More than a chicken, fewer than a grape

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Exactly how many genes make up the human genome remains a mystery, even though scientists announced the completion of the Human Genome Project a decade ago. The project to decipher the genetic blueprint of humans was supposed to reveal all of the protein-producing genes needed to build a human body.

“Not only do we not know what all the genes are, we don’t even know how many there are,” Steven Salzberg of the University of Maryland in College Park said October 11 during a keynote address at the Beyond the Genome conference, held in Boston. Most estimates place the human gene count in the neighborhood of 22,000 genes, which falls between the number of genes in a chicken and the number in a grape.

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The most accurate estimate of the human gene count is the RefSeq database maintained by the U.S. National Institutes of Health, Salzberg said. He laid out arguments for favoring this estimate, such as its inclusion of all confirmed genes to date, in a paper published in May in Genome Biology. By the RefSeq count, humans have 22,333 genes. But another government database lists 38,621 human genes. And a different project called Gencode currently recognizes 21,671.

Such disparate numbers stem from the fact that genes comprise only about 1 percent of the 3 billion As, Ts, Gs and Cs that make up the human genetic instruction book. And the genes aren’t conveniently laid out as single, continuous stretches of genetic code. Instead, human genes are found in protein-encoding pieces called exons, interspersed with stretches of DNA that don’t make protein. These spacers are called introns.

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