With respect to the ABO group, there are four major blood types becauses this blood group is determined by

Options

A. Three alleles, all of which are recessive
B. Three alleles, of which, two are recessive and the third is dominant
C. Three alleles, of which two are co-dominant and the third is recessive.
D. Three alleles, all of which are co-dominant

Answer

To answer the question regarding the ABO blood group system and the reason for the existence of four major blood types, we can break down the explanation step by step:

### Step-by-Step Solution:

1. **Understanding Blood Types**: The ABO blood group system consists of four major blood types: A, B, AB, and O. Each blood type is determined by the presence or absence of specific antigens on the surface of red blood cells (RBCs).

2. **Identifying the Antigens**:
- Blood type A has A antigens on the surface of RBCs.
- Blood type B has B antigens.
- Blood type AB has both A and B antigens (showing co-dominance).
- Blood type O has no A or B antigens.

3. **Genetic Basis**: The ABO blood group is determined by a single gene known as the I gene, which has three alleles:
- IA (which produces A antigens)
- IB (which produces B antigens)
- i (which does not produce any antigens, hence is recessive).

4. **Allelic Combinations**:
- Individuals inherit two alleles for the ABO blood group, one from each parent.
- The possible combinations of these alleles lead to the four blood types:
- AA or Ai results in blood type A.
- BB or Bi results in blood type B.
- AB results in blood type AB.
- ii results in blood type O.

5. **Co-dominance and Recessiveness**: The alleles IA and IB are co-dominant, meaning that if both are present (as in AB blood type), both antigens are expressed. The allele i is recessive, meaning it will only express the O blood type if both alleles are i.

6. **Conclusion**: The existence of four major blood types is due to the presence of three alleles (IA, IB, and i) and the co-dominance of the A and B alleles, along with the recessive nature of the i allele.

### Final Answer:
The four major blood types in the ABO group are determined by the presence of three alleles (IA, IB, and i) and the co-dominance of the A and B alleles.

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