Hypo is polygenetic, meaning that the traits are probably located on multiple genes. Geckos that have reduced spotting can produce offspring with a range of body spotting from none to lots. People that want to breed for reduced spotting will usually hold back the least spotted geckos and breed them back to the parent or to each other (with later breedings to unrelated geckos so they don't get too inbred) to get even less spotted geckos. This is called "line breeding".
Aliza