Unlike most other blood groups, the two antigens are not encoded by different alleles; rather, I-branching enzyme converts i antigen to I antigen by adding branches. The gene encoding I-branching enzyme is located on chromosome 6.
The two antigens in this blood group are not produced by different alleles but are modified by an I-branching enzyme, which adds branches to the i antigen to create the I antigen. This gene for the I-branching enzyme is found on chromosome 6.