Editing Chromosome (section)

== Aberrations ==
{{Main|Chromosome abnormality}}
[[File:Chromosome 21.png|thumb|In Down syndrome, there are three copies of chromosome 21.]]

Chromosomal aberrations are disruptions in the normal chromosomal content of a cell. They can cause genetic conditions in humans, such as [[Down syndrome]],<ref>{{Citation |title=Chromosomal Abnormalities |date=8 July 2009 |url=https://www.ncbi.nlm.nih.gov/books/NBK115545/ |work=Understanding Genetics: A New York, Mid-Atlantic Guide for Patients and Health Professionals |access-date=27 September 2023 |publisher=Genetic Alliance |language=en}}</ref> although most aberrations have little to no effect. Some chromosome abnormalities do not cause disease in carriers, such as [[Chromosomal translocation|translocations]], or [[chromosomal inversion]]s, although they may lead to a higher chance of bearing a child with a chromosome disorder.{{citation needed|date=April 2024}} Abnormal numbers of chromosomes or chromosome sets, called [[aneuploidy]], may be lethal or may give rise to genetic disorders.<ref>{{cite journal | vauthors = Santaguida S, Amon A | title = Short- and long-term effects of chromosome mis-segregation and aneuploidy | journal = Nature Reviews. Molecular Cell Biology | volume = 16 | issue = 8 | pages = 473–85 | date = August 2015 | pmid = 26204159 | doi = 10.1038/nrm4025 | hdl = 1721.1/117201 | s2cid = 205495880 | url = http://dspace.mit.edu/bitstream/1721.1/117201/1/Amon1.pdf }}</ref> [[Genetic counseling]] is offered for families that may carry a chromosome rearrangement.

The gain or loss of DNA from chromosomes can lead to a variety of [[genetic disorder]]s.<ref>{{Cite web |title=Genetic Disorders |url=https://medlineplus.gov/geneticdisorders.html |access-date=27 April 2022 |website=medlineplus.gov}}</ref> Human examples include:
* [[Cri du chat]], caused by the [[Genetic deletion|deletion]] of part of the short arm of chromosome 5. "Cri du chat" means "cry of the cat" in French; the condition was so-named because affected babies make high-pitched cries that sound like those of a cat. Affected individuals have wide-set eyes, a small head and jaw, moderate to severe mental health problems, and are very short.
* [[DiGeorge syndrome]], also known as 22q11.2 deletion syndrome. Symptoms are mild learning disabilities in children, with adults having an increased risk of [[schizophrenia]]. Infections are also common in children because of problems with the immune system's T cell-mediated response due to an absence of hypoplastic thymus.<ref>{{Cite web |title=DiGeorge Syndrome |url=https://www.ncbi.nlm.nih.gov/books/NBK549798 |access-date=8 August 2023 |website=www.ncbi.nlm.nih.gov}}</ref>
* [[Down syndrome]], the most common trisomy, usually caused by an extra copy of chromosome 21 ([[trisomy 21]]). Characteristics include decreased muscle tone, stockier build, asymmetrical skull, slanting eyes, and mild to moderate developmental disability.<ref>{{cite book|last=Miller|first=Kenneth R. | name-list-style = vanc | title=Biology|url=https://archive.org/details/biology0000mill|url-access=limited|publisher=Prentice Hall|location=Upper Saddle River, New Jersey|year=2000|edition=5th |pages=[https://archive.org/details/biology0000mill/page/194 194]–5|chapter=Chapter 9-3|isbn=978-0-13-436265-6}}</ref>
* [[Edwards syndrome]], or trisomy-18, the second most common trisomy.<ref>{{cite web|title=What is Trisomy 18?|url=http://www.trisomy18.org/what-is-trisomy-18/|website=Trisomy 18 Foundation|access-date=4 February 2017|archive-date=30 January 2017|archive-url=https://web.archive.org/web/20170130142121/http://www.trisomy18.org/what-is-trisomy-18/|url-status=dead}}</ref> Symptoms include motor retardation, developmental disability, and numerous congenital anomalies causing serious health problems. Ninety percent of those affected die in infancy. They have characteristic clenched hands and overlapping fingers.
* [[Isodicentric 15]], also called idic(15), partial tetrasomy 15q, or inverted duplication 15 (inv dup 15).
* [[Jacobsen syndrome]], which is very rare. It is also called the 11q terminal deletion disorder.<ref>{{Cite web|url=https://chromosome11.org/en/disorders/11q-long-arm/terminal-deletion/jacobsen-syndrome/ |title=Terminal deletion |website=European Chromosome 11 Network |access-date=20 February 2023}}</ref> Those affected have normal intelligence or mild developmental disability, with poor expressive language skills. Most have a bleeding disorder called [[Paris-Trousseau syndrome]].
* [[Klinefelter syndrome]] (XXY). Men with Klinefelter syndrome are usually sterile, and tend to be taller than their peers, with longer arms and legs. Boys with the syndrome are often shy and quiet, and have a higher incidence of [[speech delay]] and [[dyslexia]]. Without testosterone treatment, some may develop [[gynecomastia]] during puberty.
* [[Patau Syndrome]], also called D-Syndrome or trisomy-13. Symptoms are somewhat similar to those of trisomy-18, without the characteristic folded hand.
* [[Small supernumerary marker chromosome]]. This means there is an extra, abnormal chromosome. Features depend on the origin of the extra genetic material. [[Cat-eye syndrome]] and [[isodicentric chromosome 15 syndrome]] (or Idic15) are both caused by a supernumerary marker chromosome, as is [[Pallister–Killian syndrome]].
* [[Triple-X syndrome]] (XXX). XXX girls tend to be tall and thin, and have a higher incidence of dyslexia.
* [[Turner syndrome]] (X instead of XX or XY). In Turner syndrome, female sexual characteristics are present but underdeveloped. Females with Turner syndrome often have a short stature, low hairline, abnormal eye features and bone development, and a "caved-in" appearance to the chest.
* [[Wolf–Hirschhorn syndrome]], caused by partial deletion of the short arm of chromosome 4. It is characterized by growth retardation, delayed motor skills development, "Greek Helmet" facial features, and mild to profound mental health problems.
* [[XYY syndrome]]. XYY boys are usually taller than their siblings. Like XXY boys and XXX girls, they are more likely to have learning difficulties.

