In 1986, DNA was used in court for the first time to acquit a 17-year-old boy accused of raping and murdering two teenage girls. Three decades later, it has allowed investigators to track down the alleged Golden State Killer. Analyzing crime scene DNA has become a vital tool in solving so many cases, but not everyone's DNA is totally unique. When an identical twin is involved, things get complicated.
Identical – aka monozygotic – twins make up just 0.3 percent of the world’s population. They develop from a shared zygote (an egg that’s been fertilized by one sperm) to form two separate, but genetically identical, embryos.
This similarity makes it virtually impossible to tell which twin committed a crime, or which is the father of a specific child, using current DNA tests. But now, scientists are edging closer to telling twins apart based on their genetic material, a breakthrough that could have numerous applications.
The new technique involves comparing twins' genomes and looking for small segments of DNA called short tandem repeats (STRs). These segments mutate as we develop, and do so at a much faster rate than the rest of our DNA. And this applies to identical twins too – their STRs can differ.
Back in 2012, Michael Krawczak of Kiel University published a paper outlining calculations for using STR differences to determine paternity. Then, scientists from Eurofins Scientific – an international group of labs with headquarters in Brussels – tested it out. By looking at the STRs of two male identical twins, along with the DNA of one’s wife and child, they determined who was the father. They published a paper outlining their proof of concept in 2014.
Then the method was used to distinguish between identical twins in Boston, one of whom was suspected of rape. The test determined the perpetrator to be the suspected twin, but since the approach had not yet been replicated or peer-reviewed, the evidence was dismissed. However, other evidence confirmed the suspected twin to be responsible.
Now, Krawczak and his colleagues have published a paper in PLOS Genetics, which outlines the approach in mathematical detail. While this is a step in the right direction, before the technique can have real-life applications it needs to be tested on large samples of identical twins to determine exactly how accurate it is.
“It would be really nice to know that we could do this kind of analysis over and over and over again and never get it wrong,” Harvard geneticist Steven A. McCarroll told The New York Times.
So, while the concept is certainly exciting, more research is needed before it can be confidently relied upon in courtrooms.
“In conclusion, [the technique] has rendered genetic discrimination between the germlines of monozygotic twins a realistic option, fit for practical forensic casework,” the authors write in their paper.
[H/T: The New York Times]