This overview of the science of familial DNA searching and its use in criminal investigations is intended to assist State, local, and tribal justice agencies that are currently or considering using familial DNA searching in criminal investigations.
"Familial DNA searching" involves using CODIS (Combined DNA Index System) to search for DNA profiles that identify a biological family member of a suspect when an exact match for the DNA of a suspected perpetrator cannot be found in CODIS. This would suggest that the perpetrator is biologically related to the person whose DNA is recorded in CODIS. This information is helpful in narrowing the investigation's focus to the members of a particular family. CODIS, however, is not designed to facilitate such familial searching. This requires States that develop familial search protocols using independently validated methods and programming. A familial DNA search is based on the number of shared genetic characteristics (i.e., alleles) and the rarity of those shared alleles in human populations. Unlike a search for a direct match, a familial search will allow for matching subsets of alleles at any given genetic markers as a basis for comparison. Because alleles in humans are inherited in a one-for-one relationship from the father or mother, close relatives of a targeted perpetrator can be expected to share more alleles, especially rare alleles, than would unrelated individuals. A familial search relies on mathematical modeling specific to the DNA database being used. This overview outlines considerations in implementing a familial search protocol.