"We can look at how variations in the genome travel together with different diseases," [Kari Stefansson] says. The data produced patterns that led to genes involved with specific diseases. DeCODE also is looking for genes involved with longevity. Stefansson says the company is not working on genes associated with personality. But another company could in the near future.

Advancing from single-gene simplicity, scientists are now discovering specific patterns of gene expression associated with diseases. Last month, scientists at the Dana Farber Cancer Institute reported that the potential of a tumor to spread is associated with a specific gene-expression pattern, which basically consists of genes that are active against the background of genes that are inactive. Can gene-expression patterns for behavior be far behind?

Gene-expression profiles are identifiable through a technology called the gene chip, a tiny device that can read information on thousands of genes. Gene-expression profiles from the chips can then be analyzed by computer. Scientists can compare known expression profiles to DNA from a person or even an embryo. The information could be adapted for genetic screening tests.