I think some of the confusion stems from the assumption that "one trait = one gene."
There isn't one single gene that controls border thickness. We see border thickness as "one thing" but that is our own mind's way of simplifying things. It makes it easier for us to deal with describing a pattern we recognize, but it doesn't mean that there's one "
the" gene controlling border thickness. It's like seeing a hurricane as "one thing" and looking for the butterfly as "
the thing that caused the hurricane."
It's controlled by a lot of different factors, or "polygenic," meaning controlled by many genes.
If genes a through h affect border thickness, you might get one snake with genes a thru d being major contributors to huge borders. Meanwhile you get another snake with e thru h giving it thick borders. If they all were recessive, you could cross the two and get normal or thin borders from two really thick-bordered parents.
Or, if they were all dominant (and cumulative) and the one was het for a thru d, and the other was het for e thru h, you would get a bell curve of different degrees of border thickness in the offspring.
In reality some might be recessive, codominant, and/or dominant. You will get all kinds of mixtures between what is seen in the two parents, and even some that are more extreme than either parent. It's definitely more like crossing a collie with a poodle. There's no "poodle gene" or "collie gene," the differences lie in a bunch of different genes, and you will see blending of various degrees in the offspring, with some collie-like traits, and some poodle-like traits. In order to get what you want, you select the ones with the most desireabe phenotypes and continue to the next generation.
