Just to add a bit to what David has already explained. I tend to think of dosage as relating to the amount of antigens present on the RBCs that you are using to ID the patient's antibody, and if the reagent RBC has lots of antigens of the type in question, then the reaction will be stronger. This is really important for a patient whose antibodies are just developing--you want to use a reagent RBC with the strongest expression possible, and these are the homozygous cells.
For example, at our hospital, we use the 3 by 3 method for antibody ID (for each type of significant antibody, if the antibody is present, we want to rule in with 3 positive RBCs, and to rule out all the other antibodies, we want to have 3 negative reactions for those.) So for antigens that "show dosage", we want at least one of those three rule out RBCs to be homozygous.
Scott