How DNA Analysis Led Police to the Golden State Killer

[INTRO ♪] Back in the ‘70s and ‘80s, a man known
as the ‘Golden State killer’ terrorized California, committing a string of more than 100 burglaries,
45 rapes, and a dozen murders. Definitely a bad dude. Even though the police had DNA evidence from
crime scenes, it never matched any DNA records on file,
so the case went cold … Until last week. By this point, you’ve probably heard about
this man’s capture. But you might not realize that the science
used to crack the case is totally different from standard DNA forensics. So by understanding how DNA analysis is typically
done for crime scenes, we can dive into why the Golden State killer
case is so special. Usually, forensic scientists process DNA with
a method called short tandem repeat, or STR, analysis. The basic idea is that little stretches of
short repeated sequences, like TAGA, are scattered throughout your genome, in specific places on specific chromosomes. Because the number of repeats at each location
tends to vary from person to person, counting them can be useful. Like, they can help match the semen in a rape
kit to a suspect. For instance, you might have 6 TAGA repeats
on a chromosome that you got from one parent, and 10 repeats
from your other parent, while I might have 12 and 15. Now, if the number of repeats at one spot
in someone’s DNA happens to match evidence from a crime scene, that doesn’t necessarily mean much. But if you look at a bunch of repeats and
they all match, you might be onto something. In the US, forensic scientists have traditionally
tested for repeats at 13 different spots in the genome, although
recently they upped it to 20. That’s what’s happening when you hear
about hits in CODIS, the FBI’s database of DNA records. A match is all about probability, and there
are a lot of factors to consider, like how common certain STRs are in a given
population. But if your suspect’s DNA repeats in the
same 13 ways as a sample from the crime scene, the odds that the suspect isn’t the source
are typically about one in a billion. In other words, combined with other evidence, you can be pretty confident that you’ve
got the right person. Of course, DNA analysis isn’t perfect. Contamination is a concern, and there’s
room for doubt in cases where there’s a small amount of
DNA, or the DNA is degraded. These days, standard STR analysis uses a process
called PCR to amplify the DNA sections the police are
interested in. But that can lead to errors, and they might
not get solid data for all 20 repeat locations. Another challenge is that many samples include
DNA from multiple people, like both a victim and the criminal, making
the analysis more complicated. And any positive match still needs to be interpreted. After all, an innocent person might have scraped
their finger and left a few drops of blood behind at a
future crime scene. So that’s how standard forensic DNA testing
works. And if it’s all done right, it can be really
persuasive, as you probably know from shows like CSI. But with the Golden State killer, the cold
case heated up because of a different approach to DNA— one that’s a lot closer to spitting in a
tube to find out your ancestry. Detectives had some DNA evidence that was
collected from a double murder in 1980 and frozen, so
it was especially well-preserved. We don’t know the specifics of what they
did next, but they eventually uploaded data from that
sample onto an open-source genealogy website called
GEDmatch. While GEDmatch isn’t a power player in the
commercial DNA industry like 23andMe or, it runs on the
same kind of information. So someone might use one of those services
to get raw genome data, then submit it to GEDmatch to help find long-lost
relatives and piece together their family trees. The FBI created the DNA profile of the Golden
State killer in their own labs, from that well-preserved sample. And they probably generated the same kind
of data, by looking for single nucleotide polymorphisms,
or SNPs. These are individual As, Ts, Gs, and Cs throughout
the genome that we know vary between people and can be
passed down from parents to kids—and because of that,
they’re useful biological markers. In a way, SNP analysis is similar to STR analysis, except it’s cheap and easy to test for tens
of thousands of these puppies at once. So that’s what’s going on when you mail
in your DNA. Most companies aren’t directly sequencing
your entire genome. Instead, they’re using technology that checks
your DNA for a whole bunch of SNPs. Most importantly for detectives, because there
are so many SNPs, and they change less over time from mutation, SNP testing is much better than STR analysis when it comes to identifying far-flung relatives. So, once the detectives on the Golden State
killer case uploaded the genetic profile of their suspect, they looked for similarities among GEDmatch’s
900,000 other profiles. Their killer hadn’t uploaded his own data,
but some of his distant relatives— like, third and fourth cousins—had. These are people who would share about 2%
of their DNA with him at most. But it was enough of a lead to build potential
family trees—some 25 in all. Within 4 months of getting some initial hits
on the genealogy site, police officers had painstakingly narrowed
their focus from thousands of relatives to one man. He was the right age, and had lived in California
during the crime spree. To confirm that he was the Golden State killer, they needed some of his DNA to test for a
match. So they put him under surveillance and grabbed
his trash. They used something with some of his cells
on it: maybe a straw, soda can, or used Kleenex—we
don’t know what exactly. And then they did a DNA test, probably STR
analysis, and got a match. Just to be sure, they checked again. Another garbage item, another test—and another
match. After 44 years, they had identified
the Golden State killer. He was a former cop named Joseph James DeAngelo, now age 72, living in a suburb of Sacramento, California. Now, this case isn’t the only time DNA has
been used like this, but it’s one of the highest profile cases. And it’s likely to have people talking for
a while, especially about data privacy issues. We won’t get into that here, but it’s
definitely something people are thinking more about, as investigators
realize how many ways they might be able to use DNA. Even if it involves way more work than TV
crime dramas ever show. Thanks for watching this episode of SciShow News! If you want to learn more about forensics, you can check out our episode that dives into
a lot more crime scene science. And if you just want to keep thinking about
the world more complexly with us, you can go to youtube/scishow and subscribe. [OUTRO ♪]


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