The following pair of human blood group alleles are codominant.

Options

A. `I^(O) I^(O)`
B. `I^(A)I^(O)`
C. `I^(B)I^(O)`
D. `I^(A)I^(B)`

Answer

To determine which pair of human blood group alleles are co-dominant, we need to understand the concept of co-dominance and the specific alleles involved in human blood groups.

### Step-by-Step Solution:

1. **Understand Co-dominance**: Co-dominance occurs when two different alleles are expressed equally in the phenotype of a heterozygote. This means that both traits are fully and separately expressed.

2. **Identify the Blood Group Alleles**: In humans, the blood group system is determined by three alleles:
- IA (which codes for blood type A)
- IB (which codes for blood type B)
- IO (which codes for blood type O, and is recessive)

3. **Analyze the Given Allele Pairs**:
- **Pair 1: IO and IO**: This pair would result in blood type O. Since both alleles are the same, this does not represent co-dominance.
- **Pair 2: IA and IO**: This pair would result in blood type A. Here, IA is dominant over IO, so this does not represent co-dominance.
- **Pair 3: IB and IO**: This pair would result in blood type B. Similar to the previous case, IB is dominant over IO, so this does not represent co-dominance.
- **Pair 4: IA and IB**: This pair would result in blood type AB. In this case, both IA and IB are expressed equally, which is the definition of co-dominance.

4. **Conclusion**: The only pair that represents co-dominance is the fourth option, IA and IB, which results in blood type AB.

### Final Answer:
The pair of human blood group alleles that are co-dominant is **IA and IB**.

---