polymorphisms is the first line of attack.8 The results are entered into the Federal Bureau of Investigation’s (FBI’s) Combined DNA Indexing System (CODIS) and are searched against DNA profiles already in one of three databases: a convicted felon database, a forensic database containing DNA profiles from crime scenes, and a database of DNA from unidentified persons.

Sometimes the evidence dictates testing just for Y STRs, which assesses only the Y (male) chromosome. In sexual assaults for which only small amounts of male nuclear DNA are available (e.g., a large excess of vaginal DNA), it is possible to obtain a Y STR profile of the male who left the semen. Unlike the 13 core loci used in CODIS searches, where a match of all 13 is a strong indicator that both samples come from the same individual, Y STR testing is not as definitive with respect to identifying a single person. A third nuclear test involves the analysis of single nucleotide polymorphisms (SNPs). Although no public forensic DNA laboratory in the United States is routinely analyzing forensic evidence for SNPs, the utility of this genomic information for cases in which the DNA is too damaged to allow standard testing has garnered attention since its use in the World Trade Center identification effort.9

If insufficient nuclear DNA is present for STR testing, or if the existing nuclear DNA is degraded, two options potentially are available. One technique amplifies the amount of DNA available, although this technique is not widely available in U.S. forensic laboratories. A second alternative is to sequence mitochondrial DNA (mtDNA). Since 1996, it has been possible to compare single-source crime scene samples and samples from the victim or defendant on the basis of mtDNA. Four FBI-supported mtDNA laboratories and a few private mtDNA laboratories conduct DNA casework. This technique has been particularly helpful with regard to hairs—which do not contain enough nuclear DNA to enable analysis with current methods unless the root is present—and bones and teeth. Because it measures only a single locus of the genome, mtDNA analysis is much less discriminating than nuclear DNA analysis; all people with a common female ancestor (within the past few generations) share a common profile. But mtDNA testing has forensic value in its ability to include or exclude an individual as its source.

Laboratories entering the results of forensic DNA testing into CODIS must meet specific quality guidelines, which include the requirement that

8

Some laboratories are now using 16 loci, 13 of which are the original core loci.

9

B. Leclair, R. Shaler, G.R. Carmody, K. Eliason, B.C.Hendrickson, T. Judkins, M.J. Norton, C. Sears, and T. Scholl. 2007. Bioinformatics and human identification in mass fatality incidents: The World Trade Center disaster. Journal of Forensic Sciences 52(4):806-819. Epub May 25, 2007.



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