AAAS Geneticists are shedding light on why some people can't handle their beer, or rather may eschew a noble chalice of foaming chestnut ale in favour of a less challenging sweeter beverage.

The genetic basis of sensitivity to bitter compounds has a special significance in evolution - scientists hypothesise that the ability to distinguish bitter tastes, which are often associated with poisonous molecules, was crucial to our early ancestors.

Scientists are approaching the question by investigating a gene called PTC, which confers either sensitivity or insensitivity on taste buds to phentlythiocarbamide.

Tiny amounts of this synthetic compound are extremely bitter to people with one form of the gene, while those with the other experience little or no effect. This presents an evolutionary mystery: why hasn't natural selection eliminated genes for insensitivity to bitter tastes - which could render people unaware they were poisoning themselves - from the population?

University of Texas professor Stephen Wooding found genetic signatures in the PTC gene which point to an evolutionary phenomenon called balancing selection. Balancing selection does exactly what you would expect; the push to delete the non-tasting form of the gene is balanced by the pull to keep it.

Presenting the work at the AAAS meeting in San Francisco, Wooding said: "In the absence of this type of natural selection, you would expect one form to dominate. That hasn't happened here because for some reason, there is not a strong advantage of one over the other. It's an unusual situation."

The source of the pull that kept the non-taster form of the PTC presenting early human populations is, for now, a matter of conjecture. Wooding said: "One explanation could be that, long ago, it conferred some sort of protection from a different compound in these people." Similarly, there is evidence that insensitivity to the bitterness of the quinine family of compounds, which are used against malaria, is more common in people from regions where mosquitos carry the amoeba which causes the illness.

Nurture as well as nature plays a role in people's penchant for beverages which can be disgustingly bitter to friends though. Wooding said: "I think it would be an interesting question for psychologists."

Indeed, it could be said that the Teutonic cultural sweet tooth was indirectly responsible for the destruction of the Austrian wine industry in the 1980s when it was found vineyards were adding antifreeze to their wines to up the sweetness. ®