Monozygotic ("identical") twins

A twin is one of two offspring produced in the same pregnancy.^[1] Twins can either be monozygotic ("identical"), meaning that they develop from one zygote that splits and forms two embryos, or dizygotic ("fraternal") because they develop from two separate eggs that are fertilized by two separate sperm.

In contrast, a fetus which develops alone in the womb is called a singleton, and the general term for one offspring of a multiple birth is multiple.

Statistics[link]

The twin birth rate in the United States rose 76 percent from 1980 through 2009, from 18.9 to 33.3 per 1,000 births.^[2] The Yoruba have the highest rate of twinning in the world, at 45-50 twin sets (or 90-100 twins) per 1,000 live births,^[3][4][5] possibly because of high consumption of a specific type of yam containing a natural phytoestrogen which may stimulate the ovaries to release an egg from each side.^[6][7]

The world's highest rates of twinning are found across Central Africa, where there are 18-30 twin sets (or 37-62 twins) per 1,000 live births.^[8] In Latin-America and South Asia and South-East Asia the lowest rates are found; only 6-9 twin sets per 1,000 live births. North America and Europe have intermediate rates of 9–16 twin sets per 1,000 live births.^[8]

Due to the limited size of the mother's womb, multiple pregnancies are much less likely to carry to full term than single births, with twin pregnancies lasting only 37 weeks (3 weeks less than full term) on average.^[9]

Women who have a family history of fraternal twins have a higher chance of producing fraternal twins themselves, as there is a genetically linked tendency to hyper-ovulate. There is no known genetic link for identical twinning.^[10] Other factors that increase the odds of having fraternal twins include maternal age, fertility drugs and other fertility treatments, nutrition, and prior births.^[11]

Zygosity[link]

Zygosity is the degree of identity in the genome of twins. There are five common variations of twinning. The three most common variations are all dizygotic (fraternal):

• Male–female twins are the most common result, 50 percent of dizygotic twins and the most common grouping of twins.
• Female–female dizygotic twins (sometimes called "sororal twins")
• Male–male dizygotic twins

The other two variations are monozygotic ("identical") twins:

• Female–female monozygotic twins
• Male–male monozygotic twins (less common)

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,^[12] while it is 1.07 males/female in Italy.^[13] 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.

Fraternal/Sororal (dizygotic) twins[link]

Eight-month-old sororal twin girls napping

Actresses Mary-Kate and Ashley Olsen, possibly the best-known dizygotic twins.

Fraternal or dizygotic (DZ) twins (also referred to as "non-identical twins", "dissimilar twins", "biovular twins", and, in cases of females, sororal twins) usually occur when two fertilized eggs are implanted in the uterus wall at the same time. When two eggs are independently fertilized by two different sperm cells, fraternal twins result. The two eggs, or ova, form two zygotes, hence the terms dizygotic and biovular.

Dizygotic twins, like any other siblings, have an extremely small chance of having the same chromosome profile. Like any other siblings, dizygotic twins may look similar, particularly given that they are the same age. However, dizygotic twins may also look very different from each other. They may be of different sexes or the same sex. The same holds true for brothers and sisters from the same parents, meaning that fraternal and/or sororal twins are simply siblings who happen to be the same age.

Studies show that there is a genetic basis for dizygotic twinning. However, it is only their mother that 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.^[6]

Dizygotic twins are also more common for older mothers, with twinning rates doubling in mothers over the age of 35.^[14] With the advent of technologies and techniques to assist women in getting pregnant, the rate of fraternals has increased markedly.

Monozygotic ("identical") twins[link]

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 sacs. 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 (MZ) or identical twins occur when a single egg is fertilized to form one zygote (hence, "monozygotic") which then divides into two separate embryos.

