**Core Concept**
ABO blood group inheritance is determined by three alleles (I^A, I^B, and i) at a single gene locus on chromosome 9. The expression of A and B antigens on red blood cells depends on the combination of alleles inherited from both parents, with both alleles being fully expressed when present.

**Why the Correct Answer is Right**
In ABO blood group system, both the I^A and I^B alleles are expressed equally in heterozygous individuals (e.g., I^A I^B), resulting in the presence of both A and B antigens on the red blood cell surface. This phenomenon is known as **codominance**, where neither allele is masked by the other. The A and B antigens are synthesized via specific glycosyltransferases encoded by these alleles, and both are visible in the phenotype, confirming full expression.

**Why Each Wrong Option is Incorrect**
Option A: X-linked inheritance is incorrect because the ABO gene is located on chromosome 9, not an X chromosome.
Option B: Recessive inheritance is incorrect because A and B antigens are expressed in heterozygotes, not masked by recessive alleles.
Option C: Mitochondrial inheritance is incorrect because ABO gene is nuclear, not mitochondrial, and follows Mendelian patterns, not maternal inheritance.

**Clinical Pearl / High-Yield Fact**
Remember: ABO blood group is a classic example of **codominance**—in I^A I^B individuals, both A and B antigens are expressed. This is crucial in transfusion medicine and can lead to hemolytic reactions if mismatched.

✓ Correct Answer: D. Codominance