How many blood group types in humans can have only similar alleles ?

Options

A. Four
B. Three
C. Two
D. One

Answer

To solve the question of how many blood group types in humans can have only similar alleles, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Blood Groups**:
Humans have four main blood groups: A, B, AB, and O.

2. **Determine the Genotypes**:
- **Blood Group A**: Can be represented by the genotypes:
- \( I^A I^A \) (homozygous)
- \( I^A i \) (heterozygous)
- **Blood Group B**: Can be represented by the genotypes:
- \( I^B I^B \) (homozygous)
- \( I^B i \) (heterozygous)
- **Blood Group AB**: Can only be represented by the genotype:
- \( I^A I^B \) (heterozygous)
- **Blood Group O**: Can only be represented by the genotype:
- \( ii \) (homozygous)

3. **Identify Similar Alleles**:
- For blood group A, the similar alleles are \( I^A I^A \).
- For blood group B, the similar alleles are \( I^B I^B \).
- For blood group O, the similar alleles are \( ii \).
- Blood group AB does not have similar alleles since it consists of two different alleles \( I^A \) and \( I^B \).

4. **Count the Blood Groups with Similar Alleles**:
- Blood group A: 1 genotype with similar alleles (\( I^A I^A \))
- Blood group B: 1 genotype with similar alleles (\( I^B I^B \))
- Blood group O: 1 genotype with similar alleles (\( ii \))
- Blood group AB: 0 genotypes with similar alleles.

5. **Total Count**:
- Adding these, we find that there are 3 blood groups (A, B, and O) that can have only similar alleles.

### Conclusion:
Thus, the answer to the question is **3** blood group types in humans can have only similar alleles.

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