Comments on: Things I Don’t Understand About Genetics (A Non-Exhaustive List)

By: Douglas Knight

Douglas Knight — Fri, 18 Jul 2014 05:02:08 +0000

How big a “relatively large” effect do you expect? Why?

Steve Hsu estimates half a point per negative mutation.

By: Anders

Anders — Fri, 18 Jul 2014 04:46:00 +0000

A high number of very rare alleles each with relatively large negative effect. This is exactly what you would expect under purifying selection, most differences should be caused by uncommon ways of messing up. The formula for binomial variance V[x] = np(1-p), if we set the expected value (np) to a constant (since we know median IQ is 100) then we find that the smaller p is the higher the variance is. The reason these alleles wouldn’t be identified as having strong effect is that are individually too rare to be statistically significant.

It is perfectly possible that there are also many common genes with with low effect, but by your own argument they wouldn’t contribute much to variance.

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By: Bob Stuart

Bob Stuart — Mon, 26 Aug 2013 22:09:13 +0000

My guess is that notable intelligence arises primarily from an efficient pattern in the synapses for processing a specialized type of problem. There are many different kinds of smarts, but a genius tends to focus on one. So, in genetics, one might look for patterns of genes that will work symbiotically.

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By: Douglas Knight

Douglas Knight — Mon, 26 Aug 2013 21:01:58 +0000

Two comments about mutational load. First about correlation, then about a single trait.

We should find the same people being very smart and very tall and very athletic and very healthy.

As I said before, you seem to reify mutational load. If the genome has separate genes for height and IQ, there should be no correlation between them, even if there are a lot of genes for each and both traits are well-described in terms of mutational load. Then a person would have a height load and an IQ load, which are unrelated.

The more genes that contribute to the two traits, the more likely that one gene contributes to both traits. Indeed, that is probably why the two traits are correlated. In that sense, mutational load is about correlation. For example, if IQ is affected by 1% of genes, and height by 1% of genes, independently, so that .01% of genes affect both, the correlation between the two traits would be 0.1.

As for mutational load resolving your confusion about quantitative traits, it appears to me that you have some intermediate idea (maybe involving correlations) that you think must explain quantitative traits and so when you want mutational load to explain quantitative traits via mutational load, you claim that mutational load leads to this intermediate idea and that the intermediate idea solves your confusion. I think both steps are wrong. Moreover, my guess as to why you think mutational load is relevant because the same people talk about both ideas. But that is because understanding quantitative traits is a prerequisite to understanding mutational load, not vice versa.

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By: Randy M

Randy M — Mon, 26 Aug 2013 14:49:16 +0000

Although that makes one wonder how such things were selected upon in the first place!

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By: Scott Alexander

Scott Alexander — Mon, 26 Aug 2013 03:27:14 +0000

Can you explain sperm selection? If all mutations are uncorrelated, sperm that have mutations in their essential reaching-the-egg genes should have exactly the same number of mutations in their person-they-will-be-when-they-grow-up genes as baseline.

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By: Douglas Knight

Douglas Knight — Sun, 25 Aug 2013 18:00:31 +0000

If you are a farmer, providing a uniform environment and breeding at random, narrow root-heritability h is simply the correlation between parental trait and child trait. It’s trickier in humans.

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By: Kaleberg

Kaleberg — Sun, 25 Aug 2013 04:42:49 +0000

There are also all sorts of tradeoffs. Look at the recent study showing how rams trade of longer horns which offer better breeding opportunities and longer life which offers more opportunities for breeding.

Human brains grow rapidly in youth, so why haven’t we evolved to maintain this growth into adulthood? Well, this high speed growth introduces risks of epilepsy and other disorders, many of which vanish with maturity. Having added neural flexibility is not all it is cut out to be. Many of these genes may be self limiting and best expressed in moderation.

More seriously, these genes don’t encode for intelligence or 0.1% intelligence or whatever. They encode for structural, regulatory and developmental things. If your Lego set has 1,000 different shaped blocks, you can usually build more or less anything with 985 of them. Of course, some bastards are just out of luck.

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By: Kaleberg

Kaleberg — Sun, 25 Aug 2013 04:33:11 +0000

That’s right. There is actually very little variation in human intelligence. Almost everyone can pick up some language or another, figure out how to pick things up, learn to navigate after a fashion, recognize emotions and intentions and so on. This is the big reason Wallace, who, with Darwin, developed the theory of evolution by natural selection, never accepted that humans evolved from non-human animals. Wallace had spent years in the field in Indonesia and New Guinea, and recognized that the people there were not all that different than those he knew at home in England. The gap between the smartest human and the dumbest human as compared with the gap between the dumbest human and the smartest animal was just too small to be considered as bridgeable.

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By: Douglas Knight

Douglas Knight — Sun, 25 Aug 2013 04:19:15 +0000

It’s true that sperm selection is tremendous, but the measurement that the typical Icelander has 15 new mutations from the mother and 2A-15 mutations from the father of age A is based on people not sperm. This is how many mutations remain in living children, not sperm or fetuses. So this mechanism does not answer the question.

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