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My dad has 27 CAG repeats, what are my chances?

April 27, 2017

Huntington's Disease Youth Organization

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My dad has 27 CAG repeats, what are my chances?

Q. My dad got his results back to and got the all clear but he scored 27 on the test what is the chance I will get it

Ashley, Young Adult, UK

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A. Hi Ashley,

Many thanks for your email. I assume that your dad was tested because of a family history of HD rather than because he had symptoms? And that the 27 is referring to the number of CAG repeats in the HD gene?

If so, the chance that you would get HD will be small. 27 repeats is only just into the ‘intermediate’ range (27-35). Individuals with an intermediate CAG size are not at risk of developing symptoms of HD themselves, but the gene can be a little more prone to expansion when passed to a child. However, the level of instability is associated with the size – an HD gene with 35 repeats is much more likely to expand into the ‘full’ HD range than a gene with 27 repeats. You can find more information about the different HD gene sizes here https://en.hdyo.org/you/articles/53. In that article you’ll read that the chance that a child of someone with an intermediate HD gene would develop HD is around 5%. However, for the reasons above, the risk is likely to be quite a bit lower than this for you.

I note you live in the UK, and if you wanted to discuss your dad’s result in more detail I would recommend an appointment at your local genetics clinic. You could ask your GP for a referral. If you have any difficulty with this or have any other questions or concerns in the meantime, please do not hesitate to get back in touch.

Best wishes