monogenic vs polygenic

Imagine that there was a mutation [Syndrome X] that, by itself, made you ten centimeters shorter than you otherwise would be. It works by messing up iodine metabolism.

What environment influences could modify its course?  Maybe growth hormone could make a difference, but that’s not super-safe.

How stable with respect to environmental variation would syndrome X be?  How much money you have, where you go to school, status –  the usual stuff sociologists worry about – wouldn’t make a difference.  But perhaps a diet with with lots and lots of iodine – cod for breakfast, swordfish for lunch – would.   So the mutation’s consequence might be entirely different in fishermen and cowboys.  There’s a sense in which its effects are unstable.

Now imagine genotype Y – it makes you ten centimeters shorter, but it’s due to many small-effect  mutations.  Hundreds of them.  No – thousands! , counting all the small-effect alleles. Maybe they’re slightly deleterious, or maybe they’re fitness-neutral (each one makes you a little shorter but does something else helpful)

These short-people alleles don’t all operate through the same mechanisms –  so there are tens of different metabolic paths involved.  Make that hundreds of metabolic paths, hundreds of mechanisms.

What environmental effect would strongly influence syndrome Y? Make you tall? Well, in order to do much, that environmental factor would have to influence many different mechanisms probably most of them.  That’s not easy: I’d rather have to put Humpty Dumpty back together..   I suppose we could give the kid plenty of chicken* soup – it couldn’t hurt.  In practice, I can’t imagine any realistic environmental factor having much influence, short of nanites programmed by tnuctipun.

*As long as it’s nobody we know.



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45 Responses to monogenic vs polygenic

  1. dearieme says:

    The Guardianistas would mock that because “the gene for Y” has not been discovered, Y doesn’t really exist.

  2. Syndrome Y is highly influenced by the systematic multivariate structural invisible forces ( oppressing people transgenerationally. Social psychology will uncover these forces with a new and very powerful social particle accelerator to detect the micro and the nanoaggressions that affect all the different mechanisms and developmental pathways at the same time.

  3. jb says:

    Hmm. So it isn’t true that corn grown in bad soil without enough water will end up shorter than the same corn grown in good conditions?

    • MM says:

      I think the point is that a negative polygenic “syndrome” will cause the same 10 cm deficit compared to the average genotype in any environment, not that environments don’t affect trait levels.

      • jb says:

        But how do you tell if the deficit is due to genetics (if there are that many genes involved I wouldn’t call it a “syndrome”) or environment? We all know we are talking about IQ here, and the claim that is generally made is that any observed group deficit is due to shared environmental disadvantage. While I’m skeptical of that claim, I don’t see how Greg’s point is relevant.

        • gcochran9 says:

          We know that shared family environment has little effect on adult IQ, and most of the environmental differences that are supposed to explain lower group IQ are the same kind of stuff, income or SES etc – either that or just don’t exist. The fact that known socioeconomic differences can’t explain the IQ difference has been known for a long time – sometime people invoke the “X factor”.

          Turkheimer has invented a new argument: if group differences are also highly polygenic, caused by differences in the frequency of many small-effect alleles, as within-group differences mostly are, why the polygenic nature somehow means that if you put blacks and whites into a new environment, the difference might disappear or even reverse, because of the especial instability of polygenic traits.

          The thing is, polygenic traits are not especially unstable. Turkheimer MADE IT UP. Probably many genes explain the difference between Shetland ponies and Thoroughbreds, but that hardly means that in some new environment, Shetlands will win the Kentucky derby. Other than one in which we break the legs of all Thoroughbreds.

          In the same vein. Kevin Mitchell ( and others) have said that that it would be difficult for human populations to differ much in IQ, since they have only been separated for 100,000 or 200,000 years and it’s really hard to evolve differences in polygenic traits. But that’s nonsense: selection on highly polygenic traits isn’t slow. We know a lot about this, from agricultural genetics. Again , a new, invalid ‘principle’ that was simply made up out of whole cloth.

          • jb says:

            Ah, I hadn’t realized Turkheimer was involved in this. Carry on.

            Turkheimer’s argument sounds interesting though. Is he actually conceding that in this environment at least — the one we are all actually living in — the IQ difference between blacks and whites is in fact genetic, and not due to some mystic environmental X factor that floats around in the air and individually targets members of disadvantaged groups regardless of their family circumstances? That would be quite a concession, and speculation that maybe in some other environment blacks might be smarter doesn’t seem like much compensation.

            • gcochran9 says:

              I don’t think Turkheimer is conceding that. But with GWas happening. he may be afraid that he’ll soon have to. It’s a backup strategy: how can you say a group is genetically dumber if that were liable to sudden change, say when the Martians invade?

