Editing Twin (section)

==Types and zygosity==
The vast majority of twins are either dizygotic (fraternal) or monozygotic (identical). In humans, dizygotic twins occur more often than monozygotic twins.<ref>{{cite journal |last1=Nylander |first1=Percy P. S. |title=The Factors That Influence Twinning Rates |journal=Acta Geneticae Medicae et Gemellologiae: Twin Research |date=July 1981 |volume=30 |issue=3 |pages=189–202 |doi=10.1017/s0001566000007650|pmid=6805197 }}</ref>  Less common variants are discussed further down the article.

Fraternal twins can be any of the following:
* Female–female twins: Sometimes called sororal twins (25%).
* Male–male twins: Sometimes called fraternal (unrelated to zygosity) twins (25%).
* Female-male twins: This is the most common pairing (50%), encompassing both "female-male" (25%) and "male-female" (25%) twins.

Among non-twin births, male singletons are slightly (about five percent) more common than female singletons. The rates for singletons vary slightly by country. For example, the sex ratio of birth in the US is 1.05 males/female,<ref>{{cite web |url=https://www.cia.gov/the-world-factbook/countries/united-states/ |title=United States: People |access-date=2008-10-02 |work=[[The World Factbook]] |publisher=[[Central Intelligence Agency]] |date=2008-09-04 |archive-date=2021-12-12 |archive-url=https://web.archive.org/web/20211212224932/https://www.cia.gov/the-world-factbook/countries/united-states/ |url-status=live}}</ref> while it is 1.07 males/female in Italy.<ref>{{cite web |url=https://www.cia.gov/the-world-factbook/countries/italy/ |title=Italy: People |access-date=2008-10-02 |work=[[The World Factbook]] |publisher=[[Central Intelligence Agency]] |date=2008-09-04 |archive-date=2021-07-01 |archive-url=https://web.archive.org/web/20210701235642/https://www.cia.gov/the-world-factbook/countries/italy/ |url-status=live}}</ref> However, males are also more susceptible than females to die ''in utero'', and since the death rate ''in utero'' is higher for twins, it leads to female twins being more common than male twins.<ref name="WSTR">{{cite web |title=Are there more male twins or female twins? |date=October 2015 |publisher=Washington State Twin Registry |url=https://wstwinregistry.org/2015/10/01/are-there-more-male-twins-or-female-twins/ |access-date=24 July 2018 |archive-date=24 July 2018|archive-url=https://web.archive.org/web/20180724183635/https://wstwinregistry.org/2015/10/01/are-there-more-male-twins-or-female-twins/|url-status=live}}</ref>

[[Zygosity]] is the degree of identity in the [[genome]] of twins.

==={{anchor|Fraternal|Fraternal twins|Dizygotic|Fraternal (dizygotic) twins}}Dizygotic (fraternal) twins===
[[File:Red-haired Siblings.jpg|thumb|right|Adult fraternal twins]]
[[File:Fraternal twin brothers.jpg|thumb|Fraternal twin brothers as young babies]]
[[File:Mirjam Weichselbraun and Melanie Binder, ROMY 2009.jpg|thumb|Austrian TV host [[Mirjam Weichselbraun]] (right) and her fraternal twin Melanie look very similar but with a significant difference in height.]]
'''Dizygotic''' ('''DZ''') or '''fraternal''' '''twins''' (also referred to as "non-identical twins", "dissimilar twins", "biovular twins", and, informally in the case of females, "sororal twins") usually occur when two [[fertilisation|fertilized]] eggs are implanted in the uterus wall at the same time. When two eggs are independently fertilized by two different [[Spermatozoon|sperm cells]], fraternal twins result. The two eggs, or ''ova'', form two [[zygote]]s, hence the terms ''dizygotic'' and ''biovular''. Fraternal twins are, essentially, two ordinary [[sibling]]s who happen to develop in the womb together and who are born at the same time, since they arise from two separate eggs fertilized by two separate [[sperm]], just like ordinary siblings. This is the most common type of twin.<ref name="multi">{{cite web |url=https://multiples.about.com/cs/funfacts/a/twinzygosity.htm |title=Identical and Fraternal Twins – Determining Zygosity in Twins |publisher=Multiples.about.com |date=2013-07-16 |access-date=2013-09-16 |archive-date=2016-03-04 |archive-url=https://web.archive.org/web/20160304130136/http://multiples.about.com/cs/funfacts/a/twinzygosity.htm |url-status=dead}}</ref>

Dizygotic twins, like any other siblings, will practically always have different sequences on each chromosome, due to [[chromosomal crossover]] during [[meiosis]]. Dizygotic twins share on average 50 percent of each other's genes, the same as siblings that are conceived and born at different times. Like any other [[sibling]]s, dizygotic twins [[family resemblance (anthropology)|may look similar]], particularly as they are the same age. However, dizygotic twins may also look very different from each other (for example, be of opposite sexes).

