The HBB gene codes for a protein known as β-globin, and mutation in both alleles of the HBB gene causes sickle cell disease (SCD). The allele for wild-type β-globin protein is designated by β^A, and that for mutant β-globin protein by β^S. How many protein bands do you expect to see if you run a sample from a person with SCD through gel electrophoresis?