An imprinting of certain genes that takes place during early embryonic development, depending on whether they are of maternal or paternal origin. Depending on the parental origin of the genes, imprinting causes their different gene activity, i.e. a few genes are only active on the chromosomes inherited from the mother, other genes only on the paternally inherited chromosomes. On the biochemical level, imprinting is probably based on methylation of DNA (see also DNA methylation).