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

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.

ABO grouping, ABO genotyping, karyotyping, human leukocyte antigen (HLA) typing, and short tandem repeat (STR) analysis were performed.

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 O1, A201 and B alleles in the ABO locus; O1 was a maternal allele, while A201 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, TPOX, D8S1179, D19S433, and D21S11) analyzed in the proposita's blood- and skin fibroblast-derived DNA. The additional alleles of TPOX, D8S1179, and D21S11 were of paternal origin.

The genetic findings suggest that this proposita was produced by dispermic fertilization of two identical haploid ova formed by parthenogenetic activation.