The Ahm phenotype is distinguished by the absence of H from the cells and anti-H from the serum, the presence of Ax antigen on the cells and A1 and H antigens in the secretions. Similarly, Ohm has no cellular H or serum anti-H and normal quantities of H substances are found in the saliva.

Both phenotypes appear to result from the action of a recessive gene, possibly sex-linked, suppressing H and A1 on the cells but with no effect on the Ax antigen. Ax is considered to be largely independent of the biosynthetic pathway that proposes H substance as a precursor to A and B antigens.

Quantitative hemagglutination studies also disclosed evidence of other genes modifying the quantitative expression of cellular A and H.

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