The chances of inheriting a faulty gene

Most people in the UK are unlikely to have inherited a known fault in breast cancer gene that increases their risk of breast cancer. However, if one of these faulty genes has been identified in one of your relatives, other people in the family might also have the fault too, but it’s unlikely all have. The tables below show, on average, how likely it is that other relatives would have inherited the fault.

If you carry a faulty gene

The more closely related someone is to you, the more likely it is that they also have the faulty breast cancer gene:

Person’s relation to you Chance that they also have the faulty gene Chance that they don’t have the faulty gene
Your mother or father 1 in 2 or 50% 1 in 2 or 50%
Your sister or brother (including non-identical twins) 1 in 2 or 50% 1 in 2 or 50%
Your identical twin 100% (definitely do) 0
Your aunt, uncle, niece, nephew, grandparent, half-brother or half-sister 1 in 4 or 25% 3 in 4 or 75%
Your great aunt, great uncle or cousin 1 in 8 or 12.5% 7 in 8 or 87.5%

These figures are only estimates and the likelihood will vary depending on the patterns of breast cancer in your family. For example, if your family history of breast cancer is all on your father’s side of the family, it is very unlikely that relatives on your mother’s side would carry a faulty gene. Your genetic counsellor can provide more accurate estimations for your own situation.

If a relative carries a faulty gene

The more closely related you are to your relative with a confirmed faulty gene, the more likely it is that you have also inherited that faulty gene.

Relative who has a faulty gene Chance that you also have the faulty gene Chance that you don’t have the faulty gene
Mother or father 1 in 2 or 50% 1 in 2 or 50%
Sister or brother (including non-identical twins) 1 in 2 or 50% 1 in 2 or 50%
Identical twin 100% (definitely do) 0
Aunt, uncle, niece, nephew, grandparent half-brother or half sister 1 in 4 or 25% 3 in 4 or 75%
Your great aunt, great uncle or cousin 1 in 8 or 12.5% 7 in 8 or 87.5%

These figures are only estimates and the likelihood may vary depending on the patterns of breast cancer in your family. Your genetic counsellor can provide more accurate estimations for your own situation.

Related information
People found to carry genetic faults are at high risk of developing breast cancer – find out what this means

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Learn how faulty genes are passed down generations

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Get advice on having discussions with relatives about genetic test results

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