About 5–10% of bowel and womb cancers are believed to be hereditary, ie can be linked to a distinct mutation in one of the known bowel cancer genes. This means that about nine out of ten people with bowel cancer will not have the inherited type of bowel cancer. In families who have a change in one of the known bowel cancer genes there will be a number of family members with bowel cancer. There will probably be two or more affected relatives with bowel cancer in at least two generations. (See the sample family tree).

So far, three types of inherited bowel cancer have been found:

Familial adenomatous polyposis (FAP)

With FAP, hundreds of initially benign (non-cancerous) polyps are found in the bowel. This happens at a relatively young age: people are often in their teens. If not treated, some of these polyps will eventually develop into cancer. The gene involved is called the APC gene and if a person has inherited a mutation in this gene, they are almost certain to develop multiple polyps as a young adult.

Family members who are at risk may be screened yearly from their early teens by colonoscopy. Colonoscopy is where a thin flexible tube containing a tiny camera is inserted into the bowel to look at the inside of it. People affected by FAP may choose to have preventative surgery (removal of the bowel).

Hereditary non-polyposis colorectal cancer (HNPCC)

With HNPCC there are fewer polyps growing in the bowel, but there is still a tendency to develop tumours early in life. In women, a mutation in one of the HNPCC genes can also cause endometrial cancer (cancer of the lining of the womb). The HNPCC genetic mutations have also been associated with a higher risk of stomach, pancreas, ovarian and kidney cancer.

MYH associated polyposis (MAP)

A third inherited bowel cancer syndrome recently identified is called MYH associated polyposis (MAP). Here, a person needs two faulty copies of the MYH gene to be at increased risk (in the above cases only one is needed). This makes it far less likely for children to inherit the susceptibility from their parents, because both their parents would need to carry the same genetic mutation.

It is quite likely that more genes will be identified in the future which, together with environmental factors like diet, contribute to the development of bowel cancer.

Bowel cancer is a common cancer. It is the third most common cancer in the UK for men and the second most common cancer for women. Every year more than 30,000 people will develop it. So if you just have one elderly relative who had bowel cancer, it is unlikely that you will inherit any increased risk.

You might want to talk to your GP, particularly if:

• you have a close relative who developed bowel cancer at a young age (under 45), or
• you have two or more close relatives on the same side of the family with bowel cancer (or cancer of the womb, kidney, ovary, stomach or pancreas) at any age.

Close relatives are your parents, children, brothers, sisters, aunts, uncles and grandparents. The GP will assess your family history and might refer you to a family cancer clinic or for genetic counselling.

If, due to your family history, doctors think that you are at significantly higher than average risk of developing bowel cancer, you will be offered screening. Screening can help to detect polyps, or bowel cancer, at an early stage, when treatment is more successful. Screening for bowel cancer is done using a technique called colonoscopy, where a thin flexible tube with a camera is inserted into your bowel through your rectum (back passage). This allows the doctor or nurse to see whether there are polyps or tumours developing inside your bowel.

Small polyps can be painlessly removed during the procedure. If the screening picks up a tumour or any pre-cancerous changes, doctors will usually recommend that the affected section, or even the whole bowel, is removed by surgery.

If you have bowel cancer, or you have a relative with bowel cancer who is willing to be tested, you might be offered genetic testing. This will depend on whether your counsellor thinks that your family is likely to have a genetic mutation in one of the known bowel cancer genes. Testing is a two-step process: looking for the genetic mutation running in your family (mutation searching) and then using that information to look for the mutation in other family members who don't have cancer (predictive testing).

A different type of genetic test is also available for bowel cancer. The tumour cells can be tested for a genetic trait called instability of the DNA. If this is present in a tumour from yourself or one of your family members, it makes it more likely that the bowel cancers in your family are hereditary. However, it is not a precise testing method and is used more as a sign that the cancer may be caused by an inherited cancer gene, rather than as a conclusive genetic test.