The gene causing HD has been shown to have a region in which three of the bases (CAG) are repeated many times.
A normal gene contains 35 or fewer CAG repeats, while the Huntington disease-causing version has 36 or more repeats. A gene with 36–39 repeats falls into a “reduced penetrance” (RP) range, which may or may not be associated with the onset of HD symptoms. A gene with 27-35 repeats falls into an “intermediate allele” range, which means that this individual will not develop HD, but their children may be at risk. Further advances in genetic studies of HD may provide additional or revised information in the future. More information about the meaning of the different CAG repeat ranges can be found here.
Parents both pass one copy of each of their genes on to their child. Because our genes come in pairs, each person will have a certain number of CAG repeats in one HD gene and another number of repeats in the other gene. The CAG repeat sizes can be the same (such as 18 in both genes) or different (such as 18 repeats in one gene and 42 in the other).