The process by which some genes are rendered non-equivalent. The paternal or maternal allele is not expressed (allelic exclusion), or is expressed differently in different tissues. The phenomenon is possibly directed by an imprinting box, a parent-specific polynucleotide sequence that instructs an imprinting factor, which may act by methlyation of differentially methylated regions (DMRs), and which will reversibly activate or inactivate the gene inherited from only one parent. Imprinting passes through erasure of methylation of both parental and maternal chromosomes during germ cell development, establishment of methylation, according to the sex of the gamete, and after fertilization, maintenance of the methylation status of the genome during the mitoses of embryogenesis.