This is a question of semantics(?). Biologists are not known to be gods of well-definedness, but if there actually exists an argument over the following question (even a possibility), then I might get some credit back on some classwork. What is the definition of a dominant mutation (of a nucleotide of an allele at some locus)? 1. That the mutation yields a new allele which is necessarily dominant w.r.t. to the un-mutated (old) allele. 2. That the mutation yields a new allele which is necessarily dominant to "other" alleles (or perhaps co-dominant with the unmutated version). Co-dominance of two alleles at a locus implies that they are both equally dominant over other alleles.