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If a mother has the gene is the son's chance 50-50?

October 28, 2015

Huntington's Disease Youth Organization

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


If a mother has the gene is the son's chance 50-50?

Q. Hello there.. Love this site, very informative but I have friend actually who asked if I could do research for her on this diseases because he may have a chance of having it. So my question is if his (my friend’s) mother has the gene and shows signs of the disease but the father does not carry the gene will my friend have a 50-50 chance of having it or is it more or less likely that his sister carries it? Can he carry the gene but never show symptoms?

Heather, Young Adult, USA

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A. Hi Heather,

Thank you for your email. It is great that you are doing some research for your friend, and I am glad you found the HDYO website useful.

To answer your main question, if your friend’s mother does carry the HD gene expansion, he will have a 50-50 chance of having it. This is because HD follows what is called ‘autosomal dominant’ inheritance. As you may 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.

Because the chance of passing on the expanded gene is the same regardless of the sex of the parent who carries it (and regardless of the sex of the child), your friend and his sister would each have the same 50-50 chance. This does not mean that one would definitely have it and the other would definitely not (see this HDYO article for a more detailed explanation of why this is the case).

To come to the final part of your question, unfortunately we expect that someone who carries an HD gene expansion (over 40 ‘CAG repeats’) will always develop symptoms of HD at some stage in their life. However, the age at which symptoms start can be very variable. The average age to start with symptoms is around 35-50, but a minority of individuals with HD can have symptoms earlier or later in life. Medication can help with some of the symptoms, and there is lots of research going on which is trying to develop treatments to slow down the progression of the illness.

It is perhaps worth mentioning that many people at risk of HD find it helpful to see a genetic counsellor or doctor, as they could give more information about HD that can be tailored to the individual family. This website may help you/your friend to find your local Genetics service.

I hope this is helpful, and please do not hesitate to get back to us if you/your friend have any other questions.

Best wishes