I haven't developed JHD, has my risk of HD fallen?
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Q. If my father has HD but I have not developed JHD does that mean that the chances of me developing HD falls slightly? If so, what percentage? Thank you
Ashok, Young Adult, UK
A. Hi Ashok, Thanks for the email. You ask a very good question. You are right that you are effectively born with a 50% risk of carrying the HD gene, and for every year that passes with no symptoms, the risk goes down slightly. You are also right when you say that this decline in risk is a bit quicker if your father was affected (when compared to if your mother was affected). This is because of a phenomenon called “anticipation” (the tendency for CAG repeats to elongate in children of fathers who carry the gene). Putting a percentage on this is a bit harder and I don’t know of anyone who has done this work.
If you are 26 years old with no symptoms, I guess your risk of carrying the gene will be reduced by between 2% and 5%. This is based on my clinical experiences, rather than statistical evidence.
I have assumed a certain knowledge of HD and statistics in answering your question, but from the way you worded it, I think you have quite a bit of knowledge.
Please get back to me if I’m wrong and you need more explanation.