Family genetics

While all cancers are caused by the disruption of genes, we know that approximately 5 to 10% of cancers are related to specific inherited genetic abnormalities.


There are scientific methods to identify some of these inherited defects and show whether some people have an increased chance of developing certain types of cancer. These cancers happen more often in some families than in others.


There are tests available that can identify if you are at increased risk. Let your healthcare provider know if any of your close relatives (parents, brothers, sisters and children) have ever been diagnosed with cancer. Genetic testing should always be carried out at a clinic that also provides supportive counselling and education. Some people may decide not to take genetic testing when they understand the implications to their personal situation.


The fact that 1 or 2 family members have been diagnosed with cancer doesn’t mean that you will also develop cancer. This is especially true if the family member is not a first-degree relative or if the cancers are of different types. It is important to discuss screening with your doctor if you have a family history of cancer.

Understanding genes and inheritance

It is not entirely clear whether a family’s pattern of cancer is due to chance, similarities in the lifestyle choices of family members or hereditary factors passed from parents to children through genes.


People from the same family share many characteristics. For example, parents pass on physical traits to their children like eye colour, hair colour and body shape. Also, brothers and sisters may have similar personalities, and they may be good at the same kinds of activities. Our parents pass characteristics on to us through units of information called genes.


Our bodies are made up of cells. Genes are present in every cell and guide how each cell develops and functions. Over the past few years, scientists have learned a great deal about how changes in our genes (gene mutations) can influence our health.


Gene mutations can occur in 2 ways. They can be inherited from a parent or they can be the result of changes that occur during a person’s lifetime. Every cell has the ability to spot these changes and fix them before they are passed on to new cells through the normal process of cell development. Sometimes a cell’s ability to make these repairs fails and the altered gene may be passed on. Certain gene mutations that are passed on may increase the risk of developing cancer.

Understanding your risk

It is important for you to know the level of your risk for developing cancer because it may help you to decide what type of healthcare action to take. We talk about risk of developing cancer as:

  • risk factors which may contribute to the random development of cancer over our lifetime
  • the chance of having an inherited gene mutation

Lifetime risk
Lifetime risk is often discussed in relation to the risk of individuals in the general population. Many of us have heard that 1 in 9 Canadian women will develop breast cancer in her lifetime. This statement means that if we kept track of a group of 1000 Canadian girl babies all through their lives, we would find that 111 of them (1 in 9) would have been diagnosed with breast cancer at some point during that time. It does not mean that if 9 Canadian women were together in a room then 1 of them would have breast cancer at that moment.


Lifetime risk may be stated in percentages. For example, you may be told that you have a 15% risk of developing cancer. This statement means that you have a 15% chance of being diagnosed with cancer sometime during your lifetime.


Risk factors
Risk factors are anything that might increase our chance of developing a disease like cancer. Cancer is not a single disease, but a large group of diseases. Most cancers develop as a result of gene mutations that develop in the cells during a person’s lifetime.


Cancer develops gradually as a result of a complex mix of risk factors related to:

  • lifestyle (tobacco use, diet, exposure to sunlight)
  • the environment
  • heredity
  • chance
  • age
  • sex

Some risk factors can be avoided – we can choose not to smoke. Some cannot – we cannot choose the genes that we inherit through birth.


Many people who develop cancer have no known risk factors. Even if a person has 1 or more risk factors, it is impossible to know exactly how much they may contribute to developing cancer later in life.


Inherited gene mutations
While all cancers could be considered to be genetic – as they are triggered by altered genes – only a small portion of cancers (up to 10%) are passed on from 1 family generation to another. The risk of having an inherited gene mutation is generally talked about in relation to 1 of 3 groups – low, medium or high. A cancer might be considered hereditary for a family if:

  • the cancer is present in a number of generations
  • family members have developed cancer when they were younger than 50 years of age or at a younger age than usual for that type of cancer
  • family members have had more than 1 type of cancer

Depending on your risk of developing cancer, cancer specialists may advise certain preventive and/or early detection measures. These recommendations may be made with or without genetic testing.

Taking action to reduce your risk

Preventive measures help reduce the chance to develop cancer. Research into cancer risk prevention provides us with information about how to lower our risk for developing cancer. You may:

  • choose to have surgery
  • be given medication that may reduce your risk
  • wish to make some lifestyle changes

Early detection measures help to discover signs of cancer as early as possible. These methods could include a combination of:

  • examinations done by a doctor
  • self-examinations you do on your own
  • clinical tests (like mammography, ultrasound)