When the police initiate a search for a person based on a witness report, they usually put out a description of the person based on just a few facts, such as sex, age, height, weight, distinct features and the colours of hair, eyes and skin. Some of these traits (phenotypes) are genetically determined and can, in theory, be predicted by typing the person's DNA, if the genetic markers causing the phenotype are known. In the reverse situation, where the police have no witnesses and no suspect, but only a biological sample left at the crime scene, information on phenotypes can be pivotal for the police inquiry.
Special attention has been addressed to genetic markers determining the colours of hair, eyes and skin, mainly because animal models have been studied for decades and many genes involved in the regulation of melanin synthesis have been identified. Melanin is a complex polymer synthesized from the amino acid tyrosine via a number of toxic intermediates (Rees, 2003; Nordlund et al., 2006). The synthesis is performed in specialized compartments (organelles known as melanosomes) in the melanocytes to protect the cell from the destructive intermediates. The mature melanosomes filled with melanin are transported from the melanocytes to keratinocytes in the skin and hair, where melanin shields the body from damaging UV light. The regulatory mechanisms involved in melanocyte differentiation, melanin synthesis and melanosome transport are very complicated and not fully understood, and since genetic variations in any of the dozens of regulatory and functional proteins involved in these processes may influence melanin synthesis, there are many candidate genes to investigate and many possible combinations of alleles that eventually make up the haplo-type responsible for a certain colour. The melanocortin 1 receptor (MC1R) is one of the key regulators of melanogenesis in humans and several SNPs in MC1R are associated with red hair colour, number of freckles, nevus count and increased risk of skin cancer (melanoma) (Sturm, 2002; Duffy et al., 2004). In one model, various combinations of seven SNPs in MC1R were shown to account for 67% of the red-haired individuals in an Australian population (Duffy et al., 2004) and a similar model was proposed for north Europeans (Flanagan et al., 2000). Four SNPs in two of the genes commonly deleted in albinos, OCA2 and MATP (two membrane proteins without any known function), are associated with dark colours of hair, skin and eyes in Caucasians (Duffy et al., 2004; Graf et al., 2005), and recently an SNP in the SLC24A5 gene (encoding a putative Na+/Ca+ exchanger) was estimated to account for 30% of the difference in skin melanin between Caucasians and Africans (Lamason et al., 2005). Many of these SNPs have large differences in allele frequencies in different populations and some of them were originally categorized as ancestry informative markers (AIMs), which can be used to divide the entire human population into groups of ethnical origin. This is not particularly surprising, since one of the most prominent traits distinguishing major population groups is the colour of the skin, and it has been suggested to use AIMs for the prediction of traits related to ethnic origin instead of identifying and typing the causative markers (Frudakis et al., 2003a). However, for forensic purposes, it will be more prudent to report on genetic markers with known effects on specific pheno-types. Today, there is insufficient knowledge about such markers, but after the completion of the human genome (sequencing) project, genetic markers for complex human traits are rapidly being identified and it is expected that tools for the prediction of human phenotypes can become sufficiently accurate to be used in forensic investigations.
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