Twins

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Twins ( William Adolphe Bouguereau : Caritas , 1859)

Twins (Latin: gemini ) are medically precisely formulated two children of a mother and a father who were conceived on the same day (during the same sexual intercourse). Colloquially, however, all children are referred to as twins who grew up within the same pregnancy and are usually born in the same birth process .

In the rare case of excessive impotence , two children are also born in one birth process, but no twins. The word twin, older also zwiniling , gezwinele , is a derivation of the numerical word two and originally means "what occurs twice", "of which there is a second".

Forms of twinning

Sonogram of identical human twins (4th week pc ) - it shows dichorionic twins who have neither chorion nor amnion in common.

The two most well-known forms of twin formation are:

Identical twins ( monozygotic )

The fertilized egg cell ( zygote ) divides in the course of development, the cells resulting from the division also and so on. If there is a separation into two cell populations at a very early stage of development, two embryonic systems can arise. Both are then emerged from a single fertilized egg monozygotic with the same genetic makeup and thus the same genetic material . Depending on the time of separation, twin forms with separate or common membranes of chorion or amnion arise .

Fraternal twins ( dizygotic )

Within one cycle, two egg cells have matured and each have been fertilized by a different sperm . The two zygotes result in dizygotic twins with different genetic material and each with their own chorion and amnion.

Three other extremely rare ways in which twins can be formed are:

Polar body twins

Polar bodies formed during the meiosis of the egg in the fallopian tube , usually do not have a cell body and of the uterus repelled. As an exception, polar bodies can retain a small cell body which, in addition to the mature egg, can also be fertilized by sperm. These twins are no more similar than normal dizygoti twins, since the polar bodies were created through meiosis and differ from the egg cell in terms of genetic material.

Twins from a binuclear egg cell

In rare cases, an egg cell has two cell nuclei. In such a case, both nuclei can be fertilized by sperm. Such twins therefore have the same genes on the mother's side, but differ in the traits inherited from the father. Therefore, this form is not counted among the classic identical twins.

Sesquizygotic twins

This possibility has only been observed twice worldwide (as of 2019). Two sperm penetrate the same egg cell. Three cells are formed: An XX cell (female) from the genetic material of the egg cell and one sperm, an XY cell (male) from the egg and the other sperm, and an XY cell that only contains paternal genetic material . While the latter cell dies, the other two cells divide and multiply. At some point the cluster of cells divides into two with different proportions of XX or XY cells, from which two twins develop. Because of this, the twins exhibit intersex characteristics.

Forms of identical twins

Identical twins (the Carlsons )

The cause of the different developments of identical twins is the specialization of the cells in the first days after fertilization. If the cells are the same in the first three days, they then specialize: Some form the nourishing part ( trophoblast , forms the placenta ), others the embryonic part ( embryoblast ). With this specialization, the outer membrane ( chorion ) is formed, a few days later the inner membrane ( amnion ) is formed. These membranes cannot be split up afterwards. Depending on whether the separation in two germinal systems was completed before or after the formation of the outer membrane, each embryo has its own chorion (dichorial twins) or both twin embryos are in the common chorion. The same applies to the inner membrane.

Distribution of the germ up to the 3rd day after fertilization ( dichorial twins )

There is a full division. Both parts of the germ implant themselves in the uterus independently of one another. Two placentas and two amniotic sacs develop .

Division between the 3rd and 7th day after fertilization ( monochorial-diamniote twins )

There is a complete division of the embryonic part, but no longer the nourishing part. The two embryos nestle together in the womb and there are two amniotic sacs . However, the children are supplied from a common placenta , which means that there is a risk of fetofetal transfusion syndrome .

Division after the 9th day after fertilization ( monochorionic-monoamniotic twins )

The two embryos share the placenta and the amniotic sac. If the division does not take place until after the 13th day after fertilization, the division can often no longer take place completely; this is how Siamese twins are formed .

Different sexes

In rare cases, identical twins show different sexes. From a fertilized egg cell with the sex chromosomes XY, a cell with a nucleus that has not received a Y chromosome can develop during the subsequent cleavage divisions . This loss of chromosomes in a stem cell is due to faulty mitosis . A twin without a Y chromosome develops into a girl who can suffer from the effects of Turner syndrome .

