Familial adenomatous polyposis (FAP) is a rare, inherited condition that causes extra tissue (polyps) to form in your large intestine (colon) and rectum.Polyps can also occur in the upper gastrointestinal tract, especially the upper part of your small intestine (duodenum).f untreated, the polyps in the colon and rectum are likely to become cancerous in your 40s. Most people with familial adenomatous polyposis eventually need surgery to remove the large intestine to prevent cancer.