I have another argument for my theory that blood type is dominance and not co-dominance: Dominant until proven otherwise. This is the way I treat any mutation. For my example, assume for a moment that the O allele is actually telling the cell not to bond any antigens (I am not saying I believe this, but this is the easiest example I can give to portray my argument). If this were the case, in a type AO individual, the concentration of A antigens would be lower than in a type AA individual due to an equal number of conflicting signals. (However, this may have consequences because it is possible AO wouldn't be compatible with AA in this example if the O allele would be sending signals not to bond antigens, which is not the case in reality - basically why I do not believe this to be true.) This would be the sort of evidence I would need to agree that blood type is co-dominance and not dominance. If any studies have been done that would conclude that there is a lower concentration of antigens present on the surface of blood cells in a heterozygote than a homozygote, I would agree it is co-dominance. However, if the concentration of antigens was roughly equal, it would seem to support my theory of dominance over co-dominance. Again, I am only expressing that I think it is impossible to say for sure if blood type is co-dominance, which is why I disagree with calling it such.