New to BP's but not new to genetics.

The question that needs to be asked is the phenotype (the way it looks) different between the heterozygous Spider and the homozygous Spider. Since as far as I know there is no living homozygous Spider. I would have to say that there is a big difference. Het Spiders look awesome and homozygous spiders start to smell bad really quickly and don't move much.

So I would say that the Spider mutation is a codominent one with its super form being lethal. This is not unheard of in other species. If a living homozygous spider can be produced that looks the same as a heterozygous spider and all previous failures turn out not to be related to the genotype of the animal then its a dominant mutation.