Researchers have located a gene that triggers a form of depression, establishing biological proof of the theory that it is an inheritable, genetically based disease.

The findings, announced at a news conference yesterday by researchers from three universities, represent the first time that a major mental disorder, manic-depression, has been traced to a defective gene. The research has been published in the Feb. 25 issue of the British journal Nature.

Locating the genes that cause disorders can lead to an understanding of their underlying brain chemistry. It may also enable scientists, through genetic screening, to determine when someone is prone to a disease.

Rex Cowdry, clinical director of the National Institute of Mental Health, which funded the research and sponsored the news conference in Bethesda, said that "if there was anyone who doubted still the biological nature of mental illness, this is a critical demonstration of that fact."

He said that it will help patients and others understand that mental illness is not a problem of will or a failure of self-control, but a medical condition like heart disease or high blood pressure.

"This finding heralds a new era in the application of genetics to psychiatric disorders," said Dr. Herbert Pardes, director of the New York State Psychiatric Institute at Columbia University.

He said he expects the finding to encourage many laboratories to work on the genetics of mental disorders, which researchers have shied away from because of the complexity of symptoms and disagreement in diagnosis of psychiatric illness.

"The elegance of this approach," Pardes said, is that by manipulating the gene "you can literally see the exact {chemical} effects it has . . . like when you turn off a switch in a fuse box you can immediately see just which lights go off."

Manic-depression is one form of severe, or clinical, depression, as distinguished from everyday mood swings or grief, which are normal. Symptoms of depression include prolonged loss of interest in food, sex, and other important parts of life, along with disruption in normal sleep and obsessive thoughts of suicide.

Manic-depressives swing between depression and the hyperactive state of mania, characterized by such symptoms as unfounded euphoria, rapid talking, abundant physical energy, and attempts to carry out grandiose ideas.

Yesterday's report comes within a week of a similar discovery for Alzheimer's disease, a degenerative disease of the brain cells that often leads to mental impairment as well.

The studies reported yesterday were based on the oldest Amish community in America, in Lancaster County, Pa., said Dr. Janice A. Egeland, professor of psychiatry at the University of Miami, leader of the several research teams that carried out the work.

Depression occurs at about the same rate among the Amish as in the rest of the U.S. population, but the Amish were better subjects for her 10 years of research because they have a closed community, with complete genealogical records and few factors that might confuse diagnosis, such as alcoholism or violence.

About one in a hundred among the Amish and in the general population -- between 1 million and 2 million people in the United States -- suffer from manic-depressive psychosis.

That depressive illness runs in families has been suspected for many years, but this study demonstrated that fact by isolating a piece of DNA common in all the manic-depressives in one extended Amish family. (DNA, deoxyribonucleic acid, carries the genetic code that provides instructions for the daily operation of all the cells in the body.)

The gene was passed to children half the time. Of those receiving it, as many as 80 percent were shown to have had at least one manic-depressive episode or related mental problem in their lifetimes, said Dr. David E. Housman, a biologist at the Massachusetts Institute of Technology and one of the leading authors of the study.

Having the gene does not always cause manic-depression, but it makes people highly vulnerable to it, the researchers said. It is unclear what keeps some people with the gene from having depression, whether it is the action of some other chemical or the fact that those unaffected have not had the necessary combination of stressful events in their lives.

The paper in Nature reported finding two "markers" in the genetic code that were found to be inherited along with the predisposition for the disease. These markers are believed to be quite near the gene causing the manic-depressive susceptibility.

Dr. Kenneth K. Kidd, professor of genetics and psychiatry at Yale University School of Medicine, said yesterday that the finding can be used to locate the defective gene and experiment with it to determine what it does that creates the problem.

The researchers have now identified one suspect gene, one which makes a chemical called tyrosine hydroxylase. The chemical affects the production of dopamine, which has previously been suspected as a key chemical related to some mental disorders, especially schizophrenia.

Kidd also suggested that genetic markers may be used to identify people who have the defective gene and thus are susceptible to manic-depression. The gene implicated in the study of the Amish families was found to be on chromosome 11, one of the 23 human chromosomes that contain the entire human genetic code.

Another study reported at the news conference yesterday showed that chromosome 11 is not responsible for all manic-depression.