Genes play a role in the development of some cancers, and the link is strongest for breast, bowel and stomach cancers. Even so, current research suggests that environmental factors, such as tobacco, diet, infection, alcohol, drugs, radiation and chemicals, are more important than inherited factors in determining whether most cancers develop.
All cancer is triggered by altered genes, but only about 5 to 10 per cent of cancers are actually hereditary. Heredity can sometimes appear to be the cause, especially when the same type of cancer runs in a family. The cancers that most often show a genetic link include breast, bowel, stomach and prostate cancers.
Genetics and breast cancer#
Breast cancer is one of the most common cancers in women. Many factors can influence a woman’s chance of developing it, but the most common risk factors are simply being a woman and getting older. A family history of breast or ovarian cancer is also an important risk factor. Even so, hereditary factors are responsible for only about one in 20 cases of breast cancer (around 5 per cent).
Genetics and bowel cancer#
Bowel cancer is also a common cancer in both men and women. The biggest single risk factor is age: more than eight out of 10 bowel cancers are diagnosed in people over 60, and the risk increases as you get older. It is estimated that about two out of three bowel cancers could be prevented with changes in diet and lifestyle.
Assessing a family history of cancer#
When a person has a “strong family history” of cancer, it usually means they have several relatives diagnosed with the same cancer across different generations on the same side of the family. It can also mean that one or more relatives were diagnosed at an unusually young age, or that a relative had two or more separate cancers.
People with a strong family history may be offered a referral to a specialist genetics service. Understanding someone’s family history usually involves confirming relatives’ cancer diagnoses, for example by obtaining pathology reports, so that an estimate of the person’s own risk can be worked out.
If indicated, certain additional tests can help with the risk assessment. In some situations genetic testing may be appropriate, and it might be offered after a careful discussion about what the results could mean.
People assessed as having a higher than average risk are referred to a specialist, who can discuss screening so that any signs of cancer are detected as early as possible. Early detection helps ensure prompt treatment and the best possible outcome.
Key points#
- Genes can contribute to some cancers, but environmental factors are more important in most cases
- Only about 5 to 10 per cent of cancers are hereditary
- Breast cancer is common in women, and the main risk factors are being a woman and getting older
- A family history of breast or ovarian cancer is an important risk factor, though heredity accounts for only about one in 20 breast cancer cases
- The biggest single risk factor for bowel cancer is age, and many cases could be prevented with changes in diet and lifestyle
Where to get help#
Sources & further reading
For evidence-based global guidance on this topic, consult authoritative public-health bodies such as the World Health Organization (WHO), CDC, NHS, and ECDC.