An epigenetic mechanism of regulation of gene expression in which epigenetic modifications (imprints) are established during gametogenesis. For a given gene to show parentally biased expression, the imprint are established exclusively in one of the two parental genomes, thus generating an asymmetry between the maternal and paternal alleles. [ http://www.ncbi.nlm.nih.gov/pubmed/11498578 http://www.ncbi.nlm.nih.gov/pubmed/7502071 http://www.ncbi.nlm.nih.gov/pubmed/31896690 ]