Dr. James V. Neel, whose work helped uncover the genetic basis of sickle cell anemia and who led extensive studies of radiation's effects on the survivors of the atomic bombing of Hiroshima and Nagasaki, died on Tuesday at his home in Ann Arbor, Mich. He was 84.

The cause was cancer, said the University of Michigan, where he was an emeritus professor of human genetics and internal medicine.

In an approach that seems brilliantly prescient now that scientists are poised to map the entire human genome and so lay bare the genetic basis for many diseases, Dr. Neel foresaw the importance of inherited factors in understanding and treating a wide range of disorders, said Dr. William J. Schull, an emeritus professor at the University of Texas Human Genetics Center.

Dr. Neel's most recent research focused on viruses perhaps associated with genetic mutations that some researchers speculate could lead to cancer. He is also known as the founder of one of the nation's first departments of human genetics, at the University of Michigan, and as the director of an early clinic for counseling patients about genetic diseases they may carry.

For all those reasons, said Dr. Francis S. Collins, director of the National Human Genome Research Institute, ''Dr. Neel can be said to have birthed the field of human genetics.''

James Van Gundia Neel was born in Hamilton, Ohio, on March 22, 1915. Reared largely by his mother, after his father's death, he earned an undergraduate degree from the College of Wooster in Ohio in 1935. He moved to the University of Rochester in New York, where he received a Ph.D. and an M.D.

Soon after Dr. Neel completed his residency in medicine at Rochester, President Harry S. Truman authorized the National Research Council and the Atomic Energy Commission to ''undertake a continuing study of the medical and biological effects of the atomic bomb on man.'' This meant studying survivors of Hiroshima and Nagasaki, both of which Truman had ordered destroyed with atomic weapons.

In September 1947, Dr. Neel was named acting director of field studies for the council's Atomic Bomb Casualty Commission, which set about examining thousands of survivors for evidence of genetic damage caused by radiation.

''While we knew that DNA was the ultimate source of genetic information, we clearly didn't have the tools to explore variation in DNA in any kind of biochemical way,'' said Dr. Schull, who joined the study team two years later.

So, Dr. Schull said, the commission looked for evidence of genetic mutations through basic physical exams of exposed people and their children. The study was especially thorough because, at a time of severe food shortages in Japan, the commission registered people for examination when they signed up for food rations.

Aside from the scientific reasons for the study, Dr. Schull said, it was undertaken to ''give us some sense of the potential public health problem'' and how to prepare for it, if it occurred. As it turned out, once people had recovered from the immediate effects of radiation sickness, they did not show an unusually large number of catastrophic results like early death or stillborn children. In reporting on the findings, Dr. Neel emphasized that the children of survivors showed no abnormalities that might be attributable to damaged genes.

Years later, however, increased incidence of leukemia and tumors did turn up in people who had themselves been exposed to the radiation of Hiroshima and Nagasaki.

In the late 1940's, Dr. Neel also began studying families of African-Americans particularly prone to the blood disease called sickle cell anemia. He was the first to show that people with the most severe form of the disease had inherited two genes for the trait, a discovery that proved immensely valuable in providing couples reliable statistics on the likelihood that their offspring would have the disease.

Later, during the 1960's, Dr. Neel introduced the concept of the ''thrifty gene.'' That concept held that genes for disorders like obesity and diabetes were actually useful at an earlier stage of human history, when food was much less abundant. More recently he and colleagues studied severe genetic damage in what he called ''rogue cells'' in some human populations, including the Yanomama tribe in the Amazon.

''He asked, 'Could viruses have something to do with these chromosomal aberrations?' '' said Dr. Eugene O. Major of the National Institute of Neurological Disorders and Stroke, who collaborated with Dr. Neel on the work. ''The answer seems to be, 'Yes, they could.' ''

The evidence came by studying people who had been infected with the viruses and carried the rogue cells, and by directly observing genetic damage caused by viruses in the lab.

One speculative possibility is that viruses related to those could cause genetic damage that leads to cancer, Dr. Major said.

Dr. Neel is survived by his wife, Priscilla; a daughter, Frances Neel of Ann Arbor; two sons, James, of Santa Rosa, Calif., and Alexander, of Dodge City, Kan.; a sister, Mary Ann Blackwood of Atlanta, and three grandchildren.

Dr. Neel's influence is likely to grow with today's intense worldwide interest in genetics. In fact, as the genome project maps out the entire length of human DNA with the goal of laying bare its workings, and its disorders, Dr. Neel's focus on inherited diseases is looking more and more like a major theme of clinical medicine in the new century, Dr. Schull said.

''The problems that were once associated with infectious diseases have become, in the main, significantly less,'' he said, adding that this development supports ''very strongly Jim's vision that genetic disease was going to become a very large and very significant fraction of the health care problems to which persons are prey.''