Joe Hin Tjio, 82, a National Institutes of Health geneticist who more than 45 years ago became the first scientist to correctly count the number of chromosomes in a normal human cell, died of respiratory failure Nov. 27 at Asbury Methodist Village in Gaithersburg.

In the early morning hours of Dec. 22, 1955, in a laboratory at the Institute of Genetics of Sweden's University of Lund, Dr. Tjio was able to obtain a clear chromosome count from a microscopic photograph of human embryonic lung tissue, finding to his surprise that the cell had 46 chromosomes, not 48 as scientists had believed for 50 years.

This finding was published a month later in the Scandinavian journal Hereditas, one of the leading genetics journals. Because it was the first definitive count, it is considered to have been a critical development in the study of human chromosomes, which essentially are packages containing genes, which in turn are the units of heredity, directing all the basic body chemistry and the inheritance of characteristics from parents.

Body chemistry can easily malfunction in cases where even one gene is at variance from the norm. When whole packets of genes, chromosomes or chromosome parts are at variance, there can be extreme abnormalities. For example, not long after Dr. Tjio's chromosome count, it was discovered that Down syndrome in humans is the result of one extra chromosome.

For the last 37 years of his career, Dr. Tjio was a geneticist at NIH, where he specialized in cytogenetics, which is the microscopic study of chromosomes. In the later years of his career, his work involved the cytogenetics of cancer. He also had applied the ramifications of his work on chromosomes to the study of leukemia and mental retardation. He officially retired in 1992 but continued to work in his NIH lab for five more years.

Dr. Tjio was born in 1919 to Chinese parents in Java, which then was part of the Dutch East Indies. He was educated in Dutch colonial schools, which required him to learn to speak French, German, English and Dutch. He graduated from the Indonesian School of Agronomy.

During World War II, he was imprisoned in a Japanese concentration camp in Java, where he "knitted sweaters and underwear for the other prisoners," according to an article in the NIH Record in 1997.

When the war ended, he boarded a Red Cross displaced persons ship for the Netherlands, where the government had awarded him a fellowship for study in Europe. After six months at the Royal Danish Academy studying plant breeding, he became an associate at the Institute of Genetics in Sweden, where his work expanded to include mammalian tissues.

After publishing several scientific articles, Dr. Tjio was invited by the Spanish government to head a new program on plant improvement in Zaragoza, where from 1948 to 1959 he was director of cytogenetic research. But he returned to Sweden during holidays and in the summer to continue his work at the Institute of Genetics.

There, building on previously known techniques for separating chromosomes on glass slides, Dr. Tjio developed further improvements in the process that produced a clear microscopic photograph of an embryonic cell from human lung tissue containing the clear count of 46 chromosomes. "The number was just an incidental finding, like serendipity," he was quoted as saying in the NIH Record. "I was just surprised that it was not 48, as they had thought for so many years."

Urged by colleagues to publish his findings immediately, Dr. Tjio found himself in a conflict with his boss, Albert Levan, director of the Institute of Genetics, who, according to protocol, should have been listed as the first author in any publication resulting from research in his laboratory. But Levan had been on vacation when Dr. Tjio made his discovery.

"I told him, 'No.' I wouldn't allow him to be first author. I said, 'If you want to be the author, you do the work.' " Eventually, Levan relented, and the finding was published with Dr. Tjio as first author.

In 1958, Dr. Tjio came to the United States, studied for a year at the University of Colorado, where he received a doctorate in cytogenetics, then came to the Washington area and joined NIH. He became a U.S. citizen in 1966.

Until 1997, he lived on the Bethesda campus of NIH. "I wanted to remain within walking distance of my lab," he said.

His wife, Inga, said, "It was really because my husband never drove a car."

Dr. Tjio had consulted at universities in the United States, Germany and France. His awards included the Kennedy International Award in 1962, the Japan Society for the Promotion of Science Fellowship Award in 1984 and 1989 and honorary doctorates from Claude Bernard University in France and the University of Zaragoza in Spain. In 1990, China made him an honorary professor at the Third Army Medical College in Chungking.

In addition to his wife, of Asbury Methodist Village, survivors include a son, Yu-Hin Tjio of Silver Spring.