This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Pathology
Inheritance pattern of ABO blood group system and HLA system 
Select and explain. 
A. Pseudodominance, B. Autosomal dominant, C. Autosomal recessive, D. Codominant
D: Codominant
Answer: D: Codominant 
Explanation: "The genes that determine the A and B phenotypes are found on chromosome 9p and are expressed in a Mendelian codominant manner."