Chapter 28: DNA Forensics

DNA Forensics special topics chapter provides a comprehensive examination of forensic DNA profiling (also known as DNA typing or forensic DNA fingerprinting), outlining how scientific technologies are used to establish identity, prove innocence or guilt, and verify familial relationships in legal contexts. Forensic science utilizes these approaches to answer questions in civil or criminal cases, moving far beyond earlier methods like blood typing and the unreliable bite mark comparisons. The era of DNA identification began in 1985 with the development of Variable Number of Tandem Repeats (VNTRs), or minisatellites, which are long repeated sequences exhibiting high allelic variability. VNTR analysis requires large, non-degraded samples, necessitating DNA extraction, restriction enzyme cleavage, gel electrophoresis, and Southern blotting for fragment detection. Today, the standard technique is Short Tandem Repeat (STR) profiling (microsatellites), a method revolutionized by the Polymerase Chain Reaction (PCR), which allows scientists to generate millions of copies of specific DNA sequences from trace, degraded, or old biological material. STRs feature much shorter, repetitive motifs (2–9 base pairs) and are highly sensitive, using core sets of autosomal STR loci, such as the 20 utilized by the FBI, to achieve high discrimination power. These amplified samples are measured using capillary electrophoresis, which separates fragments by size and presents the results as fluorescent peaks on an electropherogram. Supplementary techniques include Y-chromosome STR profiling, useful for distinguishing male DNA in mixed samples and tracing paternal ancestry, although it cannot differentiate between closely related males because the Y chromosome is inherited as a single, non-recombining unit. Mitochondrial DNA (mtDNA) profiling is critical for identifying victims of mass disasters or analyzing severely degraded samples, owing to its high copy number and maternal inheritance pattern. For the most compromised samples, Single-Nucleotide Polymorphism (SNP) profiling offers an advantage because it requires only short DNA fragments for analysis, although many SNP loci must be examined to match the discriminatory power of STRs. Interpreting matching profiles involves calculating the profile probability (random match probability) using population genetics and the product rule to multiply the frequencies of individual heterozygous (2pq) or homozygous (p 2 ) genotypes across all analyzed loci. These calculated probabilities suggest extreme uniqueness when using large core sets of loci, though identical twins share identical profiles. Law enforcement relies on systems like the national Combined DNA Index System (CODIS) to store and compare profiles from convicted offenders and crime scenes. Finally, the chapter addresses technical limitations, including human error (sample switching, seen in the Kennedy Brewer case), sample backlogs, accidental DNA transference (the Lukis Anderson case), and the potential for deliberate profile tampering using synthetic DNA. Ethical scrutiny is applied to practices such as collecting DNA without consent, using partial matches to identify relatives (familial DNA testing), and the controversial application of DNA phenotyping—using sequence information to predict physical features and ancestry for investigative leads.