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 will analyze the ABO blood group system, which is determined by the alleles of the I gene. Here’s a step-by-step breakdown of the solution:

### Step 1: Understanding Blood Groups and Alleles
- The ABO blood group system is controlled by three alleles: IA, IB, and i.
- IA produces A antigen, IB produces B antigen, and i produces no antigen (O blood group).

**Hint:** Remember that alleles can be dominant, recessive, or co-dominant.

### Step 2: Identifying Genotypes
- The possible genotypes for the ABO blood groups are:
1. IAIA (Blood group A)
2. IAi (Blood group A)
3. IBIB (Blood group B)
4. IBi (Blood group B)
5. IAIB (Blood group AB)
6. ii (Blood group O)

**Hint:** List the combinations of alleles that can produce each blood type.

### Step 3: Analyzing Similar Alleles
- We need to find blood groups that can be formed by similar alleles (homozygous conditions).
- The blood groups that can be formed with similar alleles are:
1. Blood group A can be formed by IAIA (homozygous).
2. Blood group B can be formed by IBIB (homozygous).
3. Blood group O can be formed by ii (homozygous).

**Hint:** Focus on the genotypes that consist of the same allele repeated.

### Step 4: Conclusion
- The blood groups that can have only similar alleles are A, B, and O.
- Therefore, there are **three blood group types** in humans that can have only similar alleles.

**Final Answer:** 3 blood group types (A, B, O).