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Huntington's Disease Youth Organization

What if both parents have the expanded gene?

HDYO has more information about HD available for young people, parents and professionals on our site:

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Q. What if both parents have the expanded gene for Huntington’s disease?

Patrick, Young Adult, USA

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A. Dear Patrick,

Many thanks for your question.

Because HD is quite rare, both parents having an expansion in the HD gene is not a scenario that we encounter very frequently at all. If both parents have an expanded HD gene, the first thing to say is that it alters the risks for their children. The risk of a child inheriting at least one HD expansion would be 75% (3 in 4), including the possibility that they would inherit two expanded copies. Research (based on a small number of cases) has shown that having two copies of the HD expansion does not appear to change the age of onset of symptoms, but it may be that once symptoms start, the disease progresses more quickly. Also, if a child does inherit both expanded genes, when they have children themselves there would be a 100% chance of passing on an expanded HD gene.

I hope this makes sense and helps answer your question, but please do not hesitate to get back in touch if you have any further queries.

Best wishes

Bill