Forms of fraternal twins

Two fertilized human egg cells

Dizygoti twins are formed when two egg cells mature and are fertilized within one cycle. Therefore, dizygoti twins can also be of different sexes. These do not have to be fertilized in the same sexual act , but if they develop on different days, they are not medically twins. The two ovulations do not necessarily have to occur at the same time, but they usually happen within around 24 hours, as the woman's hormonal situation then changes so that no further ovulation is possible.

Due to the time interval between the two ovulations, there is a possibility that the egg cells will be fertilized by two different fathers. There is already an example of such “half twins” in Greek mythology with the two Dioscuri Castor and Pollux .

In contrast to identical twins, a familial clustering has been demonstrated in dizygotic twins. The tendency to have twin births is inherited on the maternal side: the probability of twin births is more than doubled for sisters of twin mothers. In addition to family tree analyzes, the population differences also show a genetic component.

Frequency of twin births in humans

Two identical pairs of twins:
the handball referees Bernd and Reiner Methe (left) and Andreas and Marcus Pritschow (right).

Worldwide, on average every 40th birth is a twin birth. There are, however, regional differences: For the Yoruba , for example, it is every 20th birth, in Japan only every 100. In Europe, a significant decrease in frequency from north to south has been demonstrated. In 2006, one in 64 pregnant women in Germany gave birth to twins. The number of twin births is steadily increasing due to hormone treatment and artificial insemination. For example, in Andalusia the number of twin births rose from 10.9 per 1000 births in 2000 to 16.2 per 1000 births in 2009.

According to Statistics Austria, there were 1,292 twin births in Austria in 2015 , in Germany there were 10,538 out of a total of 673,000 births according to the Federal Statistical Office and in Switzerland, according to the Federal Statistical Office, there were 1,208 twin births in 2007. That meant for Germany in 2006 that a pregnant woman had a 1.57% chance of having twins, the chance in Austria 2015 was 1.53%.

The distribution of the birth rates of identical to dizygotic twins changes significantly by year and region between 1: 4 and 1: 1. The birth rate of identical twins has proven to be almost constant over the years and regions. Dizygoti multiple births have increased in rich countries in recent years. The main causes of the increase are the increasing age of the parturient and the fertility treatments.

Mothers of twins

Analyzes of data from the 19th century from Utah on women from monogamous marriages who have been married once and who have reached the age of 50 or more allow the conclusion that the almost eight percent mothers of twins generally have a better constitution. For twin mothers born before 1870, the risk of death was 7.6% lower, for those born between 1870 and 1899 it was 3.3% lower. In the course of life they also had more single children, the time between two births was shorter by an average of two weeks, the period between the first and last birth was a few months longer, and the time of the last birth was on average later. Large women in good nutrition are 25 to 30 percent more likely to have twins.

Although fertility decreases in older women, they are much more likely to have multiple twins or multiple births. According to a study published in the Netherlands in 2006, the cause could be the increased production of a hormone that triggers egg maturation. This may be the reason for more, double ovulations within a cycle.

Views on Twins in Ancient Times

Procreation theories

Several theories of procreation for twins circulated in Greco-Roman antiquity. The oldest views on twin births are found in mythological accounts. There paternity receives special attention. Causes of twin births are on the one hand the special intervention of a god (Godfatherhood), on the other hand a certain double fatherhood: Either a deity and a mortal or two mortal people are accepted as fathers. The attribution of the twins to a single mortal father is less common.

Empedocles ( Frg. A 81 Diels-Kranz I, 300, line 10. 15) offers the oldest scientific and medical attempt at explaining twin births: twins result from the division of excessive seminal fluid in a single coitus. Democritus ( Frg. A 151 Diels-Kranz II, 125, lines 21-25) also brings the condition of the uterus into play. Hippocrates ( De natura pueri 31; De victu 30 ) combines both theories. The question he left open, how many sexual acts are required for a twin birth, is discussed in the following years with different solutions. Aristotle ( De generatione animalium 772b; Historia animalium 584b - 585a) enlivens the discussion with his theory of a post-conception ( superfecundatio ) and, together with Hippocrates, has a decisive influence on the whole further discussion of the ancient problem (cf. Pliny the Elder , Naturalis historia VII, 47 -49). Augustine ( De civitate dei V, 1.6) and other Christian authors reject the Aristotelian theory of post-conception and teach a single procreation in a single act of procreation, so that from then on the theory of Hippocrates clearly regained importance. The influence of these medical and scientific theories on the consciousness of the general population should by no means be overestimated. According to many evidence of ancient folk belief, the older mythical ideas prevail there.

