Cancer and genes

Diagram of DNA helix molecule

People often worry that cancer might run in the family because a number of relatives have had it. However, only 5-10% of cancers are clearly linked to a genetic cause. 

People are born with an increased risk of cancer because they have inherited a change in the genes that help protect against cancer. 

For more information see our factsheet Cancer and genes, which gives details about genetic testing, genetic counselling, pros and cons of testing, emotional support and what happens after genetic testing.

Did you know? Only 5-10% of cancers are clearly linked to a genetic cause.

Assessing your risk of cancer

You may have an inherited genetic cause for cancer in your family if: 

  • You have a close relative who has been diagnosed with two separate cancers. For example, bowel cancer and endometrial (womb) cancer.
  • Two or more close relatives on the same side of your family have had the same cancer.
  • You or one of your relatives have a cancer most commonly seen in the opposite sex (for example, male breast cancer).
  • You have Ashkenazi Jewish ancestry.
  • You have had cancer at a younger age than commonly seen (for example, bowel cancer under the age of 50).

The list is a guide only.

Worried about inherited cancer?

If you are worried that cancer might run in your family, visit your GP. If your doctor thinks that there may be an inherited cause for cancer, based on your family history, they will refer you to a genetics clinic. 

The clinic will do a detailed review of your family history of cancer. Following this review, they will tell you about you level of risk and whether you and your close blood relatives need genetic testing.

Genetic counselling

Finding out that you have inherited a gene change that increases your cancer risk can have a huge impact on your future and that of your family. For this reason, genetic counselling is a very important part of your referral to the genetics clinic. 

Your genetic counselling sessions will include:

  • A detailed look at the medical history of your close blood relatives. The counsellor will draw up a family tree and use this to assess your cancer risk. 
  • A discussion about genetic testing, including the risks, benefits and limitations 
  • Information on current legislation about genetic testing, insurance and confidentiality
  • Information on cancer screening recommendations and referrals
  • A discussion about informing relatives of a positive result and the best way to do this
  • Support and advice for the future if the genetic test identifies an inherited genetic cause for cancer in your family
  • Information about relevant patient support groups 
  • A written summary of the consultation

What is genetic testing?

Genetic testing is when the laboratory examines a blood sample for an inherited cause for cancer in your family.

There are 2 main types of tests:

  1. Diagnostic testing looks for a genetic change in families with a strong history of cancer. It is usually done on a family member who has had cancer. If a genetic change is found, other members of the family can then choose to have a predictive genetic test.
  2. Predictive genetic testing is carried out in a family where there is a known genetic change. The genetics team therefore knows which specific gene change to look for and where to find it. 

If a genetic change is found

If a genetic change is found, this means that you are at a higher risk than the general population of developing cancer. 

Telling your family can be a difficult task. The genetics team will give you a letter to give to your family. They can then decide to have genetic counselling and possibly have genetic testing themselves. Most genetic changes are not usually associated with cancer risk in children or teenagers, so genetic testing is not usually done until after the age of 18.

Inherited cancers

Inherited breast cancer and ovarian cancer

Breast cancer is the most common cancer in women in Ireland. Having one or even two older relatives with this cancer is unlikely to increase your risk. 

The genes most commonly associated with a high risk of developing breast or ovarian cancer are called BRCA1 and BRCA2. 

BRCA gene changes are also linked to other cancers, such as male breast cancer or prostate cancer. 

Read more about breast cancer. 

Inherited bowel cancer

Almost 2,500 Irish people are diagnosed with bowel cancer each year. Having one older relative with bowel cancer is unlikely to increase your risk. Only 5% of bowel cancers are related to inherited genetic changes. 

The 2 main types of inherited bowel cancer are: Lynch syndrome / hereditary non-polyposis colorectal cancer (HNPCC) and familial adenomatous polyposis (FAP).

  • Lynch syndrome / HNPCC is the most common gene change that causes bowel cancer. It causes non-cancerous polyps that can lead to cancer if left untreated. Patients with Lynch syndrome tend to get bowel cancer at a younger age. It also increases the risk of other cancers. 
  • Familial adenomatous polyposis (FAP) is a rare genetic condition where hundreds or thousands of polyps develop in the large bowel (colon). These polyps or adenomas start to grow during teenage years. If left untreated, they can develop into cancer.

Read more about bowel cancer.

For more information:
Cancer and genes factsheet
Cancer and genes factsheet
This factsheet is for people who are worried that cancer might run in their family because a number of relatives have had it. It is also for people who are considering genetic testing.

For more information

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