Mechanism[link]

Regarding spontaneous or natural monozygotic twinning, a recent theory posits that monozygotic twins are formed after a blastocyst essentially collapses, splitting the progenitor cells (those that contain the body's fundamental genetic material) in half, leaving the same genetic material divided in two on opposite sides of the embryo. Eventually, two separate fetuses develop.^[15] Spontaneous division of the zygote into two embryos is not considered to be a hereditary trait, but rather a spontaneous or random event.^[14][16]

Monozygotic twins may also be created artificially by embryo splitting. It can be used as an expansion of IVF to increase the number of available embryos for embryo transfer.^[17]

Incidence[link]

Monozygotic twinning occurs in birthing at a rate of about three in every 1000 deliveries worldwide.^[16]

The likelihood of a single fertilization resulting in monozygotic twins is uniformly distributed in all populations around the world.^[14] 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^[18] and up to over 20 in some Central African countries.^[8] The exact cause for the splitting of a zygote or embryo is unknown.

In-vitro fertilization techniques are more likely to create dizygotic 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.^[19]

Genetic and epigenetic similarity[link]

Monozygotic twins are genetically nearly identical and they are always the same sex unless there has been a mutation during development. The children of monozygotic twins test genetically as half-siblings (or full siblings, if monozygotic twin sisters reproduce with monozygotic twin brothers, or if monozygotic twin brothers reproduce with the same woman), rather than first cousins. Identical twins do not have the same fingerprints, due to the fact that even in a small space inside the womb, people have contact with different parts of this environment, which gives small variations in the same digital, making them unique.^{[citation needed]} On rare occasions, monozygotic twins may express different phenotypes, 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 phenotypes, normally from an XXY Klinefelter's syndrome zygote splitting unevenly.^[20][21]

Even with nearly identical DNA, epigenetic modification, caused by differing environmental influences throughout their lives, affects which genes are switched on or off. A study of 80 pairs of human monozygotic twins ranging in age from three to 74 showed that the youngest twins have relatively few 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.^[22] However, certain characteristics become more alike as twins age, such as IQ and personality. This phenomenon illustrates the influence of genetics in many aspects of human characteristics and behavior.^[23][24][25]

Semi-identical twins[link]

Half-identical or semi-identical twins (also referred to as "half twins") are the result of a very rare form of twinning in which the twins inherit exactly the same genes from their mother but different genes from their father. Although examples of half-identical twins have been found, the exact mechanism of their conception is not well-understood, but could theoretically occur in polar body twinning where sperm cells fertilize both the ovum and the second polar body.

This situation is not the same as the common form of fraternal twinning, in which two genetically different ova are fertilized by two genetically different sperm. In this case, the ova are genetically identical.

Types[link]

There are two mechanisms by which this might happen:

• Polar twins (or "polar body twins"), where two sperm fertilize an ovum, one of the two fertilizing a polar body;^[26] or where an ovum splits into identical copies, one containing a polar body, prior to fertilization, allowing it to be fertilized by two different sperm.^[27][28]
• Sesquizygotic twins, where two sperm fertilize the one ovum, forming a triploid, and then splitting.^[29]

A 1981 study of a deceased triploid XXX twin fetus 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 likely a polar body twin. The authors were unable to predict whether a healthy fetus could result from a polar body twinning.^[30] In 2003 a study argued that many cases of triploidity arise from semi-identical twinning.^[31] In 2007, a study reported a case of a pair of living twins, one a hermaphrodite and one a phenotypical male. The twins were both found to be chimeras and to share all of their maternal DNA but only half of their father's DNA. 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.^[32]

Degree of separation[link]

Various types of chorionicity and amniosity (how the baby's sac looks) in monozygotic (one egg/identical) twins as a result of when the fertilized egg divides

Dichorionic diamniotic twins in the 4th week after fertilization

The degree of separation of the twins in utero depends on if and when they split into two zygotes. Dizygotic twins were always two zygotes. Monozygotic twins split into two zygotes at some time very early in the pregnancy. The timing of this separation determines the chorionicity and amniocity (the number of sacs) of the pregnancy. Dichorionic twins either never divided (i.e.: were dizygotic) or they divided within the first 4 days. Monoamnionic twins divide after the first week.

