Familial adenomatous polyposis is a rare, inherited condition that causes extra tissue (polyps) to form in large intestine (colon) and rectum. Polyps can also occur in the upper gastrointestinal tract, especially the upper part of small intestine (duodenum). If untreated, the polyps in the colon and rectum are likely to become cancerous in 40s. Familial adenomatous polyposis is caused by a defect in the adenomatous polyposis coli (APC) gene. Most people inherit the genetic abnormality from a parent. But in about 25 percent of cases, the genetic mutation occurs spontaneously. The abnormal gene causes hundreds or even thousands of polyps to grow in colon and rectum, usually starting by mid-teens. The polyps are nearly 100 percent certain to develop into colon cancer or rectal cancer by the time you're in 40s.