Trisomy 21

Hot on the heels of the previous post on human-mouse syntenies comes another fascinating entry by PZ Myers, this time on trisomy 21, or as it's commonly known, Down Syndrome. In particular, he discusses what's been learnt from mouse models of this particular genetic aberration, drawing on the synteny between human chromosome 21 and various bits of the mouse genome: the (not really) surprising conclusion?

Reeves and his colleagues posit that genes contained in the DSCR interacting with other genes could be to blame. “The simplistic idea that just one of the hundreds of genes on chromosome 21 affect development no longer holds up,” Reeves remarks. “Now researchers can take a deep breath, accept that the syndrome is complex, and move forward.”

And this despite having to scrutinize a mere 225 genes, and having far tighter control of environments than would ever be ethically feasible with human subjects.

Considering that it's estimated that up to half of all ~25,000 human genes are expressed in the brain, anyone who thinks it's going to be easy to map human genetic variation to complex personality traits like aggressiveness or sexual orientation, let alone something as amorphous as "intelligence", will likely have a surprisingly long wait ahead. The cosy assumption that genes which influence complex traits will tend to interact in a pleasantly additive fashion is one that owes more to our need for tractable models than it does to any empirical evidence.

Comments

[João da Costa]

Abiola:

"The cosy assumption that genes which influence complex traits will tend to interact in a pleasantly additive fashion is one that owes more to our need for tractable models than it does to any empirical evidence."

I agree.
I worked many years as a computer system analyst and a thought that oft occured to me was that it´s more natural to compare the performance of two systems (for example two chess programs) looking at them as whole working systems (which one of the programs proves to be better at playing the game against us humans or against each other, for example) than to look at the details of their programming (this routine of programm "A" works more efficiently than the corresponding routine of program "B", for example).
The reason is that, in the end of the day, whatever difference in details "Ai" and "Bi" you can show, it will be always necessary to prove, beyond a reasonable doubt, that such difference will not be cancelled out somehow by other differences in other parts of the systems you want to compare.
One should not forget that we are working with complex systems here, both in the case of computer systems and in the case of genomes.
Looking at a distance it seems to me (attention: I can be wrong) that the modern paradigm of genetics oft sees the action of each gene somehow separately. But because the genes surely work together one cannot escape to take into account their mutual interference.
An example:
Not so long ago a genetic experiment was carried out which produced the so called super muscled mouse. Apparently better than "normal" mice, but... wait a minute: they were much slower and less timid than "normal" mice.
A thought I had to myself: why those mice had not appeared spontaneously in Nature? Maybe less timid mice were more easily caught by their natural predators and such muscled "advantage" didn´t worked out as such!
I really think that if someone wants to believe that White people are smarter than Black people, for example, the best argument they could use should come from the study of History (macroscopic view) not Genetics (microscopic view). Something like: "the Whites contributed more to Civilisation in a thousand years than Blacks, therefore they should be considered higher life forms" (not that I think in this way!) or similar arguments.

P.S.:
You said circa 25000 genes? I thought the number of genes in the human genome were in excess of 34000!

[Abiola Lapite]

"Something like: "the Whites contributed more to Civilisation in a thousand years than Blacks, therefore they should be considered higher life forms" (not that I think in this way!) or similar arguments."

The problem with that sort of argument is that it's trivial to point out that Egyptians and Assyrians could have said the same about all Europeans about 1500 BC, Greeks and Italians could have made the same sorts of arguments about Germanic and Celtic barbarians around 50 A.D, and the Arabs could also have said much the same of Norsemen and Russians around 1100 A.D. The brighter of the white supremacists would know better than to take up such a losing argument.

"You said circa 25000 genes? I thought the number of genes in the human genome were in excess of 34000!"

They've recently slashed the count yet again, and for all one knows it might still have slightly more to fall. I doubt it'll go below 22,000 or so, though.