Imprinted genes are those which are only expressed when they are inherited from one parent rather than the other. The classic example is IGF2, a growth factor gene only normally expressed when inherited from the father, but silent when inherited from the mother. According to the most widely-accepted theory, genes like IGF2 are silenced by mammalian mothers because only the mother has to pay the costs associated with gestating and giving birth to a large offspring. The father, on the other hand, gets all the benefit of larger offspring, but pays none of the costs. Therefore his copy is activated. The symbolism of a tug-of-war represents the mother's genetic self-interest in countering the growth-enhancing demands of the father's genes expressed in the foetus—the mother, after all, has to gestate and give birth to the baby at enormous cost to herself.

-- Christopher Badcock

from "The Imorunted Brain Theory"