Jorge,
There are currently two theories about Appy colors. The old school believes that there is one gene complex that results in the pattern plus the rat tail, and roaning. The appy complex is like gray, one parent must be appy for the pattern to appear, and "cropouts" are caused by fraud or fence jumping.
That perhaps is a bit overstated, but pretty much states the case.
The newer view is that there are at least two separate genes involved. LP which is the eyes, striped hooves, roaning, rattail, and odd spotting on the face. The patterns, blanket, and leopard are different genes on a chromosome not related to LP, and so a horse can carry just LP (Appy roans) or just the Pattern (solid horse) it takes the combination of both LP and Pattern to produce a leopard, or blanket. True fewspots carry two copies of LP, so they always produce some color.
Given that the pattern gene does not express unless LP is present, then the "hidden" pattern genes aren't selected for or against, and may be present in a whole range of breeds.
"Cropouts" or Unexpected Appys by this theory are what happens when a LP carrying horse meets a solid horse (no LP) that carries the Pattern.
The two gene theory is being embraced ( in my observation) by folks what have some understanding of genetics.
And a perfect example of another two gene color is Bay. Hidden on red based horses, the Agouti gene only "lights up" when the horse has at least one copy of the black base coat.
Anyone interested in keeping up with the research might want to take a look at The Appaloosa Project
http://www.appaloosaproject.info/index. ... sition=5:5
There are some very interesting discoveries that haven't been published yet, so I'm not comfortable in sharing.
As for LP being present in other breeds, there is the case of Mesaoud the Spotted Wonder, and pure Arabian stallion that exhibited minimal expression of LP. I've seen photos of a modern descendant that clearly had the white eye, but since it was a head shot was impossible to tell if there were other characteristics.
He's on the allbreed site, with a pretty good photo and article.