Students frequently and mistakenly think the allele i codes for a type of antigen. Take out two A-negative red blood cells and place them on one end of the whiteboard. Ask a student to come up and write, using an overhead marker, a genotype that would produce this blood type. Remind the students about the alleles which cause antigen formation. Then ask if there are any other genotypes which would produce the phenotype.
Students can now see very clearly the relationship between genotype and phenotype.
Take out a couple of A-positive RBC's. Place them on the opposite end of the whiteboard. Ask students about the difference between these cells and the A-negative cells. Students can clearly see that while both types of cells have the A antigens, one has the Rh antigens as well. You may want to go into a good amount of detail about B-cells, etc., or you may just mention that antibodies are proteins produced by certain white blood cells and leave it at that.