## **Core Concept**
The ABO blood group system is a classification of blood types based on the presence or absence of specific antigens on the surface of red blood cells. These antigens determine an individual's blood type according to the ABO blood group system. The system is controlled by a single gene with three types of alleles.

## **Why the Correct Answer is Right**
The ABO blood group is a classic example of a multiple allele system, where more than two alleles exist for a gene in a population. In this case, the ABO blood group gene has three alleles: A, B, and O. The A and B alleles are codominant, while the O allele is recessive. This results in four possible phenotypes: A, B, AB, and O. The multiple allele system allows for the diversity of blood types seen in the human population.

## **Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the ABO blood group is not an example of a dominant-recessive relationship alone; it's more specifically an example of multiple alleles.
* **Option B:** This option is incorrect because while the ABO blood group does involve interactions at the cellular level, describing it merely as "cellular interaction" does not capture its genetic basis.
* **Option C:** This option is incorrect because the ABO blood group system is indeed genetic and not an acquired trait.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that individuals with type AB blood are considered universal recipients in emergency situations because their immune system does not react with A or B antigens. However, they can only receive blood from AB donors. This highlights the importance of understanding the ABO blood group system in transfusion medicine.

## **Correct Answer:** . Multiple alleles.