ABO blood groups in human beings are controlled by the gene `I`. The gene `I` has three alleles `- I^(A),I^(B)` and `i`. Since there are three different alleles, six different genotypes are possible
How many phenotypes can occur ?

Options

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

Explanation

To determine how many phenotypes can occur for the ABO blood groups controlled by the gene `I`, we will follow these steps:

### Step 1: Understand the Alleles
The gene `I` has three alleles:
- `I^(A)`
- `I^(B)`
- `i`

### Step 2: Identify the Genotypes
Since there are three alleles, we can have the following combinations (genotypes):
1. `I^(A)I^(A)` (homozygous for A)
2. `I^(A)i` (heterozygous for A)
3. `I^(B)I^(B)` (homozygous for B)
4. `I^(B)i` (heterozygous for B)
5. `I^(A)I^(B)` (co-dominant, AB)
6. `ii` (homozygous for O)

Thus, the six possible genotypes are:
- `I^(A)I^(A)`
- `I^(A)i`
- `I^(B)I^(B)`
- `I^(B)i`
- `I^(A)I^(B)`
- `ii`

### Step 3: Determine the Phenotypes
Now, we need to determine the phenotypes corresponding to these genotypes:
1. `I^(A)I^(A)` → Blood group A
2. `I^(A)i` → Blood group A
3. `I^(B)I^(B)` → Blood group B
4. `I^(B)i` → Blood group B
5. `I^(A)I^(B)` → Blood group AB
6. `ii` → Blood group O

### Step 4: Count the Distinct Phenotypes
From the above genotypes, we can identify the distinct phenotypes:
- Blood group A
- Blood group B
- Blood group AB
- Blood group O

Thus, there are a total of **4 distinct phenotypes**.

### Conclusion
The answer to the question is that there are **4 phenotypes** possible for the ABO blood groups.

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