DNA fingerprinting

DNA fingerprinting is the conversion of biological material into a genetic profile that can identify a possible source and connect evidence across cases. In the book, its significance lies not in a single laboratory breakthrough but in a chain of changes: more sensitive testing, databases capable of comparing profiles, and investigators learning to revisit old evidence with new tools. The method can give an investigation a powerful lead, but it depends on usable samples, careful comparison, and the patient work of turning a match into a case.

Discovery and Principle

In September 1984, British scientist Alec Jeffreys discovered DNA fingerprinting, a development the book presents as a lasting transformation of forensic science and criminal justice. The account briefly reaches back to Jeffreys’s boyhood in Luton, where his early scientific curiosity took shape; the decisive change, however, came when his work made individual genetic patterns available for comparison.

DNA, or deoxyribonucleic acid, is found in the nucleus of nearly every cell in the body, with red blood cells the exception. A forensic scientist extracts available DNA from a biological sample, then isolates, amplifies, and analyzes it. The profile is determined by the sequence of DNA’s repeating units. The book compares the resulting markers to a human barcode: early testing could develop and analyze only a few, while the standard CODIS profile described in the text uses thirteen. The stated chance that two people other than identical twins would share the same barcode is roughly one in a billion.

That statistical distinctiveness gives the method its force, but a profile is not a complete account of an offense. It can identify a biological source or show that profiles correspond; investigators must still interpret what that connection means in the circumstances of a case. The book’s emphasis on cellular certainty describes the aim of forensic comparison, while its broader account shows how that result must be carried into an investigation.

From Laborious Tests to Cold Cases

The first generation of DNA testing differed sharply from current methods. When the Orange County Crime Lab began incorporating testing in the early 1990s, working a single case could take a criminalist up to four weeks. The sample had to be substantial and in good condition: a bloodstain about the size of a quarter is the book’s example. By contrast, later techniques could derive a genetic fingerprint from a small scattering of skin cells in hours.

The older method, RFLP analysis, was slow and required ample, undegraded DNA. It remained in use by the state database during the period described, but its requirements made it poorly suited to evidence that had deteriorated over time. The Orange County Crime Lab adopted PCR-STR, or polymerase chain reaction with short tandem repeat analysis. The book likens the change to replacing the labor of copying numbers by hand with a high-speed copying machine: PCR-STR was faster and could work with smaller or degraded samples. It became the backbone of forensic testing described in the account.

The value of these improvements became apparent as laboratories began reexamining old cases. A June 1996 cold hit in Orange County identified prison inmate Gerald Parker as a serial killer responsible for five women’s deaths. The database match also led to the immediate exoneration of a man who had spent sixteen years in prison for an attack on a surviving victim; Parker was a month from release when the match was made. For investigators, the result demonstrated that preserved evidence could outlast both an original investigation and the assumptions built around it.

Databases and the Forensic Network

The DNA Identification Act of 1994 established the Federal Bureau of Investigation’s authority to maintain a national database, and CODIS—the Combined DNA Index System—was created. The book describes a layered process: local crime laboratories enter unknown crime-scene profiles and eligible suspect samples into state databases; the state systems verify and compare them within the state, then pass profiles upward for national comparison. In California, samples entered into the state system were automatically uploaded every Tuesday, as described in the account.

The system was not immediately the fast, comprehensive network it would later become. In the mid-1990s, databases were still being developed, and laboratories relied on demanding methods such as RFLP. A December 1995 report described Orange County prosecutors, detectives, and criminalists submitting evidence from old unsolved cases to a new California Department of Justice laboratory in Berkeley, where four thousand profiles of known violent criminals were filed. The state database was still in its infancy, and cold-case submissions helped it grow.

The scale of the unresolved work was considerable. From 1972 to 1994, Orange County investigated 2,479 homicides and cleared 1,591, leaving nearly 900 unsolved. The strategy for reviewing them prioritized homicides involving sexual assaults, because such offenders could be repeat perpetrators and might leave biological material suitable for DNA typing. A profile from one case could be useful even without an immediate match: comparison with evidence from other cases might reveal a shared offender and give investigators a new direction.

Promise, Limits, and the Work Around a Match

The book places laboratory analysis alongside the less visible labor of organizing evidence. Michelle began indexing people, places, and objects named in official reports to help locate investigative details, distinguish individuals, and find possible connections among victims. In January 2016, she and Paul Holes were allowed to review sixty-five Bankers Boxes of case files held by the Orange County Sheriff’s Department. Michelle bought two high-volume scanners so the reports could be made searchable and shared with investigators. DNA testing could produce a comparison; archives and organized records could help investigators understand where that comparison belonged.

The prospect of using consumer genetic databases extended the idea of comparison beyond CODIS. The book notes that 23andMe held 1.5 million profiles and Ancestry.com 2.5 million, raising the possibility that a crime-scene profile might point toward a perpetrator through a relative. But the companies would not work with law enforcement, citing privacy and their terms of service. The legal question included privacy and the Fourth Amendment to the United States Constitution. Michelle had a profile on paper, not a physical sample to submit, and the account distinguishes the possibility she contemplated from an established route investigators could simply use.

The unresolved murder of Kathleen Lombardo offers a further reminder that a compelling investigative question is not itself a solution. Her case remained unsolved, despite the attention later given to it. DNA fingerprinting changed what laboratories could recover from biological traces and how widely those traces could be compared; it did not guarantee that every case would yield a usable sample, a match, or an answer. Its power depends on the whole system around it: preserved evidence, workable techniques, databases, legal boundaries, and investigators able to connect a result to the human record.