source: WebMD

Familial adenomatous polyposis (FAP) is a rare condition characterized by the presence of more than hundreds or even thousands of benign polyps, or growths in the large intestine. It is thought to be present in about 1% of all people diagnosed with colorectal cancer each year. The polyps occur early in life, with 95% of people with FAP developing polyps by age 35, and are often detected in patients in their teens, with 50% developing polyps by age 15. If the colon is not surgically removed, there is almost a 100% chance that some of the polyps will develop into cancer, usually by age 40.
While most cases of FAP are inherited, nearly a third of the cases are the result of a spontaneous (newly-occurring) gene mutation, or abnormality. For those who develop a new gene mutation, it is possible to pass the FAP gene onto their children.