You Probably Already Have Cousins in Your DNA Database
The DNA database led police to the Golden State suspected murderer through data uploaded by distant relatives. Now, population genetics researchers have calculated the likelihood that your relatives transferred their genetic information to a similar database.
According to their calculations , most of us are likely to have multiple second cousins in a database per 1 million people, about a hundred if the database contains 5 million people, and more than 200 in a database per 10 million people. At any of these sizes, the chances that the database will contain at least one person who is your fourth cousin or higher is almost 100 percent.
The GEDmatch database, used by law enforcement in the Golden State Killer case, currently contains about 650,000 records. AncestryDNA has about 5 million and 23andMe has about 2 million. ( These numbers were compiled by genealogist Leah Larkin last year.)
Larger databases are not currently used by law enforcement for DNA searches as they do not accept data files but simply spit out samples. But interest in genealogy only grows over time, so it’s worth keeping an eye on these numbers.
Putting these calculations together, it’s perfectly reasonable to expect most of us to have distant cousins at GEDmatch and perhaps some closer cousins at AncestryDNA and 23andMe.
These numbers have important limitations. They assume that the population in these databases is a random sample from the population as a whole, which is probably not the case. (I bet this is a skew of whites, wealthy and Mormons .) They also don’t involve inbreeding and that people choose their partners quite randomly. Finally, they are average; It is possible that by chance you have no relatives interested in genealogy, or, on the other hand, your mother and sister are working on a family tree and have convinced all your close relatives to take part in it.
But the bottom line, these scientists say, is that law enforcement who found the suspect’s family DNA in a public database was probably not a lucky find at all, but an expected one.
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