My grandparents didn't get HD but their children did
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Q. 3 out four children of my grandparents developed Huntington disease, but neither of them had shown any symptoms. I was wondering Since Huntington is dominant allele genetic disorder, how my aunts and uncle developed it from both healthy parents?
Pelareh, Young Adult, Canada
A. Dear Pelareh,
Thank you for your question.
As you know, HD follows what is known as ‘dominant’ inheritance, and therefore in most cases an individual with HD has a parent who also has/had the condition, although this is not always the case. There are a number of possible explanations how this can happen.
Firstly, the age at which an individual with an HD gene expansion develops symptoms can vary, even within the same family. The average age for symptoms to start is around the 40’s, but they can sometimes start much earlier, or much later in life, so some times a parent who carries the expansion may die of something else before HD symptoms develop.
Also, in the past, when HD was perhaps 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 disease, Alzheimer’s or dementia).
Another possibility is that one of your grandparents had an ‘intermediate’ or a ‘reduced penetrance’ sized expansion of the HD gene. Individuals who have expansions in this range do not usually develop any symptoms of themselves (a small number of people with reduced penetrance expansions do develop symptoms, but usually late in life). However, when they have children, the expansion can sometimes get bigger, so their children would be at risk of HD. This HDYO article gives more information about this.
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.