[cba9]

> This is what you would expect for any complex system under multi genetic control

Not necessarily. This is something you might expect if intelligence is a net fitness advantage and it is in a mutation load situation where rare variants need to be purged to keep things constant. But if intelligence is only worthwhile up to a certain point and it is controlled by frequency-dependent selection, or there is heterozygote advantage, or if intelligence is not necessarily reproductively fit at all, or other situations, then there could certainly be rare variants of large positive effect. (I would not have bet on their existence for many reasons, but not because it's impossible.)

To give an example, your claim would predict that there is no such thing as a single mutation which increases muscle mass a lot because it's a complex system affected by a lot of genes; yet nevertheless, there is a single mutation affecting myostatin which makes humans and pigs and dogs much more muscular, and it's even been edited into pigs with CRISPR this year and last year into sheep and cows. Presumably the reason that not all animals are ultra-strong thanks to the mutation is that it causes birthing difficulties and increases metabolic demands considerably, and so despite the obvious advantages of being ultra-strong, it's not actually fit.

[danieltillett]

Of course it is possible that there are alleles with a large positive effects on intelligence, but is very unlikely because of the complexity of human intelligence.

To use your myostatin mutation example, the increase in muscle mass does not lead to significantly faster animals as the supporting structures are not there to utilise this increased muscle. Human intelligence is an emergent trait like speed determined by the co-ordination of many sub-systems.

These sort of emergent traits are almost never positively controlled by single large positive alleles. The one major exception are systems that are under different selection in males and females (e.g. height, plume color in birds, etc). In these examples two different systems have emerged controlled by the sex of the individual, but you don’t tend to find single genes that contribute massively in a positive way towards variation within each sex.

As an aside I remember reading a paper from long ago that suggested that high intelligence was the result of the relatively absence of mutant alleles at the various intelligence loci. When you look at the effect of null mutations at these loci the effect on intelligence is very low (less than a point for most). We all carry a large number of mutant alleles so the suggestion is that those with a high intelligence just happen by chance to have a lower frequency of negative mutant alleles. The interesting thing about this hypothesis is it would suggest that high intelligence is the default and low to normal intelligence is the result of mutational load. This is defiantly something that we can explore in the future as we get whole genome data from large numbers of people.

[cba9]

> To use your myostatin mutation example, the increase in muscle mass does not lead to significantly faster animals as the supporting structures are not there to utilise this increased muscle.

It leads to significantly stronger animals. That's not useless, for either prey or predators.

> Human intelligence is an emergent trait like speed determined by the co-ordination of many sub-systems.

We don't know what human intelligence is on a neurological basis. Imaging studies are linking it to a number of things, but it's still preliminary and so it's premature to baldly assert that there's no possible single mutation which might help.

> As an aside I remember reading a paper from long ago that suggested that high intelligence was the result of the relatively absence of mutant alleles at the various intelligence loci.

Mutation load hasn't been found to be strongly correlated with intelligence in studies which have looked at it directly, and the GCTAs already upper bound any such effect: "The total burden of rare, non-synonymous exome genetic variants is not associated with childhood or late-life cognitive ability", Marioni et al 2014 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3953855/ "A genome-wide analysis of putative functional and exonic variation associated with extremely high intelligence" http://www.nature.com/mp/journal/vaop/ncurrent/full/mp201510... , Spain et al 2015

[Mz]

Of course it is possible that there are alleles with a large positive effects on intelligence, but is very unlikely because of the complexity of human intelligence.

Some years ago, I read something that suggested otherwise. IIRC, it suggested that we had some idea of what alleles positively influenced intelligence based on the fact that Ashkenazi Jews were something like 2% of the population but had won something like 10% of Nobel Prizes and about 60% of the population had one or more alleles for serious genetic disorders known to impact neurology.

I remember this in part because, although I am not Jewish, I have one of the genetic disorders they tend to carry.

[danieltillett]

It is in principle possible for a sub-popualtion like Ashkenazi Jews to have a higher mean intelligence and hence many more individual with extremely high intelligence without there being alleles with large affects on intelligence. It would just mean that they have a higher frequency of the small positive effect alleles. More importantly, people have looked for such alleles in the Ashkenazi population and have failed to find them.

As far as I am aware there in no genetic disorder that only Ashkenazi’s suffer from that is not also present in the rest of the human population, the only difference is frequency.

[cba9]

> More importantly, people have looked for such alleles in the Ashkenazi population and have failed to find them.

They have? I'm not aware of any GWASes targeting intelligence using Ashkenazi samples.

[Mz]

It is in principle possible for a sub-popualtion like Ashkenazi Jews to have a higher mean intelligence ...

As far as I am aware there in no genetic disorder that only Ashkenazi’s suffer from that is not also present in the rest of the human population, the only difference is frequency.

Do you honestly see no possible connection between these two things?

That's a serious question, not intended as argumentative per se.

[danieltillett]

I think what you are asking is do I think there is any connection between the sorts of mutations found more frequently in the Ashkenazi population and intelligence and the answer is there may well be some, but the contribution for any one mutation is very small.

I don’t think anyone has shown that being a carrier for any of the common mutations found in the Ashkenazi population increases intelligence in a measurable way, but if anyone knows of such a study I would love to read it.

[Mz]

One of my best search skills appears to be ego surfing, so here is a link from a previous discussion I participated in about ten years ago:

www.kuro5hin.org/story/2005/7/29/20293/9910

Edit: It contains the stats I was trying to reference above but was misremembering. They are 3% of the population and 27% of U.S. Nobel Prize winners.

[sawwit]

Which lectures did you attend to learn about things like "net fitness advantage", "mutation load situations", "frequency-dependent selection", "heterozygote advantage". I always wanted to learn about these things. If you can refer me to an online lecture or a book I would be very thankful.