Twin
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[[Image:TwinGirls.jpg|right|thumb|200px|Monozygotic twins.]]
'''Twins''' are two [[offspring]] resulting from the same [[pregnancy]], either of the same or opposite [[sex]].
The general term for more than one offspring from the same pregnancy is ''multiples'', for example ''triplets'' refers to cases of three offspring from the same pregnancy. A [[fetus]] alone in the womb is called a ''singleton''.
Twins are usually, but not necessarily, [[childbirth|born]] in close succession. Due to the limited size of the mother's womb, multiple pregnancies are much less likely to carry to full term than singleton births, with twin pregnancies lasting only 37 weeks on average, 3 weeks less than full term.<ref>{{cite journal
|journal=Clinical Perinatology
|year=2007
|month=Dec
|issue=4
|pages=599–609
|last=Elliott
|first=JP
|title=Preterm labor in twins and high-order multiples.
|quote=Unlike singleton gestation where identification of patients at risk for PTL is often difficult, every multiple gestation is at risk for PTL, so all patients can be managed as being at risk.
|pmid=18063108
|volume=34
|doi=10.1016/j.clp.2007.10.004}}</ref> Since [[Premature labour|premature births]] can have health consequences for the babies, twin births are often handled with special precautions.
Twins can either be monozygotic (MZ, colloquially, "identical") or dizygotic (DZ, colloquially, "fraternal"). There are estimated to be approximately 125 million human twins and triplets in the world (roughly 1.9% of the [[world population]]), and just 10 million monozygotic twins (roughly 0.2% of the world population and 8% of all twins).<ref>{{cite web
|url=http://www.esrc.ac.uk/ESRCInfoCentre/ about/CI/CP/Our_Society_Today/News_Articles_2006/twins.aspx?ComponentId=16191&SourcePageId=13438
|title=Twin Resources
|publisher=Economic and Social Research Council
|year=2006
|last=Oliver
|first=Judith
|accessdate=2008-02-06}}</ref> The current rate in the United States is 31 twin births per 1,000 women.<ref>{{cite web|url=http://rush.edu/rumc/page-1098987340603.html|title=Rush University Medical Center|accessdate=2008-03-06}}</ref>
==Types of twins ==
There are five common variations of twinning. The three most common variations are all dizygotic: (1) male-female twins are the most common result, at about 40 percent of all twins born; (2) female DZ twins (sometimes called sororal twins); (3) male DZ twins. The other variations are monozygotic twins: (4) female MZ twins and (5) (least common) male MZ twins. Male singletons are slightly (about five percent) more common than female singletons. (6) There is also the mirror image variations. This is where the twins develop reverse asymmetric features. About 25% of monozygotic twins are mirror image twins.The rates for singletons vary slightly by country <ref>{{cite web | url =https://www.cia.gov/library/publications/the-world-factbook | title=The CIA World Factbook|publisher=CIA.gov}}</ref>. For example, the sex ratio of birth in the [https://www.cia.gov/library/publications/the-world-factbook/geos/us.html#People US] is 1.05 males/female, while it is 1.07 males/female in [https://www.cia.gov/library/publications/the-world-factbook/geos/it.html#People Italy]. However, males are also more susceptible than females to death in utero, and since the death rate in utero is higher for twins, it leads to female twins being more common than male twins.
Another variety of twins, "[[polar body]] twins," (one egg fertilized by two different sperm) is a phenomenon that was hypothesized to occur and may recently have been proven, very rarely, to exist. Polar body twinning would result in [[half twin|"half-identical" twins]].<ref>{{cite web |url=http://www.bellaonline.com/articles/art32872.asp |title=Rare Forms of Twinning |publisher=bellaonline.com |author=Kris Bigalk |accessdate=2007-03-22}}</ref>
[[Image:Being a twin means you always have a pillow or blanket handy.jpg|thumb|left|Eight month old dizygotic twin girls napping.]]
===Dizygotic twins===
'''Dizygotic twins''' (commonly known as '''fraternal twins''', but also referred to as '''non-identical twins''' or '''biovular twins''') usually occur when two [[fertilization|fertilized]] eggs are implanted in the uterine wall at the same time. When two eggs are independently fertilized by two different [[sperm]] cells, DZ twins result. The two eggs, or 'ova', form two [[zygote]]s, hence the terms 'dizygotic' and 'biovular'.
