No, You Don’t Really Have 7,900 4th Cousins: Some DNA Basics for Those With Ashkenazi Jewish Heritage

Jennifer Mendelsohn
10 min readMay 23, 2017


Perhaps you’ve been lured by the siren song of the commercials (“We thought we were German! But it turns out we’re Scottish!”) and had your DNA tested by a company like Ancestry, FamilyTreeDNA, MyHeritage or 23andme. You’re certainly not alone: Ancestry currently boasts some (2022 update!) 22 million people in its DNA database. Perhaps, like me, your secret hopes of learning you were Inuit or Maori were dashed and the test instead confirmed what you pretty much already knew: that your ancestors were, overwhelmingly, Eastern European Jews. (“You could have paid me a hundred dollars to tell you that,” deadpanned my brother-in-law.) Or perhaps the test turned up Jewish ancestry that you had no idea you had.

While the commercials tout the ability to illuminate your ethnic heritage, perhaps less well-known is the fact that these tests provide a list of people with whom you share DNA, ranked in order of predicted closeness to you: your genetic “cousins.” When I first got my results, I excitedly reached out to many of those listed as probable “second to third cousins,” certain I would soon be trading kugel recipes with them. Over time, I became increasingly puzzled and frustrated why I couldn’t connect a single one of them to my known family tree. I quickly discovered that Jewish DNA is, well, different from other DNA. If you are expecting that your DNA test will create a clear-cut breadcrumb trail taking your family tree back to the days of King David, think again. You can definitely use your DNA results to expand your family tree; without DNA, for example, I couldn’t have confirmed that my grandfather’s first cousin played Major League baseball with Moe Berg or found the families of two of my mysterious great-grandmothers. Sometimes DNA turns up people thought lost in the Holocaust or can be used to solve decades-old mysteries. But it takes a little legwork, and there is a bit of a learning curve.

Here are a few basic things I wish I’d known before I got started:

  1. Learn the meaning of “endogamy:” For genetic purposes, Jews are what’s known as an “endogamous” population. We stayed in a relatively limited geographical area and typically married only within our own culture — and not infrequently within our own families. (A 2014 study suggested that all Ashkenazi Jews alive today trace back to the same 330 people. Try fitting all those “relatives” at your seder table.) Endogamy means that Jews share much more DNA with each other than average, which grossly inflates our relationship predictions. You might quite literally have thousands and thousands of people listed as being “4th cousins or closer;” my first cousin has more than 194,000 Ancestry DNA matches. But that doesn’t mean those people are actually related to you in the way we typically think of relatives. Virtually every Jewish person shares enough DNA to suggest that they are a “cousin” to every other Jewish person, regardless of whether those people are actually supposed to be blood related. I share DNA, for instance, not only with an old family friend, but with both of his son-in-law’s parents. And when we polled the members of the Jewish DNA group on Facebook, 62% of those who responded were DNA matches to me, though none of them are my actual relatives.
Um, help?

2. Know Your Numbers: The DNA companies predict a relationship between two people in part by looking at the amount of DNA they share, which is measured in a unit called centimorgans (cM). The closer your relationship, the more centimorgans of DNA you typically share. (23andme uses a “percentage” of shared DNA.)

Those numbers are important because there are known ranges of shared DNA that people at various relationship levels typically fall into. Here, for example, is the chart used by the popular Facebook group “DNA Detectives:”

credit: DNA Detectives

But here’s where endogamy comes in and makes thing tricky: People with significant Jewish ancestry can’t really go by the charts when it comes to anything past second cousins or so. That’s because for the average person, it’s pretty straightforward: you share 80 cM of DNA with third cousin Harry because you both inherited it from great-great-grandpa Ebenezer Humdinger. End of story. But the DNA Jews share may come from multiple shared ancestors in various parts of your tree, which inflates our relationship predictions.

How? Well, let’s say two Jews share 80 cM, which for anyone else might suggest that they are third cousins. In our case, it might mean we are third cousins, but it’s also very possible that only 40 cM of that matching DNA was inherited from your great-great-great-great-grandma Chaja Rosenschmidt on your mother’s mother’s side. The other 40 may have been inherited from your father. So that total number of shared centimorgans, rather than being evidence of a single relationship, is really the product of more than one smaller relationships. Complicate that even further by realizing that each of those smaller amounts of shared DNA could potentially be divided further: your father may have inherited 20 of those cM from his mother and the other 20 from his father. And so on and so on. Suddenly it’s easy to see why that “second to fourth cousin” match — based on a sum total that would be typical of a second to fourth cousin who wasn’t Jewish — is really someone only very very distantly “related” to you — a sixth or seventh or tenth cousin in two or three or more different ways. And for many Jews, whose trees often only trace back two or three generations due to lack of easy access to Eastern European records and the fact that most Eastern European Jews did not take surnames until roughly 1800 or so, tracing cousins at that distance can be virtually impossible.

3. Be Careful With “Shared Matches:” Most companies have a nifty feature that lets you see what matches you have in common with someone or even which matches you don’t have in common. This tool would help the average person neatly sort which side of your family a stranger is matching on; your mother’s relatives won’t typically also match your father’s, and your mother’s mother’s relatives won’t typically also match your mother’s father’s. But here endogamy strikes again to muddy the waters. “T” is my mother’s first cousin; their mothers were sisters. But when I look at which matches T and my mother have in common, they include both of my mother’s paternal first cousins, the children of her father’s siblings. If those paternal first cousins were strangers on my match list, I would be wrong to assume that they were somehow also related to my maternal grandmother simply because they also share DNA with T. Similarly, my shared matches with my Dad’s second cousin include my mother and all three of her first cousins, both maternal and paternal. When and if you get really granular, you can start looking at exactly which segments of DNA you share to find out how a stranger might be related to you, but for starters, don’t make any assumptions based on common matches.

