How DNA Helped Crack the Case

With the help of an at-home DNA testing site, one criminal investigator was able to identify the Golden State Killer.

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One Woman’s Shocking DNA Test Result (3:17)

This past summer, DNA helped crack the case of one of the biggest crime mysteries of all time – who was the Golden State Killer? Using science, the Internet, and a little bit of saliva, investigator Paul Holes identified former police officer Joseph Deangelo as the most likely culprit, and he currently stands charged with the crimes. But how exactly did Paul Holes do it and how can DNA actually help us catch criminals? 

Fingerprinting is one way detectives can identify people. Every single human fingerprint is unique – you don’t even have the same two fingerprints on your own fingers! And ever since 1892, fingerprint technology has been used to identify people and determine if they were at a crime scene. For example, if your fingerprint was found where a crime was committed, detectives could be fairly certain that you were present at that scene because nobody else could have left that fingerprint there. In one study, forensic fingerprint technology was so accurate it only accidentally attributed a fingerprint to somebody it shouldn’t have 0.1 percent of the time, and it only missed attributing a fingerprint to somebody it should have 7.5 percent of the time.

But when we use DNA evidence, we have to shrink down way smaller than our fingerprints. DNA, short for deoxyribonucleic acid, is the blueprint for our cells and is the building block that contains all of our genes. Human DNA contains approximately three billion pairs of information – but only a small percentage of that actually makes us unique. We share over 90 percent of our DNA with chimpanzees, and we share over 99 percent of our DNA with fellow humans. It is the small, less-than-one percent of our DNA that makes each of us who we are. And this is the percent of the DNA detectives are focused on. If your specific DNA is found at a crime scene, there is a very good chance that you were there – because nobody else has the exact same genetic makeup as you.

What makes the case of the Golden State Killer interesting, is that the police don’t just have a database of everybody’s DNA to compare evidence to, which was the case in this criminal investigation. Detectives had samples of the Golden State Killer's DNA from back when he was committing the murders and rapes, but they had no way to figure out who the DNA actually belonged to. However, this is all changing with the advent of genealogy companies. The idea behind these companies is simple – you send in a sample of your DNA (usually saliva), and they give you information about your genetics, certain traits you may have, and diseases you may be at risk for. But there’s something else important to this case that these websites can tell you about – your ancestors and your family members.

Even though less than one percent of our DNA makes us unique, much of that less than one percent is the same within families. In scientific terminology, we say that DNA is “conserved.” This means that, within families, certain sequences stay the same. And scientists can use this to figure out who you are related to. Distantly, scientists can use a type of DNA called mitochondrial DNA to figure out who your maternal ancestor is. And they can use the Y-chromosome from males to determine a distant paternal ancestor. But scientists can also see who your living relatives are. For example, one of the websites may report that you share three percent of your human-specific DNA with another person. This means that there is a good chance you and that person are cousins. And this is exactly how the Golden State Killer was found.

Using GEDmatch, investigator Paul Holes uploaded a sample of the Golden State Killer’s DNA. Luckily, relatives of the Golden State Killer had also uploaded DNA to GEDmatch in the past. As a result, GEDmatch was able to tell Paul Holes all of the people who shared segments of their DNA with the Golden State Killer – and therefore, all of the people who were related to him. He then used this information to focus his investigation until he was able to figure out exactly who the suspect was. He obtained one last DNA sample from Joseph Deangelo to confirm, and the arrest was made!

DNA can also play one more crucial role in the criminal justice system. While this case was all about using DNA to help catch a criminal, it can also be used to acquit innocent people who have been wrongly charged with a crime. For example, The Innocence Project is a nonprofit legal organization that uses DNA evidence to prove people did not commit a crime. To date, 362 people have been exonerated of crimes because DNA subsequently ended up showing that they had been wrongly convicted. Of these people, 20 had spent time on death row. It goes to show that DNA plays a major role in all aspects of criminal investigations. 

Related:

How to Protect Yourself Before Taking a Home DNA Test

How Learning Your Family History Can Save Your Health

How Genetic Testing Helped Save One Woman's Life