How Modern DNA Testing Solves Decades-Old Unsolved Murders
For cold cases originating in the 1970s and 1980s—such as the 1985 Rhode Island murder case recently resolved by forensic geneticists—investigators routinely encountered a major barrier: the biological evidence collected from the crime scene was either miniscule, degraded by environmental humidity, or failed to yield hits in the FBI's National DNA Index System (NDIS / CODIS).
1. From CODIS Short Tandem Repeats (STR) to Single Nucleotide Polymorphisms (SNP)
Traditional law enforcement forensic analysis relies on Short Tandem Repeat (STR) markers. The FBI standard expanded CODIS core consists of 20 autosomal STR loci (such as D3S1358, TH01, D21S11, and FGA). Because each locus has repeated sequences of 3 to 5 base pairs, STR testing works like an exact biological fingerprint. However, CODIS requires an identical match against an already-arrested or convicted offender in the state database. If the perpetrator was never convicted of a felony, CODIS returns a dead end.
Investigative Genetic Genealogy (IGG) bypasses this limitation. Rather than checking 20 STR loci, genetic genealogy laboratories utilize SNP microarray hybridization or whole-genome sequencing (WGS) to read between 600,000 and 1,000,000 single-nucleotide polymorphisms across the entire human genome.
2. Centimorgans and Identity By Descent (IBD)
When an unknown perpetrator's SNP file is uploaded to law-enforcement accessible genealogical databases (GEDmatch Pro and FamilyTreeDNA), the search engine scans for segments of DNA shared identical by descent (IBD). Genetic linkage is measured in centimorgans (cM):
- Parent / Child or Full Sibling: ~3400 cM (Parent/Child is exactly 50% across every chromosome; Full Siblings average 2500–3400 cM with fully identical regions).
- 1st Cousin: ~850 cM average (Range: 550–1200 cM). Shared Most Recent Common Ancestor (MRCA): Grandparents.
- 2nd Cousin: ~230 cM average (Range: 45–400 cM). Shared MRCA: Great-Grandparents.
- 3rd Cousin: ~73 cM average (Range: 15–185 cM). Shared MRCA: Great-Great-Grandparents.
3. Reverse Genealogy and Suspect Identification
Once two distant cousin matches are identified (e.g., a 2nd cousin on the paternal branch and a 3rd cousin on the maternal branch), forensic genealogists construct family trees backwards in time to identify the common ancestral couples. From those ancestors, investigators build the tree forwards to the 1980s, identifying all living descendants whose age, sex, and geographic proximity match the homicide timeline. Once a person of interest is pinpointed, field investigators obtain a fresh, discarded DNA sample (such as a discarded drinking cup, cigarette butt, or stamp) for direct CODIS STR verification to obtain proof beyond a reasonable doubt in court.