Studies show that there is a genetic proclivity for dizygotic twinning. However, it is only the mother who has any effect on the chances of having such twins; there is no known mechanism for a father to cause the release of more than one [[ovum]]. Dizygotic twinning ranges from six per thousand births in Japan (similar to the rate of monozygotic twins) to 14 and more per thousand in some African countries.<ref>{{Cite book
|title=Multiple Pregnancy: Epidemiology, Gestation & Perinatal Outcome
|editor=Louis G. Keith
|author=R. Derom
|author2=J. Orlebeke |author3=A. Eriksson |author4=M. Thiery
 |isbn=978-1-85070-666-3
|publisher=Taylor and Francis
|chapter=The epidemiology of multiple births in Europe
|year=1995
|page=145
}}</ref>

Dizygotic twins are also more common for older mothers, with twinning rates doubling in mothers over the age of 35.<ref name="Bortolus">{{Cite journal
|title=The epidemiology of multiple births
|journal=Human Reproduction Update
|year=1999
|volume=5
|issue=2
|pages=179–187
|last=Bortolus
|first=Renata
|author2=Fabio Parazzini |author3=Liliane Chatenoud |author4=Guido Benzi |author5=Massimiliano Maria Bianchi |author6=Alberto Marini
 |issn=1355-4786
|pmid=10336022
|doi=10.1093/humupd/5.2.179
|doi-access=free
}}</ref> With the advent of technologies and techniques to assist women in getting pregnant, the rate of fraternals has increased markedly.{{citation needed|date=August 2022}}

==={{Anchor|IdenticalTwins|Identical twins|Identical (monozygotic) twins}}Monozygotic (identical) twins<!-- This section is linked from [[Rheumatoid arthritis]] -->===
'''Monozygotic''' ('''MZ''') or '''identical''' '''twins''' occur when a single [[ovum|egg]] is fertilized to form one [[zygote]] (hence, "monozygotic") which then divides into two separate [[embryo]]s.

====Mechanism====
Regarding spontaneous or ''natural'' monozygotic twinning, a 2007 theory related to in vitro fertilization (IVF) proposes that monozygotic twins may be formed when a [[blastocyst]] contains two inner cell masses (ICM), each of which will lead to a separate fetus, rather than by the embryo splitting while hatching from the [[zona pellucida]] (the gelatinous protective coating around the blastocyst).<ref>{{Cite news |title=Study: Time-lapse recordings reveal why IVF embryos are more likely to develop into twins |url=https://www.eurekalert.org/pub_releases/2007-07/esfh-trr070107.php |work=[[EurekAlert!]] |date=2007-07-02 |access-date=2019-08-05 |archive-date=2019-08-05 |archive-url=https://web.archive.org/web/20190805221117/https://www.eurekalert.org/pub_releases/2007-07/esfh-trr070107.php |url-status=live}}</ref>

Monozygotic twins may also be created artificially by embryo splitting. It can be used as an expansion of [[in vitro fertilization]] (IVF) to increase the number of available embryos for [[embryo transfer]].<ref>{{Cite journal|vauthors=Illmensee K, Levanduski M, Vidali A, Husami N, Goudas VT |title=Human embryo twinning with applications in reproductive medicine |journal=Fertil. Steril. |volume= 93|issue= 2|pages= 423–7|date=February 2009 |pmid=19217091 |doi=10.1016/j.fertnstert.2008.12.098 |doi-access=free}}</ref>

====Incidence====
The chance of identical twins is approximately 3 to 4 in every 1,000 births.<ref name="mlp">{{cite web |title=Is the probability of having twins determined by genetics? |url=https://medlineplus.gov/genetics/understanding/traits/twins/ |publisher=MedlinePlus, US National Library of Medicine |access-date=9 July 2023 |date=11 July 2022}}</ref>

