Editing Twin (section)

====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>