If several cases of a rare condition affect the same family, and if the condition manifests at a relatively early age, these may be symptoms of a hereditary condition. Families with indications of a serious hereditary condition may be offered genetic diagnostic testing from the hospital. Such testing would examine whether the cause of the condition is a genetic mutation.
If a genetic test uncovers a mutation that is causing a patient’s condition, the closes relatives will also be offered a test. Those who do not have the mutation can then be spared worrying unnecessarily, while those who do have the mutation will know for certain and will have a chance to consider preventive measures.
You may ask why it is that genetic mutations do not always cause disease in the people who have the mutation. In general terms, genetic conditions are divided into three main categories:
Monogenic conditions, which are caused by a mutation in a single gene. These are, in turn, grouped into a) recessive genetic conditions, where you inherit the genetic mutation from both parents and b) dominant genetic conditions, where it is enough that you inherit the genetic mutation from one of your parents.
Polygenic conditions, which are caused by the cumulative effect of multiple genes.
Complex conditions, which are caused by the cumulative effect of multiple genes and environment factors, such as diet, pollution, physical activity, stress, cleanliness, etc. all the way back to the embryonic stages also have an impact.
So, to return to the question of why genetic mutations not always cause illness in the people who have a specific mutation, it is a question of whether it is a monogenic condition (dominant or recessive), polygenic condition, or complex condition. Please note that in the case of a recessive monogenic condition you can be a healthy carrier. In the case of polygenic conditions simply carrying the mutations is not enough to get sick, meaning that, although you may be predisposed to a genetic condition, other external factors may prevent that you actually get the condition.
Read more about genome testing.