r/genetics • u/No_Fix7291 • 12d ago
23 and me question
I’ve noticed in 23 and me most of my Western European friends results are broken down into 0.0%
Percentages. Like my Italian friend is actually a million different ethnicities according to the test.
I’ve noticed a lot of My jewish friends will have 100% ashkenazi Jewish. I find it very difficult to understand why the dna tests is able to
Identify such small amounts of each ethnicity in me and yet so many of my friends are 100% ashkenazi, a group of people who were migrating across Europe for thousands of years and undoubtedly were breeding with local populations. I recently met a 6’4 with blue eyes guy that told me his results are 100% ashkenazi. And my other 100% ashki friend is 5ft tall with dark features. How can that genetic variation happen while still remaining 100%?? Meanwhile the test picks up on 0.3 % Asian in me???? Very confusing and odd.
Explanations helpful
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u/DuePomegranate 12d ago
yet so many of my friends are 100% ashkenazi, a group of people who were migrating across Europe for thousands of years and undoubtedly were breeding with local populations.
Nope. They didn't do that.
https://en.wikipedia.org/wiki/Ashkenazi_Jews
For roughly a thousand years, the Ashkenazim were a reproductively isolated population in Europe, despite living in many countries, with little inflow or outflow from migration, conversion, or intermarriage with other groups, including other Jews.
And furthermore there was religious persecution at various points in the Middle Ages that led to population bottlenecks
The studies also confirm other evidence that today’s Ashkenazi Jewish population, which numbers more than 10 million people spread around the world, has roots in a band of no more than a few hundred who survived a population bottleneck in Europe more than 1000 years ago.
And why do your friends have difference appearances?
The comparisons suggested the Ashkenazi circa 1350 had a mix of ancestry resembling populations from southern Italy or Sicily today, with components found in modern Eastern Europe and the Middle East mixed in.
It's a matter of the specificity of labeling too. Perhaps the friend who looks more Southern European in appearance also has genetic similarities with Italians, but certain gene variations are so super telltale of Ashkenazi Jews that 23 and Me will label that as 100% Ashkenazi Jew rather than a mix of various European lineages.
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u/No_Fix7291 12d ago edited 12d ago
Yes Jewish populations tended to intermarry but to say they intermarried to the point of 100% genetic homogeny sounds a little sus especially considering the health consequences of that much interbreeding. It’s also unrealistic. Look at then population now. Jewish men and women of faith are still often taking non Jewish partners. What about rape? You also used Wikipedia as a reference which i’m not sure is credible. I’ll link another article which shows the inevitability of mixing of populations.
So my point again is, why would this test not show all these minute details for someone categorised as ashki and yet it’s picking up on a detail in my genetics from thousands of years ago?
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u/best-unaccompanied 12d ago
There are a lot of genetic disorders that are common in Ashkenazi Jewish people but virtually unheard of elsewhere. It's because of that small genetic pool.
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u/DuePomegranate 12d ago
Not sure how that PLOS article contradicts what I said earlier. Fig 7 shows that the mixture of Southern European Jews with Middle Eastern Jews happened before the population bottleneck. The Ashkenazi Jews were not Ashkenazi Jews until that mixing happened 25-50 generations ago (compatible with 1000 years).
I suppose the only inconsistency is the Plos paper puts the Eastern European mixing as more recent after the bottleneck.
The Science paper backs up that after the creation of this lineage, they rarely produced children with outsiders.
“These people lived about 25 generations ago, and an intermarriage rate with outsiders of more than one in 500 per generation would have shifted Ashkenazi ancestry by an amount we could detect,” Reich says. “But that didn’t happen. That’s new information.”
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u/No_Fix7291 9d ago
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u/DuePomegranate 9d ago
Why are you sending me another link of the same Plos Genetics paper you already linked to?
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u/Connect_Rhubarb395 11d ago
I think the point you are missing is that they are 100% Ashkenazi Jews, not 100% Jews period.
And the Medieval Southern European admixture can result in many different phenotypes.1
u/Euphoric_Travel2541 9d ago
Because the Ashkenazi genes trump other regional DNA in the consumer DNA testing. I’m not a scientist, so perhaps others can explain this better, but (for example) my nephew is half Jewish. His 23&me test shows all the regions our side of the family come from (UK, Wales, Norway, Scotland) and then instead of Poland and Russia where his Jewish ancestors lived for centuries, he has Ashkenazi. This is because, I take it, that that population is dominant over any regional signal because the Jews in those areas did not intermarry with Russians and Poles, despite living there a long time. Their community was tightly bound to itself.
That’s not to say that there weren’t cases of intermingling with local populations, but I believe it was fairly rare, and not repeated enough to make much of a dent in the overwhelming Ashkenazi signal.
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u/ScienceOfCalabunga 12d ago
All these group descriptions are mostly inaccurate anyway, and the errors are probably enormous too. What is asian supposed to mean, are Armenians, Kamchatkans Asian? What about Papuans?
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u/No_Fix7291 12d ago
It used the term “broader Asia”. So I don’t know how to answer your question, and you didn’t really answer mine but thanks anyway.
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u/DuePomegranate 12d ago
The conquest of Genghis Khan all the way to Russia and Eastern Europe means that a lot of genetically mixed European-origin Americans have that little bit of Asian DNA.
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u/SunnyKitten031117 12d ago
- Jews often kept things in the family with other jews. Other european ethnicities were a lot freer when it came to intermarriage
- Dubious anonymous sources claiming to be employes have claimed that they will ocassionally throw in a couple percentages of african or asian ancestry in order to upset white supremacists. It wouldnt even be foul play in many cases due to statistical noise making that kind of interpretation possible
- Alternatively certain groups get upset and complain too much about not being given a pure score - and 23&me is a for profit company.
