in humans and dogs alike eye color is detirmined by the production of melanin. melanin is produced by special cells called 'melanocytes'. within these cells are 'organelles' that actually make the melanin. What determines color is not how many of these organelles are present, but rather their activity levels. the presence of the gene, (for example OCA2) would create blue or brown eyes=its presence (or whicever gene happens to be present in the body)would then, 'tell' the cells in the iris to make melanin. however, when something goes wrong, (trauma, perhaps), and an iris does not get the 'signal' to produce melanin, the result is two different colored eyes. so yeah, it seems some breeds are predisposed to this, but actually any dog from any breed (humans as well) can have this occur.