ABO blood groups are controlled by the gene I. The gene has three alleles `I^(A), I^(B)` and i.
Because humans are diploid organisms, each person possesses any two of the three I gene alleles. `I^(A) " and " I^(B)` are completely dominant overi, in other words when `I^(A)` and i are present `I^(B)` expresses. But when `I^(A) " and " I^(B)` are present together, they both express their own types of sugars : this is because of co-dominance.
