Everyone is constantly shedding DNA. DNA is in nearly every cell in the body, so leaving behind a hair, some skin cells, or saliva can be enough to match to a person.
Criminals often leave behind even more. A struggle may leave behind the attacker’s blood or skin under the victim’s nails. In the case of a rape, the attacker may leave semen. This DNA can be used like a fingerprint to match a suspect to a crime: if the suspect’s DNA was there, he must have been, too.
Police don’t normally look at all of the information in the DNA. The DNA has all the information for making an entire person. But the police just need enough to match the crime scene DNA to the suspect’s DNA.
Since the late 1980’s, police have been using a “Combined DNA Indexing System” (CODIS for short). CODIS looks at 20 different parts of the DNA where there are different numbers of repeats. Some people have the same 20 DNA letters repeated 5 times. Another person might have them repeated 15 times. The different number of repeats don’t really matter for a person’s life, since they don’t affect any of the person’s genes. But they do make a unique DNA fingerprint. CODIS markers are akin to lowering the resolution on a picture: you won’t be able to tell exactly what the old picture looked like, but you would be able to identify an exact match.
Everyone gets one copy of their DNA from their dad and one from their mom. So every person ends up with two sets of repeats. For one CODIS marker, a person can have 5 repeats from mom and 15 repeats from dad. With two copies of 20 different markers that can have many different lengths, it’s really unlikely for two unrelated people to have exactly the same CODIS markers. Even siblings only share about 75% of them. So CODIS gives a pretty unique DNA fingerprint.
But there was no match for the Golden State Killer’s DNA in the CODIS database. So police looked more closely at the DNA.
One way to get a few more pixels to your DNA picture is to look at single nucleotide polymorphisms (commonly called SNPs, pronounced “snips”). Most people’s DNA is pretty similar, which is why, on the grand scheme of things, most people are pretty similar. But everyone has a few letters different in the giant book that is our genome. A SNP is when a single letter (scientifically, a “nucleotide”) is swapped for another . Scientists don’t even need to look at every single SNP to get a better picture of someone’s DNA, although they usually look at hundreds of thousands of them.
But this still leaves the same problem: if police didn’t have DeAngelo’s DNA, how could they match it to the Golden State Killer’s?
Family members can be identified because they share similar stretches of DNA
Everyone has 23 pairs of chromosomes (hence the company name, “23andme”). A chromosome is just one really long, continuous stretch of DNA. So one chromosome comes from your mom and one from your dad. Your mom got one chromosome from your grandma and one from your grandpa. But it’s not like your mom just gave you your grandpa’s chromosome.
Before parents pass on their DNA, it goes through a process called “crossing over”. Part of grandma’s chromosome swapped with part of grandpa’s chromosome inside your mom. This means that the chromosome mom gave you has both some of grandpa’s and grandma’s DNA. What parts get swapped is kind of random. Your sibling will end up with some of the same and some different parts of your grandparents’ DNA.
"Crossing over" mixes up chromosomes
The important part here is that you end up with stretches of DNA from one grandparent or the other. All the SNPs on that part of the chromosome are linked together -- in a line on the “rope” of the chromosome. It’s pretty easy for a computer to find the stretches of SNPs that are the same between two people. You can even figure out how closely related two people are by measuring how long these stretches of matching SNPs are.
On top of all that, only the very tips of the Y chromosome can cross over with the X. This means fathers give almost the same Y chromosome to their sons as their fathers gave to them. All men on the same paternal line (think: have the same last name) have nearly identical Y chromosomes.
Essentially, if the police could get some DNA from the family of the Golden State Killer, they could get a major lead.
People who are related share blocks of DNA