The ABO locus is on chromosome number 9. There are three major allelic genes and numerous rare genes. The three principle genes are A, B, and O. The A gene determines the product N-acetylgalactosaminyltranferase activity. The B gene determines galactosyltransferase activity. The O gene does not produce a functional enzyme. The enzyme products of the A and/or B genes act on H substance to convert it to A and/or B antigens. Not all H substance is converted; thus, all cells normally contain some H substance along with the A and/or B antigens. If both the A and B genes are present, some H antigen sites are converted to A antigen and other H antigen sites are converted to B antigen. (A single antigen site does not have both A and B antigens.) The O gene is an amorph and doesn’t act on H substance, therefore group O cells contain only H substance. See the diagram on the next page.