There not anymore proof of the spider mutation being dominant than it being co-dominant. We list morphs as dominant until proven otherwise.

Here is a part of a write up I did on the spider gene

The Homozygous Spider

If the Spider is a dominant gene and I bred a Spider with another Spider, I should statistically get 25% Homozygous Spider, 50% Heterozygous Spider, 25% Normal. The homozygous Spider should look exactly like a heterozygous Spider. The only difference would be that the homozygous Spider, when bred to any other ball python, would result in offspring that are all Spiders. While the heterozygous Spider would result in offspring that would only have a 50% chance for getting the Spider gene.

Now here's the problem, the Spider was first established at NERD in 1999, and we still have yet to see a proven homozygous Spider. Now before we get into all the theories, Kevin (the owner of NERD), the guy that imported the first, and has dealt with, more Spiders than anyone else, has to say about the issue. Kevin said that he has not produced and does not believe that there is a homozygous Spider nor that Spider is homozygous lethal. From what I understand he is saying it just simply does not exist for an unknown reason and is not a simple dominant gene.

Another theory is that homozygous Spider is lethal, which is based on the fact that we haven't seen one yet. Which is a pretty weak theory in my opinion, but not entirely dismissible. I do think this theory could be put to rest by watching some Spider x Spider pairing and keeping an eye on the follicles of the females, if somewhere around 25% of them die/reabsorb/not hatch, it may strengthen this theory, but I think people would want to see a lot of eggs before we call it fact. This would be a lot of resources to waste for a big breeder to try, so don't expect it any time soon. I think the only chance of proving this, is by us small breeders trying it out and all of us putting our data together.

Another theory, there hasn't been enough Spider x Spider breedings to prove or disprove a homozygous Spider. Well here's the deal with that, you first have a breed a Spider x Spider. Every Spider offspring only has a 33% chance to be homozygous, then you have to raise up the offspring and do multiple breedings to prove it is in fact homozygous, rather than just getting really lucky on a clutch. So, yes very difficult to prove.

Not many people have the resources to do this, NERD has tired to prove it but as far as I know, they don't have any public statement about what they did to try to prove the homozygous Spider. That leaves this theory open ended. More info would sway my opinion one way or another. Though if Kevin said he did enough breedings, he most likely did enough breedings.

Now the question, "If I bred a Spider x Spider, statistically what % of Spider should I get?" Well If homozygous Spider does exist and is dodging us all these years, 25% homozygous,50% heterozygous, and 25% normal. If it's homozygous lethal then 66% heterozygous and 33% normal. If it doesn't exist for some reason, depending on how you look at it, it could either be 66% heterozygous and 33% normal or 75% heterozygous and 25% normal. So go ahead, pick one.