A case of dispermic chimerism with normal phenotype identified during ABO blood grouping.

Background: Human chimerism with normal phenotype derived from the fusion of two different zygotes is a rare phenomenon. We describe a case of a phenotypically normal 17-year-old diagnosed with dispermic chimerism during routine ABO blood grouping. Methods: ABO grouping, ABO genotyping, karyotyping, human leukocyte antigen (HLA) typing, and short tandem repeat (STR) analysis were performed. Results: Forward typing with anti-A and anti-B sera resulted in mixed-field agglutination of red blood cells. The mother and father were blood group O and AB, respectively. The proposita had O-1, A(201) and B alleles in the ABO locus; O-1 was a maternal allele, while A(201) and B were the paternal alleles. The proposita karyotype was 46,XX/46,XY. HLA typing revealed that the proposita had three alleles (46,51,54) at the HLA-B locus, with the additional allele of paternal origin. STR analysis identified three alleles for five of the 15 markers (D2S1338, TPDX, D8S1179, D19S433, and D21S11) analyzed in the proposita's blood- and skin fibroblast-derived DNA. The additional alleles of TPDX, D8S1179, and D21S11 were of paternal origin. Conclusions: The genetic findings suggest that this proposita was produced by dispermic fertilization of two identical haploid ova formed by parthenogenetic activation.