‘True’ Genotype of Chimeric Twins Revealed by Blood‐group Gene Products in Plasma

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Abstract
A discrepancy between red cell and serum grouping in the ABO system found in routine testing of a healthy blood donor led to recognition of blood group chimerism. The red cells of the propositus grouped as A1 and his serum as AB. He was found to have a mixture of 99% A1 cells and 1% B cells, and the blood of his twin sister also contained 99% A1 and 1% B cells. In each twin the two populations of red cells also differed in Rhesus, MN, and Xg groups. The genotype of the twins was established by the presence of blood group substances in their saliva, the presence or absence of A1Leb substance on their red cells, the distribution in their serum of transferases determined by the A and B genes, and by chromosome studies on lymphocytes.