Disabled genes dull sense of smell.

You wouldn't guess from the proliferation of perfumes and underarm deodorants, but people have a woeful sense of smell compared with many other residents of the animal kingdom. Simply put, we offer no competition to bloodhounds or bunnies.

With little data to go on, scientists have proposed several theories to explain this olfactory inequality. The latest blames it on our genes. In 1998, Dominique Giorgi of the Institute of Human Genetics in Montpellier, France, and colleagues reported that more than 70 percent of the human genes encoding olfactory receptors--the cell-surface proteins used to detect smells--possess disabling mutations. The researchers hypothesized that this unexpected predominance of so-called pseudogenes accounts for the poor human sense of smell.

Giorgi's group predicted that animals with better olfactory talents, such as dogs and rodents, would have fewer pseudogenes among their olfactory-receptor genes. A new study by the team in the March 14 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES supports that contention.

The scientists compared the fraction of disabled olfactory-receptor genes among 10 primate species and the mouse. The primates most closely related to humans had the highest percentages of olfactory-receptor pseudogenes. The gorilla topped the list with 50 percent, and the chimpanzee followed with 48 percent. More distantly related species had lower percentages. The baboon showed 19 percent, and the marmoset had no pseudogenes.

As for the mouse, the investigators didn't find a single disabled olfactory receptor gene. They speculate, on the basis of their study, that olfactory prowess correlates directly with the number of working olfactory-receptor genes.