Antigen Expression - Variation in Antigen Expression

        RBCs from individuals who are homozygous for an allele typically have a greater number of antigen sites than do individuals who are heterozygous. Consequently, their RBCs can react more strongly with antibody. This difference in expression and antigen–antibody reactivity because of zygosity is known as dosage. For example, RBCs from a homozygous MM individual carry a double dose of M antigen and react more strongly with anti-M than do RBCs from a MN heterozygous individual carrying only a single dose of M. Antithetical antigens C/c, E/e, M/N, S/s, and Jk^a/Jk^b commonly show dosage. Dosage is less obvious with D, K/k, and Lu^a/Lu^b antigens. It typically is more apparent within a family than between families. Dosage within the Duffy system also may not be serologically obvious because Fy(a+b–) or Fy(a–b+) phenotypes are seen in either homozygous (Fy^aFy^a or Fy^bFy^b) or heterozygous (Fy^aFy or Fy^bFy) individuals.