In very rare cases, twins become conjoined twins. Furthermore, there can be various degrees of shared environment of twins in the womb, potentially leading to pregnancy complications.

It is a common misconception that two placentas means twins are dizygotic. But if monozygotic twins separate early enough, the arrangement of sacs and placentas in utero is indistinguishable from dizygotic twins.

Demographics[link]

A recent study has found that insulin-like growth factor present in dairy products may increase the chance of dizygotic twinning. Specifically, the study found that vegan mothers (who exclude dairy from their diets) are one-fifth as likely to have twins as vegetarian or omnivore mothers, and concluded that "Genotypes favoring elevated IGF and diets including dairy products, especially in areas where growth hormone is given to cattle, appear to enhance the chances of multiple pregnancies due to ovarian stimulation."^[39]

From 1980–97, the number of twin births in the United States rose 52%.^[40] This rise can at least partly be attributed to the increasing popularity of 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 hormones in food.^[39]

Ethnicity[link]

About 1 in 90 human births (1.1%) results from a twin pregnancy.^[41] The rate of dizygotic twinning varies greatly among ethnic groups, ranging as high as about 45 per 1000 births for the Yoruba to 10% for Linha São Pedro, a tiny Brazilian settlement which belongs to the city of Cândido Godói.^[42] In Cândido Godói, one in five pregnancies has resulted in twins.^[43] The Argentine historian Jorge Camarasa has put forward a theory that experiments of the Nazi doctor Josef Mengele could be responsible for the high ratio of twins in the area.^[44] His theory was rejected by Brazilian scientists who had studied twins living in Linha São Pedro; they suggested genetic factors within that community as a more likely explanation.^[45] A high twinning rate has also been observed in other places of the world, including Igbo-Ora in Nigeria^[46] and Kodinji in India.^[47]

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 births". In 2001, for the first time ever in the US, the twinning rate exceeded 3% of all births. Nevertheless, the rate of monozygotic twins remains at about 1 in 333 across the globe.

In a study on the maternity records of 5750 Hausa women living in the Savannah zone of Nigeria, there were 40 twins and 2 triplets per 1000 births. Twenty-six percent 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 the country. The incidence of multiple births was related to maternal age but did not bear any association to the climate or prevalence of malaria.^[48]

Predisposing factors[link]

The predisposing factors of monozygotic twinning are unknown.

Dizygotic twin pregnancies are slightly more likely when the following factors are present in the woman:

• She is of West African descent (especially Yoruba)
• She is between the age of 30 and 40 years
• She is greater than average height and weight
• She has had several previous pregnancies.

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 fertilization (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, increasing the likelihood of multiples.

Delivery interval[link]

A 15-year German study^[49] of 8,220 vaginally delivered twins (that is, 4,110 pregnancies) in Hesse yielded a mean delivery time interval of 13.5 minutes.^[50] The delivery interval between the twins was measured as follows:

• Within 15 minutes: 75.8%
• 16-30 minutes: 16.4%
• 31-45 minutes: 4.3%
• 46-60 minutes: 1.7%
• Over 60 minutes: 1.8% (72 instances)

The study stated that the occurrence of complications "was found to be more likely with increasing twin-to-twin delivery time interval" and suggested that the interval be kept short, though it noted that the study did not examine causes of complications and did not control for factors such as the level of experience of the obstetrician, the wish of the women giving birth, or the "management strategies" of the procedure of delivering the second twin.

Complications of twin pregnancy[link]

Vanishing twins[link]

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 has not been detected or recorded.^[51] 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.

Chang and Eng Bunker, born in Siam (now Thailand) in 1811, were the origin of the term "Siamese twins".

