Years ago BHB reported to have an apparent homozygous pinstripe that looked like a normal (heterozygous) pinstripe but produced a large number of only pinstripe offspring. Pinstripe is the only proven dominant ball python mutation I can think of right now. It seems to me very likely that spider is homozygous lethal and hence co-dominant. Problem is it's nearly impossible to prove a homozygous lethal scenario if the homozygous babies never hatch. Sure it could still turn out that there has just been too little interest in breeding spider X spider and trying to prove the offspring homozygous to find one but with each passing year with still no public proven homozygous spider it seems less likely.