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Far fewer human genes exist than we once thought, scientists say

Ten years ago, scientists thought humans had about 100,000 genes. Today, they have trimmed that estimate by more than three quarters.

The findings are part of a newly published analysis of the human genetic code. The authors of the analysis, an international group of researchers, say that in carrying out the project they have also begun to systematically pinpoint which genes set us apart from other animals. 

The findings are published in the Oct. 21 issue of the research journal Nature. In it, the researchers described the final product of the Human Genome Project, a 13-year effort to map out the genetic code, that ended in 2003. 

Although researchers are describing the new findings as the results of the "finished" project, it is not really quite finished. There still remain 341 gaps in scientists' description of the human genome sequence, although this is far fewer than the 150,000 gaps that there were four years ago. Today's technology cannot readily close the remaining gaps; this will require more research and new technologies, the researchers said. 

In addition to reducing the count of human genes, scientists reported that the improved quality of the finished human genome sequence, compared with earlier drafts, provides a much clearer picture of certain phenomena such as duplication of DNA segments and the "birth" and "death" of genes. 

Segmental duplications are large, almost identical copies of DNA present in at least two places in the human genome. In some cases these are duplicate copies of genes that, in other animals, exist in only one copy. 

Researchers think that although these duplications may arise as evolutionary accidents, they can serve an important purpose because the organism uses them as a "laboratory" for creating genes with new functions. Once two identical genes appear where just one used to be, that frees one of them to mutate and take on new functions. 

The high proportion of segmental duplication in the human genome shows our genetic material has undergone rapid innovation and change during the last 40 million years, presumably contributing to unique characteristics that separate us from our non-human primate ancestors. 

But some diseases are also associated with mutations in segmentally duplicated regions. 

"Now, through the unstinting efforts of groups around the world, this important and rapidly evolving part of our genome is open for scientific exploration," said Robert H. Waterston, chair of the Department of Genome Sciences at the University of Washington in Seattle. 

Segmental duplications cover 5.3 percent of the human genome, significantly more than in the rat genome, which has about 3 percent, or the mouse genome, which has between 1 and 2 percent. Segmental duplications provide a window into understanding how our genome evolved and is still changing. 

The accuracy of the finished human genome sequence produced by the Human Genome Project has also given scientists some initial insights into the "birth" and "death" of genes. 

Scientists have identified more than 1,000 new genes that arose in the human genome after the lineage leading to humans separated from the one leading to rodents, some 75 million years ago. Most of these new genes arose through recent gene duplications and are involved with immune and reproductive functions, among others. 

Researchers also used the finished human genome to identify 33 nearly intact genes that have recently acquired mutations, causing them to stop functioning, or "die." Scientists found that 10 of these appear to have coded for proteins involved in smell, which helps to explain why humans have a poorer sense of smell than rodents.

The researchers also reported that five of the genes inactive in humans were still functional in chimpanzees. This could help researchers identify the key differences between humans and other primates, providing "fertile ground for future research projects," said Richard Gibbs, director of Baylor College of Medicine's Human Genome Sequencing Center in Houston, Texas. 

More than 2,800 researchers who took part in the International Human Genome Sequencing Consortium share authorship on the Nature paper, which expands upon the group's initial analysis published in Feb. 2001.

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