Dizygotic twins, like any other siblings, have an extremely small chance of having the exact same chromosome profile. Like any other [[sibling]]s, DZ twins may look similar, particularly given that they are the same age. However, DZ twins may also look very different from each other. They may be different sexes or the same sex. The same holds true for brothers and sisters from the same parents, meaning that DZ twins are simply brothers and/or sisters who happen to have the same age.
Studies show that there is a genetic basis for DZ twinning. However, it is only the female partner that has any influence on the chances of having DZ twins as the male cannot make her release 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 name="Keith">{{cite book
|title=Multiple Pregnancy: Epidemiology, Gestation & Perinatal Outcome
|editor=Louis G. Keith
|author=R. Derom
|coauthors=J. Orlebeke, A. Eriksson, M. Thiery
|isbn=1850706662
|publisher=Taylor and Francis
|page=145
|chapter=The epidemiology of multiple births in Europe
|year=1995
}}</ref>
DZ 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
|coauthors=Fabio Parazzini, Liliane Chatenoud, Guido Benzi, Massimiliano Maria Bianchi, Alberto Marini
|publisher=European Society of Human Reproduction and Embryology
|issn=1355-4786
|pmid=10336022
|doi=10.1093/humupd/5.2.179
}}</ref> With the advent of technologies and techniques to assist women in getting pregnant, the rate of fraternals has increased markedly. For example, in [[New York City]]'s [[Upper East Side]] there were 3,707 twin births in 1995; there were 4,153 in 2003; and there were 4,655 in 2004. Triplet births have also risen, from 60 in 1995 to 299 in 2004.
===Monozygotic twins===<!-- This section is linked from [[Rheumatoid arthritis]] -->
[[Image:Identical-fraternal-sperm-egg.png|thumb|300px|Comparison of zygote development in monozygotic and dizygotic twins. In the uterus, monozygotic twins share the same [[placenta]] and [[amniotic sac]], while dizygotic twins do not.]]
'''Monozygotic twins''', frequently referred to as '''identical twins''', occur when a single [[ovum|egg]] is fertilized to form one [[zygote]] ('''monozygotic''') which then divides into two separate [[embryo]]s. Their traits and physical appearances are not exactly the same; although they have nearly identical DNA[http://www.msnbc.msn.com/id/23276953/], environmental conditions both inside the womb and throughout their lives influence the switching on and off of various genes. Division of the zygote into two embryos is not considered to be a hereditary trait, but rather an anomaly that occurs in birthing at a rate of about three in every 1000 deliveries worldwide,<ref>{{cite web |url=http://www.wonderquest.com/TwinsTrigger.htm |title=What triggers twinning? |author=April Holladay |date=2001-05-09 |publisher=WonderQuest |accessdate=2007-03-22}}</ref> regardless of ethnic background. The two embryos develop into [[fetus]]es sharing the same womb. When one egg is fertilized by one sperm cell, and then divides and separates, two identical cells will result. If the zygote splits very early (in the first two days after fertilization), each cell may develop separately its own [[placenta]] ([[chorion]]) and its own sac ([[amnion]]). These are called '''dichorionic diamniotic''' ('''di/di''') '''twins''', which occurs 20–30% of the time. Most of the time in MZ twins the zygote will split after two days, resulting in a shared placenta, but two separate sacs. These are called '''monochorionic diamniotic''' ('''mono/di''') '''twins'''.
In about one percent of MZ twinning the splitting occurs late enough to result in both a shared placenta and a shared sac called '''monochorionic monoamniotic''' ('''mono/mono''') '''twins'''. Finally, the zygote may split extremely late, resulting in [[conjoined twins]]. Mortality is highest for conjoined twins due to the many complications resulting from shared organs. Mono/mono twins have an overall in-utero mortality of about 50 percent, principally due to cord entanglement prior to 32 weeks gestation. If expecting parents choose hospitalization, mortality can decrease through consistent monitoring of the babies. Hospitalization can occur beginning at 24 weeks, but doctors prefer a later date to prevent any complications due to premature births. The choice is up to the parents when to start hospitalization. Many times, monoamniotic twins are delivered at 32 weeks electively for the safety of the babies. In higher order multiples, there can sometimes be a combination of DZ and MZ twins.