===Sperm aneuploidy===

Exposure of males to certain lifestyle, environmental and/or occupational hazards may increase the risk of aneuploid spermatozoa.<ref name="Templado-2013">{{cite journal | vauthors = Templado C, Uroz L, Estop A | title = New insights on the origin and relevance of aneuploidy in human spermatozoa | journal = Molecular Human Reproduction | volume = 19 | issue = 10 | pages = 634–43 | date = October 2013 | pmid = 23720770 | doi = 10.1093/molehr/gat039 | doi-access = }}</ref> In particular, risk of aneuploidy is increased by tobacco smoking,<ref name="Shi-2001">{{cite journal | vauthors = Shi Q, Ko E, Barclay L, Hoang T, Rademaker A, Martin R | title = Cigarette smoking and aneuploidy in human sperm | journal = Molecular Reproduction and Development | volume = 59 | issue = 4 | pages = 417–21 | date = August 2001 | pmid = 11468778 | doi = 10.1002/mrd.1048 | s2cid = 35230655 }}</ref><ref name="Rubes-1998">{{cite journal | vauthors = Rubes J, Lowe X, Moore D, Perreault S, Slott V, Evenson D, Selevan SG, Wyrobek AJ | title = Smoking cigarettes is associated with increased sperm disomy in teenage men | journal = Fertility and Sterility | volume = 70 | issue = 4 | pages = 715–23 | date = October 1998 | pmid = 9797104 | doi = 10.1016/S0015-0282(98)00261-1 | doi-access = free }}</ref> and occupational exposure to benzene,<ref name="Xing-2010">{{cite journal | vauthors = Xing C, Marchetti F, Li G, Weldon RH, Kurtovich E, Young S, Schmid TE, Zhang L, Rappaport S, Waidyanatha S, Wyrobek AJ, Eskenazi B | title = Benzene exposure near the U.S. permissible limit is associated with sperm aneuploidy | journal = Environmental Health Perspectives | volume = 118 | issue = 6 | pages = 833–9 | date = June 2010 | pmid = 20418200 | pmc = 2898861 | doi = 10.1289/ehp.0901531 | bibcode = 2010EnvHP.118..833X }}</ref> [[insecticide]]s,<ref name="Xia-2004">{{cite journal | vauthors = Xia Y, Bian Q, Xu L, Cheng S, Song L, Liu J, Wu W, Wang S, Wang X | title = Genotoxic effects on human spermatozoa among pesticide factory workers exposed to fenvalerate | journal = Toxicology | volume = 203 | issue = 1–3 | pages = 49–60 | date = October 2004 | pmid = 15363581 | doi = 10.1016/j.tox.2004.05.018 | bibcode = 2004Toxgy.203...49X | s2cid = 36073841 }}</ref><ref name="Xia-2005">{{cite journal | vauthors = Xia Y, Cheng S, Bian Q, Xu L, Collins MD, Chang HC, Song L, Liu J, Wang S, Wang X | title = Genotoxic effects on spermatozoa of carbaryl-exposed workers | journal = Toxicological Sciences | volume = 85 | issue = 1 | pages = 615–23 | date = May 2005 | pmid = 15615886 | doi = 10.1093/toxsci/kfi066 | doi-access = free }}</ref> and perfluorinated compounds.<ref name="Governini-2015">{{cite journal | vauthors = Governini L, Guerranti C, De Leo V, Boschi L, Luddi A, Gori M, Orvieto R, Piomboni P | title = Chromosomal aneuploidies and DNA fragmentation of human spermatozoa from patients exposed to perfluorinated compounds | journal = Andrologia | volume = 47 | issue = 9 | pages = 1012–9 | date = November 2015 | pmid = 25382683 | doi = 10.1111/and.12371 | hdl = 11365/982323 | s2cid = 13484513 | doi-access = free }}</ref> Increased aneuploidy is often associated with increased DNA damage in spermatozoa.