Biological-physiological, family-sociological and slave-law views about twins

In Greco-Roman antiquity, some fundamental biological-physiological, family-sociological and slave law considerations about twins also emerged.

Biological-physiological aspects

An inheritance of the disposition to twin births is clearly affirmed in antiquity for animals, very rarely only for humans. It is worth mentioning that an accumulation of twin births can be observed in individual mythological family trees. The climatic-geographic favoritism of twin births is attributed to certain landscapes, especially Egypt. The survival chances of twins are seen from information from the ancient testimonies as clearly endangered, especially if a lower birth weight is present in one of the twins.

Family sociological aspects

Mythological evidence often suggests a sexual relationship between the children in the womb in the case of same-sex twins. The treatment of the twins depends on the assessment of this prenatal incest (cf. Byblis in Ovid , Metamorphosen 9, 447-665). If this relationship was judged negatively, one of the twins was killed, mostly the female, or the twins separated. At the same time there was - especially in Egypt - the tradition, which can be documented until the turn of the ages, of marrying such twins together. The attitudes towards twins and their mother handed down in ancient myths are closely related to the above-mentioned views on twin fatherhood. The mother and twin children are sometimes threatened with abandonment, killing or displacement from the community. Only Christianity, with its consistent affirmation of the child, adopted from Judaism, and with its strict ban on killing and abortion, ensures the unrestricted right to life of both twins and their mother. The question of how far the psychic identity or parallelism of twins goes is answered in the direction that there is no such thing as complete indistinctness ( Cicero , Academica 2,54-57). Social behavior patterns such as inseparability, violent quarrels between twins and the dominance of one of the two twins, which modern twin research also picks up on, are often documented for antiquity (on this Rathmayr 2000, 89-100). The explosive problem of the firstborn (primogeniture) arises particularly for dynastic families. Because the eldest son usually succeeds in power, the family marks the firstborn of the twins e.g. B. by characters or by naming. If this priority marking was omitted, problems of rule were inevitable, as the legend of Romulus and Remus in particular attests. The story of Jakob and Esau , however, also knows that the younger are given priority over the older.

Aspects of slave law

Slavery twins were of considerable value to their owner in ancient times, so large sums of money were willingly paid for them. However, twins only retained their value as a couple; if one of them was killed, the price was always to be paid for both ( Gaius , Institutiones 3, 212; Digesten 9,2,22,1).

Views About Twins In Other Cultures

In many traditional cultures, the birth of twins was considered unnatural for humans. Twins were despised by some peoples and partly (sometimes with their mother) killed. In Africa, same-sex twins in particular should cause harm because they are supposed to be associated with evil bush spirits who also appear in pairs. On the Siberian peninsula of Kamchatka , according to a description from 1774, it was believed that the father of twins was always a wolf.

“If [the children] are twins, wrong notions of decency and family honor require that one of the children be killed. To bring twins into the world means to surrender to the general mockery, it means to act like rats, pouch animals and the lowest creature that gives birth to many young at the same time. But even more: 'Two children born at the same time cannot be of one father'. […] For the sake of domestic peace, the mother's old bases or the mure japoic-nei [midwife] take it upon themselves to put one of the children aside. "

- Alexander von Humboldt on the Sáliva in: Journey to the equinoctial regions of the new continent . Volume 3, p. 154

In traditional African beliefs, twins are predominantly considered to be unusual people, whose shadow soul (in the West African Malinke and Bambara it is called dya ) is closely related to the Creator God. Twins are a sign of blessing for their parents, the Yoruba see them as healing. Their appreciation in West Africa is also related to the couples who created the African cosmogony . Since twins are often more susceptible to disease and are more thin, they are only carried out later than other children. Only when both have survived the first few years does their mother take them to the market, because she now assumes that the sensitive souls of the children cling tightly to their bodies and do not get lost. At Homowo Festival, a harvest festival celebrated by the Ga in Ghana at the beginning of the rainy season in spring, there is a special ritual program for twins, where they receive sacred food, which at the same time preserves their magical abilities and protects them from danger should.

