Joan Marks, a pioneer in the field of  genetic counseling, the practice of helping patients understand their risk of an inherited medical condition, died Sept. 14 at her home in Manhattan. She was 91.

Ms. Marks was the director of the graduate program at Sarah Lawrence College in Bronxville, NY, for 26 years. During her tenure, she developed it into the largest such program in the country, which it remains, and helped to establish a new health care field. Today, there are thousands of certified genetic counselors in the U.S., trained to help patients and their families confront a variety of inherited conditions.

Genetic testing was once primarily used for diagnosing genetic defects in fetuses and newborns. But by the mid-1990s, testing was able to predict the risk of developing a variety of adult conditions, including breast cancer and ovarian cancer. Today, more advanced tests can detect more than 6,000 genetic disorders, according to the Genetic Disease Foundation. The need for trained counselors to help patients understand the test results has increased exponentially.