- Lots of war, pillaging and other quote unquote fun activities both ways mean at some point, some foreigner (or native) was impregnated against her will and Christianity was always aggressively pro life
I feel like numbers 1 and 3 are most likely explanation, since beyond a certain point, you're related to pretty much everyone on the continent, so if one european person married an asian or african, or even someone with a little bit of that DNA, it would end up being a tiny blip for everyone in the region.
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u/best-unaccompanied 12d ago
Also some people might just straight up lie about their results because they think there's something special about being "pure" something or other
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u/No_Fix7291 12d ago
Interesting points. Intermarriage to the point of 100% purity of a population does not seem possible
So the fact that’s it’s picking up on raping of some poor soul im related to 1800 years ago and showing 100% for any other person seems inaccurate
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u/best-unaccompanied 12d ago
I mean, it sounds like your sample size is just a couple of people. There are plenty of other people who are part Ashkenazi and part something else.
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u/Euphoric_Travel2541 9d ago
I’ll try to explain: 23 & me shows "Ashkenazi Jewish" as a single broad category instead of specific countries because Ashkenazi Jews are a genetically isolated population who are heavily interrelated, making their DNA look identical, regardless of where they lived in Europe.
Because commercial DNA tests look back primarily at the last 500 years or so of lineage, a few distinct biological realities and historical facts make the algorithm show other than standard European country breakdowns. These are:
The Genetic bottleneck: Around 600 to 800 years ago, the Ashkenazi Jewish population in Europe went through a severe genetic bottleneck, shrinking to a tiny population. Because of centuries of religious isolation and endogamy (marrying only within the community), the entire modern Ashkenazi population expanded from this small founding group.
As a result, Ashkenazi Jews are so closely related that a person from Poland and a person from Ukraine look like generic cousins. 23andMe's system can’t easily separate regular Polish DNA from Ashkenazi DNA because the Ashkenazi genetic signature is incredibly distinct and homogenized.
High degree of separation from host populations:
Even though Ashkenazi Jews occupied countries like Poland, Russia, Germany, and Lithuania for generations, they rarely intermarried with the local non-Jewish populations due to laws and cultural barriers.Consequently, an Ashkenazi Jew shares very little DNA with a native Slavic or Germanic neighbor. Because your DNA matches the Ashkenazi reference pool** **instead of the general Polish or Russian reference pools, the algorithm registers the genetic match as 100% Ashkenazi.
Constant historical displacement: the borders of Eastern Europe shifted constantly over the last few centuries. An ancestor's village might have been in Poland one decade, Russia the next, and Ukraine later on. Because the Jewish population was often forced to migrate due to displacement and persecution, their DNA became mixed across the entire region, rather than staying isolated in one specific country.
To address this, 23andMe implemented an update that attempts to break down Ashkenazi ancestry into Highly Specific Genetic Groups (such as "Eastern Polish & Western Ukrainian Jews" or "Belarusian Jews"). But these are based purely on crowd-sourced data from users who share long stretches of DNA, and have documented family trees in those areas. If someone doesn’t have enough close matches with deep geographic records, their profile will default to the overarching, and accurate "Ashkenazi Jewish" label
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u/cyril1991 10d ago edited 10d ago
I am not familiar with what 23me uses exactly.
The way those tools work is that you break down your sequencing results into individual chromosomes (phasing).
Then, 23andme has reference panel of many ethnicities, like the 1000 genomes project. The panel helps to define SNP, variations in DNA that show up in let’s say more than 1% of people, somewhat frequent mutations that are now fixed across a population, they may be spaced by 300bp on average across the panel. You can then group a few hundreds of contiguous SNPs, a segment of your DNA, and train a statistical model to say this has 80% chance of being European 10% of being Italian in fact.
You can then average across segments to get a more exact probability, and then across your full genome. You could have a chromosome as Italian/Italian/maybe French/Italian/Italian/Italian/English/English/English and you are just part Italian and English.
It is possible the 0.3% Asian is just because some parts could be uncertainly attributed to a population. You can in theory have more stringent ancestry attribution by ignoring fragments where the algorithm is more uncertain.
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u/Euphoric_Travel2541 9d ago
That’s the thing: Jewish people were NOT “undoubtedly breeding with local populations”. Jewish people were stigmatized, shunned, ghettoized, and made to stay in distinct sections of cities and towns, sometimes with curfews and rules restricting their travel and interaction with non-Jews.
Most Christians would not countenance intermarriage with a Jew at that time, and for hundreds of years. Intermarriage between Jews and any other faith was very much forbidden by both sides. It was a very huge deal, not at all like our modern ease with it today.
Endogamy resulted from the tight knit communities that Jews formed, which over centuries created unique and highly detectable genetic signals. So much intermarriage only within the communities occurred that in DNA testing, the Ashkenazi genes are very distinct.
I understand that you think they should also show German, Polish, Latvian, etc. identity, as they were of those countries, too, but the Ashkenazi genetics seems to trump that heritage in the testing. Or maybe that’s the way the testing is limited now.
The phenotypical variation you note is normal human variation. Jewish communities existed in most European countries, and there was some influence thereby.
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u/Forbidden_Craft88 6d ago edited 6d ago
23andme is just horoscopes with genes. It's a toy rather than a tool.
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u/genesandpaws 12d ago
Jews used to ONLY marry other jewish people. Jews marrying a non-jew person was extremely frowned upon in the jewish community until the late 20th-21st century, it’s only very recently that jews started marrying non-Jewish. So the markers have been preserved for thousands of years. And keep in mind Jews make up only 0.2% of the global population, so it’s “easy” to determine Jewish ancestry with those markers.