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Genetic ancestry is basically a horoscope?

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zodiac from 6th c/NASA

So says conservation biologist Ross Pomeroy at RealClearScience:

Think about it. As you travel back in time though your family history, the number of ancestors you have roughly doubles with every generation. Using the most conservative estimate of generation time — 32 years — in the year 1152, you had as many as 134,217,728 potential ancestors. And since genes are scrambled with every generation, it’s very likely you share little to no genetic relation to most of them. They might as well be strangers!…

DNA testing companies often take this ambiguity and fill in the blanks with impressive stories that you can show your friends and relatives. Though fascinating, these tales share more in common with astrological horoscopes than historical accounts.

Mark Thomas, a Professor of Evolutionary Genetics at University College London is one of the most vocal advocates of this criticism. On a recent episode of the BBC radio show The Infinite Monkey Cage, he said that that appeal of both horoscopes and genetic ancestry tests arises from the Forer effect.

“If you tell somebody something that seems like it’s highly personalized but in fact is very generic — you can apply it to anybody — then people are much, much more likely to believe it. More.

<em>coffee</em> cup with moustache by thegreatmoustache on DaWanda.com
Last one this morning, promise!

Also, though Pomeroy doesn’t dwell on it, very few such services would stay in business long if they informed their customers: “Your genes show that you come from a long line of habitually dishonest people who were forced to reform because their lifestyle was dangerous on account of the fact that they weren’t—from what we can tell—very bright.”

No, no, it’s for sure got to be cool stuff like royalty or African genesis or Neanderthal man.

See also: Use salt? We thought one needed to do more to be a denialist

and

Eating up the “science” of Whole Foods

When one considers all the “science”-based scams aimed directly at the public these days, it’s a wonder there aren’t more anti-science types and denialists than there actually are.

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One Reply to “Genetic ancestry is basically a horoscope?

  1. 1
    Eric Anderson says:

    Coincidentally, I was thinking about this issue last night when I saw a 23andMe commercial on television.

    For full disclosure, let me say that I am a big fan of 23andMe and the work they are doing. I became a customer years ago, before the whole FDA flap and long enough ago to be grandfathered under their exception to the FDA agreement.

    The genetic sequencing data I received has been helpful in identifying a disease-causing genetic trait that came from my mother’s family line. Nothing of huge significance, but of enough import that I value being aware of it and having some information about lifestyle changes that can help mitigate risk.

    The ancestry data and analysis is fun, but I have always viewed it with a huge grain of salt. There are lots of issues with trying to identify ancestry, and those caveats often don’t make it into the commercials or the claims of ties to famous people or royal lines.

    Using the most conservative estimate of generation time — 32 years — in the year 1152, you had as many as 134,217,728 potential ancestors. And since genes are scrambled with every generation, it’s very likely you share little to no genetic relation to most of them.

    This raises a couple of interesting points:

    1. The 134,217,728 number is inaccurate and misleading. Yes, in theory this is the number of “potential” ancestors you have going back 27 generations. In practice, however, you will likely have far fewer.

    Anyone who has done significant geneaology work will be familiar with the phenomenon of “pedigree collapse.” For those who aren’t familiar, basically it happens anytime you have ancestors on your family tree who are, themselves, related. This happens more often than the lay person might think, and the farther back you go, the greater the likelihood that you will see pedigree collapse in your family tree.

    Indeed, (regardless of whether one takes the Biblical view seriously or the evolutionary view seriously), the pedigree ultimately collapses to a small set of ancestors, or theoretically even a single progenitor. That is a very interesting topic in itself — perhaps for another time.

    2. Notwithstanding the importance of the prior point, the author is correct that you may share very little DNA — potentially none — with some of your ancestors.

    This raises the very interesting question of how important DNA is in the concept of a family tree in the first place.

    Please note, I’m not suggesting DNA doesn’t play a central biological role. But the old family concepts of “bloodline” and “I’m a direct descendant of X” need to be taken with a huge caveat, given our current understanding. Ancient genealogy records, including those in the Bible, make much of the fact that “Person Y is a descendant of Person X”. And yet, going back even a modest 10 generations means that Person Y is also a descendant of potentially 1,023 other individuals as well. Indeed, it is very possible that the famous Person X they are bragging about contributed nothing to Person Y’s current genetic makeup.

    With the exception of Y chromosome DNA flowing exclusively from father to son and mitochondrial DNA flowing exclusively from mother to child (although even the exclusive nature of the latter is under some question now), the rest of one’s DNA becomes very difficult to trace and assign to any particular ancestor.

    It is thus indeed possible that I am related to some famous person centuries ago — documented, verified, confirmed by all reasonable historical evidence — and yet at the same time share absolutely no DNA with that person.

    Contrary to what many lay people tend to assume, a family tree and a genetic tree are not one and the same.

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