Decades after he sat by the man’s bedside, Henry T. Lynch remembered the patient whose story helped persuade him — despite widespread doubt across the medical establishment at the time — that there had to be a hereditary component to some forms of cancer.

The man was an alcoholic admitted to the hospital in Omaha where Dr. Lynch was a medical resident in 1962. “Everyone in his family had died of colon cancer,” Dr. Lynch recalled. “It was why he drank. He was convinced he was going to die.”

That patient’s family was one of more than 3,000 families around the world — from the farming communities of Nebraska to the Middle East — that Dr. Lynch studied over his nearly six-decade career, painstakingly charting the spread of cancer through the branches of family trees and down through generations.

Years before the advent of the genetic testing that later proved his theories, such handmade family histories allowed Dr. Lynch to identify hereditary links to certain types of colon cancer, breast and ovarian cancers, and other forms of the disease.

Known as the father of cancer genetics, he was credited with saving thousands of lives by championing screening practices that help physicians catch cancers early in their course, as well as preventive surgeries that in some cases forestall the disease’s onset altogether. Anyone who has undergone a physical examination and completed a questionnaire about family history of cancer is the beneficiary of his research.

Dr. Lynch died June 2 at a hospice center in Omaha at 91. His death was announced by Creighton University in Omaha, where he spent nearly his entire career, and where he founded the Hereditary Cancer Center in 1984. The cause was congestive heart failure, said his son, Patrick Lynch.

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“Henry Lynch occupies a distinguished place in the pantheon of the greatest cancer geneticists of the modern era,” Kenneth Offit, chief of the Clinical Genetics Service at the Memorial Sloan Kettering Cancer Center in New York City, said in an interview. “He defined the hereditary basis of common human cancers during a period when these views were considered heretical and lived to see the genetic basis of cancers become part of the practice of preventive medicine.”

Dr. Lynch took a roundabout path into the annals of medicine. Born to what he described to the Omaha World-Herald as a “downcast Irish” family, he grew up in a rough-and-tumble neighborhood of New York City and lied about his age to join the Navy when he was 15 or 16. (Accounts vary.)

He served in the South Pacific during World War II before becoming a prizefighter known, at 6-foot-5, as “Hammerin’ Hank.” After obtaining a high school equivalency diploma, he pursued university studies culminating in a medical degree.

When Dr. Lynch began his career, most scientists blamed cancer on environmental causes, such as the presence of carcinogenic chemicals, and viruses. Hereditary explanations were dismissed, even shunned. Some doctors feared that if patients were told that cancer ran in their families, they would become resigned to what they perceived as their fate and resist lifestyle changes such as quitting smoking or adopting a healthier diet.

“Nobody believed me,” Dr. Lynch later remarked, according to Creighton University. “But I knew we had something here. I knew we could potentially save lives.”

Dr. Lynch was credited with providing, in the 1960s, the first complete description of hereditary non-polyposis colorectal cancer, a form of colon cancer eventually renamed Lynch syndrome. Lynch syndrome patients also have an increased risk of endometrial, ovarian, stomach, pancreatic and other forms of cancer.

In 1971, he identified a hereditary form of breast and ovarian cancers, which in the 1990s was linked to the BRCA genes.

Recent research has shown up to 10 percent of cancers to be inherited, said Sapna Syngal, founder of the newly launched Lynch Syndrome Center at the Dana-Farber Cancer Institute in Boston, describing Dr. Lynch as a “visionary” with a “sense of something that nobody else [knew].” But in the critical early years of his career, he often struggled to secure funding for his radical research.

He attributed the resistance he encountered at least in part to “the prejudice of major East-coast institutions against a small, mid-western university that challenged the orthodoxy,” David Cantor, a historian of medicine, wrote in the journal Medical History. But whatever its distance from major research institutions, his Nebraska community presented advantages.

Rural Nebraska families “kept careful records of their kindred” and rarely strayed far from their communities, even over generations, Cantor noted. Furthermore, they had what Dr. Lynch called a “deep-seated pioneering tradition and philosophy in support of worthwhile pursuits.”

“Critics may have looked down on Nebraska,” Cantor observed, “but Nebraska was to be the saving of Lynch.”

Dr. Lynch conducted his family medical histories with obsessive devotion, convening reunions and going door to door to locate relatives. He pored over family bibles, medical records, autopsy records and census data. The result of his efforts was one of the world’s largest databases of family cancer histories.

Besides colon and breast and ovarian cancers, Dr. Lynch documented hereditary forms of melanoma and prostate and pancreatic cancers.

He promoted early detection techniques, such as frequent colonoscopies for patients with Lynch syndrome, as well as preventive mastectomies and oophorectomies, or the removal of the ovaries, for some patients with hereditary forms of breast and ovarian cancer. The actress Angelina Jolie brought widespread attention to such procedures when she announced in 2013 that she carried a mutation in the BRCA1 gene and had undergone a preventive mastectomy.

“In these families,” Offit said, “preventive surgeries save lives.”

Henry Thompson Lynch was born in Lawrence, Mass., on Jan. 4, 1928, and grew up in New York City, where he honed his future boxing skills.

After abandoning his postwar fighting career — he “didn’t have the stamina in the legs to be a serious boxer,” his son said — he received a bachelor’s degree from the University of Oklahoma in 1951 and a master’s degree in clinical psychology from the University of Denver in 1952.

He began a doctoral degree in human genetics at the University of Texas at Austin before moving to the University of Texas Medical Branch at Galveston, where he received a medical degree in 1960. He joined Creighton in 1967.

The former Jane Smith, Dr. Lynch’s wife of 60 years and a psychiatric nurse who assisted him in his research, died in 2012. Survivors include three children, Patrick Lynch of Houston, Kathy Pinder of Corona, Calif., and Ann Kelly of Redondo Beach, Calif.; two brothers; 10 grandchildren; and nine great-grandchildren.

When Dr. Lynch counseled families with high incidence of cancer, he sought to assuage their anxieties and fears, emphasizing that early detection of the disease could save their lives. He took phone calls and requests from patients around the world, often reporting to work at 3:30 a.m. for the fullest day of research.

In the 1980s, according to his university, Dr. Lynch severely injured his back but refused to cancel a scheduled lecture. An assistant rolled him into the classroom on a gurney so he could speak to the students.

An earlier version of this obituary incorrectly reported that the actress Angelina Jolie underwent a preventive mastectomy because she carried the BRCA1 gene. She underwent the procedure because she carries a mutation in the BRCA1.