Without forensic genetics, the two Zurich investigators Isabella Grandjean (Anna Pieri Zuercher, 45) and Tessa Ott (Carol Schuler, 37) would probably never have solved their eighth “crime scene” case “Of Apes and People”. A twin kills the identical sister and then takes over her identity. Actually a brilliant move, if it weren’t for science. As explained in the film, the DNA of twins can be distinguished and the murderer can be convicted. An alibi is no longer tenable. But is that actually true or was it just a twist in the script? What is it like in reality?
Twins have fascinated people for centuries, not only because of their often uncanny resemblance, but also because of the biological and genetic questions they raise. For example, do identical twins have identical DNA? Even if it does, can modern forensic technologies now even distinguish between them? Are twin alibis still a problem for law enforcement today, or can modern forensic scientists – as portrayed in the film – help them with such cases today?
First things first: Identical twins occur when a single fertilized egg (zygote) divides into two separate embryos early in development. These twins initially always share the same DNA because they come from the same zygote. They are always of the same gender and usually look extremely similar, which is due to their identical genetic codes.
Standard DNA analysis, which has now been used in forensic laboratories for decades, is very effective in identifying people because it is based on the analysis of sections of DNA that vary in most people. These sections, known as short tandem repeats (STRs), provide a unique genetic profile of a person, with the exception of identical twins. Even in identical twins, the STRs are initially identical. In the past, numerous procedures had to be stopped because it could not be proven beyond doubt which twin the DNA found ultimately came from.
For this reason, identical twins need to be looked at more closely than other suspects. As already mentioned, initially the DNA is identical, a standard STR analysis cannot differentiate between them. This poses a particular challenge for forensic science, particularly in criminal cases where DNA evidence plays a role and identical twins are possible suspects or in victim identification.
However, as scientific research and technological development have progressed, researchers have now managed to develop complicated methods that make it possible to differentiate between identical twins. These have been used regularly for about ten years and often rule out twin alibis. One of these modern methods is the study of epigenetic patterns.
Epigenetics deals with changes in gene function that can be inherited without changing the DNA sequence. These changes can be influenced by environmental factors or lifestyle and also vary between identical twins, providing a possible basis for distinguishing them. For the first time, British scientists analyzed the methylation of DNA, i.e. chemical deposits on the DNA double helix. This methodology is now a common procedure used by forensic geneticists.
Another potential method is to analyze mutations that may occur in monozygotic twins after the original zygote divides. Although these mutations are very small and rare, they can be used to differentiate twins at the DNA level – and ultimately convict. However, in contrast to searching for environmental influences, this method has a decisive disadvantage: it is extremely time-consuming and expensive.
The “Crime Scene: Of Apes and People” definitely reflects reality. Although DNA analysis procedures for identical twins are more complicated than for other people, there are now ways to distinguish between them. However, using the traditional methods of DNA analysis, which have been used as standard since the 1980s, a conviction is impossible.
