Forensic DNA typing currently focuses on STR markers that are highly polymorphic and are thus able to readily resolve unrelated individuals. While many more SNPs than STRs are required to obtain similar random match probabilities (Chakraborty et al. 1999), SNPs have the potential to be used in other ways to aid investigations, such as predicting a perpetrator's ancestral background (Frudakis et al. 2003a).
SNPs have a much lower mutation rate than STRs and therefore are more likely to become 'fixed' in a population. SNPs change on the order of once every 108 generations (Brookes 1999) while STR mutation rates are approximately one in a thousand (see Chapter 6). Because of their low mutation rate, SNPs and Alu insertions (discussed later in this chapter) are often found to be population-specific (Bamshad et al. 2003). These loci could thus be useful in predicting a perpetrator's ethnic origin to aid criminal investigations (see Cyranoski 2004).
The presence of rare STR or SNP alleles in particular population groups can be used to estimate the ethnic origin of a sample. Although efforts have been made with STRs (Lowe et al. 2001, Rowold and Herrera 2003), estimating ethnic origin is far from foolproof. Individuals with mixed ancestral background may not possess the expected phenotypic characteristics (e.g., dark colored skin for African-Americans). Thus, results from genetic tests attempting to predict ethnic origin or ancestry should always be interpreted with caution and only in the context of other reliable evidence.
A company in Florida named DNAPrint (Sarasota, Florida) is trying to predict an individual's ethnic/racial background with a panel of 56 SNPs (Frudakis et al. 2003a). DNAPrint has targeted pigmentation and xenobiotic metabolism genes in their search for ancestrally informative SNPs. Much of their work is based on the research efforts of Dr. Mark Shriver who is looking for ancestry informative markers (AIMs) that possess alleles with large frequency differences between populations (Shriver et al. 1997, Shriver et al. 2003). 'Population-specific alleles' (PSAs) have been found in both STR and SNP markers. While presently used AIMs are not 100% accurate for predicting ancestral background of samples (and perhaps never will be), the DNAPrint SNP typing approach was used to aid the investigation of an important serial rapist case in 2003 demonstrating the forensic value of this type of approach (see D.N.A. Box 8.1).
Aiding a criminal investigation by predicting the ethnic origin of a biological sample
DNA typing tests with the standard 13 STRs linked five murders and rapes in the Baton Rouge, Louisiana area that occurred over an 18-month period. Based on an eyewitness report that a white male was seen leaving one of the crime scenes in a pickup truck, a police dragnet was initiated to collect DNA samples from more than 1000 white males in the area. However, the dragnet and seven long months of investigative work failed to find the culprit.
The police then turned to DNAPrint Genomics Inc. (Sarasota, Florida) to perform a genetic test with single nucleotide polymorphisms (SNPs) to predict the ethnic ancestry of the biological samples obtained from the crime scenes. The DNAPrint test revealed that the samples came from a person who had 85% African-American ancestry and 15% American-Indian ancestry. Authorities turned their attention to black males and within two months arrested Derrick Todd Lee, an African-American resident in the area with an extensive criminal record. Confirmatory testing with the standard 13 STRs matched Derrick Todd Lee's DNA profile with the ones found at the crime scenes.
In the future, this type of analysis to predict ethnicities and even phenotypic characteristics of perpetrators may be used in conjunction with DNA intelligence screens (see D.N.A. Box 18.3) to help narrow the list of possible suspects. Currently SNP tests, like the DNAPrint one, consume too much DNA material to be used routinely on precious crime scene samples. More validation studies will be needed in the future before such ethnicity tests become widely accepted.
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