The "IA" allele gives type A, "IB" gives type B, and "i" gives type O. As both "IA" and "IB" are dominant over "i", only "ii" people have type O blood. Individuals with "IAIA" or "IAi" have type A blood, and individuals with "IBIB" or "IBi" have type B. "IAIB" people have both phenotypes, because A and B express a special dominance relationship: codominance, which means that type A and B parents can have an AB child. A couple with type A and type B can also have a type O child if they are both heterozygous ("IBi","IAi"). The "cis-AB" phenotype has a single enzyme that creates both A and B antigens. The resulting red blood cells do not usually express A or B antigen at the same level that would be expected on common group A1 or B red blood cells, which can help solve the problem of an apparently genetically impossible blood group.