Relatedness and DNA: are we taking it seriously enough?

In forensic DNA testimony most DNA laboratories report the match probability for an unrelated person from some relevant population. These laboratories typically make available the match probability for relatives when requested. This practice has served well for many years. However, as the discrimination power of our multiplexes has increased the estimated match probabilities for both related and unrelated people have become markedly smaller. Associated with this general reduction in match probabilities have been the observations that the relative balance between the match probabilities of the many unrelated people and the few relatives of a suspect has changed. We suggest that we should now report routinely the match probability for a sibling whenever the suspect has a non-excluded sibling.