Rubocki R J, McCue B J, Duffy K J, Shepard K L, Shepherd S J, Wisecarver J L
Department of Pathology and Microbiology, University of Nebraska Medical Center, Omaha 68198-5454, USA.
J Forensic Sci. 2001 Jan;46(1):120-5.
Analysis of multiple genetic loci using short tandem repeats (STR) is widely used in human identity testing because the extensive polymorphism at these loci allows for a high degree of discrimination among individuals. We recently received a forensic case that included several pieces of evidence and reference blood samples. Upon initial testing, one of the suspects had a DNA 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 to 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 identical to that of the original specimen at each locus. One of the evidence samples also displayed an identical mixed DNA profile matching that of the reference specimens mentioned above. The relative peak heights of the two "major" and one "minor" allele remained constant in all three samples. Additional background information revealed that the suspect had not received a bone marrow transplant or blood transfusion. However, it was disclosed that this individual is a fraternal (dizygotic) twin. We hypothesize that an exchange of blood cells between the fetuses occurred in utero and that the additional alleles present in these reference samples are derived from cells contributed by his twin sibling. No additional specimens from the suspect or his twin could be obtained for confirmation, and our hypothesis remains untested. Forensic scientists should be aware of this possibility when faced with a DNA profile in which extra alleles at multiple loci are detected.
使用短串联重复序列（STR）分析多个基因座在人类身份鉴定中被广泛应用，因为这些基因座广泛的多态性使得个体之间具有高度的区分度。我们最近收到一个法医案件，其中包含几件证据和参考血样。初步检测时，一名嫌疑人的DNA图谱在检测的9个基因座中的4个（vWA、FGA、TH01和D5S818）包含三个等位基因。在每个基因座上，其中两个等位基因似乎是“主要”等位基因，同时存在第三个“次要”等位基因。该图谱似乎是两个人的混合图谱。怀疑第一个参考样本受到污染，因此要求该个体提供第二个未开封的血样。来自第二个参考样本的DNA图谱在每个基因座上与原始样本相同。其中一件证据样本也显示出与上述参考样本相同的混合DNA图谱。在所有三个样本中，两个“主要”等位基因和一个“次要”等位基因的相对峰高保持不变。额外的背景信息显示，该嫌疑人未接受过骨髓移植或输血。然而，据披露该个体是异卵双胞胎。我们推测，胎儿在子宫内发生了血细胞交换，这些参考样本中存在的额外等位基因来自他的双胞胎兄弟姐妹贡献的细胞。无法获得该嫌疑人或其双胞胎的其他样本进行确认，我们的推测仍未得到验证。法医科学家在面对检测到多个基因座存在额外等位基因的DNA图谱时，应意识到这种可能性。
