An unusual case of blood group ABO inheritance: O from AB X O.

An unusual blood group inheritance, that is, a phenotype O child from AB X O parents, was found in a Japanese family. Since two other children from the parents are blood type B, this is not a case of Cis-AB inheritance. The mother is not blood A/B chimera, and normal levels of blood group N-acetylgalactosaminyltransferase (A-enzyme) and galactosyltransferase (B-enzyme) were detected in her plasma. Therefore, the mother is genetically true AB heterozygous. The two sons with phenotype B had normal levels of plasma B-enzyme, but had no A-enzyme, and the father and the daughter with phenotype O had neither A- nor B-enzyme in their plasma. The analyses of 24 genetic marker systems indicated that the O daughter was a true child of the parents. The affirmative probability of parentage on the O daughter was calculated to be .9999999917 by Bayes' theorem. We concluded that the genotype of the O daughter was not the usual 00, and that this rare O expression might be due to a new structural mutation or a deletion in either maternal A or B gene during oogenesis.