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value that can then be used in making decisions about whether to go ahead with a particular action.

The basic formula for risk has two entries, Frederick explained: the probability that a particular undesirable event will occur, and the hazard or damages that would accompany that event. Risk is calculated by multiplying the probability of an event by its peril. Thus, to estimate the risk posed by transgenic crops, one must have values for both numbers— probability and hazard.

Power said, “in much of the work done so far in risk assessment of genetically modified crops, we have focused on the probability of the event, trying to get a handle on the number. But the extent of the hazard— what kind of hazards these traits actually confer—is probably more important. An example is the gene-flow literature, in which we now have pretty good estimates of the probability of gene flow for a lot of different crops into their wild relatives, but we still don't have many studies on the extent of the hazard: What does it mean if the gene flow occurs? Does it actually present a hazard?”

Because of that uncertainty, Power said, researchers should focus more on understanding the potential hazards posed by genetically modified crops—the “So what?” question. “Laboratory experiments have shown a variety of examples of hazards from viral recombination that do occur in the laboratory, such as increased virulence, increased host range so that the virus can infect hosts that it would not normally have infected, and changes in transmission, such as viruses that can now be transmitted by an aphid although formerly they were not transmissible by an aphid. Those have been shown under laboratory conditions. The challenge is to figure out how to monitor for them under field conditions.”

As an example, Power described studying the effects of putting viral genes into oats to make the oats resistant to a virus. Research showed that the viral genes did indeed make their way into a companion weed, wild oats, and that the genes made the wild oats resistant to the virus as well. “The question is, Once it becomes resistant, is it likely to become more of a weed? Wild oats are a weed both in agroecosystems and in natural habitats in the sense that they outcompete a lot of native perennial grasses in many parts of the West. The existence of risk depends on such factors as the co-occurrence of domesticated and wild oats, gene flow between them, and the occurrence of viable hybrids of oats and wild oats; and all these have been shown quite extensively. We have been working on whether viral-resistance gives wild oats a selective advantage, and the answer is yes. We can see substantial effects on growth, reproduction, and all those things that you would associate with fitness traits. The next step is to ask about it in the field, and that is essentially where we are now.”