## Core Concept
The ABO blood group system is controlled by a single gene with three types of alleles: A, B, and O. The A and B alleles are codominant, while the O allele is recessive. The genotype of an individual determines their ABO blood group phenotype.

## Why the Correct Answer is Right
The correct answer, , indicates that the person with blood group 'A' can have either the genotype AA or AO. This is because the A allele is dominant over the O allele. When an individual has the genotype AA or AO, they express the A antigen on their red blood cells, resulting in blood group 'A'. The presence of the A allele is sufficient to produce the A phenotype.

## Why Each Wrong Option is Incorrect
* **Option A:** . This genotype corresponds to blood group 'B' because the B allele is expressed, not the A allele.
* **Option B:** . This genotype corresponds to blood group 'AB' because both A and B alleles are expressed.
* **Option D:** . This genotype corresponds to blood group 'O' because only the O allele is expressed, and there is no A or B allele present.

## Clinical Pearl / High-Yield Fact
A key point to remember is that individuals with blood group 'A' can have either AA or AO genotypes, and those with blood group 'B' can have either BB or BO genotypes. This is crucial in transfusion medicine and for understanding inheritance patterns of ABO blood groups.

## Correct Answer: .