The cause is in the genes and nowhere else. Either you have the Huntington's mutation and will get the disease or not. This is determinism, predestination and fate on a scale of which Calvin never dreamed. The age at which the madness will appear depends strictly and implacably on the number of repetitions of the 'word' CAG in one place on one gene....No horoscope matches this accuracy. No theory of human causality, Freudian, Marxist, Christian or animist, has ever been so precise. No prophet in the Old Testament, no entrail-gazing oracle in ancient Greece, no crystal-ball gypsy clairvoyant on the pier at Bognor Regis ever pretended to tell people exactly when their lives would fall apart, let alone got it right.
We are dealing here with with a prophecy of terrifying, cruel and inflexible truth. There are a billion three-letter 'words' in your genome. Yet the length of just this one little motif is all that stands between each of us and mental illness....Huntington's disease is at the far end of a spectrum of genetics. It is pure fatalism, undiluted by environmental variability. Good living, good medicine, healthy food, loving families or great riches can do nothing about. Your fate is in your genes.
-- Matt Ridley, Genome