The Math Behind a Lack of Genetic Privacy #dna


I dunno about the “how easy” part when the individual is Ashkenazi Jewish.  I noted that on the Finding Your Roots episode with George R. R. Martin as a subject, they determined from his DNA that his grandfather wasn’t the Italian fellow expected, but some unknown Ashkenazi Jewish man. At that point in the program, with folks of English/German/random Western European descent, they usually whip out the other DNA relatives they’ve found and their determination of who the unknown ancestor really was. But here with presumably the resources to use the best DNA detectives, they couldn’t identify the grandfather.

Susan Slusky
Highland Park, NJ

Jan Meisels Allen


The Wall Street Journal posted an article in their May 21-22, 2022 issue entitled, The Math Behind a Lack of Genetic Privacy. In the online edition it is called, Its too Late to Protect Your Genetic Privacy. The Math is Explaining Why.



While you personally may not have taken a DNA test, the article explains that they can track you down from a cousins’ DNA that was submitted to one of the genetic DNA testing companies.


The article explains, “people have about 6,800 cMs. A child inherits half their DNA—one set of chromosomes—from each biological parent. So child and parent will have around 3,400 cMs of DNA that match… For every “degree of relatedness,” the length of shared cMs halves. An uncle or grandparent, one degree removed from parents, shares half as much DNA on average. That is 25%, or about 1,700 cMs. One more degree removed: A first cousin or great-grandparent shares half again, or around 850 cMs. And so on.”


The article includes a graphic depicting how much DNA you share with distant relatives-going to the third great-grandparents. “Even with all these halvings, very distant relatives out to fifth cousins share so much identical DNA that a common ancestor is the only possible source.”


“It is easy to find distant relatives, because a typical individual has so many: according to various methods, around 200 third cousins, upward of 1,000 fourth cousins and anywhere from 5,000 to 15,000 fifth cousins… An adopted child who doesn’t know his biological parent still shares 3,400 cMs with that person, and hundreds of centiMorgans with numerous cousins from that parent’s family. The child, or generations from now that child’s descendants, could upload their DNA to a database and by looking for matches with others who have uploaded theirs, discover some of those distant cousins. That would be enough to reconstruct his family tree and identify the parent, even though the parent never uploaded their DNA—the exact same process used to identify DNA in cold cases.”


According to data from the International Society of Genetic Genealogy, the scale of testing is enormous: around 21 million samples on AncestryDNA, 12 million at 23andMe, 5.6 million at MyHeritage and 1.7 million at FamilyTreeDNA.


To read previous articles on genetic testing and more see the IAJGS Records Access Alert archives at:  You must be registered to access the archives. To register for the IAJGS Records Access Alert go to:  and follow the instructions to enter your email address, full name and which genealogical  organization with whom you are affiliated   You will receive an email response that you have to reply to or the subscription will not be finalized.


Jan Meisels Allen

Chairperson, IAJGS Public Records Access Monitoring Committee