**Core Concept**
The ABO blood group system is a classic example of an autosomal codominant inheritance pattern. This means that an individual's ABO blood group phenotype is determined by the interaction of two alleles (different forms) of the ABO gene, which encodes for the enzyme responsible for synthesizing the A and B antigens on red blood cells.

**Why the Correct Answer is Right**
The ABO gene has three alleles: A, B, and O. The A and B alleles are codominant, meaning that they express equally in the presence of each other. The O allele is recessive, meaning that it will be masked by the presence of either the A or B allele. When an individual inherits two A alleles (AA or AO), they express the A blood group phenotype. When they inherit two B alleles (BB or BO), they express the B blood group phenotype. If they inherit one A and one B allele (AB), they express the AB blood group phenotype. When an individual inherits two O alleles (OO), they express the O blood group phenotype.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because the ABO blood group system does not follow a simple autosomal dominant or recessive pattern.
**Option B:** This choice is incorrect because the ABO blood group system is not influenced by sex chromosomes (X and Y).

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the ABO blood group system is not the only blood group system; there are 35 recognized human blood group systems. Understanding the ABO blood group system is crucial for transfusion medicine and ensuring compatibility between blood donor and recipient.

**Correct Answer: A. Autosomal codominant inheritance pattern**