Due to the unique properties discussed above and the expected increases in the data content of Y-chromosome reference databases it can be expected that Y-chromosomal markers will be increasingly used in forensic casework in the future, particularly in cases where autosomal STRs do not provide useful information. If an autosomal STR profile can be obtained, e.g. in difficult cases through LCN analysis in combination with laser dissection microscopy, the existing national DNA databases together with essentially individual identification provided by autosomal STRs will always make autosomal STR profiles more informative than those from Y-STRs (given their lack of identifying individuals). However, if no interpretable autosomal STR profile can be obtained from a crime scene sample, Y-STRs are the markers of choice for male lineage identification, including low male and multi-male components in mixed stains.
The forensic use of Y-chromosomal markers for geographical origin identification is also expected to increase but will depend not only on the construction of enlarged worldwide reference databases, but also on non-scientific issues such as the adaptation of national DNA laws or the practical interpretation of existing laws. The forensic application of DNA markers to geographical origin identification is not in agreement with the legislation in those countries where the use of DNA for law enforcement purposes is restricted towards markers that allow DNA identification based on non-coding number codes, as usually obtained from autosomal STRs, and do not allow the use of markers that can reveal other kinds of information. Such countries will have to adjust their legislation if they wish to take advantage of the new scientific possibilities offered by new Y-chromosomal (and other genetic) markers. The Netherlands is one (if not the only) country that has modified its DNA legislation and, since 2003, under specific conditions allows the use of DNA markers for genetic ancestry identification as well as for the identification of externally visible characteristics. However, assumptions about the externally visible characteristics of an individual based on geographical origin identification will always be indirect. Furthermore, the accuracy of the assumptions is highly dependent on the level of correlation between the geographical region and the visible trait and therefore such tests are currently limited to a small number of geographical regions where a high correlation exists. In the future it will be important to understand the genetic basis of externally visible human characteristics, which is challenging due to the complex nature of many genes as well as environmental factors being most likely involved. Such research might provide DNA markers to be used by forensic laboratories as direct predictors of human appearance and thereby help to trace unknown suspects (so far, scientifically possible only for red hair; Grimes et al., 2001).
Finally, the Y-chromosomal markers available today only allow male lineage but not male individual identification. Not being able to differentiate between members of the same male lineage is clearly a major limitation when a Y-chromosomal profile match is obtained (although excluding male individuals is highly valuable too and can be done with a high degree of certainty based on existing Y markers). Future research will determine whether it will be possible to find Y chromosome markers that allow differentiation between close male relatives and thus allow the identification of male individuals and not 'only' groups of male relatives as possible today.
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