A, B, and O Genes

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The page below is a sample from the LabCE course Introduction to the ABO Blood Group System. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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A, B, and O Genes

The ABO locus is on chromosome number 9. There are three major allelic genes and numerous rare genes. The three principal 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 the H substance, therefore group O cells contain only H substance. See the diagram on the next page.