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Leveraging forensic genetic genealogy to solve cold cases

Detective Ashley Nolan of the Chandler Police Department in Arizona details how her PD solved a sexual assault cold case from 1986


Genetic genealogy can be used to identify perpetrators in sexual assault cases.


The National Sexual Assault Kit Initiative (SAKI), which provides grants for processing sexual assault kits and other funding to solve current and cold case sexual assaults, recently hosted a webinar that detailed how to potentially utilize genetic genealogy to identify perpetrators in sexual assault cases.

Detective Ashley Nolan of the Chandler Police Department in Arizona detailed how her PD solved a sexual assault cold case from 1986. Here are some top takeaways from the webinar.


1. How it works

Some of the companies available for ancestry data include, 23andMe, MyHeritage DNA, HomeDNA and Living DNA. Two that are notably law enforcement friendly are Family Tree DNA and GEDmatch, which give users the option to opt in to allow police to use genetic genealogy in cold cases.

These websites upload what’s called a single nucleotide polymorphism (SNP profile) to their databases to produce the information investigators are looking for. DNA labs use DNA from your case to make an SNP profile, which you can then use to upload to genealogy websites.

What you’re looking for is centimorgans (cM), a unit of measure for DNA. It tells you how much DNA a person shares with another individual. The higher the centimorgans someone shares with a person, the closer related they are to that person. You can use that information to then build out a family tree of the person who shares centimorgans with the perpetrator and look for the common ancestor, then narrow down your potential suspects based on the case data you have (too old, too young, etc.). These trees can become very large, so ask your genealogy vendor for assistance.

2. Assessing your case

Before doing anything, review the case file and make sure it’s appropriate for sending out for genetic genealogy testing:

  • What’s the victim’s status? Is the victim still living? Out of state?
  • What’s the statute of limitations? The case may be too old to proceed.
  • Are all leads exhausted? Important to do so before going down this road.

Another thing to consider is if you want to notify the victim you’re moving forward with testing. You want to limit additional trauma for the victim. Keep in mind that if you tell the victim you’re planning to test the DNA and the testing ultimately isn’t successful, it may cause unnecessary pain for them.

3. Assessing your DNA

Before you send DNA off to a lab, you need to know what the source of your DNA is. Is it a single-source profile, a mixed profile, or a degraded profile? This may determine a particular lab to conduct the testing.

Next, it’s vitally important to assess how much DNA you actually have. The last thing you want to do is consume all the DNA you have for the case for a potential SNP profile, end up not being able to develop one, and then not have any DNA left for other types of testing. According to Nolan, there are real-life examples of cases that weren’t able to go forward because of this issue. Consider how much DNA you have before sending it off to a lab for an SNP profile.


  • You’re going to have DNA leaving your lab to go to another lab – make sure to monitor the chain of custody. During this process a lot of things can go back and forth and you need to keep track of your evidence.
  • One tool Nolan uses is DNA Painter, which lets you type in the number of centimorgans and tells you the closest relationship match.
  • Nolan’s three crucial elements for a genetic genealogy cold case are:

- DNA (genealogy match + DNA confirmation [surreptitious sample])

- Geographical location (present in the jurisdiction during the date and time of the crime)

- Historical information (family, friends and witness statements regarding suspect)

NEXT: How DNA on an ancestry site identified the Golden State Killer

Cole Zercoe previously served as Senior Associate Editor of Lexipol’s and His award-winning features focus on the complexity of policing in the modern world.

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