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before, they simply saw it as bad for the individual and good for the transposons.7 Muggers and outlaws do not exist for the benefit of society, but to its detriment, and for the benefit of themselves. Perhaps transposons were, in Richard Dawkins’s words, ‘outlaw genes’. Hickey then noticed that transposons were much commoner among outbreeding sexual creatures than among inbreeding or asexual ones. He ran some mathematical models which showed that parasitic genes would do well even if they had a bad effect on the individual they inhabited. He even found some cases of parasitic genes of yeast that spread quickly in sexual species and slowly in asexual ones. Such genes were on ‘plasmids’ or separate little loops of DNA, and it turns out that in bacteria such plasmids actually provoke the very act of conjugation by which they spread. They are like the rabies virus making dogs bite each other. The line between a rogue gene and an infectious virus is a blurred one.8

Nobody is Descended from Abel

Despite this little rebellion, life is fairly harmonious in the bacterial team. Even in a more complicated organism such as an amoeba, formed by an agglomeration of ancestral bacteria some time in the distant past,9 there is little difference between the interests of the team and the individual members. But in more complicated creatures the opportunities for genes to thrive at the expense of their fellows are greater.

The genes of animals and plants turn out to be full of half-suppressed mutinies against the social harmony. In some female flour beetles there exists a gene called Medea that kills those offspring that do not inherit it:10 it is as if the gene booby-traps all the female’s young and defuses only those that it itself inhabits. Whole selfish chromosomes called B chromosomes exist that do nothing except ensure their transmission to the next generation by invading every egg the insect makes.11 Another insect, a scale insect, has an even more bizarre genetic parasite. When its eggs are fertilized, sometimes more than one sperm penetrates the egg. If this happens, one of the sperm fuses with the egg’s nucleus in the normal way; the spare sperm hang around and begin dividing as the egg divides. When the creature matures, the parasitic sperm cells eat out its gonads and replace them with themselves. So the insect produces sperm or eggs that are barely related to itself, an astonishing piece of genetic cuckoldry.12

The greatest opportunity for selfish genes comes during sex. Most animals and plants are diploid: their genes come in pairs. But diploidy is an uneasy partnership between two sets of genes and when partnerships end, things often get acrimonious. The partnerships end with sex. During meiosis, the central genetic procedure of sex, the paired genes are separated to make haploid sperm and eggs. Suddenly each gene has an opportunity to be selfish at its partner’s expense. If it can monopolize the eggs or sperm, it thrives and its partner does not.13

This opportunity has been explored in recent years by a group of young biologists, prominent among them Steve Frank, of the University of California at Irvine, and Laurence Hurst, Andrew Pomiankowski, David Haig and Alan Grafen at Oxford University. Their logic goes like this. When a woman conceives, her embryo gets only half of her genes. They are the lucky ones; the unlucky other half languish in obscurity in the hope of another toss of the coin when she next breeds. For, to recapitulate, you have 23 pairs of chromosomes, 23 from your father and 23 from your mother. When you make an egg or a sperm you will pick one from each pair to give a total of 23 chromosomes. You could give it all the ones you inherited from your mother or all the ones from your father, or more likely a mixture of the two. Now a selfish gene that loaded the dice so that it stood a better than fifty-fifty chance of getting into the embryo might do rather well. Suppose it simply killed off its opposite number, the one that came from the other grandparent of the embryo.

Such a gene exists. On chromosome