Monday, September 15, 2008

The battle of the sexes

Ever since Richard Dawkins' The Selfish Gene we have known that a goodly part of what we are is the result of little snippets of DNA trying to have their own way. We have 20,000 or so genes and everyone of them is out for itself. My genes are in competition with your genes. You may think I'm a nice guy, but my genes are self-serving and vainglorious. I might invite you onto the porch for a drink, but my genes would kick your genes over the railing into the shrubbery.

The idea of the selfish gene was not original with Dawkins, but he gave it popular currency. He did not mean genes are willfully "selfish" in the human meaning of the word, only that the genes that get passed on to the next generation are the ones that serve their own replication. Our physical selves, and a good part of our behaviors, are a result of genes jostling their way to the front of the queue. Start with a blob of protoplasm, add genetically-determined reproduction, variation and natural selection, and you end up with Chartres Cathedral and the Large Hadron Collider. Who would have thunk it?

And now the competition gets nastier. In the August 28 issue of Nature, sociologist Christopher Badcock and evolutionary biologist Bernard Crespi argue that the genes we inherit from our parents do battle with each other. Some genes are expressed when inherited from one parent but not the other. This is achieved by a process called imprinting, in which genes in the sperm and egg are marked for expression or silencing in a later embryo or child. For example, a fetus inherits a growth factor gene called IGF2 from both mother and father. Ordinarily, only the father's copy of the gene is expressed. It is to a father's genetic advantage to have big babies -- they are healthier and live longer at no personal cost to the father. The mother pays the cost of giving birth and suckling a larger child, and therefore she silences her copies of the growth-factor gene.

But the competition is real, and sometimes the mother's gene is expressed, and sometimes both parents' copies are silenced -- to ill effect for the child in both cases. According to Badcock and Crespi, many mental disorders from autism to schizophrenia might be the result of a tug-of-war between a mother's and a father's genes.

What's the moral of this story? What we are and who we are is to a large extent the result of a dynamic of which, until recently, we didn't have a clue. Oh, we had names for it. The will of God. The Devil's work. Luck. Fate. And all along, down there among the genes, the elbowing, the pushing, the playground squabbles -- the all-pervasive invisible molecular commerce of life on Earth.