Washington ? In a major step toward one day offering gene-based customized medicine, scientists Thursday unveiled the first map of common human genetic variations — patterns of DNA differences that may help forecast people’s disease risks and best treatments.

This map, created by California-based Perlegen Sciences Inc., is essentially the first chapter in the quest to identify tiny bits of genetic information, known as SNPs or “snips,” believed key in creating gene-based medicine.

“It is a dramatic advance,” said Barbara Jasny, a senior editor at the journal Science, which published the research Thursday. The study also was presented at a meeting of the American Association for the Advancement of Science.

Public health experts view it “with a lot of excitement,” agreed Lawrence Lesko, who heads the Food and Drug Administration’s division of gene-based medicine.

Even unrelated people share DNA that is 99.9 percent similar. Variations in that last remaining bit are what make us individual, determining traits from our hair color to our risk of various diseases.

Until now, most genetics breakthroughs have come when a mutation in a single gene causes illness. But the most common health problems, such as heart disease, diabetes or depression, are caused by complex interactions between numerous genes and environmental or behavioral risks. Teasing out the genetic culprits under such conditions has been almost impossible.

Enter SNPs, single-nucleotide polymorphisms. DNA is made up of precise orders of chemicals identified with the letters A, T, C or G. SNPs are the most common type of genetic variation, sort of a spelling error — one of those letters gets out of order.

Even that tiny a difference may have profound effects.