The majority of mammalian genes are present in two copies that are expressed and regulated. A small number of mammalian genes, however, are subject to special regulation by a process called gene imprinting. The imprint is a chemical mark attached to genes during egg or sperm development. Imprinting physically marks a gene in such a way that the parental origin of the gene can be distinguished and expressed accordingly. Most imprinted genes seem to govern fetal growth regulation, explained Shirley M. Tilghman, an HHMI investigator at Princeton University.
Some researchers speculate that imprinting evolved in order to establish boundaries between species.
Imprinting is also thought to provide a barrier to unisexual reproduction and the interbreeding of species. According to the theory, fathers contribute genes that enhance growth because their best interests are served if their progeny extract as many maternal resources as possible in order to ensure survival. Mothers, on the other hand, then silence their copy of growth-promoting genes in retaliation because they value all of their progeny equally. In a research article published in the May 2000 issue of the journal Nature Genetics, Tilghman and colleagues at Princeton report that crossing two related mouse species, Peromyscus polionotus and Peromyscus maniculatus, results in abnormalities in gene imprinting and growth abnormalities in the hybrid offspring. This suggested to us that the hybrids may have defects in imprinting, and our first idea was that one of the species was not imprinting normally.