THE RECENT ANNOUNCEMENT that the human genome contains approximately 30,000 genes, rather than the previously estimated 100,000, took the world by surprise. In the days following, much was made of humanity's "new" relationship with the worm, which, with approximately 20,000 genes, has only 10,000 fewer than us. To many commentators, it seemed that our position at the center of the genetic universe was being overturned. In addition to this newfound and much ballyhooed humility, the first draft sequencing of the human genome has raised many questions that will impact the development of medicine, agriculture, evolutionary studies, and perhaps even the nature of science for years to come. Right now, some of the most important questions coalesce around the surprise over the gene number itself.

That neither of the two groups competing to sequence the genome first -- the publicly funded Human Genome Project (HGP) and the publicly traded Celera -- anticipated a gene count of 30,000 is clear. In their article published in Science on February 12, Celera called the figure of 30,000 a "major surprise," a sentiment echoed by representatives of the HGP. Why were they surprised? How can two separate multimillion dollar scientific projects be that wrong? Because, in the words of the HGP article in Nature's February 12 issue, the figure of 100,000 was "intended only as a back-of-the-envelope estimate."

But -- and this is the puzzle -- if it was just a qualified estimate, why was anyone surprised? In fact, how does an estimate become as the HGP article described, "widely quoted and adopted in many textbooks."

IN THE DAYS FOLLOWING the announcement, many reports, rather than addressing how an estimate becomes an expectation, dwelt instead on the effect of the new figure on our psyche. Most journalists were unable to resist either the comic lure of the human/worm comparison or variations on the inevitable "Does size matter?" In tones ranging from New York Times rueful ("the impact on human pride is another matter") through to dot-com gonzo ("some cosmic scriptwriter's idea of a joke"), much of the initial reporting was framed by a strong mix of anthropocentrism and mock self-deprecation. On the day of the announcement, an anchorwoman on ABC news concluded, "if DNA is the book of who you are, then it turns out to be closer to a short story." Said Australia's The Age, "We're not so special;" and the Times of London, along with many other papers, found the announcement "humbling."

But this ubiquitous hook has confusing implications; it suggests that the gene count was somehow less than it should be, that the human species isn't as complex as we thought, and, as epitomized by the New York Times headline, "Humans Survive on Low Numbers of Genes," that we merely get by on a less than adequate quantity. The fundamental implication was that the figure of 30,000 was meaningful in and of itself. While neither the Celera nor the HGP articles explicitly suggested this, the idea could be reasonably inferred from their reports. One of the Celera article's sections was titled "The Low Gene Number in Humans," and the HGP discussed the "paucity" of genes. Such language undoubtedly contributed to the confusion that 30,000 was low with respect to some absolute scale rather than with respect to expectation.

David Altshuler, director of Medical and Population Genetics at the Whitehead Center for Genome Research in Cambridge, Massachusetts, agrees that there was "a lot of misperception in the last few days about the gene number." But he says, "the bottom line is that the number of genes is the number of genes, and the number doesn't tell you anything about how important genes are or not." Finding the media reports to be largely ironic in intent, he says that "the number of genes would be a pretty stupid thing [for a species] to be proud of." Barbara Hanson, a geneticist at Canisius College in New York, also thought the suggestion that the dignity of our species had been compromised was silly. Of the media coverage, she laughs, "It's a bunch of garbage."

THE MOST IMPORTANT scientific question about the inaccurate estimate is whether it affected the search. Altshuler says that it was not possible for the sequencing techniques to be influenced by expectations about the final number. This is because the methods that were used to determine the sequence of human DNA involve taking pieces of an individual genome, breaking it up into little pieces, and then cataloging its chemical bases, typically abbreviated as G, A, T, and C.

This job is complicated (there are approximately three billion bases) but necessary because it is within the microscopic coils of DNA that the segments we call genes tell cells how to grow and what relationships to have with each other. As we all know, genes correspond to the clearly identifiable characteristics, like eye color and leg number, that make individual humans individual humans, and not cloned humans or, for that matter, individual centipedes. The vast majority of genes produce RNAs, which are then coded into proteins. "It's the protein," says Altshuler, "that goes and does the work."

The genome of a species is thus a detailed parts list of its DNA material; protein-coding genes, non-protein-coding genes, and DNA of unknown function (which, incidentally, forms the vast majority of the genome). Importantly, even though every individual has a unique genome, the two genomes assembled by the HGP and Celera are chimeras; patchwork collections of DNA from the individuals who contributed samples to the research.