Mono/di twins have about a 25 percent mortality due to [[twin-to-twin transfusion syndrome]]. Di/di twins have the lowest mortality risk at about nine percent, although that is still significantly higher than that of singletons.<ref>Benirschike K, "Multiple Gestation" in Creasy RK, Resnik R, (eds.) Maternal-Fetal Medicine: Principles and Practice; 5th ed. (2004):55-62. Philadelphia: Saunders.</ref>
Monozygotic twins are genetically identical (unless there has been a mutation in development) and they are always the same sex. On rare occasions, monozygotic twins may express different phenotypes, normally due to an environmental factor or the deactivation of different X chromosomes in monozygotic female twins, and in some extremely rare cases, due to [[aneuploidy]], twins may express different sexual phenotypes, normally due to an XXY [[Klinefelter's syndrome]] zygote splitting unevenly <ref>Edwards, J. H., Dent, K., and Kahn,J. Monozygotic twins of different sex. J. Med. Genet. 1966 3: 117-123.</ref> <ref>Machin, G. A. Some causes of genotypic and phenotypic discordance in monozygotic twin pairs. Amer. J. Med. Genet. 1996 61: 216-228</ref>). Monozygotic twins look alike, although they do not have the same [[fingerprint]]s (which are environmental as well as genetic). As they mature, MZ twins often become less alike because of lifestyle choices or external influences. Genetically speaking, the children of MZ twins are half-siblings rather than cousins. If each member of one set of MZ twins reproduces with one member of another set of MZ twins then the resulting children would be genetic full siblings. It is estimated that there are around 10 million monozygotic twins and triplets in the world.
The likelihood of a single fertilisation resulting in MZ twins appears to be a random event, not a hereditary trait, and is uniformly distributed in all populations around the world.<ref name="Bortolus" /> This is in marked contrast to DZ twinning which ranges from about six per thousand births in Japan (almost similar to the rate of MZ twins, which is around 4-5 to 15 and more per thousand in some parts of India<ref>Oleszczuk JJ, Keith DM et al. [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?&db=pubmed&cmd=Retrieve&dopt=abstractplus&list_uids=10589400 Projection of Population-based twinning rates through the year 2100]. ''J Reprod Med''. 44(11), 1999. 913–921.</ref>) and up to 24 in the US, which might mainly be due to IVF (in vitro fertilisation). The exact cause for the splitting of a zygote or embryo is unknown.
Monozygotic twins have nearly identical [[DNA]] but differing environmental influences throughout their lives affect which genes are switched on or off. This is called [[Epigenetics|epigenetic modification]]. A study of 80 pairs of human twins ranging in age from three to 74 showed that the youngest twins have relatively few epigenetic differences. The number of epigenetic differences between MZ twins 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>{{Citation
| last = Fraga | first = M.
| last2 = Ballestar | first2 = E.
| last3 = Paz | first3 = M.
| last4 = Ropero | first4 = S.
| last5 = Setien | first5 = F.
| last6 = Ballestar | first6 = M.
| last7 = Heine-Suner | first7 = D.
| last8 = Cigudosa | first8 = J.
| last9 = Urioste | first9 = M.
| last10 = Benitez | first10 = J.
| last11 = Boix-Chornet | first11 = M.
| author11-link = M. Boix-Chornet
| last12 = Sanchez-Aguilera | first12 = A.
| author12-link = A. Sanchez-Aguilera
| last13 = Ling | first13 = C.
| author13-link = C. Ling
| last14 = Carlsson | first14 = E.
| author14-link = E. Carlsson
| last15 = Poulsen | first15 = P.
| author15-link = P. Poulsen
| last16 = Vaag | first16 = A.
| author16-link = A. Vaag
| last17 = Stephan | first17 = Z.
| author17-link = Z. Stephan
| last18 = Spector | first18 = T.
| author18-link = T. Spector
| last19 = Wu | first19 = Y.
| author19-link = Y. Wu
| last20 = Plass | first20 = C.
| author20-link = C. Plass
| last21 = Esteller | first21 = M.
| author21-link = M. Esteller
| title = Epigenetic differences arise during the lifetime of monozygotic twins
| journal = [[Proceedings of the National Academy of Sciences]]
| volume = 102
| issue = 30
| pages = 10413—10414
| year = 2005}}</ref> However, certain characteristics become more alike as twins age, such as IQ and personality.<ref>Segal, K. Entwined Lives: Twins and What They Tell Us About Human Behavior, Plume Publishing, 2002</ref><ref>DeFries, J. McGuffin, P., McClearn, G., Plomin, R., Behavioural Genetics, Worth Publishers, 4th Edition, 2000</ref> This phenomenon illustrates the influence of genetics in many aspects of human characteristics and behaviour.
