Sep 13, 2026 06:05 PM
Identical twins once had the perfect DNA alibi. Here’s how forensic scientists found a way around it
https://www.zmescience.com/other/offbeat...tic-alibi/
EXCERPTS: For most of the history of forensic DNA testing, identical twins presented police and courts with a problem that seemed almost impossible to solve.
Identical twins share nearly 100 percent of their DNA, so if DNA from one twin was found at a crime scene, the police could not tell which twin it belonged to—leaving investigators unable to determine who the culprit was.
In practice, this could make DNA evidence much less useful. If investigators found a genetic profile that matched two identical twins, the result might show that one of them was involved without revealing which one. That could leave prosecutors relying on other evidence such as phone records, surveillance, witnesses, or fingerprints; and sometimes that’s just not available.
And this is not just some hypothetical. Identical twins have complicated criminal investigations for well over a century, long before DNA testing entered the courtroom. Some have gotten away with crime thanks to their twin sister or brother.
[...] Identical twins begin with essentially the same genome. However, ‘identical’ does not mean genetically frozen in perfect lockstep.
When cells copy their DNA, they occasionally make mistakes or ‘mutations’. Most have no noticeable effect, but some mutations can arise after the embryo splits into two twins. If such a mutation occurs early enough, it can spread through many cells in one twin while remaining absent from the other.
Scientists began trying to find these minute genetic differences by looking far beyond the handful of markers used in conventional forensic testing.
[...] However, the differences between identical twins can be extremely rare. They may appear in only a small fraction of their cells, making them difficult to detect, verify, and use reliably as forensic evidence. Plus, finding a difference in the twins is only half the problem. Scientists must still demonstrate that the same mutation is present in the crime-scene sample and establish how strongly that evidence favors one twin over the other.
[...] The most important change is that investigators no longer have to treat an identical twin DNA match as an absolute dead end.
A conventional DNA profile may still leave two twins looking identical. However, scientists can now go deeper, searching the genome for mutations acquired as the twins developed and using extremely deep sequencing to detect variants that may exist in only a small fraction of cells... (MORE - details, specific crime cases covered)
https://www.zmescience.com/other/offbeat...tic-alibi/
EXCERPTS: For most of the history of forensic DNA testing, identical twins presented police and courts with a problem that seemed almost impossible to solve.
Identical twins share nearly 100 percent of their DNA, so if DNA from one twin was found at a crime scene, the police could not tell which twin it belonged to—leaving investigators unable to determine who the culprit was.
In practice, this could make DNA evidence much less useful. If investigators found a genetic profile that matched two identical twins, the result might show that one of them was involved without revealing which one. That could leave prosecutors relying on other evidence such as phone records, surveillance, witnesses, or fingerprints; and sometimes that’s just not available.
And this is not just some hypothetical. Identical twins have complicated criminal investigations for well over a century, long before DNA testing entered the courtroom. Some have gotten away with crime thanks to their twin sister or brother.
[...] Identical twins begin with essentially the same genome. However, ‘identical’ does not mean genetically frozen in perfect lockstep.
When cells copy their DNA, they occasionally make mistakes or ‘mutations’. Most have no noticeable effect, but some mutations can arise after the embryo splits into two twins. If such a mutation occurs early enough, it can spread through many cells in one twin while remaining absent from the other.
Scientists began trying to find these minute genetic differences by looking far beyond the handful of markers used in conventional forensic testing.
[...] However, the differences between identical twins can be extremely rare. They may appear in only a small fraction of their cells, making them difficult to detect, verify, and use reliably as forensic evidence. Plus, finding a difference in the twins is only half the problem. Scientists must still demonstrate that the same mutation is present in the crime-scene sample and establish how strongly that evidence favors one twin over the other.
[...] The most important change is that investigators no longer have to treat an identical twin DNA match as an absolute dead end.
A conventional DNA profile may still leave two twins looking identical. However, scientists can now go deeper, searching the genome for mutations acquired as the twins developed and using extremely deep sequencing to detect variants that may exist in only a small fraction of cells... (MORE - details, specific crime cases covered)