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"According to the so-called imprinted brain theory, the paradoxes can be explained in terms of the expression of genes, and not simply their inheritance. 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."