On the consequences of DNA profile mismatches for close relatives of an excluded suspect

If the DNA profiles of a crime stain and the reference sample from the suspect do not match, the suspect is excluded as the donor of the crime stain. However, in some situations the DNA evidence can suggest that a close relative of the suspect might match the stain, in particular when the reference sample from the suspect and the crime stain share rare alleles. This finding can be important for the authorities. The forensic scientist has to decide whether or not to notify the authorities in these circumstances. To the best of our knowledge there is not yet an objective rule for making this decision. We propose such a decision rule for brothers of the suspect, investigate its performance and address some ethical, legal, and practical aspects. Our calculations can be simply adjusted for other relatives of the suspect.