In particular, the discovery of eye colour markers with large phenotypic effects has made it easy to develop pretty accurate genetic predictors of this trait. The best-known tool commonly used in the forensic field today is the IrisPlex predictive system, which includes both a genetic test for data acquisition and a mathematical algorithm for predicting the three categories of eye colour
[10]. The algorithm was developed based on the systematic selection of markers made by Liu et al., who reported 24 variants from eight genes, enabling the prediction of blue and brown eye colour with a prediction accuracy expressed by an AUC of 0.91 and 0.93, respectively
[11]. AUC, which stands for area under the ROC (receiver operating characteristic) curve, describes the general performance of the model in such a way that 1 means perfect classification and 0.5 means random assignment to the phenotype categories. For forensic purposes, the number of markers from the originally identified 24 was restricted to the six with the largest effect
[10][11]. The six crucial predictors included
HERC2 rs12913832,
OCA2 rs1800407,
SLC24A4 rs12896399,
SLC45A2 rs16891982,
TYR rs1393350, and
IRF4 rs12203592. The original IrisPlex method implements a multinomial logistic regression algorithm and a simple single base extension method based on SNaPshot minisequencing, which allows the PCR amplification and genotyping of several SNPs in a multiplex reaction. Importantly, the products of primer extension are analysed using capillary electrophoresis platforms, which are commonly used in human identification testing laboratories. Other tools based on other mathematical solutions were soon developed but, essentially, each of these algorithms relied on exploiting information in the
HERC2-OCA2 gene complex. In general, these works were limited to the development of predictive algorithms using various sets of samples and mathematical approaches, but did not present specific tools for the collection of genetic data
[12][13][14][15][16][17]. Notably, IrisPlex and other forensic methods of eye colour prediction can accurately predict blue and brown iris colours, but have difficulty with the prediction of intermediate eye colours
[3]. Moreover, in some populations, the effect of sex was noted on prediction results
[18][19][20]. The IrisPlex tool for the genotyping and prediction of eye colour evolved to HIrisPlex
[21] and finally to the HIrisPlex-S tool
[22], which were developed based on the same strategy as IrisPlex. The algorithm for hair colour prediction implemented in HIrisPlex was developed based on the investigation of a Polish population sample, which enabled the selection of 22 crucial SNPs from 11 genes for hair colour. The study showed a high level of accuracy for red and black hair colour prediction (AUC ~ 0.9) and a lower prediction accuracy for blond and brown hair colour (AUC ~ 0.8)
[23]. The skin colour predictor was proposed by Walsh et al. after a systematic study of skin colour candidate variants in a sample of 2025 individuals from 31 worldwide populations. The algorithm predicted skin colour with very high accuracy, with an AUC = 0.97 for light skin colour, 0.83 dark, and 0.96 for dark-black skin colour
[24]. Notably, it has been demonstrated that the original SNaPshot protocol can be replaced by the targeted massive parallel sequencing (MPS) method
[25], and the HIrisPlex-S method was also adopted in a tool combining pigmentation prediction capability with ancestry inference developed by the VISAGE consortium
[26]. Other studies also investigated the possibility of hair and skin colour prediction in the forensic field
[13][27][28][29][30]. The Snipper Application suite deserves more attention because it provides an online tool that allows the performance of predictive calculations based on data generated by any genotyping method. The tool was originally developed for the statistical interpretation of data in ancestry inference studies, but a number of new functionalities have subsequently been added to enable the prediction of pigmentation and even age
[31]. A more complete prediction of pigmentation will be provided by the developed algorithms for freckle prediction
[32][33]. It is worth noting that the use of extended DNA variant sets for prediction has begun to be explored, which may lead to the development of next-generation prediction tools. For example, the previously described association work of Hysi et al. was extended to predictive modelling. Hair colour prediction was compared in two independent cohorts using prediction models based on the 258 associated SNPs and the original HIrisPlex method, and these new models outperformed the previous HIrisPlex model
[34]. Further development of pigmentation predictors may also require the use of sex information, and age will naturally be needed for the final interpretation of the data
[34][18]. This issue is also addressed later in the article, as sex in particular can be important for predicting other appearance traits.