Change in how much or what kind of product is made from a gene. Somehow its RNA would have a dominant effect. If it bound another protein and remained bound, or if was an always ON sort of receptor. Because there are many more ways to screw up a protein or prevent it from being made than there are to change a protein to have a novel phenotype . Dominance has to do with phenotype.
Dominance is an effect observable with a single mutant allele.
Those that wouldn't be observable would be due to what we currently call recessive mutations.