The expression of the A and B Genes appears to depend on the action of another gene, known as H. Most individual are homozygous for this gene (HH), though since its allele, h, is an amorphic gene, the heterozygote Hh cannot be recognized. The phenotype h is extremely rare. The genetic sequence leading to the expression of ABH genes on the red cell is believed to be as a precursor mucopolysaccharide substance is converted by the H gene to H substance. This “altered’ precursor substance (H substance) is partly converted by the A and B genes into A and B antigen. Some H substance remains unconverted. The O gene, being amorphic, effects no conversion of H substance. Since no conversion of H substance takes place by the action of the O gene, the H antigen is found in greatest concentration in group O individuals.
The addition of a sugar known as L-fucose to the terminal D-galactose of the basic precursor substance (Type 1 and Type 2 chains) gives the resultant molecule “H” specificity. The Fucose molecule is bound in alpha (1-2) linkage.
In much the same way, the presence of the B gene results in the attachment of D-galactose to the substrate formed by the H gene. In the absence of H gene, no L-fucose will be added to the terminal sugar. The A and B genes in this case will not be expressed. An individual of genotype hh will therefore group as “O” even when the A or B genes have been inherited. The order of reactivity usually follows the pattern O> A2 > A2B> B> A1> A1B