Angela
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This is a note written by Joe Pickrell of DNALand, who is also a respected geneticist.
What is ancestry?
https://medium.com/@dl1dl1/what-is-ancestry-842109cb8ebd#.th2hby2si
He's honest, but I don't think he goes far enough....
"This suggests that people expect a genetic “ancestry test” to predict the geographic and/or ethnic labels of their ancestors. Unfortunately, if you sit down and try to write an algorithm to do this, you will immediately come across two huge and mostly intractable problems.
Problem #1: What time depth are we talking about?
Obviously we all have ancestors that lived at different times. You had maybe 8 ancestors living 100 years ago, but many thousands that lived 500 years ago. So whose geographic and/or ethnic labels should we try to guess — those of your ancestors living 100 years ago, or those living 500 years ago? (Or 1,000 years ago? Or…?)."
"So it’s not totally clear what time depth people generally think of when they think about their ancestry. Indeed, it seems plausible that the “correct” time depth to report in an ancestry test depends on a user’s…ancestry. This should be a hint that this is not something that can be objectively read from DNA."
"Problem #2: Ancestry identifiers are influenced by social and political factors...
Indeed, construction of a shared ancestral identity was (and remains) a method for consolidation of political power in ver6 diverse cultures (see e.g.Franco in Spain[COLOR). This is largely invisible to genetics, except after hundreds or thousands of years (if shared identities influence subsequent marriage and/or migration patterns)."
"what you would ideally like to have is a detailed list of your ancestors at different time depths, each labeled with their geographic location and any ethnic self-identifiers. You could then say, for example, that 100 years ago 25% of your ancestors lived in Illinois and identified as Jewish, while 500 years ago 5% of your ancestors lived in present-day Andalucia and identified as Muslim."
Unfortunately genetic tests are about as useful as Ouija boards for obtaining much of this information, so we’re going to have to compromise with some dramatic approximations [3]. Specifically, the approach taken by all of the commercial companies (and that we take as well) is to try to estimate the general geographic regions where your ancestors lived (and in a select small number of cases their ethnic identifiers) some indeterminate time in that past, probably something like a few hundred years ago."
"But the key is this: if we replace an impossible goal of perfectly understanding the geography and ethnicity of your ancestors with the more realistic goal of getting a general understanding about some of them, we can now make some progress. "
There's other stuff in there; definitely worth a read, but the fact is that if people really understood what he says here how many of them would actually do these tests?
The other huge problem is that a lot of people, either because they don't know these things, or because of some agenda, take the results of things like the gedmatch calculators, which are watered down versions of Admixture and Structure, absolutely literally, and even worse, actually use them to attempt to reconstruct population history on national and regional levels. It's breathtaking in its audacity, especially when done by amateurs who don't even understand how Admixture works.. So, a "Balkan" score in modern people equates to "Greek" ancestry from the Classical period. How on earth could you conclude that?
I also quibble with something he says about 23andme vs. AncestryDNA. He says he tends to think that 23andme shows more "recent" ancestry than AncestraDNA, i.e. is limited to 500 years ago. That's ridiculous in my opinion.I don't know of Ancestry DNA can figure out your ancestry at a greater time depth than five hundred years, but I do know that 23andme does as well, although they don't admit it. Northern Italians don't have 11, 20, 27, whatever, % "NWEuropean" ancestry that entered their genome in the last 500 years, and nor do Southern Italians have additional "Caucasus" ancestry since 1400 AD. So, even that isn't accurate. Plus, upthread he says they all are giving you ancestry within the last 500 years.
I take nothing away from these people in their knowledge of genetics. The problem is that they know next to nothing of European history.
Ed. The real take home is that these things cannot give people the information they really want. It really only works for people who have all four grandparents from one area, and even then all it's going to tell you is if you are roughly similar to other people of your time and place.
Yes, it can also tell you if you are really the child of your parents, and perhaps it could reunite some lost child or some "cousin", with his or her lost family, but that isn't what people are signing up for....
