Even a simple trait like acidity is controlled not by a single gene but by as many as 30 that operate in concert. Hunting down the genes that influence personality remains a dauntingly difficult business. 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 anxiety gene is postulated to affect serotonin's action.
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. Still, as critics emphasize and Hamer himself acknowledges, genes alone do not control the chemistry of the brain. Ultimately, it is the environment that determines how these genes will express themselves.