A criminal who has an identical twin poses a problem: Identical twins arise from identical DNA. The microenvironment of a developing embryo affects the formation of fingerprint patterns. As a result, identical twins (who developed in different microenvironments) have different fingerprints. What happens when a savvy criminal does not leave fingerprints at a crime scene?

Barry Starr of QUEST Northern California tackled this question in his posting entitled, “Genetic Sleuthing, or How to Catch the Right Identical Twin Criminal.” Starr highlights a sexual assault case in Marseille, France. Using video evidence and standard DNA analyses, police identified two suspects who are identical twins. Which twin is the guilty one?

DNA can offer one way to distinguish between a set of identical twins. Twins begin life with identical DNA, but DNA changes. One type of change is called an epigenetic change. “Epigenetic” refers to changes in DNA that affect gene activity without altering nucleotide sequences in DNA. Researchers have found different epigenetic markers in the DNA of identical twins. DNA mutations, caused by chemicals and UV light, also cause small changes in DNA. Why didn’t police in Marseille rush to order DNA tests for epigenetic markers and mutations? The cost of one million euros to perform the tests discouraged that approach.

If you’re planning to include a killer twin in your story, peruse the comments to Starr’s article. You’ll find plenty of ideas about how your investigators might tackle the problem.