ABO blood group in humans is controlled by gene I and shows co-dominance. The gene I has three alleles ${\mathrm{I}}^{\mathrm{A}}, {\mathrm{I}}^{\mathrm{B}}$ and i. ${\mathrm{I}}^{\mathrm{A}}$ and ${\mathrm{I}}^{\mathrm{B}}$ produce slightly different form of sugar, while allele i does not produce any sugar. ${\mathrm{I}}^{\mathrm{A}}$ and ${\mathrm{I}}^{\mathrm{B}}$ are completely dominant over allele i. But if ${\mathrm{I}}^{\mathrm{A}}$ and ${\mathrm{I}}^{\mathrm{B}}$ are present together, they both express their own types of sugars.