John W. Littlefield, a former chairman of pediatrics and physiology at the Johns Hopkins University School of Medicine whose research advanced the field of genetics, died April 20 at a retirement community in Cockeysville, Md. He was 91.

The cause was complications from dementia, the Baltimore Sun reported.

Dr. Littlefield’s accomplishments included playing a leading part in discovering the role of the ribosome in protein synthesis. He developed the technique of using amniocentesis to diagnose prenatal genetic disorders, and he helped pioneer the derivation and study of human stem cells.

John Walley Littlefield was born in Providence, R.I., on Dec. 3, 1925. He graduated from Harvard University and Harvard Medical School, and completed his medical studies in 1947 when he was 21.

Dr. Littlefield was called to active duty during the Korean War in 1952 and served as a doctor aboard a hospital ship stationed off the coast of Korea. He was a member of the Navy Reserve and was later recalled to active duty, stationed near the Arctic Circle.

After leaving the Navy, he joined the faculty at Harvard Medical School and the staff of Massachusetts General Hospital in Boston.

From 1957 to 1958, Dr. Littlefield and his wife lived in Cambridge, England, where he was a research assistant to James Watson and Francis Crick, who several years later earned the Nobel Prize in medicine for their study of the molecular structure of DNA.

After returning to Massachusetts General Hospital in 1958, he delved deeper into genetic research and achieved something family members said was a proud accomplishment for him — development of a method to isolate hybrid cells, a technique that would be used by researchers in genetic mapping.

In 1966, he was appointed chief of a new genetics unit at the Children’s Service at Massachusetts General. There he gained renown as a champion for genetics — then a new discipline that became recognized as a medical specialty in 1982.

During the 1960s, he also co-founded the Genetics Training Program at Harvard Medical School that trained scientists and clinicians and supported researchers in the field. He became a full professor at Harvard Medical School in 1970.

Three years later, Victor A. McKusick, who was known as the “father of medical genetics,” brought Dr. Littlefield to Hopkins. He assumed the position of professor and chairman of pediatrics at its school of medicine and pediatrician in chief of the children’s hospital of Johns Hopkins Hospital.

He continued his work on cultured cells while overseeing a facility that had 250 beds, 60 professors and saw more than 200,000 patients a year.

Family members said Dr. Littlefield’s greatest legacy may be his work in the use of amniocentesis — the isolation of fetal cells from the womb that diagnoses genetic disorders in fetuses. His innovation enabled women to be tested before they gave birth.

After Dr. Littlefield retired in 1992, he became an integral member of John D. Gearhart’s research team at Hopkins, which first identified and isolated human stem cells that were capable of forming all cell types in the body.

Dr. Littlefield, who also taught courses in ethics in genetics, was the author of more than 200 scientific publications.

His wife, the former Elizabeth Lascelles “Bette” Legge, died in 1995. Survivors include his companion, Nancy Warner of Baltimore; three children; and seven grandchildren.

— Baltimore Sun