How many blood group types in humans can have only similar alleles?

• The blood group in human beings are determined by a single gene called $ABO$ gene which has three different alleles.
• These alleles are- $I_A$, $I_B$, and $i$.
• The blood groups $A$, $B$ and $O$ are formed by similar alleles.
• Blood group $A$- $I_A$ $I_A$.
• Blood group $B$- $I_B$ $I_B$.
• Blood group $O$- $i$ $i$.
• $I_A$ and $I_B$ are called codominant alleles whereas, $i$ is a recessive allele.