Twin research

The Watson Twins ( The Watson Twins )

The twin research allows interesting statements regarding the question of how much of human behavior through the genome and which is due to the environment.

intelligence

In order to investigate which part of the variance of the intelligence quotients within a population is due to hereditary and which due to environmental factors, called heritability or (misleadingly) heredity by the population geneticists , one takes the additive composition of the total variance based on a classic approach by Ronald Aylmer Fisher from individual, independent variances. Then the total variance in a population can be divided into a hereditary and an environmental variance: ${\displaystyle (VAR_{G})}$ ${\displaystyle (VAR_{E})}$ ${\displaystyle (VAR_{U})}$

${\displaystyle VAR_{G}=VAR_{E}+VAR_{U}}$

Identical and dizygoti twin pairs are examined with intelligence tests to determine the hereditary component . Since dizygoti twins, like all siblings, agree on fifty percent of their genetic makeup, and identical twins one hundred percent, if part of the variance of the intelligence factor is hereditary, identical twins should be more similar to one another than dizygoti on the results of the tests. The variance between the identical twins serves as an estimator for the proportion of variance that can be attributed to environmental influences. Identical twins have an identical genetic makeup, so differences in their intelligence should only be due to environmental influences. In the case of the dizygotic twins, both genetic and environmental influences should be noticeable in the result of the tests, whereby ideally the proportion of environmental variance would be the same as in the case of the identical twins. This means that the difference in the variance can only be explained genetically.

This research method is based on the assumption that both monozygotic and dizygotic twins experience an almost identical environment (known in research as the equal environment assumption , or EES for short). However, this is often not the case - identical couples are often treated differently. In fact, it can even be shown that identical twins with development in the womb from a common placenta (see above) differ from those with separate ones, which is attributed to epigenetic influences already in the womb.

A related research approach compares identical twins who were separated as infants and grew up in different families. Unlike twins grown up together, they are therefore subject to different environmental influences, which means that the difference between socio-economic factors such as the social status of the parents and the variance in intelligence can be examined. One example is the now famous Minnesota twin study. The research approach had fallen into the twilight because the British psychologist Cyril Burt, who was highly recognized at the time , was able to prove the falsification of study results using this method.He probably invented data because it is naturally difficult to find enough twins growing up separately. The current studies on the topic are summarized by the twin researcher Nancy Segal (who is herself a twin).

Classic twin studies using the approach described above resulted in values ​​for the heritability of intelligence mostly between 60 and 80 percent. Correct interpretation of the result is very important. In such a case one often reads statements like "80% of intelligence is inherited". That is a fallacy. The studies show that in such a case approx. 80% of the variance in intelligence in a population is due to hereditary factors. Heritability is a property of populations and says nothing at all about heredity in individuals, let alone their reasons and influences, the result is only valid for the exact environment in which the study was carried out. More recent approaches call the approach into question as a whole because the basic assumptions of the model, i. H. additive distribution of the variance and attribution of the residual variance exclusively to the "genes" obviously have simplified reality far too much. A high proportion of the variance can be explained by the interaction between genes or that between genes and the environment, which in the model is erroneously assigned to the genetic component alone. In a twin study, for example, it was shown that the influence of the parents' wealth on the intelligence of their children has a non-linear effect on heredity, contrary to the model assumptions; its value drops to almost zero in the poorest children examined. This explains the apparent paradox that, despite apparently extremely high values ​​for the heritability of intelligence, it is apparently easily possible to raise the intelligence quotient of disadvantaged children by 15 points (one standard deviation) through targeted encouragement, which suggests that their environment has an overwhelming influence.

Language development

The language development in monozygotic twins has special features compared to other children.

Deficit hypothesis

On the one hand, their language often seems underdeveloped compared to children of the same age. This language development delay is captured in the deficit hypothesis . Studies show that two to four year old monozygous twins are delayed in their speech development by about half a year. In the same period of time, they show fewer utterances, use shorter and simpler constructions than only children of the same age. This slight delay, which is completely balanced in the course of development until the onset of puberty , can be attributed to various peculiarities in the growing up of twins:

• With only children, a very close bond develops with a primary caregiver who is already an adult. This means that there is a linguistic gap between the child and the caregiver, which helps the children learn the language more quickly. In the case of monozygotic and dizygotic twins, however, the closest caregiver is their own sibling. However, there is no linguistic gap between them, which could explain the delay in language development.
• Only children are in a dyadic situation with their primary caregiver. Twins, on the other hand, are in a triadic relationship with the adult caregiver. There are not two, but three people to be involved. Our language provides many simple and common forms of dyadic relationships (speaker and addressee, etc.). It gets more complicated with triads. In addition, the adult caregiver (father or mother) must split their attention, love and time together between two children. One study found that mothers of twins on average rely more on directive parenting measures. In addition, there was less mutual attention between the adult caregiver and the single child in twins than in only children.
• Monozygous twins often develop a special deictic system in the beginning . When you use the name of one of the two (sometimes a fantasy name), you either really mean one child or you mean both siblings at the same time. This phenomenon is known as the nominal dual .

