Laird G. Jackson, 89, of Philadelphia and Lambertville, N.J., a physician-scientist who was one of the country’s earliest practicing geneticists, and who went on to advocate genetic counseling as an important part of patient care, died Thursday, Oct. 17, of complications from a stroke at his home.

Dr. Jackson’s introduction to the nascent field of medical genetics was serendipitous. In 1960, while seeing patients at Thomas Jefferson University Hospital, he met a 7-year-old girl with chronic myelogenous leukemia.

Curious about what caused the illness, he collaborated with physicians and researchers David Hungerford and Peter Nowell, who were just then discovering a link between the disorder and a chromosomal mutation that became known as the “Philadelphia chromosome.”

The connection was a landmark finding because it proved for the first time that a defective chromosome was consistently present in any kind of malignancy, wrote William J. Shiel Jr. on MedicineNet, a health-care website.

"That experience got me looking into chromosomes," Dr. Jackson told an interviewer for the American College of Medical Genetics and Genomics in 2017. "I was soon noticed by pediatricians who worked in the newborn nursery, who asked me to come check on a child who appeared to have Down syndrome. And pretty soon I became a geneticist before there were official geneticists."

Not only did Dr. Jackson become an expert on medical genetics, he also championed genetic counseling as a critical aspect of ethical and effective patient care long before the field existed.

Born in Seattle, Dr. Jackson graduated from high school at age 16. He earned a bachelor’s degree from Pomona College in Claremont, Calif., in 1951 and entered medical school at the suggestion of an adviser who said that his age would be advantageous in a field that required so many years of training.

"That was my guidance into medicine, without any relatives or family friends who worked as physicians," Dr. Jackson recalled in 2017. "It was serendipitous."

He earned a medical degree in 1955 from the University of Cincinnati School of Medicine. With three years of Air Force service behind him, he served an internal medicine residency in 1959 at what was then Jefferson Medical College.

Dr. Jackson completed a series of leukemia research fellowships and taught medical students at Jefferson in 1969 and 1970. His focus, though, was shifting to pediatric genetics and prenatal genetic testing.

At first, amniocentesis was the testing mechanism for genetic defects. When questions arose about its safety, an alternative was developed — sampling of cells taken from where the placenta attaches to the uterine wall.

Dr. Jackson spearheaded a clinical trial to test it in the United States, leading to a landmark paper published in the New England Journal of Medicine in 1989. He generated the proposal for a multicenter trial comparing various types of chromosomal testing. The results were published in the same journal in 2012.

In 2017, he collaborated with researchers developing tests to sample fetal cells from the mother's circulating blood.

Dr. Jackson is best known for his work on Cornelia de Lange syndrome, a rare developmental disorder that affects multiple parts of the body. As medical director for the Cornelia de Lange Syndrome Foundation, he traveled worldwide to get blood samples, said his wife, Marie.

At Jefferson, Dr. Jackson rose from instructor in 1962 to professor of medicine, pediatrics, and obstetrics and gynecology in 1978. He ran the division of medical genetics from 1969 to 1998.

In 2001, he was appointed a professor of obstetrics and gynecology in genetics at Drexel University College of Medicine, and a research associate in pediatric human genetics at Children’s Hospital of Philadelphia.

In 2017, Dr. Jackson received the ACMG Foundation for Genetic and Genomic Medicine’s David L. Rimoin Lifetime Achievement Award.

Nancy Spinner, chief of the division of genomic diagnostics at Children’s Hospital, once said of Dr. Jackson that he “shares his ideas freely, encouraging everyone around him, and in all of the time I’ve known him, I’ve never seen him seek recognition for the ideas he generates."

When not working, Dr. Jackson liked to split wood on his farm in Lambertville or walk the family’s poodles in Fairmount Park. He retired in 2016.

Besides his wife, he is survived by sons Todd and Scott, and two granddaughters. He was previously married to Joanne Jackson, who died earlier.

Services are private.

Donations may be made to Cool Cars for Kids via https://coolcarsforkids.org, and earmarked for the Jackson Award.