          • Ripper Woo says:

            Assuming that Turkheimer is right about its being environmental.* Would one’s environmentally caused score go up by attending college? By working as a coder at Google? By working as a coder at a Google where white employees are hectored by diversity staff? If not, what is the point of affirmative action or holistic admissions? It seems just like some random tax on companies and white people, distributed very inefficiently and randomly to some black people.

            That is not actually his claim. He says that there is not yet any good evidence that it’s genetic. So you can see that he’s hedging quite a bit.

        • gcochran9 says:

          “But how do you tell if the deficit is due to genetics (if there are that many genes involved”

          we’re finding them.

  4. pyrrhus says:

    The solution to such a shortness problem is obviously to use different tape measures so that everyone measures the same, and to make any discussions of comparative height “hate speech.” After a while, those who haven’t been sent to the gulags will instinctively not notice such things….

    • Jason says:

      Perhaps operations to remove small sections of bone from the femur and tibia of the overly privileged, will ensure true equality.

      • Patrick Boyle says:

        I think adding bone to your legs must be harder than it seems. It’s sort of a Silver Blaze – ‘dog that didn’t bark’ kind of thing. If it were easy then surely Tom Cruise would be taller. It wouldn’t matter how expensive it was. Just two or three inches more height would be worth millions to his career.

    • savantissimo says:

      “Shortness problem”? I presume you mean “diversity of tallness”. Everyone is tall in their own way, which is our strength. The only problem is narratives of height supremacy.

      • Daud Deden says:

        A small human, elephant or Okapi have advantages in the rainforest. Outsiders would call that size condition a syndrome, and try to explain it.

      • moscanarius says:

        Add to that, savantissimo, that identifying “tall” people is much more difficult than the most people think.

        The public thinks they can assess someone’s height just by looking at them, but the EXPERTS know this is very unreliable, and only an official measurement can tell you how tall a person is. There are several pieces of equipment to do this, the most common being the metric tape – which is notoriously unreliable even if used carefully, let alone when misused. There are tenths of millimeters of difference in what a “centimeter” measures among different tapes, making cross-tape comparisons difficult. Metric tapes are also often mishandled: few people bother to check whether the tape is perfectly perpendicular to the ground before checking the numbers, sometimes adding a few centimeters to the estimated height. It’s also known that people tend to exaggerate their self-reported height due to the pervasive Height Supremacism in the West, making most reports about “height” unreliable. To further complicate the matter, metric tapes use two scales (centimeters and inches), which is known to cause some of confusion among the general public.

        There is also an environmental component to the measurement itself. Some people are taught from an early age to stand still with their shoulders back, which gives them an edge over their more lump-backed peers. Some wear platform shoes that may give them more than 2 inches! Experts in the field try to counter this by measuring height barefoot, but some there are credible reports of subjects (kids as young as five!) cheating by standing in the tip of their toes.

        Given all these troubles, it’s ludicrous to suggest that we know anything for sure about “height”, let alone about the supposed height differences between different groups.

        In fact, there is no such a thing as one “height”; experts heard by the NYT assert that there are Multiple Heights. One person can have a short trunk and long legs, and another a long trunk and short legs. But Heightists would say they are all the same!

  5. Lior says:

    It’s possible to break his legs and give him leg lengthening surgery,problem solved.

  6. mtkennedy21 says:

    I am interested to see the larger number of taller girls than I recall when I was young. I see girls 6 feet tall all the time. I know I’ve grown shorter the past 20 years but I don’t think it is just a matter of relative size.

  7. tc says:

    Not necessarily. Here’s an example for height I heard on Razib’s podcast with James Lee: suppose there are lots of genes, each influencing the size of a particular organ or part of an organ: some genes make your brain bigger or smaller, others affect the lungs, stomach, kidneys, legs, etc. When you add up the effect of all of these organ-specific genes, you get a polygenic basis for height. Now suppose that all of these genes respond positively to the same environmental cause, e.g. plentiful food. Then there would be a single environmental cause that affected lots of genes in the same way, causing a large phenotypic effect. Obviously this might not work for IQ, since it’s probably not as simply additive as height.

    • moscanarius says:

      It likely would; I don’t doubt starvation or a deficit in, say, iodine intake might cause the same problems in IQ that starvation / lack of calcium cause on height.

    • Greying Wanderer says:

      “Now suppose that all of these genes respond positively to the same environmental cause, e.g. plentiful food.”

      yes – or a specific nutrient like vitamin D which is used in hundreds of different body processes.

      the problem with that from the opposing point of view is a) they’d initially have to admit the IQ gap was real even if the reason meant there was an easy fix available (supplementation of some kind) and b) they want the publicly accepted cause of the IQ gap to be politically useful to them more than they want to actually fix the IQ gap.

