**Core Concept**
The ABO blood group system is determined by a single gene on chromosome 9, which encodes for the ABO blood group antigen. This gene has three alleles: A, B, and O, with the A and B alleles being codominant and the O allele being recessive.

**Why the Correct Answer is Right**
The genotype of blood group A can be either AA or AO. In individuals with the AA genotype, both alleles are expressed, resulting in the production of A antigen on the surface of red blood cells. In individuals with the AO genotype, the A allele is dominant over the O allele, also resulting in the production of A antigen. The presence of the A antigen is what determines an individual's blood group as A.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is a genotype that would result in blood group O, not A.
**Option B:** This option is incorrect because it is a genotype that would result in blood group B, not A.
**Option C:** This option is incorrect because it is a genotype that would result in blood group AB, not A.

**Clinical Pearl / High-Yield Fact**
In the ABO blood group system, the ABO gene has a high degree of polymorphism, which allows for the existence of multiple alleles and genotypes. Understanding the genetics of the ABO blood group system is crucial for transfusion medicine and forensic science.

**Correct Answer:** A. AA