The likelihood of a single fertilization resulting in monozygotic twins is uniformly distributed in all populations around the world.<ref name="Bortolus" /> This is in marked contrast to dizygotic twinning, which ranges from about six per thousand births in Japan (almost similar to the rate of identical twins, which is around 4–5) to 15 and more per thousand in some parts of India<ref>{{Cite journal |first=Jaroslaw J. |last=Oleszczuk |author2=Donald M. Keith |author3=Louis G. Keith |author4=William F. Rayburn |title=Projections of population-based twinning rates through the year 2100 |journal=The Journal of Reproductive Medicine |volume=44 |issue=11 |pages=913–921 |date=November 1999 |pmid=10589400 |url=https://www.reproductivemedicine.com/toc/auto_abstract.php?id=13594 |access-date=2008-10-02 |url-status=live |archive-date=2009-01-04 |archive-url=https://web.archive.org/web/20090104100103/http://www.reproductivemedicine.com/toc/auto_abstract.php?id=13594}}</ref> and up to over 20 in some Central African countries.<ref name="Smits"/> The exact cause for the splitting of a zygote or embryo is unknown.

IVF techniques are more likely to create dizygotic twins. For IVF deliveries, there are nearly 21 pairs of twins for every 1,000.<ref>{{cite press release |title=Time-lapse recordings reveal why IVF embryos are more likely to develop into twins. Researchers believe the laboratory culture could be the cause |publisher=[[European Society of Human Reproduction and Embryology]] |date=2007-07-02 |url=https://www.eshre.com/emc.asp?pageId=939 |archive-url=https://web.archive.org/web/20070921094734/https://www.eshre.com/emc.asp?pageId=939 |url-status=dead |archive-date=2007-09-21 |access-date=2008-09-30}}</ref>

====Genetic and epigenetic similarity====
[[File:Identical-fraternal-sperm-egg.svg|thumb|upright=1.35 |Comparison of zygote development in monozygotic and dizygotic twins. In the uterus, a majority of monozygotic twins (60–70%) share the same [[placenta]] but have separate [[amniotic sac]]s. In 18–30% of monozygotic twins each fetus has a separate placenta and a separate amniotic sac. A small number (1–2%) of monozygotic twins share the same placenta and amniotic sac. Fraternal twins each have their own placenta and own amniotic sac.]]
Monozygotic twins are genetically nearly identical and they are the same chromosomal sex unless there has been a mutation during development. The children of monozygotic twins test genetically as half-siblings (or full siblings, if a pair of monozygotic twins reproduces with another pair or with the same person), rather than first cousins. Identical twins do not have the same [[fingerprint]]s however, because even within the confines of the womb, the fetuses touch different parts of their environment, giving rise to small variations in their corresponding prints and thus making them unique.<ref>{{cite journal |vauthors=Patwari P, Lee RT |title=Mechanical control of tissue morphogenesis |journal=Circulation Research |date=2008-08-01 |volume=103 |issue=3 |pages=234–43 |doi=10.1161/CIRCRESAHA.108.175331 |pmid=18669930 |pmc=2586810}}</ref>

Monozygotic twins always have the same [[genotype]]. Normally due to an environmental factor or the deactivation of different X chromosomes in female monozygotic twins, and in some extremely rare cases, due to [[aneuploidy]], twins may express different sexual [[phenotype]]s, normally from an XXY [[Klinefelter syndrome]] zygote splitting unevenly.<ref>{{Cite journal |vauthors=Edwards JH, Dent T, Kahn J |title=Monozygotic twins of different sex |journal=Journal of Medical Genetics |volume=3 |issue=2 |pages=117–123 |date=June 1966 |pmid=6007033 |pmc=1012913 |doi=10.1136/jmg.3.2.117}}</ref><ref>{{Cite journal |last=Machin |first=GA |title=Some causes of genotypic and phenotypic discordance in monozygotic twin pairs |journal=American Journal of Medical Genetics |volume=61 |issue=3 |pages=216–228 |date=January 1996 |pmid=8741866 |doi=10.1002/(SICI)1096-8628(19960122)61:3<216::AID-AJMG5>3.0.CO;2-S}}</ref><ref name="Schmid2000">{{cite journal |pmid=11113914 |volume=20 |issue=12 |title=Prenatal diagnosis of heterokaryotypic mosaic twins discordant for fetal sex |date=Dec 2000 |journal=Prenat Diagn |pages=999–1003 |doi=10.1002/1097-0223(200012)20:12<999::aid-pd948>3.0.co;2-e |last1=Schmid |first1=O |last2=Trautmann |first2=U |last3=Ashour |first3=H |last4=Ulmer |first4=R |last5=Pfeiffer |first5=RA |last6=Beinder |first6=E| s2cid=31844710}}</ref>

