March 2 1997 3:30 AM


Cloning, Nature, and Nurture.


Cloning, Nature, and Nurture


Dolly, the world's first cloned mammal and thus probably history's most famous sheep, is likely to leave a somewhat ironic legacy. This triumph of genetic engineering, achieved last week by Scottish embryologist Ian Wilmut, might well mark the defeat of the idea that genes determine who and what we are.

A clone is an identical twin, a second creature with a genetic code identical to the first's. Dolly, in other words, is the twin sister her original never had. Identical twins are rare in nature, and because they're created when a single fertilized egg divides into two embryos, they're born at the same time. If Wilmut's techniques for cloning mammals prove to be practical, that time constraint will no longer exist. It will be possible to make twins whenever we want, to replace your old dog Pete with Puppy Pete, to replicate a deceased human infant, or to copy yourself.

But twins usually grow up in the same family and community (studies of their rearing suggest that people treat identical twins much more alike than they treat siblings or even fraternal twins). A clone's environment will be different. It will grow from a different egg, develop in a different womb, possibly grow up in a different place, and have different triumphs and disasters.

Cloning, in other words, offers the possibility of the mother of all "twin studies"--comparisons of how often a trait appears among identical twins vs. how often it appears among others. Twin studies these days are invoked to feed the popular notion that genes determine behavior, since they suggest that genes play a role in such things as schizophrenia, alcoholism, performance on intelligence tests--even proneness to divorce (a 1992 study of 1,500 sets of twins found that if one twin had been divorced, the likelihood that the co-twin had also been divorced was 45 percent in identical twins, but only 30 percent in fraternal twins).

It was Sir Francis Galton, cousin of Charles Darwin and founder of the eugenics movement, who first suggested twin studies as a means of teasing out the role of heredity in shaping human traits. Galton wanted to use the studies to establish the primacy of Nature over Nurture. In fact, he was the one who coined that cliché, seizing a snatch of verse from Shakespeare's The Tempest in which Prospero calls Caliban "[a] devil, a born devil, on whose nature/ Nurture can never stick; on whom my pains,/ Humanely taken, all, all lost, quite lost!"

Since Galton's time, the Nature-Nurture dichotomy has hardened into competing ideologies. A vigorous intellectual movement in favor of Nature in the 1920s gave rise to a countertradition emphasizing the importance of Nurture, which spawned the field of cultural anthropology. Meanwhile, the school of Nature, following the discovery of DNA in 1953, evolved into sociobiology and evolutionary psychology, whose adherents believe that the common heritage of human nature, transmitted through genes, has a greater effect on behavior than culture and history do.

The political and cultural arguments, however, lag behind the lab work. At conferences and seminars, biologists and psychologists have been overheard remarking to one another that the Nature-Nurture dichotomy can't accommodate what they're finding.

Consider, for example, oxytocin, a hormone found in the bodies of many mammals, including human beings, where it is associated with uterine contractions and breast-feeding. Add a little oxytocin to female rats' brains, and they get friendlier. Block receptors for oxytocin, and the creatures threaten each other more. Oxytocin also seems to raise tolerance for pain, and it decreases blood pressure. Sounds like the rule of Nature.

But what triggers the release of oxytocin in the rat brain when there's no researcher to inject the hormone? Warmth, touch, and friendly social interactions. So, does a hormone cause an action? Or does an action cause a hormone to be released? It seems to be a bit of both, especially in big-brained animals, like us, that have a lot of flexibility to their behavior. In primates, says Kim Wallen, a behaviorist at the Yerkes Regional Primate Center in Atlanta, hormones "act in concert with so many other factors. I think we might see these not as chemical signals that turn things on and off but as things that permit an organism to do certain things under certain circumstances."

The question of where characteristics come from also turns out to be too complicated to divide neatly into Nature or Nurture, according to Stephen Suomi, director of the Laboratory of Comparative Ethology at the National Institutes of Health. His research suggests that a rhesus monkey's sexual behavior, aggression, thinking, and responses to stress are affected by "prenatal stressing"--that the strains and pains to which the pregnant mother is subjected will have an immense impact on her infant long after it is born.

Not even DNA, that supposedly impregnable "digital" code, is viewed as platonically separate anymore. "DNA doesn't do anything," says James Shapiro, a cell biologist at the University of Chicago Medical School. A gene is an instruction for making a protein. Reading that instruction, following it, folding the protein into the shape it must have to do its job--all this is the job of the cell, and the cell is affected by its environment.

In twin studies, too, as Elizabeth Spitz, a researcher at the Université René Descartes in Paris, has noted, Nurture is nearly impossible to tease out. Identical twins, at the moment of birth, may have already had very different experiences during their nine months in the womb. This is because the number of membranes identical fetuses share depends on when exactly their single egg split into two embryos. If this divisional split occurred early (within four days after fertilization), then each twin grew to birth in its own little world. Each developed its own chorion, the outermost membrane, and its own amnion, the inner membrane that contains the amniotic fluid in which the fetus floats. If the cell division happened later (four to eight days after fertilization), each twin got its own amnion but shared a chorion. If the division happened later still (eight to 10 days after conception), the two fetuses shared both chorion and amnion.

How similar twins are depends in part on how many membranes they shared. That amazing story about how two separated twins both married red-headed engineers in the same year--the resemblance could be because they shared the same amnion, and not because of their genes. So it's a little premature to propose a genetic predisposition to divorce.

In fact, the standard laboratory procedure for sorting out one influence from another--controlling the genetics, the prenatal environment, and the rearing of the twins--turns out to be impossible. The point may seem abstruse now. But once all those millions of Dollies have been gamboling about for a few years, and it's plain that they don't all look and act alike, we'll have no choice but to get off Galton's seesaw.

David Berreby writes regularly about science and culture for Slate.

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