Huntington's disease is one of a small number of conditions that is caused by a single-gene defect (along with haemophilia, and a few others).
It's very rare.
What is known about mental "illnesses" are that they sometimes manifest when certain combinations of genes are present, but not always, and when you look at the details of cases you can pretty much always find an outer-world factor that triggers the state (e..g, abuse, trauma, neglect etc).
It's very rare.
What is known about mental "illnesses" are that they sometimes manifest when certain combinations of genes are present, but not always, and when you look at the details of cases you can pretty much always find an outer-world factor that triggers the state (e..g, abuse, trauma, neglect etc).
So we're talking about very different things.