With continued progress in the rapidly growing field known as pharmacogenetics, it will become possible to prescribe drugs based on each patient's ow… - Gary F. Marcus

"How does the body push the comparatively tiny genome so far? Many researchers want to put the weight on learning and experience, apparently believing that the contribution of the genes is relatively unimportant. But though the ability to learn is clearly one of the genome's most important products, such views overemphasize learning and significantly underestimate the extent to which the genome can in fact guide the construction of enormous complexity. If the tools of biological self-assembly are powerful enough to build the intricacies of the circulatory system or the eye without requiring lessons from the outside world, they are also powerful enough to build the initial complexity of the nervous system without relying on external lessons.

The discrepancy melts away as we appreciate the true power of the genome. We could start by considering the fact that the currently accepted figure of 30,000 could well prove to be too low. Thirty thousand (or thereabouts) is, at press time, the best estimate for how many protein-coding genes are in the human genome. But not all genes code for proteins; some, not counted in the 30,000 estimate, code for small pieces of RNA that are not converted into proteins (called microRNA), of "pseudogenes," stretches of DNA, apparently relics of evolution, that do not properly encode proteins. Neither entity is fully understood, but recent reports (from 2002 and 2003) suggest that both may play some role in the all-important process of regulating the IFS that control whether or not genes are expressed. Since the "gene-finding" programs that search the human genome sequence for genes are not attuned to such things-we don't yet know how to identify them reliably-it is quite possible that the genome contains more buried treasure."

Cell differentiation can turn neurons into everything from clocks that control circadian rhythms to photoreceptors that convert light into electrical-chemical impulses or decision makers that tally votes and decide courses of action. In the retina (often used as a case study because it can be directly and naturally stimulated), there are at least fifty different kinds of neurons specialized to different tasks, such as looking for motion, recognizing colors, detecting objects in low light, and measuring brightness and contrast. In the brain as a whole, there may be as many as 10,000 different kinds of neurons, each contributing to a different aspect of mental life.

In the years to come, some of our best minds will try to dig deeper into that computer program, to figure out its individual lines of code (the IF-THENS that we call genes), the products of those lines (what we call proteins), how all those lines of biological code fit together, and how they make room for nurture.

In the long run, the effects on society will be profound. Take, for example, the advances that our increasing understanding of genes will lead to in medicine. Because, as we have seen, the brain is built like the rest of the body, it is also amenable to many of the same types of treatment. For example, stem cell therapies originally developed for leukemia are being adapted to treat Parkinson's disease and Huntington's disease. Gene therapies developed for cystic fibrosis may someday help treat brain tumors. Both work by harnessing the body's own toolkit for development.