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Science Background Multiple Alleles and Codominance The multiple alleles considered in the ABO blood typing system do not follow basic Mendelian principles of complete dominance. Instead, the expression of the surface antigen trait in the ABO blood typing system is governed by the genetic principle of codominance, where both A and B are codominant alleles. Therefore, a person inheriting a Type A allele from their mother and a Type B allele from their father would express the codominant phenotype, Type AB. Because the only recessive allele in the ABO system is the O allele, only homozygous recessive individuals express the Type O phenotype, which is characterized by a lack of A and B surface antigens on the red blood cell. As the following table illustrates, it is possible to express Type A and Type B phenotypes in the heterozygous condition. Blood Type Chart Blood Type (Phenotype) Surface Antigen Protein(s) Possible Alleles in Genotype Type A A AA or Ao Type B B BB or Bo Type AB A and B AB Type O none oo The Rh Factor Early studies using rhesus monkeys as test subjects allowed scientists to discover what is now understood as the Rh factor. The expression