What is ancestry?
https://medium.com/@dl1dl1/what-is-ancestry-842109cb8ebd#.th2hby2si
He's honest, but I don't think he goes far enough....
"This suggests that people expect a genetic “ancestry test” to predict the geographic and/or ethnic labels of their ancestors. Unfortunately, if you sit down and try to write an algorithm to do this, you will immediately come across two huge and mostly intractable problems.
Problem #1: What time depth are we talking about?
Obviously we all have ancestors that lived at different times. You had maybe 8 ancestors living 100 years ago, but many thousands that lived 500 years ago. So whose geographic and/or ethnic labels should we try to guess — those of your ancestors living 100 years ago, or those living 500 years ago? (Or 1,000 years ago? Or…?)."
"So it’s not totally clear what time depth people generally think of when they think about their ancestry. Indeed, it seems plausible that the “correct” time depth to report in an ancestry test depends on a user’s…ancestry. This should be a hint that this is not something that can be objectively read from DNA."
"Problem #2: Ancestry identifiers are influenced by social and political factors...
Indeed, construction of a shared ancestral identity was (and remains) a method for consolidation of political power in ver6 diverse cultures (see e.g.Franco in Spain[COLOR). This is largely invisible to genetics, except after hundreds or thousands of years (if shared identities influence subsequent marriage and/or migration patterns)."
"what you would ideally like to have is a detailed list of your ancestors at different time depths, each labeled with their geographic location and any ethnic self-identifiers. You could then say, for example, that 100 years ago 25% of your ancestors lived in Illinois and identified as Jewish, while 500 years ago 5% of your ancestors lived in present-day Andalucia and identified as Muslim."
Unfortunately genetic tests are about as useful as Ouija boards for obtaining much of this information, so we’re going to have to compromise with some dramatic approximations [3]. Specifically, the approach taken by all of the commercial companies (and that we take as well) is to try to estimate the general geographic regions where your ancestors lived (and in a select small number of cases their ethnic identifiers) some indeterminate time in that past, probably something like a few hundred years ago."
"But the key is this: if we replace an impossible goal of perfectly understanding the geography and ethnicity of your ancestors with the more realistic goal of getting a general understanding about some of them, we can now make some progress. "
There's other stuff in there; definitely worth a read, but the fact is that if people really understood what he says here how many of them would actually do these tests?
The other huge problem is that a lot of people, either because they don't know these things, or because of some agenda, take the results of things like the gedmatch calculators, which are watered down versions of Admixture and Structure, absolutely literally, and even worse, actually use them to attempt to reconstruct population history on national and regional levels. It's breathtaking in its audacity, especially when done by amateurs who don't even understand how Admixture works.. So, a "Balkan" score in modern people equates to "Greek" ancestry from the Classical period. How on earth could you conclude that?
I also quibble with something he says about 23andme vs. AncestryDNA. He says he tends to think that 23andme shows more "recent" ancestry than AncestraDNA, i.e. is limited to 500 years ago. That's ridiculous in my opinion.I don't know of Ancestry DNA can figure out your ancestry at a greater time depth than five hundred years, but I do know that 23andme does as well, although they don't admit it. Northern Italians don't have 11, 20, 27, whatever, % "NWEuropean" ancestry that entered their genome in the last 500 years, and nor do Southern Italians have additional "Caucasus" ancestry since 1400 AD. So, even that isn't accurate. Plus, upthread he says they all are giving you ancestry within the last 500 years.
I take nothing away from these people in their knowledge of genetics. The problem is that they know next to nothing of European history.
Ed. The real take home is that these things cannot give people the information they really want. It really only works for people who have all four grandparents from one area, and even then all it's going to tell you is if you are roughly similar to other people of your time and place.
Yes, it can also tell you if you are really the child of your parents, and perhaps it could reunite some lost child or some "cousin", with his or her lost family, but that isn't what people are signing up for....
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