Q.Explain the genetic basis of ABO blood grouping man.

Step 1. Identify the gene and its alleles: Bernstein (1925) showed ABO blood type is controlled by a single gene (I, or L after Landsteiner) on chromosome 9, existing as three alleles — IA, IB, IO.

Step 2. Explain what each allele does biochemically: IA encodes a transferase enzyme adding N-acetyl galactosamine to a precursor (H substance), producing the A antigen; IB encodes a different transferase adding galactose to the same precursor, producing the B antigen; IO is a 'null' allele producing no functional terminal transferase, adding neither sugar.

Step 3. State the dominance relationships: IA and IB are each dominant over IO (their antigen appears even paired with a null IO), but co-dominant with each other, so an IAIB genotype expresses both A and B antigens together rather than one masking the other — written IA = IB > IO. …

Walk through the IA/IB/IO allele series, their enzyme products, and their dominance/co-dominance relation …

• Describing IA and IB as simply 'both dominant' without specifying they are co-dominant with EACH OTHER while each remaining dominant over IO. …