In the context of the $ABO$ blood group system,what happens when both alleles $I^A$ and $I^B$ are present together?

• A
  Only the $I^A$ allele is expressed.
• B
  Only the $I^B$ allele is expressed.
• C
  Both $I^A$ and $I^B$ alleles are expressed.
• D
  None of the above.