Editing Twin (section)

==={{anchor|Dizygotic (fraternal) twins|Half-identical twins}}Semi-identical (sesquizygotic) twins===
[[Monozygotic]] twins can develop differently, due to their genes being differently activated.<ref>{{Cite book |first=Scott F. |last=Gilbert |title=Developmental biology |publisher=Sinauer Associates |location=Sunderland, Mass |year=2006 |isbn=978-0-87893-250-4 |chapter=Non-identical Monozygotic Twins |chapter-url=https://8e.devbio.com/article.php?id=111 |access-date=2008-09-30 |oclc=172964621 |url-status=dead |archive-url=https://web.archive.org/web/20090106171619/https://8e.devbio.com/article.php?id=111 |archive-date=2009-01-06}}</ref> More unusual are "semi-identical twins", also known as "sesquizygotic". {{As of|2019}}, only two cases have been reported.<ref name="NEJM"/><ref>{{cite web |title=Rare new kind of twins: boy and girl are semi-identical |website=CBC News |url=https://www.cbc.ca/news/technology/semi-identical-twins-1.5036783 |publisher=Thomson Reuters |access-date=28 February 2019 |date=28 February 2019 |archive-date=28 February 2019 |archive-url=https://web.archive.org/web/20190228160101/https://www.cbc.ca/news/technology/semi-identical-twins-1.5036783 |url-status=live}}</ref> These "half-identical twins" are hypothesized to occur when an [[ovum]] is fertilized by two [[sperm]]. The cell assorts the chromosomes by [[heterogonesis]] and the cell divides into two, with each daughter cell now containing the correct number of chromosomes. The cells continue to develop into a [[morula]]. If the morula then undergoes a twinning event, two embryos will be formed, with different paternal genes but identical maternal genes.<ref>{{cite web |last1=NEJM Group |title=Sesquizygotic Twinning |url=https://www.youtube.com/watch?v=06_nsL46rEY |website=YouTube |date=4 March 2019 |access-date=2023-04-29 |archive-date=2023-04-29 |archive-url=https://web.archive.org/web/20230429015258/https://www.youtube.com/watch?v=06_nsL46rEY |url-status=live}}</ref>

[[File:Hereford twins.jpg|thumb|right|Twin calves of the [[Hereford (cattle)|Hereford]] breed in [[Miles City, Montana]]]]

In 2007, a study reported a case of a pair of living twins, which shared an identical set of maternal chromosomes, while each having a distinct set of paternal chromosomes, albeit from the same man, and thus they most likely share half of their father's genetic makeup. The twins were both found to be [[chimera (genetics)|chimera]]s. One was an [[intersex]] XX, and one a [[phenotypical|XY]] male. The exact mechanism of fertilization could not be determined but the study stated that it was unlikely to be a case of polar body twinning.<ref>{{cite journal |last1=Souter |first1=Vivienne L |last2=Parisi |first2=Melissa A |last3=Nyholt |first3=Dale R |last4=Kapur |first4=Raj P |last5=Henders |first5=Anjali K |last6=Opheim |first6=Kent E |last7=Gunther |first7=Daniel F |last8=Mitchell |first8=Michael E |last9=Glass |first9=Ian A |last10=Montgomery |first10=Grant W |title=A case of true hermaphroditism reveals an unusual mechanism of twinning |journal=Human Genetics |date=2007 |volume=212 |issue=2 |pages=179–85 |doi=10.1007/s00439-006-0279-x |pmid=17165045|s2cid=3343267}}</ref><ref name="Reuters">{{cite web |last1=Dunham |first1=Will |title=Study describes new type of "semi-identical" twins |url=https://www.reuters.com/article/us-twins-identical-idUSN2640130120070326 |website=Reuters |publisher=Thomson Reuters Trust |access-date=28 February 2019 |archive-url=https://web.archive.org/web/20190228142954/https://www.reuters.com/article/us-twins-identical-idUSN2640130120070326 |archive-date=28 February 2019 |language=en |date=26 March 2007}}</ref>

The likely genetic basis of semi-identical twins was reported in 2019 by [[Michael T. Gabbett|Michael Gabbett]] and [[Nicholas Fisk (academic)|Nicholas Fisk]]. In their seminal publication, Gabbett, Fisk and colleagues documented a second case of sesquizygosis and presented molecular evidence of the phenomenon.<ref name=NEJM/> The reported twins shared 100% of their maternal chromosomes and 78% of their paternal genomic information. The authors presented evidence that two [[sperm]] from the same man fertilized an [[ovum]] simultaneously. The chromosomes assorted themselves through [[heterogonesis]] to form three cell lines. The purely paternal cell line died out due to [[genomic imprinting]] lethality, while the other two cell lines, each consisting of the same maternal [[DNA]] but only 50% identical paternal DNA, formed a [[morula]] which subsequently split into twins.<ref name="NEJM">{{cite journal |last1=Gabbett |first1=Michael T. |display-authors=et al. |title=Molecular Support for Heterogonesis Resulting in Sesquizygotic Twinning |journal=New England Journal of Medicine |date=28 February 2019 |volume=380 |issue=9 |pages=842–849 |doi=10.1056/NEJMoa1701313 |publisher=Massachusetts Medical Society |location=Boston |language=en |pmid=30811910|doi-access=free|hdl=10072/384437 |hdl-access=free }}</ref><ref>{{cite news |last1=Molina |first1=Brett |title=Doctors say they have identified rare set of semi-identical twins in Australia |url=https://www.freep.com/story/news/health/2019/02/28/semi-identical-twins-rarity-identified-australia-doctors-claim/3013228002/ |access-date=28 February 2019 |agency=USA TODAY |publisher=[[Gannett]] |date=28 February 2019 |archive-url=https://web.archive.org/web/20190228144337/https://www.freep.com/story/news/health/2019/02/28/semi-identical-twins-rarity-identified-australia-doctors-claim/3013228002/ |archive-date=28 February 2019 |language=en}}</ref>

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