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05/25/2001 Archived Entry: "Neocortex"

For those of you who want to while away half an hour, visit Vectorpark - lots of very fun and imaginative puzzles and animation there.

I had an extremely interesting Evolution and Behaviour supervision a couple of days ago, discussing the idea of genetic imprinting. First a bit of background. In each of our cells, our DNA is made up of 23 pairs of chromosomes. You have 23 chromosomes from your mother and 23 from your father, and they are called homologous chromosomes because they essentially have the same function, but each of the chromosomes in a pair have slight differences.

On your chromosomes are specific sections called genes, which code for proteins and enzymes and so on - the cellular machinery. Whether a gene on one chromosome is expressed depends largely on the status of its 'twin' on the other chromosome - it might get overrided or it might override the other gene (this is all a vast simplification, but it'll do).

However. One pair among the 23 pairs are the sex chromosomes. If you're male, you have an X chromosome from your mother and a Y chromosome from your father, and if you're female you have two X chromosomes, one from your mother and one from your father. One of the ways in which genetic imprinting works is that (for instance) on the Y chromosome there might be a gene that always overrides the equivalent gene on the female chromosome, no matter what the female gene is. And vice versa, it can happen the other way around.

So a group of biologists in Cambridge wanted to figure out what imprinted genes there were. What they did was to get a rat embryo consisting of only a few dozen cells, and then genetically alter a few of those cells. The exact alteration they did depended on the sex of the embryo. For a female embryo, they extracted one of the X chromosomes and implanted an identical copy of the one that was remaining. For a male embryo, they extracted the X chromosome and implanted an identical copy of the remaining Y chromosome.

They only did this for a few of the cells because if they did it for all of them, the rat wouldn't grow. Anyway, the rats grew up, and the differences from the normal were astonishing. The female rat, which you'll remember has cells that have two identical X chromosomes, was normal sized, but it had a vastly larger sized neocortex volume in relation to its limbic system (I'll explain those terms in a moment). On the other hand, the male rat had a much larger limbic system than normal, with a reduced neocortex. The male rat was also physically huge.

Now, the neocortex could be said to be the 'advanced' part of the brain. It carries out what are called 'executive functions' such as decision making, inhibition of immediate desires and so on. The limbic system is the 'primitive' part of the brain - it controls the body via hormones. In 'primitive' animals such as reptiles, you'll find a limbic system but not a neocortex.

So what does all of this mean? Well, what is happening is that the female rats are getting a double dose of the genetically imprinted genes that reside on the X (the 'female') chromosome - which means that those genetically imprinted genes code for enlarged neocortex volumes. Conversely, the genetically imprinted genes on the Y ('male') chromosome code for reduced neocortex volumes. The larger your neocortex, the less dependence your behaviour has on hormones.

I should point out that we do not see these differences in normal rats or humans because we always have two non-identical chromosomes which effectively balances all of this out (I know that doesn't explain everything by far, but there you go).

What is going on (some say) is a evolutionary war between the X and Y chromosomes. The X chromosomes - the female sex - wants enlarged neocortex volumes for increased social functioning and, say, Machiavellan intelligence. The Y chromosomes - the male sex - wants smaller neocortex volumes and larger body sizes so that they can beat people up. What we have right now is a compromise.

Well. That bit of science didn't come out quite as bad as I thought it might.