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:
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 sometime in the distant past,'° 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-GENETIC MUTINY AND GENDER
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suppressed mutinies against the social harmony: In some female flour beetles there exists a gene called Medea that kills those off-•
spring that do
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.'
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 twenty-three pairs of chromosomes, twenty-three from your father and twenty-three from your mother. When you make an egg or a sperm, you pick one from each pair to give a total of twenty-three chromosomes: You could give 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 two of a certain kind of fruit fly there is a gene called 'segregation distorter, ' which simply kills all sperm containing the other copy of chromosome two.
The fly therefore produces half as much sperm as normal. But all of the sperm contains the segregation distorter gene, which has thereby ensured a monopoly of the fly 's offspring.'
Call such a gene Cain: Now it so happens that Cain is Abel's virtually identical twin, so he cannot kill his brother without killing himself. This is because the weapon he uses against Abel is merely a destructive enzyme released into the cell—a chemical weapon, as it were. His only hope is to attach to himself a device that protects him—a gas mask (though it in fact consists of a gene that repels the destructive enzyme). The 'mask of Cain ' protects him from the gas he uses against Abel. Cain becomes an ancestor, and Abel does not. Thus a gene for chromosomal fratricide will spread as surely as a murderer will inherit the Earth. Segregation distorters and other fratricidal genes go under the general name of
' meiotic drive ' because they drive the process of meiosis, the division of the partnerships, into a biased outcome.'
Meiotic-drive genes are known in flies and mice and a few other creatures, but they are rare. Why? For the same reason that murder is rare. The interest of the other genes has been reasserted through laws. Genes, like people, have other things to do than kill each other. Those genes that shared Abel 's chromosome and died with him would have survived had they invented some technique to foil Cain. Or, to put it another way, genes that foil meiotic drivers will spread as surely as meiotic drivers will spread. A Red Queen race is the result:
David Haig and Alan Grafen believe that such a response is indeed common and that it consists of a sort of genetic scrambling, the swapping of chunks of chromosomes. If a chunk of chromo-GENETIC MUTINY AND GENDER
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some lying next to Abel suddenly swapped places with the chunk lying next to Cain, then the mask of Cain would be unceremoni-ously removed from Cain 's chromosome and plonked onto Abel 's.
The result: Cain would commit suicide, and Abel would live happily ever after:'
This swapping is called 'crossing over: ' It happens between virtually all pairs of chromosomes in most species of animal and plant. It achieves nothing except a more thorough mixing of the genes—which is what most people thought its purpose was before Haig and Grafen suggested otherwise. But Haig and Grafen are implying that crossing over need not serve any such function; it is merely a piece of intracellular law enforcement. In a perfect world policemen would not exist because people would never commit murder. Policemen were not invented because they adorn society but because they prevent the disruption of society. So, according to the Haig-Grafen theory, crossing over polices the division of chromosomes to keep it fair.
This is not, by its nature, the sort of theory that lends itself to easy confirmation. As Haig remarks, in a dry Australian manner, crossing over is like an elephant repellent. You know it 's working because you don ' t see any elephants.'
