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code with two kinds of dot and two kinds of dash. Call these bases ‘letters’: A, C, G and T. The beauty of DNA is that each letter is complementary to another, meaning that it prefers to align itself opposite that other letter. Thus A pairs with T and vice versa, and C with G and vice versa. This means that there is an automatic way of copying DNA: by going along the strand of the molecule stitching together another from the complementary letters. The sequence AAGTTC becomes, on the complementary strand, TTCAAG: copy that and you get the original sequence back again. Every gene normally consists of a strand of DNA and its complementary copy closely entwined in the famous double helix. Special enzymes move up and down the strands and where they find a break repair it by reference to the complementary strand. DNA is continually being damaged by sunlight and chemicals. If it were not for the repair enzymes, it would quite quickly become meaningless gobbledegook.

But what happens when both strands are damaged at the same place? This can be quite common, for example when the two strands get fused together like a spot of glue on a closed zip. The repair enzymes have no way of knowing what to repair the DNA to. They need a template of what the gene used to look like. Sex provides it. It introduces a copy of the same gene from another creature (outcrossing) or from another chromosome (recombination) in the same creature. Repair can now refer to a fresh template.

Of course, the fresh template may also be damaged at the same place, but the chances of that are small. A shopkeeper adding up a list of prices makes sure he got it right the first time by simply repeating the task. His reasoning is that he is unlikely to make the same mistake twice.

The repair theory is supported by some good circumstantial evidence. For example, if you expose a creature to damaging ultraviolet light, it generally fares better if it is capable of recombination than if it is not, and fares better still if it has two chromosomes in its cells. And if a mutant strain appears that eschews recombination, it proves to be especially susceptible to damage by ultraviolet light. Moreover, Bernstein can explain details that his rivals cannot: for example, the curious fact that just before dividing its chromosome pairs in two to make an egg, a cell will double the number and then dispose of three-quarters of the proceeds. In the repair theory this is to find, and convert to a ‘common currency’, the errors that are to be repaired.27

None the less, the repair theory remains inadequate to the task it has set itself. It is silent on outcrossing. Indeed, if sex is about getting spare copies of genes, it would be better to get them from relatives, than to seek out unrelated members of the species. Bernstein says outcrossing is a way of masking mutations, but this amounts to no more than a restatement of the reason why inbreeding is a bad thing, and sex is the cause of inbreeding, not the consequence.

Moreover, every argument that the repair people give for recombination is merely an argument for keeping backup copies of genes; there is a far simpler way of doing that than swapping them at random between chromosomes. It is called diploidy.28 An egg or a sperm is haploid – it has one copy of each gene. A bacterium, or a primitive plant, such as moss, is the same. But most plants and nearly all animals are diploid, meaning they have two copies of every gene – one from each parent. A few creatures, especially plants that are descended from natural hybrids, or have been selected by man for large size, are polyploid. Most hybrid wheat, for example, is hexaploid: it has six copies of each gene. In yams, female plants are octoploid or hexaploid, males all tetraploid – a discrepancy that renders yams sterile. Even some strains of rainbow trout and domestic chickens are triploid – plus a single parrot that turned up a few years ago.29 Ecologists have begun to suspect that polyploidy in plants is a sort of alternative to sex. At high altitudes and high latitudes many plants seem