Editing Karyotype (section)

=== Changes during development ===

Instead of the usual gene repression, some organisms go in for large-scale elimination of [[heterochromatin]], or other kinds of visible adjustment to the karyotype.
* Chromosome elimination. In some species, as in many [[Sciaridae|sciarid flies]], entire chromosomes are eliminated during development.<ref>{{cite journal |vauthors=Goday C, Esteban MR |title=Chromosome elimination in sciarid flies |journal=BioEssays |volume=23 |issue=3 |pages=242–50 |date=March 2001 |pmid=11223881 |doi=10.1002/1521-1878(200103)23:3<242::AID-BIES1034>3.0.CO;2-P |s2cid=43718856 }}</ref>
* Chromatin diminution (founding father: [[Theodor Boveri]]). In this process, found in some [[copepods]] and [[roundworms]] such as ''[[Ascaris suum]]'', portions of the chromosomes are cast away in particular cells. This process is a carefully organised genome rearrangement where new telomeres are constructed and certain heterochromatin regions are lost.<ref>{{cite journal |vauthors=Müller F, Bernard V, Tobler H |title=Chromatin diminution in nematodes |journal=BioEssays |volume=18 |issue=2 |pages=133–8 |date=February 1996 |pmid=8851046 |doi=10.1002/bies.950180209 |s2cid=24583845 }}</ref><ref>{{cite journal |vauthors=Wyngaard GA, Gregory TR |title=Temporal control of DNA replication and the adaptive value of chromatin diminution in copepods |journal=J. Exp. Zool. |volume=291 |issue=4 |pages=310–6 |date=December 2001 |pmid=11754011 |doi=10.1002/jez.1131|bibcode=2001JEZ...291..310W }}</ref> In ''A. suum'', all the somatic cell precursors undergo chromatin diminution.<ref>Gilbert S.F. 2006. ''Developmental biology''. Sinauer Associates, Stamford CT. 8th ed, Chapter 9</ref>
* [[X-inactivation]]. The inactivation of one X chromosome takes place during the early development of mammals (see [[Barr body]] and [[dosage compensation]]). In [[placental mammals]], the inactivation is random as between the two Xs; thus the mammalian female is a mosaic in respect of her X chromosomes. In [[marsupials]] it is always the paternal X which is inactivated. In human females some 15% of somatic cells escape inactivation,<ref>{{harvnb|King|Stansfield|Mulligan|2006}}</ref> and the number of genes affected on the inactivated X chromosome varies between cells: in [[fibroblast]] cells up about 25% of genes on the Barr body escape inactivation.<ref>{{cite journal |vauthors=Carrel L, Willard H | year = 2005 | title = X-inactivation profile reveals extensive variability in X-linked gene expression in females | journal = Nature | volume = 434 | issue = 7031| pages = 400–404 | doi = 10.1038/nature03479 | pmid = 15772666 | bibcode = 2005Natur.434..400C | s2cid = 4358447 }}</ref>