One gene variant doubles the risk of age-related macular degeneration, the most common cause of blindness in developed countries, researchers said. A separate study found several common DNA variations that can raise heart disease risk in millions of people.

The findings are the latest made possible by genetic tools from two California-based companies, Affymetrix Inc. and Illumina Inc., that can analyze as many as 900,000 genetic variations in 48 hours. Scientists using the devices have found potential explanations for diseases in dozens of previously unexplored genes in the last few years. "These studies represent an important advance in medicine,'' wrote Jeffrey Drazen, editor of the New England Journal of Medicine, in an editorial. "They convey novel, unbiased information about the heritable basis of disease at a level of detail that has not been possible previously.''

Three of the studies are published in the New England Journal, and a fourth is in the journal Nature Genetics.

For some rare disorders, a faulty gene can act like an "on'' switch that determines a person's fate from conception. In most common diseases, however, genes are just one of many puzzle pieces -- along with influences such as diet and exposure to chemicals -- that result in illness.

The tools made by Santa Clara-based Affymetrix and San Diego-based Illumina identify markers that show which variant of a gene is present in an individual. These markers are called single nucleotide polymorphisms, or "snips.'' "This amounts to a full-employment act for cell biologists and physiologists to understand what these screens are telling us,'' Drazen said in a phone interview Thursday. "Understanding how these genetic variants lead to disease won't be easy.''

One study in the New England Journal pointed to a series of genes that raise patients' risk of coronary artery disease, a type of damage to the heart's blood vessels that affects 15 million Americans and often leads to heart attacks. The study confirmed earlier research from the U.K. and Germany pointing to the same genes, and identified at least four new genetic variants that may also be linked to heart disease.

The results "are likely to uncover unanticipated mechanisms in heart disease and will ultimately enhance our ability to care for patients,'' said Anthony Rosenzweig, a Harvard Medical School cardiologist in an editorial.

John Yates and Tina Sepp of the Cambridge Institute for Medical Research in the U.K. led the study of genes involved in macular degeneration, a deterioration of the macula, the most sensitive part of the eye. Comparing gene screens of 505 patients with those of 351 healthy people, they found that one version of an immune gene called C3 can increase the risk of macular degeneration from 70 percent to 160 percent.

The higher risk is associated with a variant of the gene called C3F that might increase inflammation in the eye. People who get two copies of the gene are more likely to get the disease than those who get one copy or none.

"It's exciting,'' said Margaret DeAngelis, a Harvard Medical School genetic epidemiologist at the Massachusetts Eye and Ear Institute in Boston, in a telephone interview Thursday. "Our group is looking at every gene and associated pathway, and this group got to it first.'Genome screening is also giving new insights into poorly understood disorders such as restless leg syndrome, an urge to move one's legs at night that often disrupts patients' sleep. Some researchers have questioned whether the disease, described in doctors' writings as long as 350 years ago, has any physiological basis.

Papers published in the New England Journal by researchers from DeCode Genetics Inc. of Iceland and in Nature Genetics by Max Planck Institute of Psychiatry scientists point to a gene called BTBD9 on chromosome 6 as a partial explanation for the disorder. The Max Planck paper, published in Nature Genetics, found three additional genes at other locations that raise the risk of the disorder.

Some scientists had said that restless leg syndrome was a myth, pointing out that it was far more common in developed countries in the West, where health care is more available than in developing Eastern nations. The BTBD9 variant is also more common in people of European descent, which may explain why the disease occurs in the West, said Kari Stefansson, DeCode's chief executive officer.