The KIT gene is one of the most important genes when it comes to white markings. It comes in many different versions, but any horse can only have two at the same time
For example. Tobiano is actually a reversion of the genetic code very close to the KIT gene. And dominant white can be a deletion of bits of the genetic code on this gene <sources!>. With all these little adjustments to one gene, many different variations of white patterns appear. But remember, the exact and final pattern is unique and depends on environmental factors in the mare’s womb, after the foal is conceived! For that reason, no two horse clones are the same when it comes to their white markings.
The only exception to everything above is crossover. During meiosis is it not unseen that two accompanying DNA chromosomes exchange a part of their genes. This happens when the two chromosomes form a knot and then break when they are pulled apart. The broken ends are then switched between the two chromosomes. If this breakage point happens to be around this large KIT gene, it is possible one of the sperm or egg cells ends up with two versions of 1 gene on the very same chromosome.