Typically assumptions are made as to the number of contributors possible in a mixture. The vast majority of cases most laboratories face are two-person
Mixture profile at the STR locus FGA from a three-person mixture. Genotypes and peak heights in relative fluorescence units are shown underneath each peak. The peak height variation observed can be used to tentatively associate alleles 20 and 22 as coming from the same individual, alleles 18.2 and 23 as coming from the second contributor, and alleles 24 and 27 as coming from the third contributor.
mixtures where one of the contributors is known (e.g., the victim in a sexual assault case). However, situations with gang rape become much more complicated. Examining results in the context of the entire DNA profile is important in terms of assessing the number of contributors. Frequently, the more polymorphic loci with more allele possibilities, such as D18S51 and FGA, will provide the best chance of determining the number of contributors to a mixture (Figure 22.3). Evett and Weir (1998) point out that when only a few of the possible alleles are present in a mixture profile then it becomes less probable that a large number of contributors will have that set of alleles in common between them (pp. 204—205).
Interpretation of a DNA profile can only be performed on loci that have results. Unfortunately, with degraded DNA specimens or low-copy number samples (see Chapter 7) the PCR amplification may fail to generate signals above the detection threshold of the instrument and individual alleles and entire loci may be lost from the final DNA profile. Foreman and Evett (2001) note that partial profiles occur in approximately 20% of cases seen by the Forensic Science Service. Given that it is often not possible to know what alleles should have been present had the sample not been degraded, the standard practice is to interpret only the detected alleles as described in Chapter 21.
Of course, obtaining matching alleles between a full-profile suspect and a partial-profile evidentiary sample is not as powerful as a full-profile to full-profile match. However, any data is better than none. Even if results are obtained on only a few STR loci, this information can provide ample assistance to either include or exclude the suspect and therefore aid in resolving the case.
Occasionally results from additional loci may be recovered from degraded DNA samples through the use of validated miniSTR primer sets (see Chapter 7) or other genetic systems such as single nucleotide polymorphisms (see Chapter 8) that amplify smaller regions of the DNA template. Finally, in most cases, the forensic sample has been divided into two or more parts so that unused portions are retained to permit additional tests as desired by the court according to NRC II recommendation 3.3 (see Appendix VI). These retained samples can be tested as occasion warrants in order to verify previous test results.
REFERENCES AND ADDITIONAL READING
Brenner, C.H. (1997) Proof of a mixed stain formula of Weir. Journal of Forensic Sciences, 42, 221-222.
Curran, J.M., Triggs, C.M., Buckleton, J. and Weir, B.S. (1999) Journal of Forensic Sciences, 44, 987-995.
Devlin, B. (1992) Statistical Methods in Medical Research, 2, 241-262.
DNA Advisory Board (2000) Statistical and population genetic issues affecting the evaluation of the frequency of occurrence of DNA profiles calculated from pertinent population database(s) (approved 23 February 2000). Forensic Science Communications, July 2000. Available at: http://www.fbi.gov/programs/lab/fsc/backissu/july2000/ dnastat.htm; printed in Appendix V.
Evett, I.W. and Weir, B.S. (1998) Interpreting DNA Evidence: Statistical Genetics for Forensic Scientists. Sunderland, MA: Sinauer Associates.
Foreman, L.A. and Evett, I.W. (2001) International Journal of Legal Medicine, 114, 147-155.
Foreman, L.A., Champod, C., Evett, I.W., Lambert, J.A. and Pope, S. (2003) International Statistical Review, 71, 473-495.
Fung, W.K. and Hu, Y.-Q. (2000) Forensic Science Communications, 2 (4). Available online at: http://www.fbi.gov/hq/lab/fsc/backissu/oct2000/fung.htm.
Fung, W.K. and Hu, Y.Q. (2001) International Journal of Legal Medicine, 115, 48-53.
Fung, W.K. and Hu, Y.-Q. (2002a) Statistics in Medicine, 21, 3583-3593.
Fung, W.K. and Hu, Y.Q. (2002a) International Journal of Legal Medicine, 116, 79-86.
Hu, Y.Q. and Fung, W.K. (2003a) International Journal of Legal Medicine, 117, 39-45.
Hu, Y.Q. and Fung, W.K. (2003b) International Journal of Legal Medicine, 117, 248-249.
Ladd, C., Lee, H.C., Yang, N. and Bieber, F.R. (2001) Croatian Medical Journal, 42 (3), 244-246.
Lauritzen, S.L. and Mortera, J. (2002) Forensic Science International, 130, 125-126.
Mortera, J., Dawid, A.P. and Lauritzen, S.L. (2003) Theoretical Population Biology, 63, 191-205.
National Research Council Committee on DNA Forensic Science (1996) The Evaluation of Forensic DNA Evidence. National Academy Press: Washington, DC; usually referred to as NRCII; recommendations listed in Appendix VI.
Perlin, M.W. and Szabady, B. (2001) Journal of Forensic Sciences, 46, 1372-1378.
Tomsey, C.S., Kurtz, M., Flowers, B., Fumea, J., Giles, B. and Kucherer, S. (2001) Croatian Medical Journal, 42, 276-280.
Torres, Y., Flores, I., Prieto, V., Lopez-Soto, M., Farfan, M.J., Carracedo, A. and Sanz, P. (2003) Forensic Science International, 134, 180-186.
Wang, T., Xue, N. and Wickenheiser, R. (2002) Least-square deconvolution (LSD): a new way of resolving STR/DNA mixture samples. Proceedings of the Thirteenth International Symposium on Human Identification. Available at: http://www.promega.com/ geneticidproc/ussymp13proc/contents/wang.pdf.
Weir, B.S. (1996) Genetic Data Analysis II: Methods for Discrete Population Genetic Data. Sunderland, MA: Sinauer Associates, pp. 221-225.
Weir, B.S. and Buckleton, J.S. (1996) Statistical issues in DNA profiling. In Carracedo, A., Brinkmann, B., and Bar, W. (eds) Advances in Forensic Haemogenetics 6, pp. 457-464. New York: Springer-Verlag.
Weir, B.S. (2003) Forensics. In Balding, D.J., Bishop, M. and Cannings, C. (eds) Handbook of Statistical Genetics, 2nd Edition, pp. 830-852. Hoboken, New Jersey: John Wiley & Sons.
Weir, B.S., Triggs, C.M., Starling, L., Stowell, L.I., Walsh, K.A. and Buckleton, J. (1997) Journal of Forensic Sciences, 42, 213-222.
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