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Not necessarily. Most cancers are random and not inherited; nobody knows what causes them. Only a small percentage of cancer cases are due to hereditary factors.
You don't say what type of cancer your friend’s relatives had; you can inherit a tendency to get SOME particular cancers (not all), but you won't definitely get that cancer. Some cancers are not hereditary at all – bone cancer is one.
Scientists have identified some high risk faulty genes but these are rare in the general population. One example of a high risk gene is BRCA1. Women who carry a faulty BRCA1 gene have an 80% chance of getting breast cancer at some point in their lives - but this still means that one in five of women with the gene will not get breast cancer. (Only 5 – 10 % of breast cancer cases are caused by hereditary factors)
Doctors who specialise in this area think that the younger you are when a cancer occurs, the more likely it is that an inherited genetic tendency has played a part.
It's best for your friend not to worry about it. Both my parents had cancer and my mother died of it. Their six children are now all in their 40s and 50s and only one (me) has had cancer, and my cancer was non-hereditary and unconnected to theirs - just random, like most cancers.
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