How Genetic Genealogy Helped Catch The Golden State Killer

Joseph DeAngelo has plead guilty to the murder of 13 people, the rape of around 50 women and committing burglaries across California during the 1970s and 80s.

The so-called ‘Golden State Killer’ was arrested in April 2018 after detective work was combined with DNA databases and family trees to identify potential suspects, an approach known as ‘genetic genealogy‘.

DeAngelo, now 74, used his knowledge as a former police officer to avoid leaving evidence and managed to escape justice for over 40 years — but he couldn’t hide from the clues in his own genes. Here’s how genetic genealogy is used to catch criminals.

1. Profiling the Perpetrator

Investigators collect biological material — such as blood, hair, skin or semen — from a crime scene. Those samples sometimes contain DNA that can be read through genetic sequencing, which involves cutting the DNA into tiny fragments and scattering them over a ‘genotyping chip’ to see what sticks.

The chip contains microscopic wells with about 700,000 probes that each match a unique genetic variant — a specific sequence that may (or may not) be found in a person’s DNA. Fluorescent dyes are then used to isolate the appropriate probes so a computer can identify the series of DNA letters in a sample, creating a genetic profile.

2. Finding the Relatives

The perpetrator’s sequence is added to a public database of DNA sequences, such as the genealogy website GEDmatch, which you can search to find similar profiles among its one million users. Those genetic profiles are previously uploaded by consumers who took a DNA test (sold by companies like 23andMe or Ancestry) to learn about variants that might reveal a genetic predisposition to disease and where their (often distant) relatives live around the world.

Law enforcement officials are more interested in whether their perp is closely related to other people in the database, as calculated from number of shared genetic variants. Your DNA is roughly 50% similar to each of your parents, 25% for grandparents. For each generation since two people shared a common ancestor — such as grandparents — their genetic similarity is reduced to one quarter, which means first cousins share about 12.5% of their DNA, second cousins 3.125% and third cousins less than 1%.

3. Building the Family Tree

Unless all your close relatives are obsessed with their ancestry, it’s highly unlikely you would find many first or second cousins in the GEDmatch database, but you should get thousands of third cousins. According to renowned genetic genealogist CeCe Moore, investigators in the Golden State Killer case studied third cousins.

The traditional techniques of genealogy — tracking-down records like birth and marriage certificates, census data and newspaper obituaries — along with modern methods like Facebook stalking are then combined with the DNA profiles to build a huge family tree of people who might be related to the perp. Those family members are painstakingly added to the tree, starting with the twigs of living relatives and then connecting them through branches of distant ancestors. CeCe Moore calls this process ‘reverse genealogy’.

4. Identifying the Suspect

After law enforcement agencies have identified some potential suspects via genetic genealogy, they use conventional investigative methods such as comparing present physical features to past eyewitness statements and police sketches. This narrows-down the choices to a few candidates.

In the Golden State Killer case, some of the victims who survived his attacks described him as a 5’9″, 165-pound white male — characteristics that matched the features of Joseph James DeAngelo.

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