How DNA Analysis Led Police to the Golden State Killer

Back in the 70s and 80s a man known as the Golden State Killer terrorized California committing a string of more than a hundred burglaries 45 rapes and a dozen murders.

Definitely a bad dude even though 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 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 ta GA 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 60 AGA repeats on a chromosome that you got from one parent and 10 repeats from your other parent.

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Well 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 on to 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 suspects 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 that 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 what's 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 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 GED match while GED match isn't a power player in the commercial DNA industry like 23andme or ancestry.com 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 GED match 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 snips these are individual A's T's GS and C's 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 snip 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 snips most importantly for detectives.

Because there are so many snips and they change less over time from mutation snip 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 GED matches 900,000 other profiles their killer hadn't uploaded his own data but some of his distant relatives like 3rd and 4th 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 four 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 a 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 D'Angelo
** now aged 72 and 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.

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