From: Lynn Haney, Gregory Peck: A Charmed Life, Carroll & Graf Publishers: New York, NY (2003), page 148:
Darryl Zanuck harbored a burning desire to make a breakthrough film. In the case of a movie about anti-Semitism, he had a personal reason to hand over $75,000 for the rights to Laura Z Hobson's best-selling novel on which the film was based. Although one of the few movie moguls who was not Jewish (insiders referred to Fox as the goy studio), he had an experience as a young man starting out in Hollywood in which he was the target of just the kind of anti-Semitism portrayed in the novel [that Zanuck made into the movie A Gentleman's Agreement].
When one works in the film industry, assumptions are made. How could Zanuck be in the same business as Zukor, Mayer, Cohn and Warner and not be Jewish? Yet, he clearly wasn't.