A recent theory posits that monozygotic twins are formed after an embryo essentially collapses, splitting the progenitor cells (those that contain the body's fundamental genetic material) in half. That leaves the same genetic material divided in two on opposite sides of the embryo. Eventually, two separate fetuses develop. The research was presented at a meeting of the [[European Society of Human Reproduction and Embryology]] in [[Lyon]], [[France]]. Utilizing computer software to take photos every two minutes of 33 embryos growing in a laboratory, Dr. Dianna Payne, a visiting research fellow at the Mio Fertility Clinic in Japan, documented for the first time the early days of twin development. Payne also discovered explanation for why in-vitro fertilization techniques are more likely to create twins. Only about three pairs of twins per 1,000 deliveries occur as a result of natural conception, while for IVF deliveries, there are nearly 21 pairs of twins for every 1,000.<ref>{{cite web |url=http://health.yahoo.com/news/177033 |title=Study: Twins form after embryo collapses |author=Associated Press |accessdate=2007-07-03}}</ref>
===Zygosity, chorionicity and amniocity===
The two types of twins discussed before, monozygotic and dizygotic, are generally referred to as zygocity. Zygocity reflects the genetic type of twins. Two others terms define twin types: chorionity and amniocity. Chorionity refers to the number of chorionic sacs, while amniocity refers to the number of amniotic sacs. The number of chorionic and amnionic sacs can sometimes reveal the zygocity. Monoamniotic twins indicate monozygotic twins. However, two placentas does not provide information about zygocity since monozygotic twins can have two placentas. Chorionicity and amniocity are a result of the division time. Dichorionic twins divide within the first 4 days. Monoamnionic twins divide after the first week of life.
==Demographics==
A recent study found that [[Veganism|vegan]] mothers are five times less likely to have twins than those who eat animal products.<ref name="vegan">[http://www.reproductivemedicine.com/toc/auto_abstract.php?id=22626 Gary Steinman, M.D., Ph.D. Mechanisms of Twinning: VII. Effect of Diet and Heredity on the Human Twinning Rate. Journal of Reproductive Medicine May 2006.]</ref>
From 1980–97, the number of twin births in the United States rose 52%.<ref>[http://www.multiplebirth.com/nvs47_24.pdf National Vital Statistics Reports Vol. 47, No. 24: "Trends in Twin and Triplet Births: 1980-1997"] (PDF)</ref> This rise can at least partly be attributed to the increasing popularity of [[fertility medication|fertility drugs]] like [[Clomid]] and procedures such as [[in vitro fertilization]], which result in multiple births more frequently than unassisted fertilizations do. It may also be linked to the increase of [[growth hormone]]s in food.<ref name="vegan"/>
===Ethnicity===
About 1 in 90 human births (1.1%) results from a twin pregnancy.<ref name="asch">{{cite book |last=Asch |first=Ricardo |title=Progress in Reproductive Medicine |year=1995 |publisher=Informa Health Care |isbn=1850705747 }}</ref> The rate of ''dizygotic'' twinning varies greatly among [[ethnic group]]s, ranging as high as about 6% for the [[Yoruba people|Yoruba]] or 10% for Linha Sao Pedro, a tiny Brazilian village.<ref>U. Matte et al. [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?holding=npg&cmd=Retrieve&db=PubMed&list_uids=9181177&dopt=Abstract Study on possible increase in twinning rate at a small village in south Brazil]. ''Acta Genet Med Gemellol'' (Roma). 45(4), 1996. 431–437.</ref> The widespread use of fertility drugs causing hyperovulation (stimulated release of multiple eggs by the mother) has caused what some call an "epidemic of [[multiple birth]]s". In 2001, for the first time ever in the [[United States|US]], the twinning rate exceeded 3% of all births. Thus, approximately 5.8% of children born in the US in 2001 were twins. Among [[Hausa]] of Nigeria and Niger the incidence of multiple births was
studied using the maternity records of 5750 Hausa women living in the savannah zone of Nigeria. There were 40 twins and 2 triplets/1000 births. Twenty six per cent of twins were monozygous. The incidence of multiple births, which was about five times higher than that observed in any western population, was significantly lower than that of other ethnic groups, who live in the hot and humid climate of the southern part of country. The incidence of multiple births was related to maternal age but did not bear any association to the climate or prevalence of [[malaria]].<ref name="hausa">{{cite web |url=http://www.blackwell-synergy.com/doi/abs/10.1111/j.1471-0528.1980.tb04464.x |publisher="An International Journal of Obstetrics and Gynaecology" |title="MULTIPLE BIRTHS IN HAUSA WOMEN"}}</ref> Nevertheless, the rate of ''monozygotic'' twins remains at about 1 in 333 across the globe, further suggesting that pregnancies resulting in identical twins occur randomly.
