Chapter 24: Forensic DNA Databases: Tools for Crime Investigations

CODIS operates through multiple organized indexes that serve distinct investigative functions: the Offender Index contains DNA profiles from individuals convicted of qualifying offenses, the Forensic Index maintains unknown profiles recovered from crime scenes, and the Missing Persons Index includes profiles from disappeared individuals and their relatives. The chapter explains how database searches function as primary investigative tools, particularly case-to-offender searches that attempt to match an unknown crime scene profile against known offenders, and case-to-case searches that establish connections between separate unsolved crimes by identifying matching profiles. A critical component involves understanding search parameters and stringency levels, which determine how closely profiles must align before generating investigative leads, along with procedures for evaluating and managing partial match results. The chapter also addresses familial searching, a more nuanced and contentious investigative technique employed when standard database searches fail to produce direct matches but reveal partial genetic similarities suggesting the actual perpetrator may be a biological relative of someone already in the database. Familial searching relies on statistical methods including the Kinship Index and identity-by-state calculations to estimate the probability of biological relationships, often emphasizing rare alleles that increase discriminatory power. The chapter explains Y-chromosome short tandem repeat analysis as a complementary technique particularly valuable in cases involving male suspects, enabling investigators to exclude individuals not sharing paternal lineage. Throughout the discussion, the chapter addresses the significant legal and ethical implications surrounding these technologies, particularly concerns about genetic privacy, the scope of law enforcement surveillance capabilities, and the appropriate boundaries for database searches in democratic societies.