Probabilistic Strategies for Familial DNA Searching

Abstract

Summary Familial searching is the process of searching in a deoxyribonucleic acid (DNA) database for relatives of a certain individual. Typically, this individual is the source of a crime stain that can be reasonably attributed to the offender. If this crime stain has not given rise to a match in a DNA database, then in some jurisdictions a familial search may be carried out to attempt to identify a relative of the offender, thus giving investigators a lead. We discuss two methods to select a subset from the database that contains a relative (if present) with a probability that we can control. The first method is based on a derivation of the likelihood ratio for each database member in favour of being the relative, taking all DNA profiles in the database into account. This method needs prior probabilities and yields posterior probabilities. The second method is based on case‐specific false positive and false negative rates. We discuss the relationship between the approaches and assess the methods with familial searching carried out in the Dutch national DNA database. We also give practical recommendations on the usefulness of both methods.

Does an English appeal court ruling increase the risks of miscarriages of justice when complex DNA profiles are searched against the national DNA database?

“…If there was exculpatory evidence, then this would reduce the posterior odds (see Balding [28] page 29 for a detailed discussion). A method to interpret LRs and obtain posterior probabilities can be found in [18]. (7) The emphasis of the DNA database search is exploratory and its use is focussed more towards aiding investigations by creating sequential ranked lists of suspects to be evaluated by the method in (4).…”

“…To date the debate has focussed primarily on matches with single source, unambiguous DNA profiles [18,25]. Chung et al [26] introduce the evaluation of DNA mixtures from database searches, without drop-out.…”

“…Therefore, questions are raised about the robustness of the method employed and the chance of adventitious matches. Sorting a database according to LR is routinely carried out for familial searching [18,19]. It is already known that the person of interest does not need not to rank very high.…”

Searching secrets rationally

“…[30] for the case k = 1 and, more generally, [21]) and the coverage model by Slooten and Meester [29], are in general sub-optimal if a cost structure is considered. In both cases, in fact, if more than |w * | guesses are investigated, the net reward for the additional k − |w * | investigated guesses, is negative.…”

“…Given the genetic trait y found on a corpse, an interesting question, already posed by [29] in the case of a fixed coverage approach, is if the probability the body is related to a contributor in w * y is greater than the probability it is not. If this condition happens to be true, it encourages further identification activities.…”