HERC2 and OCA2: how blue-eyed is the perpetrator?
It took a while, but now the newspapers are filled with it: at the initiative of the states of Baden-Württemberg and Bavaria, the German Bundesrat recently had a draft for a far-reaching change in the law (printed matter 117/17; in German). It’s about expanding the scope of investigations of so called “DNA-capable material”.
To date, German investigators have been allowed to compare a DNA sample saved at a crime scene with a database to determine a potential immediate match. If necessary – and only after court approval – the police can also ask a larger group of people to provide their DNA to verify their identity to the seized DNA. If this procedure does not lead to a hit, in a German criminal case usually the utilization of the DNA trace ends at this point. But, wouldn’t it be possible to gather information on the origin, stature, skin, hair or eye color of an unknown person from it’s DNA? And if so, why can’t the German officials do that so far? And isn’t it time to change this situation?
My hope is that this week’s article will enable the reader to form an opinion on this matter, which – since it is unlikely to be re-negotiated in this legislative period – is likely to be included in the electoral programs of the major German parties. And to first explain to what extent the smallest traces of DNA allow conclusions about the appearance of a person, I thought I’ll start with a very simple system, namely eye color genetics. In fact, I really only want to write about brown and blue eyes, because green eyes are indeed wonderful, but unfortunately also terribly complicated when it comes to genetics. As for brown, we assume that it was the original eye color of people. Even today, around 90% of the world’s population has brown eyes.
But then, about 6,000-10,000 years ago, a mutation led to the ability to form the brown pigment getting lost. This mutation, which was described in 2008 in two well-known journals (here and here), lies in a gene called HERC2. Unfortunately, HERC2 does not have anything to do with pigmentation. In addition, this mutation is in an intron, thus in a “non-coding” region and does not change anything in the HERC2 protein composition. However, many noncoding regions in our genome are hugely important for the regulation of transcription, that is, the activity of different genes. And exactly this mutated site in intron 86 of the HERC2 gene, has been shown to be essential to guarantee the transcription of another gene, OCA2. The OCA2 gene encodes the so-called P protein, the exact fucntions of which are still unknown. However, we know that this P protein is used specifically in pigment cells and is necessary for pigment production. Thus, when this essential regulatory sequence is mutated in the HERC2 gene, no P protein is made from the OCA2 gene and the brown pigment is absent. Mutations on the OCA2 gene itself can therefore produce the same effect: the P protein is not formed or is formed only incorrectly and the eyes remain free of brown pigment (see diagram).
In fact, the status of the HERC2 and OCA2 genes are relatively likely to tell which eye color a human has, at least when it comes to blue or brown. If one includes the status of four more genes (published as Irisplex model), then this probability increases again and one ends up with a value of about 90-95%. This value can also be found in the explanatory notes to the bill. For the prediction of the color of the skin (light or dark type) this design even gives a value of 98%. But what do these 98% mean? Well, since explanations are missing, for the moment this remains completely unclear. However, it can be assumed that the so-called likelihood ratio (LR) is meant. This implies that the test, when run onthe DNA of a light-skinned human, suggests with 98% probability that it is indeed a light-skinned human. However, this value by no means takes into account the different group sizes within the total population.
Here’s an example: Imagine in a village with 1000 light-skinned and 10 dark-skinned residents, a felony takes place and DNA is secured from the crime scene. For the sake of simplicity, we also assume that it is certain that this DNA is really from the perpetrator. The genetic analysis now sugessts, with the error given above, that the DNA is from a dark-skinned human. In 98% of all cases in which DNA would come from a light-skinned human (that is, in 980 of the fair-skinned villagers), the test would have also indicated a light skin color. So there are 20 people of light skin color, where the test falsely indicates a dark skin color. So there are 30 people as perpetrators in question, of which 20 people have light and 10 dark skin. In 66% of cases, in this example, we would have a fair-skinned perpetrator and a test that falsely suggested a dark skinned person. The a posteriori probability that the offender has dark skin color is therefore only 33%.
However, these misleading statements of probabilities are not the only points to complain about in the bill. In fact, the Federal Council (printed matter 231/17) and the Bundestag (printed matter 796/16) already have two other draft laws, which would allow these DNA analyzes not only for capital offenses, but in fact for all offenses, even without a court order. In addition, the DNA samples should be allowed to be stored until the investigation of the offense is completed. Taking into account the current detection rate of burglary theft of about 15%, then one can imagine how fast such a DNA database would grow. We would probably soon be no further from the recently ordered measure to save the DNA profile of all people living in Germany.
Such a situation would not exist in any other country, at least in Europe. The Netherlands and the United Kingdom, so often cited as role models, use such DNA analysis only for capital offenses and only after a court decision (which only happens if there are no other follow-up investigative strands). All this is currently causing concern not only among privacy advocates, but several representatives of various scientific disciplines. For example, this weekend a symposium took place here at the Freiburg Institute for Advanced Studies (FRIAS), in which these developments were examined again from various angles. In any case, it remains exciting to see what the authorities in the future may do with DNA samples and how they will be interpreted and / or stored.
NOTE: Much research work on this article was done in collaboration with the other nice people of the Freiburg initiative on these three legislative proposals, which has already published elsewhere an opinion on this. Thanks for all the exciting input!