Evaluation of the IrisPlex DNA-based eye color prediction tool in the United States

Abstract

DNA phenotyping is a rapidly developing area of research in forensic biology. Externally visible characteristics (EVCs) can be determined based on genotype data, specifically from single nucleotide polymorphisms (SNPs). These SNPs are chosen based on their association with genes related to the phenotypic expression of interest, with known examples in eye, hair, and skin color traits. DNA phenotyping has forensic importance when unknown biological samples at a crime scene do not result in a criminal database hit; a phenotype profile of the sample can therefore be used to develop investigational leads. IrisPlex, an eye color prediction assay, has previously shown high prediction rates for blue and brown eye color in a European population. The objective of this work was to evaluate its utility in a North American population. We evaluated the six SNPs included in the IrisPlex assay in an admixed population sample collected from a U.S.A. college campus. We used a quantitative method of eye color classification based on (RGB) color components of digital photographs of the eye taken from each study volunteer and placed in one of three eye color categories: brown, intermediate, and blue. Objective color classification was shown to correlate with basic human visual determination making it a feasible option for use in future prediction assay development. In the original IrisPlex study with the Dutch samples, they correct prediction rates achieved were 91.6% for blue eye color and 87.5% for brown eye color. No intermediate eyes were tested. Using these samples and various models, the maximum prediction accuracies of the IrisPlex system achieved was 93% and 33% correct brown and blue eye color predictions, respectively, and 11% for intermediate eye colors. The differences in prediction accuracies is attributed to the genetic differences in allele frequencies within the sample populations tested. Future developments should include incorporation of additional informative SNPs, specifically related to the intermediate eye color, and we recommend the use of a Bayesian approach as a prediction model as likelihood ratios can be determined for reporting purposes.