If my mother doesn't have HD could I still have it?
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Q. My grandmother has huntingtons and we are not sure if my mother has huntingtons because she won’t get tested. I understand that if my mother has it I have a 50% chance of having it. But if my mother doesn’t have it is there still a risk that I could still have it?
Seth, Teen, Canada
A. Dear Seth,
Many thanks for your question. It sounds like you have a very good understanding of the inheritance of HD. As you probably know, everyone (whether at risk of HD or not) has two copies of the HD gene, and we inherit one copy from each parent. HD is caused when there is a specific change (an expansion in the ‘CAG repeats’) in one copy of the HD gene. This HDYO article has lots more information about this.
In terms of your question, if your mother does not have the HD gene expansion, you would not be at any increased risk of HD. This is because if both copies of her HD gene have CAG repeat sizes within the normal range, regardless of which one you inherited from her, it would not cause HD. On the other hand, if your mother does have the HD gene expansion you are absolutely right that there would be a 50% chance that you inherited it.
I hope this helps answer your question, and please do not hesitate to get back to us if you have any other questions.