hexaploid, males all 'tetraploid '
—a discrepancy that renders yams sterile. Even some strains of rain-bow trout and domestic chicken are 'triploid '—plus a single parrot that turned up a few years ago:
But by mentioning ecologists we are getting ahead of ourselves: The point at issue is gene repair: If diploid creatures were to indulge in a little recombination between chromosomes every time their cells divided as the body grew, there would be plenty of opportunity for repair: But they do not: They recombine their genes only at the final peculiar division called meiosis that leads to the formation of an egg or a sperm. Bernstein has an answer for this: He says that there is another, more economical way to repair damage to genes during ordinary cell division, which is to allow the fittest cells to survive: There is no need for repair at that stage THE ENIGMA
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because the undamaged cells will soon outgrow the damaged ones.
Only when producing germ cells, which go out to face the world alone, need you check for errors.'
The verdict on Bernstein: unproven. Certainly the tools of sex seem to be derived from the tools of repair, and certainly recombination achieves some gene repair. But is it the purpose of sex? Probably not.
CAMERAS AND RATCHETS
The geneticists, too, are obsessed with damaged DNA. But whereas the molecular biologists concentrate on the damage that is repaired, the geneticists talk about the damage that cannot be repaired. They call this 'mutation. '
Scientists used to think of mutations as rare events. But in recent years they have gradually come to realize how many mutations happen. They are accumulated at the rate of about one hundred per genome per generation in mammals. That is, your children will have one hundred differences from you and your spouse in their genes as a result of random copying errors by your enzymes or as a result of mutations in your ovaries or testicles caused by cosmic rays. Of those one hundred, about ninety-nine will not matter: they will be so-called silent or neutral mutations that do not affect the sense of genes. That may not seem many, given that you have seventy-five thousand pairs of genes and that many of the changes will be tiny and harmless or will happen in silent DNA between genes. But it is enough to lead to a steady accumulation of defects and, of course, a steady rate of invention of new ideas.'
The received wisdom on mutations is that most of them are bad news and a good proportion kill their owners or inheritors (cancer starts as one or more mutations), but that occasionally among the bad there is a good mutation, a genuine improvement.
The sickle cell anemia mutation, for example, can be fatal to those who have two copies of it, but the mutation has actually increased in some parts of Africa because it gives immunity to malaria.
For many years geneticists concentrated on good mutations and viewed sex as a way of distributing them among the population, like the ' cross-fertilization ' of good ideas in universities and industries: Just as technology needs 'sex' to bring in innovations from outside, so an animal or plant that relies on only its own inventions will be slow to innovate: The solution is to beg, borrow, or steal the inventions of other animals and plants, to get hold of their genes in the
The most obvious reason to borrow genes is to benefit from the ingenuity of others as well as yourself. Sex brings together mutations, constantly rearranging genes into new combinations until fortuitous synergy results. One ancestor of a giraffe, for example, might have invented a longer neck while another invented longer legs: The two together were better than either alone: But this argument confuses consequence with cause. Its advantages are far too remote; they will appear after a few generations, by which time any asexual competitor will long ago have out- populated its sexual rivals. Besides, if sex is good at throwing together good combinations of genes, it will be even better at breaking them up. The one thing you can be sure about sexual creatures is that their offspring will be different from them, as many a Caesar, Bourbon, and Plantagenet discovered to their disappointment: Plant breeders much prefer varieties of wheat or corn that are male-sterile and produce seeds without sex because it enables them to be sure their good varieties will breed true.
It is almost the definition of sex that it breaks up combinations of genes. The great cry of the geneticists is that sex reduces
' linkage disequilibria: ' What they mean is that if it were not for THE ENIGMA
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recombination, genes that are linked together—such as those for blue eyes and blond hair—would always be linked together, and nobody would ever have blue
In the late 1980s there was one last revival of interest in theories of 'good' mutation: Mark Kirkpatrick and Cheryl Jenkins were interested not in two separate inventions but in the ability to invent the same thing twice. Suppose, for example, that blue eyes double fertility, so that people with blue eyes have twice as many children as people with brown eyes. And suppose that at first everybody has brown eyes. The first mutation in a brown-eyed person to blue eyes will have no effect because blue eyes are a recessive gene, and the dominant brown-eye gene on the person 's other chromosome will mask it: Only when the blue-eye genes of two of the descendants of the original mutant person come together will the great benefit of blue eyes be seen. Only sex would allow the people to mate and the genes to meet. This so-called segregation theory of sex is logical and uncontroversial. It is indeed one of the advantageous consequences of sex. Unfortunately, it is far too weak an effect to be the main explanation for sex 's prevalence. Mathematical models reveal that it would take five thousand generations to do its good work and asex would long since have won the game.'
In recent years the geneticists have turned away from good mutations and begun to think about bad ones: Sex, they suggest, is a way of getting rid of bad mutations. This idea also has its origins in the 1960s, with Hermann Muller, one of the fathers of the Vicar of Bray theory Muller, who spent much of his career at the University of
