To understand how invaluable SNPs are in tracking down mutations that cause disease, you have to appreciate the immense size of genome.

Consider this: If each of the DNA molecules in our genome was about the size of a ping pong ball, the long unraveled chain of molecules would circle the earth 3 times, or just over 75,000 miles. The real difficulty is that less than 2 percent of that — about 1500 miles, or a little less than the distance from Los Angeles to Chicago — is DNA that we know codes for proteins.

These protein-coding areas are what has traditionally been referred to as "genes." But those 1500 miles worth of "genes" aren't all in a row. Genes are scattered throughout the genome, and in between them is the so-called "junk" DNA. Since scientists estimate that genes are on average about 600 base pairs long, a gene on our global ping pong scale would be 24 meters (80 feet) long. Given a genome that wraps around the world three times, 24 meters is miniscule. If you were walking — or swimming — the entire trip, you'd be likely to encounter a gene an average of once every 2.5 miles (4 kilometers).

Because the genome is so immense, it is practically impossible to find a specific gene or disease-causing mutation without having a rough idea of where to begin looking. Searching for disease genes without SNPs would be like searching for an address without a postal code. The address could be anywhere in the US and you would have no clue where to start. But with a postal code, you could narrow your geographic area and then methodically search a local map to find the street. SNP analysis does the same thing, reduces the possibilities so that researchers can better focus their search and find the disease'causing mutation they are looking for within the vastness of the human genome.