The color of eyes: a study with application in forensic sciences

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The applications of genetics in the field of forensic sciences are well known, but that does not mean there is still much to be explored and new lines of related research continue to be opened. This is the case of the determination of the color of the eyes, a topic that historically has not aroused greater interest because it is considered that any study aimed at defining the phenotypic aspect of people – that is, their characteristics and traits – was stigmatizing.

In the last decade, however, it was gaining more and more momentum, summoning a large number of experts. In this favorable context, two scientists from the Multidisciplinary Institute of Cell Biology (IMBICE, CONICET-UNLP-CICPBA) (Argentina) have just published a paper in the journal Genetics and Molecular Biology.

The study investigates the genes that participate in the determination of the hue of the iris, the circular area in whose center the pupil is located. “The sample with which we work corresponds to the native population of Buenos Aires and with a minimum of five years of residence in the province,” says Diana Hohl, CONICET doctoral scholar and first author of the work, adding: “We compared the results with similar research based on Europeans, Africans, and Asians, and the differences are so many that the eye color identification protocols that are used internationally could not serve for this region of the planet. The variation of the genes involved may be different in each location. ”

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The research was based on 118 people between 18 and 50 years old, given that a very old age could influence the color of the iris, especially if there is an eye disease. In the same way, those who use the medication in droplets that could cause the darkening of the eye were excluded. Each volunteer was photographed without incident of external light and illuminated only with a flashlight, and a sample of saliva was also requested to correlate what was observed in the images with the genetic information, and thus be able to determine the genes that are involved in the precise definition of each tone.

Within the aforementioned identification protocols, in Europe there is a kit that analyzes a series of polymorphisms, that is, variations in the DNA that occur within the same population. The result allows us to estimate the color of a person’s eyes from their genetic information with a very low probability of error. “But here we find different frequencies of those gene variants, called alleles. It is likely that they are very common among Europeans but less frequent in this area due to our ethnic mix. So those markers might not work here, “explains Cecilia Catanesi, CONICET assistant researcher at IMBICE and participant in the research.

The genes that participate in the definition of the color of the iris are at least twenty, although the most involved are seven or eight. The main contribution to the forensic disciplines is that knowing them allows us to predict that trait of a suspect that is being sought, and not only to compare it with a detainee. “For example, it is very common to find biological material under the nails of a victim who defended himself. In these cases, knowing which genes give pigmentation of the skin, eyes, and hair allows us to draw an approximate profile of the characteristics of the attacker. But particularly the tone of the eyes is very little sense to the environment: it means that it is almost entirely determined by genetics and, with the exception of age, which can produce changes, there are not many other factors that influence it “,

Another application that can have this information is when estimating the identity of a person from skeletal remains. “When bones are found and there is no DNA from potential relatives to compare, genetic determination is very useful,” says Hohl, and continues: “As a forensic anthropologist can know if it was a man or a woman, or the height that he had, for example, it is also possible to know the color of eyes that he had, and it is one more fact when orienting the search “.

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At the beginning of the research, the scientists used software developed abroad to perform a quantification of the color of the iris, but they had many problems because they did not achieve a good recognition of many of the Buenos Aires’ own tones, and instead It was much more precise with light tones, which predominate in other parts of the world.

For the following trials, then, they used another program and, with the help of IMBICE staff, they are developing another measurement methodology. They also improved the procedure to take the photographs, and went on to do it with the collaboration of CONICET intern María de Los Ángeles Gutiérrez in the scope of the University Extension Environmental Program of the National University of La Plata (PAEU, UNLP), which counts with a laboratory and equipment much more adapted for the task.

The publication was based on Hohl’s thesis, a work that was awarded twice in the past year: during the Third International Scientific and Technological Congress of the Province of Buenos Aires, and at the XIV Latin American Congress of Biological Anthropology, organized by the Association of Argentine Biological Anthropology in Uruguay.

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