Non-Mendelian behavior: codominance in human blood types. The cell surface glycoprotein antigens on red blood cells (erythrocytes) determine human blood type. Two different alleles encode two variants of the enzyme glycosylase (an enzyme involved in formation of glycoproteins) and produce different cell surface glycoproteins, A (allele I^A, yellow circles) and B (allele I^B, blue circles). Both alleles are expressed in the heterozygote (AB blood type, genotype I^AI^B). Because both alleles produce functional surface glycoproteins, neither allele is dominant to the other. Individuals with O-type blood have two null alleles (ii) and thus produce no A or B surface antigens on their red blood cells.