source: Mayo Clinic
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 your colon and rectum, usually starting by your mid-teens. The polyps are nearly 100 percent certain to develop into colon cancer or rectal cancer by the time you’re in your 40s.