ON APRIL 24th police in California announced the arrest of Joseph DeAngelo. Mr DeAngelo stands accused of eight counts of murder. On April 27th some intriguing details emerged of what had prompted the arrest. The starting-point was genetic material recovered from the crime scenes. Though this directly matched no DNA held in a police database, analysis of it led investigators all the way back to the 1800s, to Mr DeAngelo’s great-great-great grandparents. The trail they followed allegedly links Mr DeAngelo to crimes committed around Sacramento in the 1970s and 1980s by an unknown man who acquired the nickname of the Golden State Killer, and who murdered at least 12 people and raped more than 50.

That a link to distant ancestors could lead to an arrest is testament to the power of modern genomics. Investigators first uploaded Mr DeAngelo’s genetic profile to a website called GEDmatch. This allows anyone to use his or her own genetic profile to search for family connections. GEDmatch’s database turned out to hold profiles, returned as weak matches, which looked as if they had come from distant cousins of the Golden State Killer. GEDmatch encourages uploaders to include their real name with their genomes, and the investigators were able to trace back through the matches’ parents and grandparents to find their most recent common ancestor. Then, having moved backward in time, they moved forward again, looking for as many as possible of this ancestor’s descendants. Using newspaper clippings, census records and genealogy websites, they discovered some 25 family trees stretching down from the common ancestor. On its own, the tree on which Mr DeAngelo appears has 1,000 members.

After that, old-fashioned sleuthing took over. From these thousands of descendants, the detectives found two who had had connections with Sacramento at the time the Golden State Killings were taking place. One was eliminated from the investigation by further DNA tests of a family member. The other, Mr DeAngelo, was arrested after police had tested the DNA on an item he had discarded.

Serial privacy

If a serial killer really has been caught using these methods, everyone will rightly applaud. But the power of forensic genomics that this case displays poses concerns for those going about their lawful business, too. It bears on the question of genetic privacy—namely, how much right people have to keep their genes to themselves—by showing that no man or woman is a genetic island. Information about one individual can reveal information about others—and not just who is related to whom.

With decreasing degrees of certainty, according to the degree of consanguinity, it can divulge a relative’s susceptibilities to certain diseases, for example, or information about paternity, that the relative in question might or might not want to know, and might or might not want to become public. Who should be allowed to see such information, and who might have a right to see it, are questions that need asking.

They are beginning to be asked. In 2017 the Court of Appeal in England ruled that doctors treating people with Huntington’s chorea, an inherited fatal disease of the central nervous system the definitive diagnosis of which is a particular abnormal DNA sequence, have a duty to disclose that diagnosis to the patient’s children. The children of a parent who has Huntington’s have a 50% chance of inheriting the illness. In this case, a father had declined to disclose his newly diagnosed disease to his pregnant daughter. She was, herself, subsequently diagnosed with Huntington’s. She then sued the hospital, on the basis that it was her right to know of her risk. Had she known, she told the court, she would have terminated her pregnancy.

That is an extreme case. But intermediate ones exist. For example, certain variants of a gene called BRCA are associated with breast cancer. None, though, is 100% predictive. If someone discovers that he or she is carrying such a variant, should that bring an obligation to inform relatives, so that they, too, may be tested? Or does that risk spreading panic to no good end?

It may turn out that such worries are transient. As the cost of genetic sequencing falls, the tendency of people to discover their own genetic information, rather than learning about it second-hand, will increase. That, though, may bring about a different problem, of genetic snooping, in which people obtain the sequences of others without their consent, from things like discarded coffee cups. At that point genetic privacy really will be a thing of the past.