BAR HARBOR — Researchers have found a genetic abnormality that could lead to the cause of the most common form of Alzheimer’s disease, which afflicts about 4 million Americans.

The finding was a surprise to most researchers studying the genetics of Alzheimer’s disease, who have been busily studying a different genetic abnormality.

“We’re betting the output of our laboratory” that the new abnormality is the cause of all but a rare form of Alzheimer’s disease, said the discoverer of the new finding, Dr. Allen Roses of the Duke University School of Medicine in Durham, N.C.

In a presentation at the Jackson Laboratory in Bar Harbor, Roses said Tuesday that Alzheimer’s disease appears to be linked to a genetic abnormality on chromosome 19, one of the 23 pairs of human chromosomes.

Previous studies had found a defect on chromosome 21 linked to a rare, early-onset form of Alzheimer’s disease in which memory loss and other symptoms begin to appear around age 40. That form of Alzheimer’s disease has been found in only 10 or 20 families in the world, Roses said.

Other researchers are now rapidly trying to determine whether their genetic samples from Alzheimer’s disease victims also reveal an abnormality on chromosome 19, said Dr. Peter St. George-Hyslop of the Massachusetts General Hospital in Boston. He is one of the discoverers of the defect on chromosome 21.

“The follow-up studies need to be done,” said St. George-Hyslop. “This area of genetics of late-onset neurological disease is extremely difficult to do.”

“We’re anxiously following this, and if it’s true, it would be quite significant,” said Creighton Phelps, vice president of medical and scientific affairs for the National Alzheimer’s Association in Chicago.

Once the genetic cause of Alzheimer’s disease is found, researchers will be able to understand precisely what is going wrong. That should lead them toward possible new treatments.

One of the problems is that Alzheimer’s disease normally appears in people who are in their 60s, 70s or 80s. Many people who carry a genetic predisposition for Alzheimer’s disease die of something else before they get old enough to develop Alzheimer’s disease, Roses said.