ABO blood groups in humans are controlled by the gene I . It 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. Four
B. Two
C. Three
D. One

Answer

To solve the question regarding the phenotypes of ABO blood groups in humans, we can follow these steps:

### Step 1: Understand the ABO Blood Group System
The ABO blood group system is controlled by a single gene, denoted as I, which has three alleles: \(I^A\), \(I^B\), and \(i\).

### Step 2: Identify the Alleles and Their Relationships
- \(I^A\) and \(I^B\) are both dominant alleles that express antigens A and B, respectively.
- The allele \(i\) is recessive and does not produce any antigen.

### Step 3: Determine the Possible Genotypes
With three alleles, the possible combinations (genotypes) are:
1. \(I^A I^A\) (homozygous A)
2. \(I^A i\) (heterozygous A)
3. \(I^B I^B\) (homozygous B)
4. \(I^B i\) (heterozygous B)
5. \(I^A I^B\) (co-dominant AB)
6. \(ii\) (homozygous O)

This gives us a total of 6 different genotypes.

### Step 4: Identify the Corresponding Phenotypes
Now, we need to determine the phenotypes (observable traits) associated with these genotypes:
- Genotypes \(I^A I^A\) and \(I^A i\) both result in phenotype A.
- Genotypes \(I^B I^B\) and \(I^B i\) both result in phenotype B.
- Genotype \(I^A I^B\) results in phenotype AB.
- Genotype \(ii\) results in phenotype O.

### Step 5: Count the Unique Phenotypes
From the above analysis, we can summarize the phenotypes:
1. A
2. B
3. AB
4. O

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

### Final Answer
The number of phenotypes that can occur from the ABO blood group system is **4**.

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