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The results of Hamer's first foray into behavioral genetics, published by the journal Science in 1993, ignited a furor that has yet to die down. According to Hamer and his colleagues, male homosexuality appeared to be linked to a stretch of DNA at the very tip of the X chromosome, the chromosome men inherit from their mothers. Three years later, in 1996, Hamer and his collaborators at NIH seconded an Israeli group's finding that linked a gene on chromosome 11 to the personality trait psychologists call novelty seeking. That same year Hamer's lab helped pinpoint another gene, this time on chromosome 17, that appears to play a role in regulating anxiety.
Unlike the genes that are responsible for physical traits, Hamer emphasizes, these genes do not cause people to become homosexuals, thrill-seeking rock climbers or anxiety-ridden worrywarts. The biology of personality is much more complicated than that. Rather, what genes appear to do, says Hamer, is subtly bias the psyche so that different individuals react to similar experiences in surprisingly different ways.
Intriguing as these findings are, other experts caution that none has been unequivocally replicated by other research teams. Why? One possibility is that, despite all of Hamer's work, the links between these genes and these particular personality traits do not, in fact, exist. There is, however, another, more tantalizing possibility. Consider the genes that give tomatoes their flavor, suggests Hamer's colleague Dr. Dennis Murphy of the National Institute of Mental Health. Even a simple trait like acidity is controlled not by a single gene but by as many as 30 that operate in concert. In the same way, he speculates, many genes are involved in setting up temperamental traits and psychological vulnerabilities; each gene contributes just a little bit to the overall effect.
Hunting down the genes that influence personality remains a dauntingly difficult business. Although DNA is constructed out of a mere four chemicals--adenine, guanine, cytosine, thymine--it can take as many as a million combinations to spell out a single human gene. Most of these genes vary from individual to individual by only one chemical letter in a thousand, and it is precisely these minute differences that Hamer and his colleagues are trying to identify. Of particular interest are variations that may affect the operation of such brain chemicals as dopamine and serotonin, which are well-known modulators of mood. The so-called novelty-seeking gene, for example, is thought to affect how efficiently nerve cells absorb dopamine. The so-called anxiety gene is postulated to affect serotonin's action.
How can this be? After all, as Hamer and Copeland observe in their book, "...genes are not switches that say 'shy' or 'outgoing' or 'happy' or 'sad.' Genes are simply chemicals that direct the combination of more chemicals." What genes do is order up the production of proteins in organs like the kidney, the skin and also the brain. Thus, Hamer speculates, one version of the novelty-seeking gene may make a protein that is less efficient at absorbing dopamine. Since dopamine is the chemical that creates sensations of pleasure in response to intense experiences, people who inherit this gene might seek to stimulate its production by seeking out thrills.