The evolutionary reason for imprinting is not yet clear, although some scientists propose that, at least in mammals, it arose from an evolutionary tug of war between males and females. In this scheme, fathers (who contribute only sperm) benefit when the embryo grows as fast as possible. Thus, silencing genes that slow down embryonic growth is in their interest, even if it depletes resources from the mother. Mothers, on the other hand, need to conserve their resources. Silencing genes that promote rapid growth is therefore in their interest. Supporting this hypothesis is the fact that many of the known imprinted genes regulate growth. Paternally expressed (maternally imprinted) genes such as IGF2 tend to promote growth, whereas maternally expressed (paternally imprinted) genes tend to inhibit it.
Everman, David B., and Suzanne B. Cassidy. "Genomic Imprinting: Breaking the Rules." Journal of the American Academy of Child and Adolescent Psychiatry 39, no. 3 (March 2000): 386-389.
Greally, John M., and Matthew W. State. "Genomic Imprinting: The Indelible Mark of the Gamete." Journal of the American Academy of Child and Adolescent Psychiatry 39, no. 4 (April 2000): 532-535.
Paulsen, Martina, and Anne C. Ferguson-Smith. "DNA Methylation in Genomic Imprinting, Development, and Disease." Journal of Pathology 195, no. 1 (2001): 97-110.
Yale University School of Medicine and Yale-New Haven Hospital. <http://info.med.yale.edu>.