Conjoined twins[link]

Conjoined twins (or the deprecated term "siamese") are monozygotic twins whose bodies are joined together during pregnancy. This occurs where the single zygote of MZ twins fails to separate completely, and the zygote starts to split after day 12^[33] 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 the brain, heart or liver.

Chimerism[link]

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 coloration in human chimeras. A chimera may be intersex, composed of cells from a male twin and a female twin.

Parasitic twins[link]

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[link]

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[link]

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, as the twin is not reabsorbed.

Low birth weight[link]

Twins typically suffer from the lower birth weights and greater likelihood of prematurity that is more commonly associated with the higher multiple pregnancies.

Twin-to-twin transfusion syndrome[link]

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[link]

Twin studies are utilized in an attempt to determine how much of a particular trait is attributable to either genetics or environmental influence. These studies compare monozygotic and dizygotic twins for medical, genetic, or psychological characteristics to try to isolate genetic influence from epigenetic and environmental influence. Twins that have been separated early in life and raised in separate households are especially sought-after for these studies, which have been used widely in the exploration of human nature. However, the utility and accuracy of these twin studies has been called into question and remains controversial. Classical twin studies have are now being supplemented with molecular genetic studies which identify individual genes.

Unusual twinnings[link]

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-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. One such pairing was born in Germany in 2008 to a white father from Germany and a black mother from Ghana.^[52][53]

Heterotopic pregnancy is an exceedingly rare type of dizygotic twinning in which one twin implants in the uterus as normal and the other remains in the fallopian tube as an ectopic pregnancy. Ectopic pregnancies must be resolved because they can be life-threatening to the mother. However, in most cases, the intrauterine pregnancy can be salvaged.

Among monozygotic twins, in extremely rare cases, twins have been born with different sexes (one male, one female).^[54] There can be identical boy/girl twins if the sex gene of the embryo has an extra x chromosome (the fertilized egg would be an xxy) then when the egg splits one can have xx (girl) genes and one can have xy (boy) genes. This is rare, but possible. Records show there are only 4 known cases of these type twins. The probability of this is so vanishingly small 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 karyotypes.

Semi-identical twins[link]

Monozygotic twins can develop differently, due to different genes being activated.^[55] 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 chimeras. This form had been speculated until only recently being recorded in western medicine.^[56][57][58]

Twin calves of the Hereford breed

Animal twins[link]

Twins are common in many animal species, such as cats, sheep, ferrets, dogs 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 female calf that is the twin of a bull becomes partially masculinized and is known as a freemartin. The nine-banded armadillo (Dasypus novemcinctus) has identical twins (usually four babies) as its regular reproduction and not as exceptional cases.^[59][60]

See also[link]

Wikimedia Commons has media related to: Twin

Look up twin in Wiktionary, the free dictionary.

• Conjoined twins
• Doppelgänger
• Gemini (astrology)
• Incest between twins
• List of multiple births
• List of twins
• Litter (animal)
• Look-alike
• Multiple birth
• Twins in mythology

References[link]

Further reading[link]

