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. THE IMPRI