Predisposition

From the best novel I've read in some time…

If a parent has it, you have a fifty-fifty chance of going down too. Chromosome four. The misfortune lies with a single gene, in an excessive repeat of a single sequence - CAG. Here's biological determinism in its purest form. More than forty repeats of that one little codon, and you're doomed. Your future is fixed and easily foretold. The longer the repeat, the earlier and more severe the onset. Between ten and twenty years to complete the course, from the first small alterations of character, tremors in the hands and face, emotional disturbance including - most notably - sudden, uncontrollable alterations of mood to the helpless jerky dance-like movements, intellectual dilapidation, memory failure, agnosia, apraxia, dementia, total loss of muscular control, rigidity sometimes, nightmarish hallucinations and a meaningless end. This is how the brilliant machinery of being is undone by the tiniest of faulty cogs, the insidious whisper of ruin, a single bad idea lodged in every cell, on every chromosome four.

Source: McEwan, Ian. Saturday. Doubleday, 2005.