Genomic imprinting

Genomic imprinting is an epigenetic phenomenon that causes genes to be expressed or not, depending on whether they are inherited from the female or male parent.^{[1][2][3][4][5]} Genes can also be partially imprinted. Partial imprinting occurs when alleles from both parents are differently expressed rather than complete expression and complete suppression of one parent's allele.^[6] Forms of genomic imprinting have been demonstrated in fungi, plants and animals.^[7][8] In 2014, there were about 150 imprinted genes known in mice and about half that in humans.^[9] As of 2019, 260 imprinted genes have been reported in mice and 228 in humans.^[10]

Genomic imprinting is an inheritance process independent of the classical Mendelian inheritance.^[11] It is an epigenetic process that involves DNA methylation and histone methylation without altering the genetic sequence. These epigenetic marks are established ("imprinted") in the germline (sperm or egg cells) of the parents and are maintained through mitotic cell divisions in the somatic cells of an organism.^[12]

Appropriate imprinting of certain genes is important for normal development. Human diseases involving genomic imprinting include Angelman, Prader–Willi, and Beckwith–Wiedemann syndromes.^[13] Methylation defects have also been associated with male infertility.^[3]