ABO blood groups is determined by

Options

A. three recessive alleles
B. three codominant alleles
C. three alleles, two dominant and one recessive
D. three alleles, two recessive and one dominant

Answer

**Step-by-Step Solution:**

1. **Understanding ABO Blood Groups**: The ABO blood group system is determined by the presence of specific antigens on the surface of red blood cells. These antigens are determined by the alleles of a single gene.

2. **Identifying the Gene and Alleles**: The gene responsible for the ABO blood group has three alleles:
- IA (which produces A antigens)
- IB (which produces B antigens)
- I0 (which does not produce any antigens, often referred to as the O allele)

3. **Dominance Relationships**: In the ABO blood group system:
- Both IA and IB are dominant alleles.
- The I0 allele is recessive. This means that if an individual has at least one IA or one IB allele, they will express the A or B blood type, respectively. The O blood type is expressed only when an individual has two I0 alleles (I0 I0).

4. **Possible Genotypes and Phenotypes**: The possible combinations of these alleles lead to four blood types:
- Type A: Can be either IA IA or IA I0
- Type B: Can be either IB IB or IB I0
- Type AB: IA IB (both antigens are present)
- Type O: I0 I0 (no antigens present)

5. **Conclusion**: Therefore, the ABO blood groups are determined by the presence of three alleles (IA, IB, and I0) of a single gene, with IA and IB being dominant and I0 being recessive.

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