, or stallons that are not accepted as breeding stock. Soon, it will go away completely.
In appaloosa colored horses, if we breed out the recessive lp gene, we completely change the appearance of the horse. All horses would end up looking like this:
And I don't know about you, but solid white is BORING!
A Note about Sugarbush Draft Horses
I see it over and over again, and no matter how many times it's said, it's still wrong. "Sugarbush Drafts are just an Appaloosa Draft Cross". Uh.... no. The Sugarbush Draft Horse was a breed created many years ago in Ohio. While the initial cross was made using Percherons to Appaloosas, in the many generations following, the breed has been solidified into a consistent type. Saying these horses are "just" a draft cross makes as much sense as saying that AQHA horses are "just" a Thoroughbred cross, American Cream Drafts are "just" a dilute Belgian, or that Morgans are "just" a grade.