If both copies of a person's HD gene contain 26 or fewer CAG repeats they will not develop HD and nor will any of their children (or their children's children and so on). 2.Gene-positive: If at least one copy of a person's HD gene contains 40 or more CAG repeats they will develop HD in their lifetime and each of their children have a 50% risk of inheriting HD. 3.'Reduced penetrance' range: People with a HD gene containing between 36 and 39 repeats are in the 'reduced penetrance' range. Some people in this range will develop symptoms of the disease, while others won't and there is still a risk of their children inheriting the disease. 4.'Intermediate alleles' range: People in this range have repeat lengths between 27 and 35 CAGs. People with an intermediate allele will not develop HD themselves, but there may be a reduced risk of HD developing in their children. If Tom's at-risk parent has been tested and is gene-negative then your unborn grandchild has no chance of inheriting Huntington's Disease. However, if they are showing no symptoms but have not been tested then they may be gene-negative but there is also still a chance that they may fall within one of the other three categories. If that is the case, Tom would be at risk (50% chance or less) of inheriting the faulty gene and your unborn grandchild would have a 25% chance or less. I hope this is helpful. ow does genetic anticipation work? parent