Can DNA fingerprints distinguish between identical twins?

The other day we were watching one of our favorite British crime shows where science and forensics plays a prominent role in catching the criminal.

This one involved a complicated story line with three identical triplets who impersonated each other.¹ DNA was present at one of the crime scenes and it matched the DNA of the triplets but the investigators weren't able to distinguish between the three triplets. I knew that monozygotic twins and triplets had identical DNA fingerprints but I also knew that it was possible to tell them apart if you had more DNA sequences.

A typical DNA fingerprint consists of amplifying selected regions of the genome using PCR. These regions contain short tandem repeats (STRs) consisting of multiple stretches of identical sequences such as AGGCCT or CGCGAT. Individuals differ in the number of these repeats and hence the length of DNA that contains them and this length can be easily detected. The best DNA fingerprint techniques look at about 30 different STR regions and the chance that all 30 will be identical in size in unrelated individuals is extremely low.

Identical twins will have identical DNA fingerprints and, by chance, I stumbled across a recent case in France that was covered in a Nature news article (Fieldhouse, 2026). Although the DNA fingerprints are identical, it's possible to tell the twins, or triplets, apart if you have a whole genome sequence. That's because of early somatic cell mutations occurring in the developing embryos that are propagated to a substantial portion of the adult tissues. The article quotes a study that identified five differences in the whole genomes of identical twins.

But that kind of analysis requires substantial amounts of DNA from the crime scene and that's often unavailable. If you had that much DNA from a known tissue (e.g. blood, sperm, skin) then you would have better luck finding a match with the same tissue from the suspects because of additional somatic cell mutations that occurred in that particular tissue.

Unlike the case on TV, I suspect that just getting a standard match to one of the twins would be sufficient because of other evidence linking one of them to the case.

---

1. Monozygotic triplets are extremely rare. They occur in approximately one in one million pregnancies.

The image is from: Kaur, L., Sharma, S.G. (2022) Forensic DNA Analysis: A Powerful Investigative Tool. In: Singh, J., Sharma, N.R. (eds) Crime Scene Management within Forensic Science. Springer, Singapore. https://doi.org/10.1007/978-981-16-6683-4_1

Fieldhouse, R. (2026). Identical twins on trial: can DNA testing tell them apart?. Nature, 651(8105), 287-288. https://www.nature.com/articles/d41586-026-00521-z