===Predisposing factors===
The cause of monozygotic twinning is unknown.
Dizygotic twin pregnancies are slightly more likely when the following factors are present in the woman:
*She is of [[West Africa]]n descent (especially [[Yoruba people|Yoruba]] or [[Hausa people|Hausa]])<ref name=Bortolus />
*She is between the age of 30 and 40 years
*She is greater than average height and weight
*She has had several previous pregnancies.
*She has a family history of dizygotic twinning, especially a mother who is a twin.
Women undergoing certain fertility treatments may have a greater chance of dizygotic multiple births. This can vary depending on what types of fertility treatments are used. With [[in vitro fertilisation]] (IVF), this is primarily due to the insertion of multiple embryos into the uterus. Some other treatments such as the drug [[Clomid]] can stimulate a woman to release multiple eggs, allowing the possibility of multiples. Many fertility treatments have no effect on the likelihood of multiple births.
There is also speculation that the West African predisposition to twinning is due to the large amount of [[Yam (vegetable)|yams]] in their diet because yams contain [[phytoestrogens]], which may stimulate the ovaries. Most likely this is a myth, as phytoestrogens would tend to suppress twinning by reducing [[gonadotropin]] levels.
==Complications of twin pregnancy==
===Vanishing twins===
{{Main|Vanishing twin}}
Researchers suspect that as many as 1 in 8 pregnancies start out as multiples, but only a single fetus is brought to full term, because the other has died very early in the pregnancy and not been detected or recorded.<ref name="Keith-2">{{cite book
|title=Multiple Pregnancy: Epidemiology, Gestation & Perinatal Outcome
|editor=Louis G. Keith
|author=L. Gedda
|isbn=1850706662
|publisher=Taylor and Francis
|page=4
|chapter=The role of research in twin medicine.
|year=1995
}}</ref> Early [[obstetric ultrasonography]] exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes. This is known as [[vanishing twin]] syndrome.
[[Image:Chang-eng-bunker-PD.gif|thumb|[[Chang and Eng Bunker]], born in Siam (now [[Thailand]]) in 1811, were the origin of the term "Siamese twins".]]
===Conjoined twins===
{{Main|Conjoined twin}}
Conjoined twins (or "Siamese twins") are monozygotic twins whose bodies are joined together at birth. This occurs where the single zygote of MZ twins fails to separate completely, and the zygote starts to split after day 13 following fertilization. This condition occurs in about 1 in 50,000 human pregnancies.
Most conjoined twins are now evaluated for surgery to attempt to separate them into separate functional bodies. The degree of difficulty rises if a vital organ or structure is shared between twins, such as [[brain]], [[heart]] or [[liver]].
===Chimerism===
{{Main|Chimera (genetics)}}
A chimera is an ordinary person or animal except that some of their parts actually came from their twin or from the mother. A chimera may arise either from monozygotic twin fetuses (where it would be impossible to detect), or from dizygotic fetuses, which can be identified by chromosomal comparisons from various parts of the body. The number of cells derived from each fetus can vary from one part of the body to another, and often leads to characteristic [[mosaicism]] skin colouration in human chimeras. A chimera may be a [[hermaphrodite]], composed of cells from a male twin and a female twin.
