Wednesday, November 19, 2008

Many genes and one g?

The October issue of Scientific American has an article on the search for genes that influence intelligence. Twin studies suggest that such genes exist, yet efforts to date have been disappointing for Robert Plomin, a behavioral geneticist at the Institute of Psychiatry in London.

Failing to find genes for intelligence has, in itself, been very instructive for Plomin. Twin studies continue to persuade him that the genes exist. “There is ultimately DNA variation responsible for it,” he says. But each of the variations detected so far only makes a tiny contribution to differences in intelligence. “I think nobody thought that the biggest effects would account for less than 1 percent,” Plomin points out.

That means that there must be hundreds—perhaps thousands—of genes that together produce the full range of gene-based variation in intelligence.

Variation in intelligence thus seems to be an accumulation of small effects from very many genes. For a long time, I had trouble reconciling this view with the concept of g—this single mental property, still unknown, that accounts for most variation in human intelligence. I now realize that this contradiction is more apparent than real. Whatever this single property might be, it doesn’t necessarily correspond to a single gene or even a single gene complex. It could be affected by an indefinite number of genes.

Reference

Zimmer, C. (2008). The search for intelligence. Scientific American, October, pp. 68-75.

10 comments:

Tod said...

"But each of the variations detected so far only makes a tiny contribution to differences in intelligence. “I think nobody thought that the biggest effects would account for less than 1 percent,” Plomin points out.

This "tiny contribution" seems surprisingly little, if there are hundreds of genes relevant to G it is hard to see how most gene variants influencing intelligence would confer enough of an advantage to make a difference.
Moreover if any variant's effect was but one among an indefinite number of genes
how could it be selected for.
From this and the Seed article genetics seems to be getting back in line with Tooby; nothing has really changed the genetic basis of intelligence since the Stone Age.

Average Joe said...

nothing has really changed the genetic basis of intelligence since the Stone Age

Then how do you explain the intelligence differences between Negroids, Caucasoids and Mongoloids? The fact that these intelligence differences exists is proof that intelligence has been subject to recent natural selection.

Peter Frost said...

Tod,

I don't follow your reasoning. Selection acts on phenotypes, not on genotypes. If we look at the end result, there is no difference between a hundred genes that each cause a 1% difference and one gene that causes a 100% difference.

It might seem architecturally cleaner if only one gene controlled variation in 'g'. But evolution is not about clean architecture. It's about what works. The genome may look like one big jerry-rigged mess, but it works all the same.

Anonymous said...

Sorry, what's g again ?
too lazy to google it.
I hope g is not just IQ renamed to please political correcteness, because it failed the purpose, g is already taken for something else, if you see my POINT.

RG

Anonymous said...

Moreover, if there is a large number of genes that contribute to G, its pretty much normal distribution becomes highly explicable.

Tod said...

Thanks for taking the time to point out the flawed reasoning in my first comment.

Why would the genes increasing an advantagious phenotypical trait as g all have such a tiny effect, some of the hundreds of genes ought to have been able to contibute say 5% as everyone in the field was expecting.

As I understand it the ASPM variant is not very different in its effect on the phenotype (inasmuch as it aids mental activity) compared to the hundreds of genes related to g. Natural selection has picked up on a single gene that makes a big difference in this case (going by the strength of selection it has been subjected to).

So why isn't there any gene with a big effect on g instead of a myriad of relatively tiny effects if not because of what you have previously explained is:-

"Tooby's argument ... that complex genetic traits, such as those underlying behavior, cannot evolve rapidly. This is because any significant change would require simultaneous adjustments to a large number of interacting genes. By the same reasoning, a complex genetic trait cannot easily vary within a single species, since gene flow would break it up and create maladaptive variants".

Peter Frost said...

Tod,

Tooby ignores the fact that complex behavioral traits can be changed by changes to one gene, typically a regulatory gene that acts as a 'switch'. In short, it isn't necessary to create a whole new complex of interacting genes to create new behaviors or new intellectual traits. You just have to alter what already exists.

In this case, we're looking at a large number of genes that influence the same mental property ('g'). There's no complex interaction involved, just a lot of finetuning.

I suspect there are a few genes with major effects on g. The probability, however, is that most g-related genes have small effects.

Anon,

'g' is not IQ. It's a hypothetical mental property that seems to account for most variation in IQ.

Anonymous said...


'g' is not IQ. It's a hypothetical mental property that seems to account for most variation in IQ.


Well, of course not, but it is highly correlated with IQ ...

Genetic variance leads to variation in g which leads to variation in IQ ...

Anonymous said...

This is off topic, but Peter, could you provide a link to your old study "Skin Color Preference, Sexual Dimorphism and Sexual Selection:a case of Gene-Culture Co-evolution?" from 1986? I lack a subscription to the typical journal archives where it's listed, so I was wondering if you could atleast provide it via email or something. I was primarily interested in knowing the various cultures you list with skin color preferences.

Peter Frost said...

Anon,

My 1986 article with Pierre van den Berghe is not online yet, even if you have university periodical access. If you wish, I could send you a Word file that contains all of the cross-cultural references. Of course, I'd need an e-mail address.