Huntington disease
Huntington disease is a progressive brain disorder that usually appears in mid-adult life, bringing involuntary jerky movements (chorea), changes in mood and thinking, and a gradual loss of control over body and mind. Picture a wire whose insulation slowly frays from the inside: the nerve cells look fine for decades, then steadily degenerate as a toxic protein builds up.
It is autosomal dominant, so a single faulty copy of the HTT gene is enough to cause it, and each child of an affected parent has a 50 percent chance of inheriting that copy. The fault is a trinucleotide repeat expansion: a stretch of the DNA sequence CAG, repeated about 10 to 35 times in unaffected people, grows to roughly 40 or more in those who develop the disease. The expanded gene makes an abnormally long huntingtin protein that gradually poisons neurons.
Because the number of CAG repeats tends to increase when passed from parent to child (especially through fathers), the disease can begin earlier and more severely in later generations, a phenomenon called anticipation. A predictive genetic test can tell an at-risk adult whether they carry the expansion, a profound decision usually made with genetic counseling. This entry is educational, not medical advice.
A 42-year-old whose father had the disease notices small involuntary movements; a genetic test counts 44 CAG repeats in one HTT copy, confirming the diagnosis.
The diagnosis comes from counting CAG repeats, not from looking for a changed letter.
Huntington disease is a rare example of a dominant disorder in which a longer repeat tends to bring earlier onset, and the expanded protein causes harm by a toxic gain of function rather than simple loss of function.