Can Forensic DNA Tell Identical Twins Apart?

by | Mar 19, 2020

Have you ever thought of using identical twins in your next crime fiction novel as a plot twist? What if your suspect had an identical twin that no one knew about? What if one twin deliberately framed the other? Or what if they were in it together? Could they get away with it?

Can current forensic DNA techniques tell identical twins apart?

In 2009 in Germany, $6.8 million worth of jewelry was stolen from a department store. The culprit left behind DNA evidence, which hit to identical twin brothers in the national database. Since it was impossible to tell which one did it using current forensic DNA technology, they were released.

This case happened 10 years ago, but current forensic DNA technology still cannot discriminate between identical twins.

However, there are documented exceptions, and new technologies are being researched as we speak, so there’s no telling what may happen in the future.

In this post, I explain why current forensic DNA techniques can’t discriminate between identical twins, documented exceptions, future technologies, and examples of actual court cases involving identical twins. Then I share how you can use these ideas when writing your crime fiction novel.

Current Forensic DNA Technology

Present day forensic DNA analysis does not analyze all the DNA in the human body. It focuses on 20 specific areas called short tandem repeats (STRs). Forensic DNA analysis counts how many repeating segments of DNA there are at these 20 locations. A forensic DNA profile is literally just a bunch of numbers.

Forensic DNA profiles from crime scene samples (unknown source) and reference samples (known source) are then compared to see if there’s a match.

Identical twins start out as one fertilized egg that splits into two separate eggs. Identical twins are DNA clones. Their DNA is the same. Fraternal twins result from two separate eggs that were fertilized at the same time. Fraternal twins share as much DNA as regular siblings. Their DNA is not identical.

When forensic DNA analysis is performed on identical twins, their results will be the exact same. There is no way to tell them apart because forensic DNA looks at a tiny fraction of the DNA found in humans.

However, there are some exceptions. In this homicide case, one of the twins had a mutation at one of the 20 STR locations. A mutation is a permanent change in the DNA. Mutations are very rare. Because only one twin had this mutation, the DNA results could differentiate between the guilty twin and the innocent twin.

In this home invasion case in Canada, they couldn’t tell which identical twin left the sample at the scene, but both twin suspects were convicted. Four people were involved in the crime. Two suspects plead guilty, and the jury convicted the twins as the other two culprits.

In this Texas case, evidence collected in a sexual assault of a child case resulted in a hit to a convicted offender who had an identical twin brother. Turns out one twin drove the car, and the other twin committed the crime. They blamed each other, but one twin confessed during a police interview. The jury found him guilty. The verdict was later appealed, but the conviction was upheld.

In the Research Lab

Scientists are always discovering new technologies, and the field of forensic DNA is no exception.

Current research on discriminating identical twins uses a technique called next-generation sequencing (NGS for short). NGS looks beyond the 20 areas that are analyzed in current forensic DNA analysis. However, NGS is expensive, and finding a mutation is hard because they’re very rare in identical twins.

This case study of a sexual assault in China used NGS and found a rare mutation in the DNA of one of the twins and not the other.

This study used NGS to find ultra-rare mutations in a paternity case to determine who the father was.

Another area of research that tries to discriminate between identical twins analyzes something called DNA methylation.

DNA methylation occurs when a protein called an enzyme adds a chemical group to the DNA sequence. You can think of it as a little DNA tag. DNA methylation analysis in a forensic context doesn’t actually look at the DNA sequence itself. It looks at how patterns differ between people. Different people should have different DNA methylation patterns.

In this study, scientists studied the differences in DNA methylation patterns on items that are commonly encountered in forensic settings (cheek swabs, saliva, and cigarette butts). Research in this area is still in the beginning stages, and analysis of DNA methylation hasn’t been used in actual forensic casework yet, but new advancements are being made all the time.

Identical Twins in Crime Fiction Books

I just threw a large amount of forensic DNA/twin talk at you, but what does it all mean when writing your crime fiction novels?

You now know that current forensic DNA technology can’t tell the difference between identical twins.

However, if you want one twin to get caught/convicted, you could have one of the twins have a rare mutation at one of the 20 locations.

You could also have someone at the crime lab recommend NGS to look for rare mutations to try and figure how which identical twin was guilty. If you use this scenario in your story, the sample would be sent to a specialized lab, so make sure you mention that in your story somewhere.

Another scenario you could use is having a witness verify that he or she was with one of the twins during the time of the murder, so the perpetrator would have had to be the other twin. A non-forensic way to tell twins apart is by looking at other unique traits. Perhaps one twin has a secret tattoo or a scar. Two of my good college friends are identical twins, but one has a tattoo on her back, and the other doesn’t.

I hope this post was helpful for you in understanding forensic DNA and identical twins and gave you some story ideas for your crime fiction book.

I am very excited to talk about forensics and share my knowledge with you. I had a big forensic DNA nerd-out session when researching this topic. Do you have any other questions about identical twins? Let me know in the comments below!



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