Autonomy Hypothesis

A second research approach is less concerned with the delay in language development than with the uniqueness of monozygous twins. This approach is called the Autonomy Hypothesis .

With very young monozygous twins one often observes a kind of special language that is only used between the two siblings and only understood by them. This is also called Kryptoglossie or Idioglossie . However, this special language disappears with increasing age.

Differences Between Identical Twins

Identical twins arise from one and the same egg cell, but they are not identical . For example, there are differences in fingerprints, moles or birthmarks. Sometimes the two siblings are even significantly different in terms of their physical and psychological characteristics. Such differences can occur, although their basic configuration with regard to the alleles of genes is the same in the early development period and also - apart from rare somatic mutations - remains the same over the subsequent cell divisions.

The epigenetic profiles of the twins can develop in different ways , which means that different expression patterns of genes occur in the siblings, which can lead to different expressions of characteristics . For example, with the same sequence of nucleotides in the DNA , i.e. the same genotype, differences in characteristics can still occur, i.e. different phenotypes . Important epigenetic mechanisms by which the gene activity without changing the DNA sequence will be affected are primarily the DNA methylation and histone - acetylation . In a study from 2005, cell samples from a total of 40 pairs of identical twins, dichorionic and monochorionic, were examined with regard to these mechanisms. In about a third of the twin pairs, noticeable differences in the epigenetic pattern were found, in older couples these were more pronounced than in young ones.

While the identical twins of a couple do not differ genetically or epigenetically from one another in the early phases of their life, differences in the pattern of gene expression can become increasingly pronounced with increasing age . Interestingly, the discrepancies in twins are greater the less lifetime the couple has spent together. This supports the assumption that environmental factors such as smoking , diet or physical activity influence the epigenetic profile. Suffering from various illnesses in the course of life also seems to play a role.

In addition, as in other people, also in twins, isolated mutations can be found in the genome, which can be detected as so-called somatic point mutations only in some body cells, for example in lymphocyte populations. In identical pairs of twins, however, they can play a special role, as they make the two twins genetically distinguishable. B. may be of interest in cases of questionable paternity.

literature

• Egg identity diagnosis in adolescent twins with questionnaires. In: Ärztliche Jugendkunde 71/1980, pp. 105–109.
• Francesca Mencacci: I fratelli amici. La appresentazione dei gemelli nella cultura romana . 1996.
• Reinhard Rathmayr: Twins in Greco-Roman antiquity (everyday life and culture in antiquity 4). Vienna: Böhlau 2000 (vi, 148 pp.). ISBN 3-205-99203-2
• Reinhard Rathmayr: Twins . In: The New Pauly . Enzyklopädie der Antike, Vol. 12/2 (2003), Col. 858-860.
• Barbara Frey: Twins and Twin Myths in Literature. Frankfurt am Main / London 2006.

Web links

Commons : Gemini  - collection of pictures, videos and audio files

• www.wissenschaft.de: Epigenetic factors cause deviations between genetically identical twins
• www.wissenschaft.de: The one-and-a-half-egg twins

