What condition has been linked to a gene that influences its symptoms?

Huntington's disease is a neurodegenerative disorder that has a direct genetic linkage to a specific mutation in the HTT gene. This gene encodes the protein huntingtin, and the mutation involves an expansion of CAG repeats in the gene. This genetic alteration leads to the progressive symptoms characteristic of Huntington's, including cognitive decline, movement disorders, and psychiatric symptoms. The inheritance pattern is autosomal dominant, meaning that only one copy of the mutated gene is sufficient to display the disease.

In contrast, while conditions like Parkinson's and Alzheimer's diseases may have genetic components that influence susceptibility or some aspects of the disease, they are more complex and often influenced by multiple genetic and environmental factors. Down syndrome is caused by trisomy 21, a chromosomal abnormality, rather than a single gene mutation affecting symptoms in a manner similar to Huntington's disease. Thus, the clear genetic basis linking the HTT gene to the symptoms experienced in Huntington's makes it the correct choice in this context.