Monozygotic twins, although genetically very similar, are not genetically exactly the same. The DNA in white blood cells of 66 pairs of monozygotic twins was analyzed for 506,786 [[single-nucleotide polymorphism]]s known to occur in human populations. Polymorphisms appeared in 2 of the 33 million comparisons, leading the researchers to extrapolate that the blood cells of monozygotic twins may have on the order of one DNA-sequence difference for every 12&nbsp;million nucleotides, which would imply hundreds of differences across the entire genome.<ref name="pmid24123875">{{cite journal |vauthors=Li R, Montpetit A, Rousseau M, Wu SY, Greenwood CM, Spector TD, Pollak M, Polychronakos C, Richards JB |title=Somatic point mutations occurring early in development: a monozygotic twin study |journal=J. Med. Genet. |volume=51 |issue=1 |pages=28–34 |date=January 2014 |pmid=24123875 |doi=10.1136/jmedgenet-2013-101712 |s2cid=6031153}}</ref> The mutations producing the differences detected in this study would have occurred during embryonic cell-division (after the point of fertilization). If they occur early in fetal development, they will be present in a very large proportion of body cells.{{citation needed|date=February 2024}}

[[File:Mark and Scott Kelly at the Johnson Space Center, Houston Texas.jpg|thumb|upright=1.15|Despite being genetically identical, twins [[Mark Kelly|Mark]] and [[Scott Kelly (astronaut)|Scott Kelly]] are distinguishable from each other.]]
Another cause of difference between monozygotic twins is [[Epigenetics|epigenetic modification]], caused by differing environmental influences throughout their lives. Epigenetics refers to the level of activity of any particular gene. A gene may become switched on, switched off, or could become partially switched on or off in an individual. This epigenetic modification is triggered by environmental events. Monozygotic twins can have markedly different epigenetic profiles. A study of 80 pairs of monozygotic twins ranging in age from three to 74 showed that the youngest twins have relatively few [[epigenetics|epigenetic]] differences. The number of epigenetic differences increases with age. Fifty-year-old twins had over three times the epigenetic difference of three-year-old twins. Twins who had spent their lives apart (such as those adopted by two different sets of parents at birth) had the greatest difference.<ref>{{Cite journal |last1=Fraga |first1=Mario F. |last2=Ballestar |first2=Esteban |last3=Paz |first3=Maria F. |last4=Ropero |first4=Santiago |last5=Setien |first5=Fernando |last6=Ballestar |first6=Maria L. |last7=Heine-Suñer |first7=Damia |last8=Cigudosa |first8=Juan C. |last9=Urioste |first9=Miguel |last10=Benitez |first10=Javier |last11=Boix-Chornet |first11=Manuel |last12=Sanchez-Aguilera |first12=Abel |last13=Ling |first13=Charlotte |last14=Carlsson |first14=Emma |last15=Poulsen |first15=Pernille |last16=Vaag |first16=Allan |last17=Stephan |first17=Zarko |last18=Spector |first18=Tim D. |last19=Wu |first19=Yue-Zhong |last20=Plass |first20=Christoph |last21=Esteller |first21=Manel |display-authors=5 |title=Epigenetic differences arise during the lifetime of monozygotic twins |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=102 |issue=30 |pages=10604–9 |date=July 2005 |pmid=16009939 |pmc=1174919 |doi=10.1073/pnas.0500398102 |bibcode=2005PNAS..10210604F |doi-access=free}}</ref> However, certain characteristics become more alike as twins age, such as IQ and personality.<ref>{{Cite book |last=Segal |first=Nancy L. |title=Entwined lives: twins and what they tell us about human behavior |publisher=Dutton |location=New York |year=1999 |isbn=0-525-94465-6 |oclc=40396458 |url-access=registration |url=https://archive.org/details/entwinedlivestwi00sega|pages=135–138}}</ref><ref>{{Cite book|last=Plomin |first=Robert |title=Behavioral genetics |publisher=Worth Pubs |location=New York |year=2001 |isbn=978-0-7167-5159-5 |oclc=43894450}}{{Page needed|date=September 2010}}</ref><ref>{{cite journal |last1=Mandler |first1=G |year=2001 |title=Apart from genetics: What makes monozygotic twins similar? |journal=Journal of Mind and Behavior |volume=22 |pages=147–159}}</ref>

