* **The 4 common blood groups of the human ABO blood group system are determined by three alleles Ia, Ib, and i (also represented in some texts as A, B, and O)**
* **ABO antigens consist of sugars attached to the red blood cell surface. These sugars provide individual antigenic properties. The alleles code for enzymes that join these sugars together.**
* **Allele i produces a non-functional enzyme that is unable to make changes to the basic antigen (sugar) molecule**
* **The other two alleles (IA, IB) each produce a different enzyme that adds a different specific sugar to the basic antigen.**
* **Any one individual possesses only two alleles and they are expressed equally.**