Natural DNA mixtures generated in fraternal twins in utero

Analysis of multiple genetic loci using short tandem repeats (STR) is widel y used in human identity testing because the extensive polymorphism at thes e loci allows for a high degree of discrimination among individuals. We rec ently received a forensic case that included several pieces of evidence and reference blood samples. Upon initial testing, one of the suspects had a D NA profile that included three alleles at four of the nine loci tested (vWA , FGA, TH01, and D5S818). At each locus, two of the alleles appeared to be "major" alleles with a third "minor" allele present. The profile appeared t o be a mixture of two people. Contamination of this first reference sample was suspected and a second, unopened blood specimen was requested from this individual. The DNA profile from this second reference specimen was identi cal to that of the original specimen at each locus. One of the evidence sam ples also displayed an identical mixed DNA profile matching that of the ref erence specimens mentioned above. The relative peak heights of the two "maj or" and one "minor" allele remained constant in all three samples. Addition al background information revealed that the suspect had not received a bone marrow transplant or blood transfusion. However, it was disclosed that thi s individual is a fraternal (dizygotic) twin. We hypothesize that an exchan ge of blood cells between the fetuses occurred in utero and that the additi onal alleles present in these reference samples are derived from cells cont ributed by his twin sibling. No additional specimens from the suspect or hi s twin could be obtained for confirmation, and our hypothesis remains untes ted. Forensic scientists should be aware of this possibility when faced wit h a DNA profile in which extra alleles at multiple loci are detected.