Editing Genomic imprinting (section)

==Overview==
In [[ploidy#Diploid|diploid]] organisms (like humans), the somatic cells possess two copies of the [[genome]], one inherited from the male and one from the female. Each [[autosomal]] gene is therefore represented by two copies, or alleles, with one copy inherited from each parent at [[fertilisation|fertilization]]. The expressed allele is dependent upon its parental origin. For example, the gene encoding [[insulin-like growth factor 2]] (IGF2/Igf2) is only expressed from the allele inherited from the male. Although imprinting accounts for a small proportion of mammalian genes, they play an important role in embryogenesis particularly in the formation of visceral structures and the nervous system.<ref>{{cite journal | vauthors = Butler MG | title = Genomic imprinting disorders in humans: a mini-review | journal = Journal of Assisted Reproduction and Genetics | volume = 26 | issue = 9–10 | pages = 477–486 | date = October 2009 | pmid = 19844787 | pmc = 2788689 | doi = 10.1007/s10815-009-9353-3 }}</ref>

The term "imprinting" was first used to describe events in the insect ''[[Pseudococcus nipae]]''.<ref name="Schrader 1921">{{Cite journal |vauthors=Schrader F |year=1921 |title=The chromosomes in ''Pseudococcus nipæ'' |url=http://www.biolbull.org/cgi/content/abstract/40/5/259 |journal=Biological Bulletin |volume=40 |issue=5 |pages=259–270 |doi=10.2307/1536736 |jstor=1536736 |access-date=2008-07-01}}</ref> In [[Pseudococcidae|Pseudococcids]] ([[mealybug]]s) ([[Hemiptera]], [[Coccoidea]]) both the male and female develop from a fertilised egg. In females, all chromosomes remain [[Euchromatin|euchromatic]] and functional. In embryos destined to become males, one [[haploid]] set of chromosomes becomes [[heterochromatin]]ised after the sixth cleavage division and remains so in most tissues; males are thus functionally haploid.<ref name="Brown and Nur 1964">{{cite journal | vauthors = Brown SW, Nur U | title = Heterochromatic Chromosomes in the Coccids | journal = Science | volume = 145 | issue = 3628 | pages = 130–136 | date = July 1964 | pmid = 14171547 | doi = 10.1126/science.145.3628.130 | bibcode = 1964Sci...145..130B }}</ref><ref name="Hughes-Schrader 1948">{{Cite book |title=Cytology of Coccids (Coccoïdea-Homoptera) |vauthors=[[Sally Hughes-Schrader|Hughes-Schrader S]] |year=1948 |isbn=9780120176021 |series=Advances in Genetics |volume=35 |pages=127–203 |doi=10.1016/S0065-2660(08)60468-X |pmid=18103373 |issue=2}}</ref><ref name="Nur 1990">{{cite journal | vauthors = Nur U | title = Heterochromatization and euchromatization of whole genomes in scale insects (Coccoidea: Homoptera) | journal = Development | volume = 108 | pages = 29–34 | year = 1990 | pmid = 2090427 | doi = 10.1242/dev.108.Supplement.29 }}</ref>