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Why did my grandpa get HD when his father didn't?

January 30, 2015

Huntington's Disease Youth Organization

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


Why did my grandpa get HD when his father didn't?

Q. Hi my Dad has HD, I was wandering why my grandpa got HD when his father never had HD?

Brad, Teen, Australia

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A. Dear Brad,

Thank you for your email, I am sorry to hear that your father has HD.

As you may know, HD is a condition that can run in families, as it is caused by a change in a gene (the type of change is called an ‘expansion’ in a specific part of the genetic code). If you like, you can find a lot more information about this on the HDYO page

In your family, it sounds like your grandpa was the first person to be diagnosed with HD. There are a number of possible explanations why this could be the case. You mention that his father never had HD, but do you know anything about his mother? HD can be inherited from either parent, so your grandpa could have inherited it from her.

The age at which an individual with the HD gene expansion develops symptoms can also vary, even within the same family. So, another possibility is that one of your grandpa’s parents carried the HD gene expansion, but perhaps they died of something else before the symptoms of HD started. Also, in the past, partly because HD was less well known, sometimes people were not diagnosed with HD even if they had symptoms, or the symptoms may have been wrongly called something else (such as Parkinson’s, Alzheimer’s or dementia).

Another possible explanation is that the HD gene expansion happened for the first time in your grandpa (likely very early on in development, before he was born). This is because very occasionally genes change between a parent and child.

A fourth possibility is that neither of his parents had HD for non-genetic reasons, for example if he was adopted by them rather than being their ‘biological’ child.

I hope this helps to answer your question, and please do not hesitate to get back in touch if there is anything else we can help with.