In January 2021, new research from a team of researchers in [[Iceland]] was published in the journal ''[[Nature Genetics]]'' suggesting that identical twins may not be quite as identical as previously thought.<ref>{{Cite journal |last1=Jonsson |first1=Hakon |last2=Magnusdottir |first2=Erna |last3=Eggertsson |first3=Hannes P. |last4=Stefansson |first4=Olafur A. |last5=Arnadottir |first5=Gudny A. |last6=Eiriksson |first6=Ogmundur |last7=Zink |first7=Florian |last8=Helgason |first8=Einar A. |last9=Jonsdottir |first9=Ingileif |last10=Gylfason |first10=Arnaldur |last11=Jonasdottir |first11=Adalbjorg |last12=Jonasdottir |first12=Aslaug |last13=Beyter |first13=Doruk |last14=Steingrimsdottir |first14=Thora |last15=Norddahl |first15=Gudmundur L. |date=2021-01-07 |title=Differences between germline genomes of monozygotic twins |journal=Nature Genetics |volume=53 |issue=1 |pages=27–34 |doi=10.1038/s41588-020-00755-1 |issn=1546-1718 |pmid=33414551|s2cid=230986741 }}</ref> The four-year study of monozygotic (identical) twins and their extended families revealed that these twins have genetic differences that begin in the early stages of embryonic development.<ref>{{cite web |title=Identical twins aren't always genetically identical, new study finds |date=8 Jan 2021 |author=Lianne Kolirin |website=CNN |url=https://www.cnn.com/2021/01/08/health/identical-twins-genome-study-scn-scli-intl/ |access-date=8 January 2021 |archive-date=11 January 2021 |archive-url=https://web.archive.org/web/20210111213750/https://www.cnn.com/2021/01/08/health/identical-twins-genome-study-scn-scli-intl/ |url-status=live}}</ref>

===Polar body and semi-identical twins===
A 1981 study of a deceased [[triploid syndrome|XXX]] twin fetus [[Parasitic twin|without a heart]] showed that although its fetal development suggested that it was an identical twin, as it shared a placenta with its healthy twin, tests revealed that it was probably a [[polar body]] twin. The authors were unable to predict whether a healthy fetus could result from a polar body twinning.<ref>{{cite journal
 | doi = 10.1126/science.7196086
 | last = Bieber
 | first = Frederick R.
 |author2=Walter E. Nance |author3=Cynthia C. Morton |author4=Judith A. Brown |author5=Fay O. Redwine |author6= Robert L. Jordan and T. Mohanakumar
 | date = 1981-08-14
 | title = Genetic Studies of an Acardiac Monster: Evidence of Polar Body Twinning in Man
 | journal = Science
 | volume = 213
 | issue = 4509
 | pmid = 7196086
 | pages = 775–777
 | jstor=1686613| bibcode = 1981Sci...213..775B
 }}</ref> However, a study in 2012 found that it is possible for a polar body to result in a healthy fetus.<ref>{{Cite journal |last1=Scott |first1=Richard T. |last2=Treff |first2=Nathan R. |last3=Stevens |first3=John |last4=Forman |first4=Eric J. |last5=Hong |first5=Kathleen H. |last6=Katz-Jaffe |first6=Mandy G. |last7=Schoolcraft |first7=William B. |date=June 2012 |title=Delivery of a chromosomally normal child from an oocyte with reciprocal aneuploid polar bodies |journal=Journal of Assisted Reproduction and Genetics |volume=29 |issue=6 |pages=533–537 |doi=10.1007/s10815-012-9746-6 |issn=1058-0468 |pmc=3370038 |pmid=22460080}}</ref>

In 2003, a study argued that many cases of [[triploid syndrome|triploidy]] arise from sesquizygotic (semi-identical) twinning which happens when a single egg is fertilized by two sperm and splits the three sets of chromosomes into two separate cell sets.<ref>{{cite journal
 | doi = 10.1093/humrep/deg060
 | last = Golubovsky
 | first = M.D.
 | date = February 2003
 | title = Postzygotic diploidization of triploids as a source of unusual cases of mosaicism, chimerism and twinning
 | journal = Human Reproduction
 | volume = 18
 | issue = 2
 | pages = 236–242
 | pmid = 12571155
 | doi-access = free
 }}
* {{cite journal |author=John Whitfield |date=26 March 2007 |title='Semi-identical' twins discovered |journal=Nature |doi=10.1038/news070326-1 |s2cid=85215225 |url=https://www.nature.com/news/2007/070326/full/news070326-1.html |access-date=7 February 2022 |archive-date=29 October 2021 |archive-url=https://web.archive.org/web/20211029052309/https://www.nature.com/news/2007/070326/full/news070326-1.html |url-status=live}}</ref><ref>{{cite web | url=https://edition.cnn.com/2019/02/28/health/rare-twins-semi-identical-australia-trnd/index.html | title=Semi-identical twins are rare, and doctors say they've identified the second case ever }}</ref>