Tag Archives: Autosomal DNA Testing

Genetic Genealogy: Using DNA for Police Investigations

Catching Culprits

The Golden State Killer terrorized California from 1974 to 1986. He killed at least 13 people and committed a series of rapes and burglaries during this twelve-year period, but was never caught. On April 24, 2018 however, Sacramento County Sheriff’s Department arrested Joseph James DeAngelo, who they believed was responsible for the crimes, before formally charging him. Genetic Genealogy was responsible for bringing him to justice and now crime solvers believe that more cold cases can be solved with this exciting method. While DNA profiling has been around since 1984, the genetic database was not large enough at the time to be as effective as it is today.  How it works is that genealogists upload DNA sequences from a crime scene to a large database consisting of genetic profiles from a mass of other individuals in order to find the relatives of an unknown suspect. Afterward, they narrow in on the possible perpetrator by constructing a family tree and pass on the information to law enforcement.

A Family Tree. Image from:                                            https://www.wikihow.com/Design-a-Family-Tree#/Image:Design-a-Family-Tree-Step-19.jpg


But First… what are SNPs?

Single Nucleotide Polymorphism (or SNPs) are markers in our entire DNA set. They signify that a portion of individuals will have one building block, called a nucleotide, at a specific position in a nucleotide sequence, but other people will have a different nucleotide (four different types) at that same position.  The two forms of SNPs are known as alleles.


This segment of a DNA strand has one SNP that does not match. Image from Molecule World: https://digitalworldbiology.com/archive/genetic-variation-i-what-snp


The Process

So how do genealogical DNA tests work? There are actually three major types of tests that can be conducted. Our focus is on the autosomal DNA test, which targets chromosomes 1-22 for analysis (the non-sexual somatic chromosomes). This testing method allows individuals to be matched within a database. An individual being tested may have a number of consecutive SNPs in common with a person already in a system’s database, in which case it can be inferred that they share a DNA segment within that section of their genomes. If this segment is longer than a threshold value set by a certain group, then the two subjects are treated as a match. Since half of the genetic information is inherited from each parent, the SNP of each individual decreases by approximately half each generation, but genealogical DNA tests generally account for about 700,000 SNPs. With this in mind, it is not expected that even somewhat distant family members will fail to match (as was the case in DeAngelo’s situation).

Video about how DeAngelo was caught. Video from CBC News: The National.


The Limitations

While this method is very effective, it does pose some limitations as well. DNA that is contaminated, degraded, or too small in amount can leave room for doubt when attempting to make a match. Also, due to privacy issues, many DNA-testing firms will not provide law enforcement any access to their database without consent from the user of the data or by a court order.

Genetic Genealogy is an effective way of solving cold and even active criminal cases. While some restrictions exist, the vast database now available can be a great resource for law enforcement.

– Hani Ghaffari