Individual evidence

1. ↑ https://www.theguardian.com/science/2019/feb/27/scientists-stunned-discovery-semi-identical-twins
2. ^ Kai J. Bühling: Risk factors in pregnancy. In: Kai J. Bühling, Wolfgang Friedmann (ed.): Intensive course in gynecology and obstetrics. 2nd Edition. Munich 2009, 164f.
3. ↑ irb / ulz / dpa: Family in Vietnam: twins have two fathers. In: Spiegel Online . March 4, 2016, Retrieved September 4, 2016 . 
4. ↑ Armin Grunwald , Peter Hocke: Castor, In: Nadine Marquardt, Verena Schreiber (Ed.): Ortregister. transcript Verlag, 2014, ISBN 978-3-8394-1968-7 , p. 57 ( limited preview in the Google book search).
5. ↑ Twins, triplets, quadruplets - that's how high the probability of multiples is, t-online.de
6. ↑ That's right / Right: Double the chance, time online
7. ↑ Number of twin births , as of 2006
8. ↑ Archive link ( Memento from October 17, 2011 in the Internet Archive )
9. ↑ EG Mesa, JA Peral: Incidence and costs of multifetal pregnancies in Andalusia (2000-2010). In: Twin research and human genetics: the official journal of the International Society for Twin Studies. Volume 14, Number 5, October 2011, pp. 484-489, ISSN  1832-4274 . doi: 10.1375 / twin.14.5.484 . PMID 21962143 .
10. ↑ Number of twin births , from 1991–2015
11. ↑ Number of twin births , as of 2006
12. ↑ Archive link ( Memento from October 17, 2011 in the Internet Archive )
13. ↑ Number of twin births ( memento from September 23, 2015 in the Internet Archive ), as of 2007
14. ↑ Increase in twin births: More luck in a double pack. Der Spiegel, December 29, 2011, accessed on August 10, 2012 . 
15. ↑ ^{a b Mothers of} twins live longer , science.orf.at, May 11, 2011
16. ↑ Twins, triplets, quadruplets - that's how high the probability of multiples is, t-online.de
17. ↑ Cf. Michael Gärtner: The family education in the old church. A study of the first four centuries of Christianity with a translation and a commentary on the work of John Chrysostom on the addiction to recognition and raising children (Cologne publications on the history of religion 7). Cologne / Vienna: Böhlau 1985, esp. Pp. 6–31.
18. ↑ Rathmayr 2000, pp. 5–41
19. ↑ Uno Harva : The religious ideas of the Altaic peoples. FF Communications N: o 125.Suomalainen Tiedeakatemia, Helsinki 1938, p. 473
20. ↑ Klaus E. Müller , Ute Ritz-Müller: Soul of Africa. Magic of a continent. Könemann, Cologne 1999, pp. 198-204
21. ↑ on the methodology cf. Albert Tenesa & Chris S. Haley (2013): The heritability of human disease: estimation, uses and abuses. Nature Reviews Genetics 14: 139-149. doi: 10.1038 / nrg3377
22. ↑ K. Richardson & S. Norgate (2005): The equal environments assumption of classical twin studies may not hold. British Journal of Educational Psychology 75: 339-350. doi: 10.1348 / 000709904X24690
23. ↑ DK Sokol, CA Moore, RJ Rose, CJ Williams, T. Reed, JC Christian (1995): Intrapair differences in personality and cognitive ability among young monozygotic twins distinguished by chorion type. Behavior Genetics 25: 457-466. doi: 10.1007 / BF02253374
24. ^ Nancy L. Segal: Born Together — Reared Apart. The Landmark Minnesota Twin Study. Harvard University Press, 2012. ISBN 978-0-6740-5546-9 .
25. ^ Nancy L. Segal: Twin Mythconceptions: False Beliefs, Fables, and Facts about Twins. Academic Press, 2017. ISBN 978-0-12-803995-3 . on page 32.
26. ↑ Dorret Boomsma, Andreas Busjahn, Leena Peltonen (2002): Classical twin studies and beyond. Nature Reviews Genetics 3: 872-882. doi: 10.1038 / nrg932
27. ^ David S. Moore & David Shenk: The heritability fallacy. Wiley Interdisciplinary Reviews Cognitive Science 8 (1-2): e400. doi: 10.1002 / wcs.1400
28. ↑ Bruno Sauce & Louis D. Matzel (2018): The paradox of intelligence: Heritability and malleability coexist in hidden gene-environment interplay. Psychological Bulletin 144 (1): 26-47. doi: 10.1037 / bul0000131
29. ↑ Eric Turkheimer, Andreana Haley, Mary Waldron, Brian D'Onofrio, Irving I. Gottesman (2003): Socioeconomic Status Modifies Heritability of IQ in Young Children. Psychological Science 14 (6): 623-628.
30. ↑ MF Fraga, E. Ballestar et al. a .: Epigenetic differences arise during the lifetime of monozygotic twins. In: PNAS. Volume 102, Number 30, July 2005, pp. 10604-10609, ISSN  0027-8424 . doi: 10.1073 / pnas.0500398102 . PMID 16009939 . PMC 1174919 (free full text).
31. ↑ GM Martin: Epigenetic drift in aging identical twins. In: PNAS. Volume 102, Number 30, July 2005, pp. 10413-10414, ISSN  0027-8424 . doi: 10.1073 / pnas.0504743102 . PMID 16027353 . PMC 1180801 (free full text).