      • another fred says:

        “b) they want the publicly accepted cause of the IQ gap to be politically useful to them more than they want to actually fix the IQ gap.”

        Mostly that.

  8. Citizen A says:

    I often wonder if the Flynn effect is simply enriched food for a couple of generations allowing better potential growth.

    After all, some of the poorest countries still refuse to require iodine:

    Click to access Household_Wealt.pdf

    Of course, I am conflating intelligence and wealth effects;-)

    But hey, less competition on top, if you keep the bottom from rising…

  9. sterling sorbet says:

    In summary: There’s no fixing stupid.

  10. TWS says:

    How much of the difference in IQ is due to the different archaic populations we ran into over the years? Could some populations have picked up advantageous but IQ lowering genes from other hominids?

    • gcochran9 says:

      Nothing obvious, but not quite inconceivable.

    • Probably close to zero. These admixture events are old. After all, IQ differences mostly reflect different strategies.

      • Greying Wanderer says:

        “IQ differences mostly reflect different strategies.”

        as well as brain development iodine is important for metabolism so i wonder if people in colder regions developed some form of more efficient iodine processing for metabolic reasons which had at least a minor side-effect on IQ?

        (if so my guess would be it happened twice and both EDAR and SLC-whatever do the same thing (i.e. not additive)(and maybe got from an archaic as if it developed for metabolism (warmth) reasons they might have needed it too?)

  11. James Shearer says:

    “… In practice, I can’t imagine any realistic environmental factor having much influence, short of nanites programmed by tnuctipun.”

    Being well fed versus poorly fed seems to be such an environmental factor so whatever argument you are trying to make here appears to be incomplete.

    • gcochran9 says:

      You don’t understand. The idea is that population A might outscore pop B when both experience environment Aleph, while B would outscore A ( or score even) when both experience environment Beth. Turkheimer was suggesting that this was somehow more likely with a highly polygenic trait. He’s wrong.

      If you think that African-Americans are malnourished today… see an optometrist.

  12. One environmental factor, consistent nutrition during development, including vitamins and trace minerals, could affect many alleles. Many, meaning oh, 10% of the whole range of deleterious alleles. Taken together, they could affect height by a centimeter or so. About what we see with the Flynn Effect. It would be easy to say “Hey, only nine centimeters to go! They must be there!” But consistent nutrition is a huge difference. Eating is one of the main things that has shaped us over thousands of years. Even in prosperous places, most of the population experienced serious hunger during part of the year in 1700. 41% of America was below the poverty level in 1949. (Still well-fed by historical standards, though.) We have done better in height since 1700, but not two feet taller, three feet taller. There is a law of diminishing returns. Even if we identify a lot of other environmental (my bet is that prenatal will be the largest, though still small) factors, or genes that express differently in different environments, we’re not going to find anything that has the same amount of influence as food. Because it’s food, dammit. It’s not Jupiter compared to the other planets, it’s the sun compared to all the planets.

    Hell, even if nutrition is on the scale of Jupiter compared to the other planets, it’s going to mean there isn’t much more gain to be found.

  13. crew says:

    In practice, I can’t imagine any realistic environmental factor having much influence, short of nanites programmed by tnuctipun.

    Well, maybe Phssthpok could figure it out.

  14. David Chamberlin says:

    The analogies between differences in height and intelligence are useful because a lot of people are rational about height and barking mad about intelligence. But here is an interesting tidbit about height genes. Africans have their own separate height genes which are at this early stage are unknown. So they are six inches taller than the present day incomplete genetic information would predict.

    Makes sense really, they have far more genetic diversity and the genetic testing complied so far is Eurocentric. When the day comes we do know the optimum gene combinations that result in higher intelligence that African gene pool while lagging behind now may be of great help to make the optimum hybrid.

  15. Martin L. says:

    Greg, don’t you consider shortness due to non-nutritional reasons the result of genetic load (like so many other things)?

    • gcochran9 says:

      There can be adaptive reasons. Being short is adaptive in Pygmies, and there’s a fairly large-effect short allele at moderate frequency in Andean Indians.

      • Martin L. says:

        I mean among “typical” peoples who are adapted to a normal society with adequate nutrition and environmental conditions.

      • Patrick Boyle says:

        Actually Dr.Cochran is wrong about the uselessness of the study of psychology. I got my undergraduate degree in Psychology and I learned something useful.

        I learned that girls want to be pretty and boys want to be tall.

        That’s pretty much it.

  16. ilkarnal says:

    Isn’t overall muscle mass probably very polygenic, like height? Something can be really complex and also possible to change radically through a simple intervention that targets one of the more important inputs.

  17. crew says:

    Or perhaps they are just lazy, although we are not allowed to say that about extant groups (and it doesn’t make much sense when applied to Homo erectus.) Sounds like victim blaming.

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