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Chimaerism, Mosaicism, and Forensic DNA Analysis

NCJ Number
223163
Journal
Canadian Society of Forensic Science Journal Volume: 41 Issue: 1 Dated: March 2008 Pages: 21-28
Author(s)
Tristan J. Walker
Date Published
March 2008
Length
8 pages
Annotation
This article explores the relevant literature and proposes some answers to the questions that arise for DNA analysis regarding the phenomenon of "chimaerism," which is the name given to organisms composed of two or more genetically distinct cell lines.
Abstract
A true human chimaera would potentially give a confounding result in forensic analysis. True chimaeras may have identifying features, including ambiguous genitalia and hermaphroditism, but may also have a normal appearance. Studies have successfully detected chimaerism through short tandem repeat PCR analysis, which showed the profile of the chimaeric individual to be a mixture that varied in cell population contribution depending on the tissue analyzed. Chimaerism should not be confused with mosaicism, which describes a phenotype where an individual has a patch of tissue that originates from the same zygote, but which differs genetically from the rest due to somatic mutation or DNA replication errors made during the mitotic cycle. True chimaerism is still believed to be a relatively rare event, but there is growing evidence that micro-chimaerism is more prevalent. Since fetal cell trafficking may be an incidental byproduct of pregnancy, this could present a widespread phenomenon that has been underreported so far in the literature. Chimaerism, micro-chimaerism, and mixed chimaeric states will only be a problem for forensic science if the scientists involved are unaware of such phenomena. Since most interpretation results are based on the comparison of allelic peak height ratios, when the peaks appear imbalanced, among the possibilities considered should be the presence of a mosaicism or chimaeric state. In cases where a chimaeric state is suspected, the analysis should be repeated to confirm the results and then followed with additional analysis, using samples obtained from the relevant family members. 1 figure and 25 references