In human beings, 'ABO' Blood group is controlled by which gene ?

Answer

**Step-by-Step Solution:**

1. **Identify the Gene Responsible for ABO Blood Group**:
The ABO blood group system in humans is controlled by a specific gene known as the I gene.

2. **Understand the Alleles of the I Gene**:
The I gene has three different alleles: IA, IB, and i (small i). Each of these alleles contributes to the blood type expressed in an individual.

3. **Function of the Alleles**:
- The IA allele produces a specific type of sugar (antigen A) on the surface of red blood cells (RBCs).
- The IB allele produces a different type of sugar (antigen B) on the surface of RBCs.
- The i allele does not produce any sugar (no antigen).

4. **Expression of Blood Types**:
- When an individual has the IA allele and the i allele, only the sugar from the IA allele is expressed, resulting in blood type A.
- When an individual has the IB allele and the i allele, only the sugar from the IB allele is expressed, resulting in blood type B.
- If both IA and IB alleles are present, they express both types of sugars simultaneously, resulting in blood type AB. This phenomenon is known as co-dominance.
- If an individual has two i alleles (ii), they will have blood type O, as there are no sugars produced.

5. **Conclusion**:
Therefore, the ABO blood group in human beings is controlled by the I gene, which has three alleles: IA, IB, and i.

**Final Answer**: The ABO blood group is controlled by the I gene.

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