text: The mouse sperm genome is 80–90% methylated at its CpG sites in DNA, amounting to about 20 million methylated sites.  After fertilization, early in the first day of embryogenesis, the paternal chromosomes are almost completely demethylated in six hours by an active TET-dependent process, before DNA replication begins (blue line in Figure).
  Demethylation of the maternal genome occurs by a different process.  In the mature oocyte, about 40% of its CpG sites in DNA are methylated.  In the pre-implantation embryo up to the blastocyst stage (see Figure), the only methyltransferase present is an isoform of DNMT1 designated DNMT1o.  It appears that demethylation of the maternal chromosomes largely takes place by blockage of the methylating enzyme DNMT1o from entering the nucleus except briefly at the 8 cell stage (see DNA demethylation).  The maternal-origin DNA thus undergoes passive demethylation by dilution of the methylated maternal DNA during replication (red line in Figure).  The morula (at the 16 cell stage), has only a small amount of DNA methylation (black line in Figure).