4. Don’t Worry About Names. I hear all too often that people look at their DNA matches and are disappointed. “I don’t think it really works,” they say with a shrug. “I don’t recognize a single name on that list.”

Wrong answer.

Think about it. Full third cousins share two out of 16 great-great-grandparents: your grandparents’ grandparents. For you to recognize the name of a possible third cousin, that means you would need to potentially recognize the surnames of all 16 of your great-great-grandparents, including, for instance, the maiden name of your mother’s mother’s mother’s mother. For the vast majority of American Jews, that’s exceedingly rare. Put another way: A third cousin is the grandchild of one of your grandparents’ first cousins. Do you know the names of all of your grandparents’ first cousins? Do you know their daughters’ married names? And all the last names of their grandchildren? All you actually need is a single woman in your tree whose maiden or married name you don’t know, or a single man whose mother’s maiden name you don’t know and boom — any of those matches with an unfamiliar name could suddenly make sense. I had a DNA match to a woman with the most quintessentially Irish name imaginable. But sure enough, she was descended from my third great-grandfather Samuel Birnbaum of Krakow, Poland. You just never know.

It might seem counter-intuitive, but in order to get the most out of DNA testing, you should try as best you can — using sites like, Family Search and JewishGen— to flesh out your tree on paper. The more family names and locations you are familiar with, the greater the chance you will recognize how your DNA matches connect to you.

OK, so now you know what not to do. What should you actually do to help find genetic relatives?

1. Test the oldest person in your family. If you have a living parent or grandparent, or anyone who is in a generation above you on the family tree, testing their DNA can help things fall into place; you want to get as close to the source as you possibly can, so to speak. If you have two living parents, test both of them and don’t even bother testing yourself; all of your DNA came from your parents, and you only inherited half of it, so you’ll learn more from their results than your own. If you have a living grandparent, test him or her before your parent. If you have a living aunt or uncle, test them. If you’re able, test a couple of additional people—aunts, uncles or cousins — on opposite sides of your family; the patterns of who matches whom might help you recognize which side a match is on.

2. 2022 update: this section used to be a specific plug for Gedmatch, but I am editing it.

Get Thee to a Chromosome Browser, Pronto. For those from endogamous populations to get any value out of the DNA testing process, it’s imperative that you can see the size of the segments you share with others. And for that you need a chromosome browser, which you’ll find on all the major services except Ancestry. (Ancestry only tells you the size of the single largest segment you share, which is a start.) You can upload results from Ancestry or 23andme for free to MyHeritage and FTDNA, both of which provide a chromosome browser. You can also consider uploading to Gedmatch, a third party tool. Before uploading anywhere please read their terms of service carefully, especially in regard to law enforcement access.

Why is the size of the segments important? Typically, the closer you are to a common ancestor, the larger the segments, or “pieces” of DNA you’ll share with someone, so segment size can be a key way to differentiate endogamous matches from real relatives Once again, in an endogamous population, the total shared can be deceptive. You might share a total that seems like you should be third cousins, but it’s really just an accumulation of many many small segments, which are likely the vestiges of centuries-old, untraceable connections.

For Ashkenazi Jewish matches, anyone with whom your total cM shared is close to or over 200–250 cM is definitely worth contacting; that’s almost certain evidence of a traceable family relationship. (But keep in mind: DNA pulls back the veil on a lot of secrets, so it may not necessarily be a relationship you knew about.)

But most of the Jewish kits I manage have hundreds if not thousands of people in the 100–150 total cM range, most of whom are suggested to be second to fourth cousins. So how do you separate the faux cousins from the ones you should invite to your son’s bar mitzvah? Well, for starters, look for anyone with a total greater than 100 cM and a longest block at least 25 to 30 cM; then open the chromosome browser which shows a list, by chromosome, of all the segments you share. If there are multiple long segments — one or two over 20–30cM and several others over 10 cM — you just might be in business. If all you see are a handful of smaller segments, none even close to 20, you may not find your connection in this lifetime.

My father’s half second cousin and I have 2 segments longer than 30 and 5 longer than 10 cM; gedmatch suggests we are about 3.3 generations away from our “Most Recent Common Ancestor,” which is roughly correct.
A “third cousin” match of my mother’s. See how there’s not a single segment over 20 cM? Not likely a traceable match and almost certainly not a third cousin. They are probably related way beyond the scope of our trees.

That’s not to say you can’t be meaningfully related to someone with whom you don’t share a huge amount of DNA; there can certainly be outliers hiding in your matches. It’s also most definitely not saying to ignore everyone under 100 cM total, especially on Ancestry. (They have an algorithm that seriously reduces the number of endogamous matches you get on your list through the third cousin category.) The key is to look for the highest ratio of total/segments. An 80 cM total match over only three segments is a good match. A 100 cM total match over 12 segments is almost certainly endogamous. But if you’re feeling overwhelmed by the size of your match list and are looking for a way to increase the odds of hitting genealogical pay dirt, start with those with whom you share the most DNA and the largest segments. And then you can fight over whether they cut the turkey without you.



Jennifer Mendelsohn

Old school journo. #resistancegenealogy creator. Recovering Long Islander. No, your name wasn’t changed at Ellis Island.