Probabilistic strategies for familial DNA searching

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.