Familial adenomatous polyposis: A genetic disease characterized by the presence of numerous precancerous polyps in the colon and rectum. The polyps usually begin to form at puberty, and colon cancer almost always develops later in life. Abbreviated FAP. FAP is inherited as an autosomal dominant trait. Most people who receive the gene manifest the disease, although the expression of FAP can vary markedly from person to person. The gene that is mutated in FAP is the APC (adenomatous polyposis coli) gene on chromosome 5. Surgery is often necessary to remove the colon in order to prevent the development of cancer. A milder type of familial adenomatous polyposis has been identified that is inherited in an autosomal recessive manner. This is referred to as autosomal recessive familial adenomatous polyposis and is caused by mutations in a different gene known as MUTYH. Also known as familial polyposis, familial polyposis coli.