A 40 year old female patient complains of excessive bleeding. Patient gives a history of road traffic accident 5 hours ago and had a lacerated wound on lower back region. Blood grouping test reveals presence of antigen A and antigen B. This system for blood grouping consists of 3 major allelic genes which are located on-

**Core Concept**
The question is testing the understanding of the **ABO blood group system**, which is one of the most important blood types in human medicine. This system categorizes blood based on the presence or absence of specific antigens on the surface of red blood cells. The ABO blood group system is determined by a single gene with three allelic forms.

**Why the Correct Answer is Right**
The correct answer is related to the genetic basis of the ABO blood group system. The ABO blood group is controlled by a single gene (the ABO gene) on **chromosome 9**, which has three main allelic forms: A, B, and O. These alleles determine the presence or absence of the A and B antigens on red blood cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not specify the correct location of the ABO gene.
**Option B:** Incorrect as it is not related to the ABO blood group system.
**Option C:** Incorrect because the ABO gene is not located on this chromosome.

**Clinical Pearl / High-Yield Fact**
It's crucial to remember that the ABO blood group system is fundamental in transfusion medicine, as mismatched blood can lead to severe reactions. The ABO gene's location on **chromosome 9** is a key piece of information for understanding genetic aspects of blood typing.

**Correct Answer:** D. chromosome 9

✓ Correct Answer: B. Chromosome 9