The trot is dominant over the pace. You will never see a double trot horse pacing. Now so far this is fairly simple. When a horse has one or two strong gait genes the result is much more complex. Gait genes belong to a class of genes that are sometimes called Poly genes.
Poly genes regulate the amount or strength of a characteristic.
These genes produce such things as height , length, size, milking ability, speed, rate of hoof growth, and so on. The characteristics produced by Poly genes are always evident in every individual of the species, but in different amounts. Let's take height for an example. Each horse has two height genes but there may be as many as a hundred different height genes in the equine species. If you breed a tall horse to a short horse, lets say a 14 hand horse to a 17 hand horse, there are a number of different heights that you may produce.