Arrest in Golden State Killer case ‘probably not a lucky find at all, but a totally expected one’

Joseph James DeAngelo
Joseph DeAngelo, Golden State Killer Suspect, and police sketch. Photo Credit: Santa Barbara County Sheriff's Office/FBI

A DNA database led police to the Golden State Killer suspect through data his distant cousins had uploaded. Now, population genetics researchers have calculated the probability that your relatives have given their genetic information to a similar database.

According to their calculations, chances are most of us would have a handful of third cousins in a 1-million-person database, about a hundred if the database contains 5 million people, and over 200 in a 10-million-person database. At any of these sizes, chances are near 100 percent that the database would contain at least one person who is your fourth cousin or beyond.

There are important limitations to these numbers. They assume that the population in these databases is a random sample of the population at large, which it’s probably not. (I’d bet money that it skews white, wealthy, and Mormon.) They also assume no inbreeding and that people select their partners totally at random. And finally, they’re averages; it’s possible that just by chance you don’t have any relatives interested in genealogy, or on the flip side that your mom and sister are working on a family tree and have convinced all your close relatives to participate.

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But the bottom line, these scientists say, is that that law enforcement finding a suspect’s family’s DNA in a public database was probably not a lucky find at all, but a totally expected one.

Read full, original post: You Probably Already Have Cousins in a DNA Database

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