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Lecture 15: Forensics: Probability of Individual Identity

Lecture 15: Forensics: Probability of Individual Identity. March 5, 2014. Genetic Typing in Forensics. Highly polymorphic loci provide unique ‘ fingerprint ’ for each individual Tie suspects to blood stains, semen, skin cells, hair Revolutionized criminal justice in last 20 years

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Lecture 15: Forensics: Probability of Individual Identity

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  1. Lecture 15: Forensics: Probability of Individual Identity March 5, 2014

  2. Genetic Typing in Forensics • Highly polymorphic loci provide unique ‘fingerprint’ for each individual • Tie suspects to blood stains, semen, skin cells, hair • Revolutionized criminal justice in last 20 years • Also used in disasters and forensic anthropology • Principles of population genetics must be applied in calculating and interpreting probability of identity

  3. http://www.cstl.nist.gov/div831/strbase//mlt_abiid.htm Markers in Genetic Typing • Standard set of 13 core loci for forensics: CODIS (Combined DNA Index System) • Sets of highly polymorphic microsatellites (also called VNTR (Variable Number of Tandem Repeats), STR (Short Tandem Repeat) or SSR (Simple Sequence Repeat)) • Most are amplified in a single multiplex reaction and analyzed in a single capillary • Very high “exclusion power” (ability to differentiate individuals)

  4. Individual Identity: Likelihood • Assume you find skin cells and blood under fingernails of a murder victim • A hitman for the Sicilian mafia is seen exiting the apartment • You gather DNA evidence from the skin cells and from the suspect • They have identical genotypes • What is the likelihood that the evidence came from the suspect? • What is H1 and what is H2?

  5. Heterozygote Homozygote for m loci Match Probability • Probability of observing a genotype at locus k by chance in population is a function of allele frequencies: • Assumes unlinked (independent loci) and Hardy-Weinberg equilibrium

  6. Homozygotes for m loci Heterozygotes Probability of Identity • Probability 2 randomly selected individuals have same profile at locus k: • Exclusion Probability (E): E=1-P

  7. Which allele frequency to use? Illinois Caucasian • Human populations show some level of substructuring • FST generally < 0.03 • Challenge is to choose proper ethnic group and account for gene flow from other groups Georgia Caucasian U.S. Black http://books.nap.edu/openbook/0309053951/gifmid/95.gif

  8. Substructure in human populations • GST is quite high among the 5 major groups of human populations for CODIS microsatellites • Relatively low within groups, but not 0!

  9. Homozygotes Heterozygotes NRC (1996) recommendations • Use population that provides highest probability of observing the genotype (unless other information is known) • Correct homozygous genotypes for substructure within selected population (e.g., Native Americans, hispanics, African Americans, caucasians, Asian Americans) • No correction for heterozygotes

  10. Why is it ‘conservative’ (from the standpoint of proving a match) to ignore substructure for heterozygotes?

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