Next time you get cut off by a another driver, consider giving the offender a break: One-third of Americans might be genetically predisposed to crappy driving.

No, really, it's not just your imagination.

In a new study of college undergraduates, those with a common genetic variation scored 20 percent worse in a driving simulator than their counterparts.

"The people who had this genetic variation performed more poorly from the get-go and learned more slowly as they went along," said Steven Cramer, a University of California, Irvine neurologist, who works on helping stroke victims recover. "Then, when we brought them back four days later, they had more forgetting."

The single nucleotide polymorphism, or SNP, is just one of millions of single-letter variations between humans' genetic codes. This one occurs in a gene that produces a protein called brain-derived neurotrophic factor, which helps regulate the formation of new synapses, and the maintenance of old ones. BDNF plays a very important role in what's called neuroplasticity, or the brain's ability to rewire itself on the fly.

As described in a paper published in the journal Cerebral Cortex, study participants were asked to drive 16 laps in a driving simulator that was essentially a screen with a steering wheel. As they drove around the course, they attempted to keep their cars on a black strip in the center of the road. The software grades their ability to complete that task quantitatively. And, of a small sample of 29 students, people with that single genetic difference, called Val66Met, performed more poorly than their demographically similar counterparts.

"It's a very nice study, well designed, and the questions they ask are good," said Clifford Nass, a co-founder of Stanford's CarLab, an interdisciplinary research institute. He was not part of the study.

Cramer considers the simulation a good proxy not just for driving, but for other complex motor skills tasks. Because it's not controlling a motor vehicle, per se, that he's interested in, but how the brain learns, or relearns complex tasks.

When people have a stroke, and a portion of their brain dies, they have to relearn tasks using different parts of their brains. Individual genes are only part of the symphony of influences that determine individual behavior, but the Val66Met variation appears to have an unusually strong influence on the brain's activity.

"There is mounting evidence that the one in three people who have this variation have less plasticity than the two-thirds of people who lack that genetic variation," Cramer said.

Results from a separate study reported earlier this year in Scientific American also found that genetic variation in BDNF helped determined people's skill at a simple computer game.

The effect is so pronounced, in fact, that Cramer said he could imagine future stroke patient routing within hospitals based on the SNP.

"I wonder if there aren't going to be treatments, when they have traumatic brain injury and you're in the rehab ward, where they test the gene and say, 'Send them to the BDNF ward,'" he said.

So, if the presence of the gene makes you a worse driver, a slower stroke-victim recoverer and possibly has other negative effects, why is the variant still present?

"Variations can stick around just for the fact that they are not that bad for you," said Bruce Teter, a geneticist who studies the brain at UCLA. "They don't kill you before you reproduce, in which case, there is no selective advantage or disadvantage."

But it also turns out that people with the Val66Met variant could be less susceptible to degenerative neurological disorders like Parkinson's and Huntington's.

"Originally people thought plasticity had to be good, as it's related to the ability of the brain to adapt and learn and things like that," Teter said. "But neuroplasticity can also be bad for you in situations where the kinds of changes that are seen are deleterious."

But if you want to stay out of car accidents, it's better to have the dominant BDNF variant, Cramer's study suggests. And if further work continues to support that idea, the question is, can or should we do anything with that information?

"Let's pretend that the one in three people are more prone to car accidents," Cramer said. "It's up to society to say, how do we deal with that fact?"

Image:__ __Justin Fantl.__

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