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 recissive
D. Three alleles , all of which are co-dominant

Answer

To determine why there are four major blood types in the ABO blood group system, we can break down the explanation into several steps:

### Step-by-Step Solution:

1. **Understanding Blood Types**: The four major blood types in the ABO blood group system are A, B, AB, and O. Each of these blood types is defined by the presence or absence of specific antigens on the surface of red blood cells.

2. **Identifying the Alleles**: The ABO blood group is determined by three main alleles: A, B, and O. These alleles correspond to the presence of specific antigens:
- **A allele** produces A antigens.
- **B allele** produces B antigens.
- **O allele** does not produce any antigens.

3. **Inheritance of Alleles**: Each individual inherits one allele from each parent. This means that the combinations of these alleles lead to the different blood types:
- **AA or AO** results in blood type A.
- **BB or BO** results in blood type B.
- **AB** results in blood type AB.
- **OO** results in blood type O.

4. **Co-dominance and Recessiveness**: The A and B alleles are co-dominant, meaning that if both are present (as in the AB blood type), both antigens will be expressed. The O allele is recessive, which means that it will only be expressed when an individual has two O alleles (OO).

5. **Conclusion**: The presence of these three alleles and their inheritance patterns result in the four distinct blood types. Therefore, the ABO blood group system's complexity arises from the combination of these alleles and their dominance relationships.

### Final Answer:
The four major blood types in the ABO group are determined by the three alleles: A, B, and O, with A and B being co-dominant and O being recessive.

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