Hematopoietic chimerism in Italian horses

A chimera is an organism whose cells derive from two or more zygotes. This phenomenon has been detected in a wide variety of organisms, including mammalians. Chimeras can be phenotypically normal, so most of them have been discovered only by chance and the frequency of spontaneous chimerism might have been greatly underestimated. Cells with the “extra” genotype might be found in any part of the body, according to the mechanism of its development. We found several cases of chimeric horses during routine genotyping: They show more than two alleles for each locus, leading to a false incompatibility between the foal and its parents. Each of these subjects was derived from twins pregnancy, and we hypothesize that they could be hematopoietic chimeras, where an exchange of blood cells between the fetuses occurred in utero and these additional alleles derived from cells contributed by his twin sibling because of the establishment of vascular anastomoses between the developing placentas. This exchange occurs in utero, when the twins are immunologically tolerant and this condition of “mixed” blood” could take place. The number of extraneous cells in every twin can change and decrease over time. In some cases, depending on the moment in which anastomoses takes place, this number could exceed the number of cells of the original genotype. The presence of blood chimerism can lead to a false interpretation of trace analysis in criminal cases or in the investigation of kinship. In suspected cases, it is advisable to determine which profile represents the real one, typing biological samples derived from other body districts.