Journal of Theoretical Biology 2

on ic W rk R ised line Genomic imprinting is the phenomenon where a gene’s (Haig, 2004). In particular, for genes that are expressed in fetal or placental tissues in mammals and that influence the fetal growth rate, an imprinted growth enhancer is expected silenced. imprinted gene. However, the establishment, maintenance, and interpretation of the epigenetic marks associated with imprinting involve interactions between cis-acting and ARTICLE IN PRESStrans-acting elements. In most cases, these trans-acting elements represent ‘‘genetic factions’ ’ whose evolutionary interests are not identical to either the maternally derive