===Parasitic twins===
{{Main|Parasitic twin}}
Sometimes one twin fetus will fail to develop completely and continue to cause problems for its surviving twin. One fetus acts as a [[parasite]] towards the other.
Sometimes the parasitic twin becomes an almost indistinguishable part of the other, and sometimes this needs to be medically dealt with.
===Partial molar twins===
A very rare type of parasitic twinning is one where a single viable twin is endangered when the other zygote becomes cancerous, or molar. This means that the molar zygote's cellular division continues unchecked, resulting in a cancerous growth that overtakes the viable fetus. Typically, this results when one twin has either [[triploidy]] or complete paternal [[uniparental disomy]], resulting in little or no fetus and a cancerous, overgrown placenta, resembling a bunch of grapes.
===Miscarried twin===
Occasionally, a woman will suffer a [[miscarriage]] early in pregnancy, yet the pregnancy will continue; one twin was miscarried but the other was able to be carried to term. This occurrence is similar to the [[vanishing twin]] syndrome, but typically occurs later than the vanishing twin syndrome.
===Low birth weight===
Twins typically suffer from the lower [[birth weight]]s and greater likelihood of [[Premature birth|prematurity]] that is more commonly associated with the higher multiple pregnancies. Throughout their lives twins tend to be smaller than singletons on average.{{Fact|date=February 2008}}
===Twin-to-twin transfusion syndrome===
{{Main|Twin-to-twin transfusion syndrome}}
Monozygotic twins who share a placenta can develop twin-to-twin transfusion syndrome. This condition means that blood from one twin is being diverted into the other twin. One twin, the 'donor' twin, is small and [[anemic]], the other, the 'recipient' twin, is large and [[polycythemic]]. The lives of both twins are endangered by this condition.
==Human twin studies==
{{Main|Twin study}}
Twin studies are studies that assess monozygotic twins for [[medical]], [[genetics|genetic]], or [[psychology|psychological]] characteristics to try to isolate genetic influence from epigenetic and [[natural environment|environment]]al influence. Twins that have been separated early in life and raised in separate households are especially sought-after for these studies, which have been invaluable in the exploration of [[human nature]].
Twin studies are called consanguity studies, observing the differences or similarities of twins in different environments to see how much of their behavior is attributable to genetics or environments influence.
===Unusual twinnings===
There are some patterns of twinning that are exceedingly rare: while they have been reported to happen, they are so unusual that most [[obstetrics|obstetricians]] or [[midwifery|midwives]] may go their entire careers without encountering a single case.{{Fact|date=March 2008}}
Among dizygotic twins, in rare cases, the eggs are fertilized at different times with two or more acts of sexual intercourse, either within one menstrual cycle ([[superfecundation]]) or, even more rarely, later on in the pregnancy ([[superfetation]]). This can lead to the possibility of a woman carrying fraternal twins with different fathers (that is, [[Half-sibling#Half sibling|half-siblings]]). This phenomenon is known as heteropaternal superfecundation. One 1992 study estimates that the frequency of heteropaternal superfecundation among dizygotic twins whose parents were involved in paternity suits was approximately 2.4%; see the references section, below, for more details.
Dizygotic twins from [[biracial]] couples can sometimes be [[mixed twins]] - which exhibit differing ethnic and racial features.
Among monozygotic twins, in extremely rare cases, twins have been born with opposite sexes (one male, one female). The probability of this is so vanishingly small (only 3 documented cases<ref>[http://jmg.bmj.com/cgi/content/abstract/13/1/64 Monozygotic twins discordant for sex - Schmidt et al. 13 (1): 64 - Journal of Medical Genetics<!-- Bot generated title -->]</ref>) that multiples having different sexes is universally accepted as a sound basis for a clinical determination that ''in utero'' multiples are not monozygotic. When monozygotic twins are born with different sexes it is because of chromosomal birth defects. In this case, although the twins did come from the same egg, it is incorrect to refer to them as genetically identical, since they have different [[karyotype]]s.
[[Image:Hereford twins.jpg|thumb|right|200px|Twin calves of the [[Hereford (cattle)|Hereford]] breed.]]
