What range of repeats is considered normal for Huntington's Disease?

In the context of Huntington's Disease (HD), the normal range of CAG (cytosine-adenine-guanine) repeats within the HTT gene is crucial for understanding the genetic basis of the disorder. The correct answer identifies the range of repeats associated with a normal allele.

The range of 9 to 37 repeats is considered normal. Individuals with this number of repeats typically do not exhibit symptoms of Huntington's Disease. CAG repeats below this range are not associated with the development of the condition, while those that fall above can lead to HD. Specifically, alleles with 36 to 39 repeats are considered to be in a "gray area", where there is a risk of the disease developing in some carriers, especially if the repeats expand in subsequent generations.

In individuals with 40 or more CAG repeats, Huntington's Disease almost certainly develops, exhibiting a more severe prognosis the higher the repeat count goes. Thus, the selection that indicates a range of 9 to 37 repeats coincides with the established understanding of genetic diagnosis in Huntington's Disease. Other ranges presented do not align with the current knowledge related to normal allelic variation for this condition.