It's completely different with reptiles (and most other wild animals for that matter). When it comes time to breed, most species don't get out a family tree to make sure they're not mating with a direct relative.
I just wasn't going to get into the semantics of dom vs co-dom...the basic idea was enough to point out that a spider x normal can yield spider offspring.
And for those mentioning it, I agree that the BEST way to get one is to breed a het-axanthic spider to a fully axanthic mate. But that's not always easy or inexpensive to come by.