===Semi-identical twins===
[[Monozygotic]] twins can develop differently, due to different genes being activated.<ref>[http://7e.devbio.com/article.php?id=111 Non-identical Monozygotic Twins]</ref> More unusual are "semi-identical twins". These "half-identical twins" are hypothesized to occur when an unfertilized egg cleaves into two identical attached ova and which are viable for fertilization. Both cloned ova are then fertilized by different sperm and the coalesced eggs undergo further cell duplications developing as a chimeric [[blastomere]]. If this blastomere then undergoes a twinning event, two embryos will be formed, each of which have different paternal genes and identical maternal genes.
This results in a set of twins with identical genes from the mother's side, but different genes from the father's side. Cells in each fetus carry genes from either sperm, resulting in [[chimera (genetics)|chimera]]s. This form had been speculated until only recently being recorded in western medicine<ref>Vivienne L. Souter, Melissa A. Parisi, Dale R. Nyholt, Raj P. Kapur, Anjali K. Henders, Kent E. Opheim, Daniel F. Gunther, Michael E. Mitchell, Ian A. Glass and Grant W. Montgomery, [http://www.springerlink.com/content/u5274762692l/?p=3240954dfd6c4c41a596820c311e2be5&pi=0 A case of true hermaphroditism reveals an unusual mechanism of twinning], Human Genetics journal (Vol 121, No 2, pp179-185), April 2007; ISSN 0340-6717 (Print), 1432-1203 (Online)</ref> <ref>[http://news.yahoo.com/s/livescience/20070326/sc_livescience/raresemiidenticaltwinsdiscovered Rare Semi-Identical Twins Discovered]</ref>
<ref>[http://www.nature.com/news/2007/070326/full/070326-1.html 'Semi-identical' twins discovered] from [[Nature (journal)|Nature]]</ref>.
==Animal twins==
Twins are common in many animal species, such as [[cat]]s, [[sheep]], [[ferret]]s and deer. The incidence of twinning among [[cattle]] is about 1-4%, and research is under way to improve the odds of twinning, which can be more profitable for the breeder if complications can be sidestepped or managed. A species of armadillo (''[[Nine-banded Armadillo|Dasypus novemcinctus]]'') has identical twins (usually four babies) as its regular reproduction and not as exceptional cases.{{Fact|date=August 2007}}
==See also==
{{Commonscat|Twins}}
*[[Evil twin]]
*[[Incest between twins]]
*[[List of multiple births]]
*[[List of twins]]
*[[Look-alike]]
== Notes ==
{{reflist|2}}
== References ==
# {{cite journal | author=Nieuwint A, Van Zalen-Sprock R, Hummel P, Pals G, Van Vugt J, Van Der Harten H, Heins Y, Madan K. | title='Identical' twins with discordant karyotypes | journal=Prenatal Diagnosis | volume=19 | issue=1 |year=1999 | pages=72–6 | pmid=10073913 | doi = 10.1002/(SICI)1097-0223(199901)19:1<72::AID-PD465>3.0.CO;2-V | doilabel = 10.1002/(SICI)1097-0223(199901)19:1<72::AID-PD465>3.0.CO;2-V}}
# {{cite journal | author=Wenk RE, Houtz T, Brooks M, Chiafari FA | title=How frequent is heteropaternal superfecundation? |journal=Acta geneticae medicae et gemellologiae | year=1992 | volume=41 | issue=1 | pages=43–7 | pmid=1488855}}
# {{cite journal | author=Girela E, Lorente JA, Alvarez JC, Rodrigo MD, Lorent M, Villaneuva E | journal=Fertility and Sterility | title=Indisputable double paternity in dizygous twins | year=1997 | volume=67 | issue=6 | pages=1159–61 | pmid=9176461 | doi = 10.1016/S0015-0282(97)81456-2}}
==Further reading==
[http://www.aftau.org/site/News2?page=NewsArticle&id=6847 Male Twins Negatively Affect Females in Utero, New Study Finds]
* Schein, E., Bernstein, P. (Random House, 2007). "Identical Strangers: A Memoir of Twins Separated and Reunited."
* Helle, S., Lummaa, V. & Jokela, J. (2004). Selection for increased brood size in pre-industrial humans. ''Evolution 52:'' 430-436. [http://www.shef.ac.uk/aps/hlhp/helleevolution2004.pdf Full text]
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