Final answer:
The HD gene mutation would be classified as dominant negative. This type of mutation occurs when the mutated gene product adversely affects the normal, wild-type gene product within the same cell. These mutations typically act as competitive inhibitors of normal protein functions.
Step-by-step explanation:
The best classification for the HD gene mutation would be dominant negative. A dominant negative mutation occurs when the gene product adversely affects the normal, wild-type gene product within the same cell. This usually happens if the mutated product can still interact with the normal product, but blocks some of its functions. These types of mutations are typically gain-of-function mutations and act as competitive inhibitors of the normal protein functions.