A family is presented in which the propositus and 2 siblings, apparently in group O, have in their sera anti-H in addition to anti-A and anti-B. Inspection of the family tree reveals apparent violations of Mendelian heredity which can be explained on the basis of a rare suppressor gene which in the homozygote state suppresses in the propositus the actions of gene B and secretion gene Se. The genes B and Se are fully expressed when transmitted to her offspring who are heterozygous for the suppression gene.

The same mechanism may be assumed to operate in the 9 other so-called group O bloods containing anti-H.

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