A Faustian bargain

HUNTINGTON’S disease is awful. It slowly robs its victims of mobility, wits and emotions. And there is no cure. The idea that it might be the obverse of something good sounds, to say the least, counter-intuitive. Yet that is the contention of a small band of neuroscientists who have been studying it. They suggest the underlying cause of Huntington’s, a strange form of genetic mutation called a triplet-repeat expansion, might also be one of the driving forces behind the expansion of the human brain. Huntington’s, these people suspect, may be a price humanity pays for being clever.

Most genetic diseases are recessive. This means a faulty gene inherited from one parent can be covered for by a healthy one from the other. For someone to suffer symptoms, both of his or her parents must have a faulty copy of the gene in question—unless the victim is a man and the faulty gene is on his single X chromosome. Huntington’s by contrast is a dominant disease. A faulty gene from either parent is enough to cause it.