NEW YORK (AP) _ Part of the chromosome linked to Down's syndrome also appears to play a key role in Alzheimer's disease, according to a study that found an extra copy of the component in Alzheimer's patients.

The report confirms and extends a recently reported finding that an extra copy of a gene on that chromosome appeared in some Alzheimer patients, scientists said.

But it is not yet known whether that gene lies within the piece of chromosome identified in the new study, said Dr. Miriam Schweber of the Boston University School of Medicine, who reported the new finding Wednesday.

Schweber said the finding holds the promise of testing for Alzheimer's before symptoms appear. Other scientists said it is too early to evaluate that potential.

Chromosomes are threadlike molecules in every cell of the body that carry genes like beads on a string. Normal individuals have two copies of each of 22 kinds of chromosomes, plus two chromosomes that determine sex.

An extra copy of chromosome 21 leads to Down's syndrome, a condition that causes varying degrees of retardation.

Schweber's research, reported at the annual meeting of the American Academy of Neurology, found an extra copy of part of chromosome 21 in each of 15 Alzheimer's patients tested. The abnormality did not appear in 12 normal people, she said.

The Alzheimer patients did not have Down's syndrome because the portion of the chromosome she identified does not appear to trigger that condition, she said.

Alzheimer's disease is a progressive, irreversible disorder afflicting an estimated 2.5 million Americans and killing more than 100,000 a year. Symptoms include gradual memory loss, impairment of judgment and of ability to perform routine tasks and loss of language skills. Victims eventually become incapable of caring for themselves.

Schweber said the extra chromosome portion was found not only in six patients who suffered an inherited form of the disease, but also in seven who appeared to have a non-inherited form. In two other patients, the form was not known.

Ten of the patients had shown onset of symptoms before age 65, and the other five after that age, she said.

The finding holds ''the promise of a diagnostic test'' to show people who will develop Alzheimer's later, she said.

But Dr. David Drachman of the University of Massachusetts, an Alzheimer expert, said he is ''exceedingly cautious about considering this to be a presymptomatic diagnostic test.''

It is not yet proven that everybody who shows the extra copy will get Alzheimer's, he said. The abnormality may be necessary for the disease but not enough to cause it by itself, or some cases may arise without the abnormality.

Dr. Dennis Selkoe of Harvard Medical School agreed, saying some environmental factors may be needed in addition to the abnormality to bring on the disease.

Nonetheless, Drachman said, ''this is an extremely interesting and very important observation that helps point our research towrd this piece of chromosome 21.''

Researchers must now find out what genes on that portion of the chromosome do, and try to relate them to Alzheimer's, he said.

He said Schweber's finding extends a recent report in Science magazine on an extra copy of a gene from chromosome 21 found in several Alzheimer patients. The gene directs the body's production of a normal protein called amyloid, and the extra copies may lead to damaging excess production, scientists say.

Amyloid is a key component of plaques found in the brains of Alzheimer and aged Down's syndrome patients.