I considered turning in a book review that was only 85 percent complete. After all, that’s essentially what Francis Collins and J. Craig Venter did earlier this year when they declared that they had decoded the human genome. The announcement was a grand event, widely publicized and celebrated, even though the “book of life” is rife with typos and missing 15 percent of its text. Great sections of it have geneticists scratching their heads in confusion. It’s in such poor shape that scientists are wagering about how many genes the genome contains—and the bets run from a few tens of thousands to a few hundred thousand. And when scientists do succeed in decoding the genome, producing a computer disk full of As, Gs, Cs, and Ts, they will still have to figure out precisely what those chemicals mean.

Genetics has gotten much more complicated in the century and a half since Gregor Mendel figured out heritability in his field of pea plants. Our genetic code contains the instructions for creating proteins, but proteins control the way the cell follows those instructions. In this vast, complicated web of cause and effect, genes control proteins that control genes, and proteins control genes that control proteins.

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