Survival and genetic disorders Evolution: What is regarded as a disorder today might have helped the species stay alive thousands of years ago.


February 08, 2002|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

If Darwin was right, and evolution relentlessly weeds out genetic traits that impede a species' survival, then why are a quarter of adult Europeans and 90 percent of Asians unable to digest milk products - a rich, year-round source of protein and energy?

Why are 3 percent of American children struggling in school, distracted by attention deficit hyperactivity disorder? Why does one in every 28 people of European descent carry the gene for cystic fibrosis?

Scientists don't yet have all the answers. But the recently completed mapping of the human genome, and the decreasing costs of the DNA sequencing technology that made it possible, have energized new research in evolutionary genetics. Scientists are gaining intriguing glimpses into the cold logic of human evolution, and the remarkably complex interplay of genetics and human history.

The new genetic toolkit "puts this whole area of research on a more scientific basis, rather than pure speculation," says biochemist Robert K. Moyzis, of the University of California at Irvine.

Consider lactose intolerance.

In the Jan. 14 issue of Nature Genetics, researchers at the University of California at Los Angeles, and in Finland, reported the discovery of the genetic coding responsible for the inability of most adults to produce lactase, the enzyme needed to digest lactose, which is the primary sugar in milk.

Nearly everyone makes enough lactase in infancy to digest breast milk. But for many, the lactase gene switches off sometime after weaning. From then on, the consumption of milk, ice cream, cheesecake and other dairy products can bring nausea, painful cramps, diarrhea, bloating or gas.

"It's easy to say it's not really a problem, but some people are suffering," says Leena Peltonen, chairwoman of human genetics at UCLA, who led the study. Worse, the symptoms may mimic a serious digestive disease or malignancy.

"That's why people are keen to get the diagnosis," she says. Identification of the gene for lactose intolerance means there will one day be an easy diagnostic blood test.

But Peltonen's team found something even more fascinating.

In blood samples from 196 lactose-intolerant people of African, European and Asian descent, they found the same intolerance gene. Koreans, Finns, African-Americans - everybody had the same coding.

"If [a genetic feature] is found around the world, genetics tells us that it must be very old," Peltonen says. "Perhaps it was even in the genome of humans before they migrated out of Africa," that is, before modern humans differentiated into today's geographical "races."

So lactose intolerance isn't really a disorder; it's "normal," Peltonen says, and the ability to produce lactase into adulthood is a recent genetic mutation.

But the "mutants" must have enjoyed a survival advantage somewhere, or their ability to digest lactose would never have become as common as it is.

Such lactose "tolerance," it turns out, is most common among people of northern European descent. Seventy-five percent to 80 percent of them have no trouble with dairy products, compared with only 10 percent to 25 percent of African and Asian populations.

The mutation may have been present in a few individuals everywhere. But Peltonen suggests it was in northern climes, with only one harvest a year, where such people would have found a survival advantage in the year-round protein and calories available in the milk of goats, sheep and cows. Over the millennia, that's where the trait would have spread.

"It's an interesting example of the interplay between environment and genetics," she says.

Another discovery reported last month may reveal an interplay of genetics with early human migration.

In the Jan. 8 issue of the Proceedings of the National Academy of Science, a team of American and Chinese scientists reported evidence that a gene strongly associated with both attention deficit hyperactivity disorder (ADHD) and "novelty-seeking" behavior resulted from a spontaneous mutation just 10,000 to 40,000 years ago.

But what really caught the team's eye was that this gene - called the 7R allele - has since spread rapidly among the world's population. It was "positively selective" because it gave its recipients a survival advantage.

But what advantage? The 7R allele is part of the dopamine neurotransmitter system involved in movement, learning and responses to psychological rewards.

Kids with ADHD today have impulsive behavior problems. They can't sit still and have difficulty concentrating. People with the "novelty-seeking" trait are thrill-seekers and risk-takers. They frequently get mixed up with addictive drugs and alcohol.

The answers are still purely speculative. But Moyzis, the California biochemist and a member of the study team, says the timing of the 7R mutation coincided with a list of cultural innovations, and a restless surge of modern humans out of Africa and across the globe, displacing earlier populations.

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