Errorness? I'm not going to hound you with my background nor do I care to partake in an argument on the interweb :-) However, see the other thread that is going on in regard to dominant traits.

The fact that noone has a spider or pinstripe that is proven homozygous, is evidence of this. The case for dominance is the exclusion of any other gene present. Co-domance is just what it says - allowing expression of multiple mutations on differnt loci, and in rare cases the same loci, but when its on the same loci it's more leaned toward incomplete dominance where the super form is the actual meant expression of that gene (my example being pastel).

Where is the proof for it? The fact that NO ONE has produced a homozygous spider or pinstripe is the proof, or at least the presented evidence I offer. If somone had a homozygous it would be known as all of its young would contain the dominate gene (not trait) allowing for expression of the trait.