Starting in the mid-1980s, a serial killer murdered at least 10 women in the Los Angeles area. Nicknamed the “Grim Sleeper” because of the long dormancy between his crimes, he eluded capture for nearly 25 years. Then, in 2010, police arrested a man in California for what appeared to be a totally unrelated felony weapons charge. State law required the man to submit a DNA sample for a national DNA database. Typically a DNA database search looks for an exact match between a profile of DNA left at a crime scene by an unknown person and the profile of a known convicted offender. It focuses on 13 places in the genome (the full complement of our DNA) where bits of genetic material vary from person to person. If the crime-scene material differs in any of those 13 places, then the samples do not match, and investigators know that they do not have their suspect.
This time, however, the search was more subtle. It aimed to find DNA profiles that were similar, but not an exact match, to that of the Grim Sleeper. Such an inquiry was possible because in 2008 California became the first state in the nation to formally authorize a new kind of database search. Known as kinship, or familial, matching, this technique looks for partial DNA matches. It is conducted after DNA found at a crime turns up no exact hit. Because related people tend to share more DNA with one another than they do with strangers, a “near miss” in the database may suggest that the search found a person related to the actual perpetrator. Police can then investigate the relatives of the person in the database with the hope of solving the crime.