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Kate - My dad is a carrier, however he does not have the illness. My other uncle is also a carrier, but he doesn't have the illness either. This is not to say that they won't get it when they are 70 or 80. There will always be a chance as they carry the gene. My Dad's youngest brother is the only one who has the illness in its severe stages. If it is proven that I am a carrier, the next step will be to see a genetic counsellor and discuss our options from there.
That can be a more difficult situation to be in. If a mutation is fully penetrant (if a male has the mutation, he will be affected) the decision is more clear. But when a mutation is not fully penetrant (like in your case), there are a lot more things to think about.