source: Cleveland Clinic

FAP is an inherited condition, where polyps develop in the intestinal tract, but primarily it affects the large intestine, including the colon and rectum. It is due to a genetic mutation that interferes with the normal function of APC, an important gene that controls how quickly cells grow. The gene mutation is most often inherited from a parent, but 30 percent of FAP patients develop the mutation during conception and do not have a parent with FAP.
Those who inherit the gene mutation develop numerous adenomatous polyps, which are precancerous growths, in the lining of the colon and rectum. If undetected and untreated, one or more of the polyps will eventually become cancerous. Fortunately, with early diagnosis and treatment, FAP patients can avoid cancer and lead normal, healthy lives.