Once you know you have a mutation in a disease causing gene such awareness cannot be unlearnt. Knowing a mutation is present doesn’t make the mutation worse; it does however change the relationship you have with yourself. The presence of the mutation could come to dominate and divert you from the life you were living. Sometimes ignorance gives us freedom not to prejudge situations or narrow ourselves to only a few choices. For example, knowing from an early age I was susceptible to early-onset Parkinson’s probably would had stopped me pursuing the long term goal of a career in Science. I think the issue of whether you want to know you have a genetic susceptibility to a disease comes down to the question: would you like to hear the clock counting down to the disease or not?
Of course, being armed with knowledge of a mutation may enable us to choose a series of actions to save our lives (e.g. individuals with mutations in breast cancer genes may choose to remove their breast tissue as a precaution). But this depends on the type of mutation you have; not all mutations are made equal. Some will have a profound affect, meaning inheriting such a mutation confers a high likelihood of disease; others will increase the chances of disease only a small amount; and some mutations won’t have any affect. The extent of the reaction to knowledge of a mutation must be related to the severity of the mutation.
The amount of DNA we share with others is dependent on how related we are to them (e.g. parents share 50% with their children). It follows that we potentially have the same mutations as our close relatives. Therefore, knowledge of my mutation may be knowledge of my relative’s mutation and their susceptibility to disease. If I was to get tested, is it right to expose this knowledge through my actions when my relative hasn’t chosen to be tested? Should I keep quiet about my results when it may help them? Does my right to know outweigh their right to not know? This also applies to any children I plan to have. What right to information do we have when it impinges on the right of others to not know? These are difficult questions.
From my point of view, if it will cause greater harm withholding information then the information should be known. For example, it is morally wrong to not tell a young child an oven is hot. In this case the harm is known whereas in genetic testing there is a problem; the test itself needs to be done to assess harm. Nonetheless, I would advocate genetic testing if the results were clearly explained because the emotional difficulty of knowing can be mitigated, whereas the potential physical harm of a mutation is properly dealt with only after it is discovered; discovery is probably worth the risk of emotional difficulty.