(Agencies)
Updated: 2008-03-31 09:27

What's made this and other hopeful findings possible is the "genome-wide association study," which lets scientists scan the entire complement of DNA from thousands of people in unprecedented detail. While the basic technique is not new, its popularity has exploded recently because of cost-cutting advances in technology and discoveries about the genome.

"It lets you go searching for that needle in the haystack," says Michael Watson, executive director of the American College of Medical Genetics.

It's a big haystack. DNA is made up of long sequences of building blocks, sort of like sentences composed from a four-letter alphabet: A, C, G and T. The human genome contains about 3 billion letters, about as many as the total number of letters and digits in more than 100 Manhattan phone books.

Scientists have identified the order of the letters in the human genome, a feat the government declared accomplished in 2003. But of course, different people have slightly different DNA sequences. People commonly differ in what letter they have at about 10 million positions along the full genome. Some folks may have a T where most people have a C, for example.

And those single-letter variations are key to the genome-wide scans. Basically, scientists compare DNA from a large number of people, some sick with a particular disease, and others healthy. They can look at a half-million or more positions to see what letter appears. If sick people tend to show a different result than healthy ones -- say, if they tend to have a T in some spot more often than healthy people do -- it's a red flag.

It suggests that some genetic influence on the risk of that disease comes from that spot or nearby. So it gives scientists a specific place to look more closely for a disease-promoting gene.

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