Until twenty years ago we had no idea which of our genes came from our father and which came from our mother. We took it for granted that our genes expressed themselves identically and that there was a 50/50 chance that they came from either parent. We also assumed that they worked in cooperation with each other. The biggest breakthrough in genetics in the past two decades has been the discovery of genomic imprinting, which allows us to trace genes to the parent of origin. David Haig has been at the forefront of theorizing these developments arguing that these paternally and maternally active...