Which term best describes the gene inheritance pattern of most blood group systems?

• A. X-linked recessive
• B. Autosomal dominant
• C. Mitochondrial
• D. Autosomal codominant

Answer: (D)

Most blood group systems are determined by genes on autosomes and show codominant expression, so/this pattern means that when two different alleles are present, both can be expressed. The ABO system illustrates this clearly: the IA and IB alleles each encode a different enzyme that adds a specific sugar to the H antigen, and the i allele encodes a nonfunctional enzyme. In individuals with IAIB, both A and B antigens are produced on the red cell surface, showing both traits at once. In IAi or IBi, only A or B antigen is expressed, respectively, and in ii, neither A nor B is expressed, leaving the H antigen intact. This simultaneous expression of multiple alleles on the same cell is what codominance looks like, and since these genes are on autosomes rather than sex chromosomes or mitochondria, autosomal codominant best describes the inheritance pattern of most blood group systems.