Dr. Peter C. Nowell, 88, formerly of Middletown Township, an acclaimed researcher and codiscoverer of the Philadelphia chromosome, which pointed for the first time to a genetic basis for cancer, died Dec. 26 of complications from Alzheimer's disease at White Horse Village, Newtown Square.

In 1960, while working in their laboratory, Dr. Nowell and colleague David A. Hungerford were "diddling around with leukemic cells in culture, and rinsing them with tap water." Dr. Nowell noticed that the cells were dividing, so he stained them with a special dye to make their chromosomes more visible, according to an account by the Lasker Foundation, an advocate for biomedical research.

The two men then made the startling observation that the number 22 chromosome in the tumor cells of individuals suffering from chronic myelogenous leukemia (CML) was abnormally small.

The discovery came at a time when genetics as a basis for cancer was not generally accepted, according to the foundation. The research broke new ground because it was the first consistent chromosome abnormality found in any kind of malignancy.

"The tiny Philadelphia chromosome became a clear and consistent marker of CML, a cancer of the myeloid or bone marrow cells, with broad implications for diagnosis and prognosis of disease," the foundation observed when, years later, it honored Dr. Nowell for his contribution to science.

At the time of the discovery, he was employed by the University of Pennsylvania School of Medicine, and Hungerford by Fox Chase Cancer Center's Institute for Cancer Research.

"It would take doctors and researchers around the world more than three decades to unravel the implications of this landmark discovery," wrote Jessica Wapner in her book The Philadelphia Chromosome: A Genetic Mystery, a Lethal Cancer, and the Improbable Invention of a Lifesaving Treatment.

"He lived long enough to see it developed into treatment to allow individuals to lead longer lives," said his son, Michael. Hungerford, of Jenkintown, died of lung cancer in 1993 at age 66.

Born in Philadelphia to Foster Nowell and Margaret Moore Matlack Nowell, Dr. Nowell grew up in Rose Tree, Delaware County.

He attended the progressive School in Rose Valley and graduated in 1945 from Swarthmore High School before earning a fast-tracked bachelor of science in biochemistry from Wesleyan University in Middletown, Conn.

Dr. Nowell told his family that after reading the 1926 book Microbe Hunters by Paul Henry de Kruif, he chose the field of science. Later, he narrowed his focus to medicine, his son said.

He earned his medical degree from the University of Pennsylvania School of Medicine in 1952, did a rotating internship at Philadelphia General Hospital, and trained in pathology at Presbyterian Hospital.

He spent two years at the U.S. Naval Radiological Defense Laboratory in San Francisco, studying radiation and bone marrow transplantation, before returning to the University of Pennsylvania School of Medicine as an instructor, and later as a professor in the Department of Pathology. He served as chairman of the department from 1967 to 1973 and was the first director of the University of Pennsylvania Cancer Center, now known as the Abramson Cancer Center.

He received the Lindback Distinguished Teaching Award, the 1998 Albert Lasker Clinical Medical Research Award, and the Benjamin Franklin Medal in Life Science. In 2015, the endowed Peter C. Nowell, M.D. Professorship was established in his honor at the University of Pennsylvania School of Medicine.

Despite the accolades, Dr. Nowell was short on hubris and long on self-effacing humor, his family said.

He liked to remind listeners that he was always a third stringer on any athletic bench; that his ineptitude with hand tools was legendary; and that his medical students had to suffer through his frequent puns.

"The teasing, humorous patter that became his signature, be it on the tennis court or in the boardroom, was with him to the end," his family said.

He was married in 1950 to Helen Walker Worst Nowell, a high school classmate. They moved to Middletown Township, Delaware County, in 1957 and reared five children. His wife died in 2004, and their daughter, Sharon, died in 2000.

Besides his son, he is survived by another son, Timothy; daughters Karen King and Kristin; a brother; and seven grandchildren. A sister and brother also died earlier.

Plans for a memorial service were pending. Interment will be private.

Contributions may be made to the School in Rose Valley, 20 School Lane, Rose Valley, Pa. 19063; or to Elwyn, 111 Elwyn Rd., Media, Pa. 19063.