ABSTRACT
This chapter reviews the basic elements of DNA evidence interpretation, describes current areas of development, and speculates on future trends. In many ways, DNA profiling has been the poster child for attaching statistical calculations to forensic evidence. This may reflect the early involvement of statistical geneticists who were experienced in performing calculations for genetic data, and it may also reflect the absence of a culture in which a forensic analyst would examine DNA evidence, declare it to have the same type as that for a person of interest, and then claim this person was the source of the evidence. There was also widespread acceptance of Mendel's Laws, by which a parent is equally likely to pass on their maternal or their paternal copy of a genetic element to a child, with these transmissions being independent for different genetic elements. Early statistical treatments, however, did not pay attention to the field of evidence interpretation and apparently simple concepts such as “random match probability” or “random man not excluded'' may have led to unsatisfactory analyses in the early days of forensic DNA profiling.
Both the nature of DNA profiling and of the methods used to quantify the strength of DNA evidence have evolved. Forensic DNA typing is changing from the examination of a small number of discrete genetic elements or markers (currently STRs) to the sequencing of increasingly large regions of the genome. In addition, the categorical the match/no-match (or “excluded, inconclusive or included'') characterization is being replaced by “probabilistic genotyping” in which a likelihood ratio for the presence of an given individual’s profile in a questioned sample is computed with respect to alternative hypotheses.
