The Minnesota studies of the inheritance of IQ and other traits were “hugely important in changing people’s minds about the roles of genes and the environment,” says Nick Martin, an Australian who edits the journal Twin Research. “These studies had great cumulative weight.” Nancy Segal, who trained with Bouchard, points out that the study comprises 134 sets of twins at present, 80 of whom are identical. Objections sometimes are raised that twins from the same household are not innately similar but merely copy each other or learn similar behaviors from their parents or peers. The Minnesota design takes copying and learning off the table.
|
In her recent work at California State University at Fullerton, Segal has gone Bouchard one better. She has assembled a group of what she calls “virtual twins.” These are unrelated children of the same age—most often both are adopted—who are being brought up together. “Virtual twins share environments but not genes,” she says, “and that makes them the inverse of monozygotic twins reared apart.” Both kinds of twins—fraternal as well as identical twins such as the Heim girls—are compared with sets of virtual twins.
In her study, Segal has found that the virtual pairs are most dissimilar in IQ and temperament, especially in contrast to the biologically identical sets. “Genes versus environment—people say it’s been asked a zillion times,” Segal says, “but the question hasn’t been looked at in quite this way before.”
In the new century the gene once again is riding high. If twin researchers are going to nail down the heritability of human traits, they must uncover specific genes: the pieces of the DNA molecule that are important to, say, altruism and cooperation or that set our blood pressures and baldness patterns. Finnish geneticist Leena Peltonen says: “Twin researchers are a species of their own, and they haven’t been integrated with the molecular geneticists as much as they should. They’ve been making their bread on monozygotic versus dizygotic differences. It’s a bit trivial.”
Nancy Segal and Nick Martin acknowledge that progress in finding genes through twin studies has been next to nil. Therefore they look to the Human Genome Project and to scientists like Peltonen who manipulate DNA. Their traditionally low-budget science, which needs only pencil and paper to tally shared traits in the classical twin studies, has entered the high-tech world of genetic epidemiology, which traces the power of genes to influence the health of populations. “Our work will become less inferential,” says Segal, “but it’s going to take some time.”
A good example of the evolution can be seen in approaches to cigarette smoking. Twin researchers in Sweden and the United States started to study smoking behavior in the 1950s, and in due course they demonstrated that identical twins were more likely than fraternal twins to have in common either smoking or nonsmoking tendencies. Here was simple evidence of behavioral genes at work. As they continued to follow the twins in the databases, researchers showed that a twin who smoked was likely to die earlier than a twin who didn’t. Whether the twins were identical or fraternal didn’t have an effect; smoking usually hastened death no matter what amount of genes were shared. Here was evidence that the environment, in the form of cigarettes, could prevail over any genes for longevity.
At SRI International in Menlo Park, California, twin researchers have delved into the physiology of smoking, acting on the theory that certain people can metabolize nicotine faster than others. The idea is that the fast metabolizers may become addicted more readily. About 70 percent of this trait has been determined to be heritable, at least according to the different responses of the pairs in the SRI study.
And some relevant DNA has been uncovered. A team at Virginia Commonwealth University in Richmond recently found that three genetic polymorphisms—variants in the “spelling” of a certain gene—are connected to nicotine addiction. For their raw material the team analyzed the DNA of 688 twins from the Mid-Atlantic Twin Registry, one of the venerable collections in the United States.
The gene that the Virginia team identified is not the whole story behind nicotine addiction, because all such conditions and the majority of common diseases involve multiple genes and multiple interactions with the environment. Yet as a small, discrete piece of the biology of smoking, the DNA finding moves twin work far past such survey questions as, Does your father smoke? Your mother?
Fraternal twins are the best choices for molecular analysis. Say that a pair of dizygotic twins is discordant for smoking. Biologists will focus the gene hunt on the stretches of DNA where the two siblings differ. Of course, the approach won’t work with monozygotic twins, whose genes are identical. But if a promising gene shakes out of the fraternal study, it’s simple enough to look for the same gene in the identical pairings.
That is the strategy behind the world’s largest twin scan just under way in Europe: GenomEUtwin, it’s proudly called, the middle letters standing for the European Union. Directed by Peltonen, the study represents a collation of national twin registries from Denmark, Finland, Italy, the Netherlands, Norway, and Sweden. Recent entries from Great Britain and Australia make for a grand total of 800,000 twins. The siblings in the twins’ families will extend the health database even further. The first run-through of GenomEUtwin will search for genes that control height, a trait everyone can relate to, but the genes for humanity’s common diseases are the real prizes. By 2010 the scientists hope to have some of them.
A universal truth about twins is that they seem to like the attention of scientists. No country requires them to enroll in studies or make available their DNA and family histories, but over the years most twins have been eager to help. Twin Research editor Martin says: “Twins are sufficiently rare as to feel themselves special and pleased that researchers take an interest in them but not so rare as to be freaky. So their cooperation is often surprisingly good.” Then again, cooperation is in their genes.