• Bacon, Kate. Twins in Society: Parents, Bodies, Space, and Talk (Palgrave Macmillan; 2010) 221 pages; explores the experience of child twins, adult twins, and parents of twins, with a focus on Britain.
• Nieuwint, Aggie; Rieteke Van Zalen-Sprock, Pieter Hummel, Gerard Pals, John Van Vugt, Hans Van Der Harten, Yvonne Heins and Kamlesh Madan (1999). "'Identical' twins with discordant karyotypes". Prenatal Diagnosis 19 (1): 72–6. DOI:10.1002/(SICI)1097-0223(199901)19:1<72::AID-PD465>3.0.CO;2-V. PMID 10073913. 
• Wenk RE, Houtz T, Brooks M, Chiafari FA (1992). "How frequent is heteropaternal superfecundation?". Acta geneticae medicae et gemellologiae 41 (1): 43–7. PMID 1488855. 
• Girela, Eloy; Jose A. Lorente, J. Carlos Alvarez, Maria D. Rodrigo, Miguel Lorente and Enrique Villanueva (1997). "Indisputable double paternity in dizygous twins". Fertility and Sterility 67 (6): 1159–61. DOI:10.1016/S0015-0282(97)81456-2. PMID 9176461. 
• Shinwell ES, Reichman B, Lerner-Geva L, Boyko V, Blickstein I (September 2007). ""Masculinizing" effect on respiratory morbidity in girls from unlike-sex preterm twins: a possible transchorionic paracrine effect". Pediatrics 120 (3): e447–53. DOI:10.1542/peds.2006-3574. PMID 17766488. http://pediatrics.aappublications.org/cgi/pmidlookup?view=long&pmid=17766488. Retrieved 2008-10-06. 
• Lummaa V, Pettay JE, Russell AF (June 2007). "Male twins reduce fitness of female co-twins in humans". Proceedings of the National Academy of Sciences of the United States of America 104 (26): 10915–20. DOI:10.1073/pnas.0605875104. PMC 1904168. PMID 17576931. http://www.pnas.org/cgi/pmidlookup?view=long&pmid=17576931. Retrieved 2008-10-06. 
• Schein, Elyse; Paula Bernstein (2007). Identical Strangers: A Memoir of Twins Separated and Reunited. New York: Random House. ISBN 1-4000-6496-1. OCLC 123390922. 
• Helle, Samuli; Virpi Lummaa and Jukka Jokela (2004). "Selection for Increased Brood Size in Historical Human Populations" (PDF). Evolution 58 (2): 430–436. DOI:10.1111/j.0014-3820.2004.tb01658.x. PMID 15068359. http://www.huli.group.shef.ac.uk/helleevolution2004.pdf. Retrieved 2008-10-02. 
• "TWINS Guide to the First Year" (PDF). Fort Collins, Colorado: TWINS Magazine. 2008. http://www.twinsmagazine.com/media/images/Guide2008.pdf. Retrieved 2008-10-06. 
• Samson, Jennifer. "Facts About Multiples: An Encyclopedia of Multiple Birth Records". http://www3.telus.net/tyee/multiples/index.html. Retrieved 2008-10-18. 
• Am J Med Genet C Semin Med Genet. 2009 May 15;151C(2):136-41. Not really identical: epigenetic differences in monozygotic twins and implications for twin studies in psychiatry. Haque FN, Gottesman II, Wong AH.

Abigail Loraine Hensel and Brittany Lee Hensel (born March 7, 1990) are dicephalic parapagus twins, meaning that they are conjoined twins of whom each has a separate head, but whose bodies are joined. They are highly symmetric, giving the appearance of having just a single body with little variation from normal proportion. In fact, several vital organs are doubled up, each twin having a separate heart, stomach, spine and spinal cord.

Each twin controls her half of their body, operating one of the arms and one of the legs. This means that as infants, the initial learning of physical processes that required bodily coordination, such as clapping, crawling, and walking required the cooperation of both children. While each is able to eat and write separately and simultaneously, activities such as running and swimming must be coordinated and alternate symmetrically. Other activities as diverse as brushing hair and driving a car require that each twin perform a sequence of quite separate actions that coordinate with the other.

Despite the curiosity that their condition has generated, the Hensel twins have managed to live private lives with relatively little press attention. At the age of 16, they gave an interview on The Learning Channel on December 17, 2006, in which they discussed aspects of their daily lives and plans for the future.

Background[link]

Abigail and Brittany Hensel were born in Carver County, Minnesota, the daughters of Patty, a registered nurse, and Mike Hensel, a carpenter and landscaper. The twins have a younger brother named Dakota, or Koty for short, a younger sister named Morgan, and a dog named Sadie. They were raised in New Germany and attended Lutheran High School in Mayer, Minnesota.

Physiology[link]

The Hensel twins have a single body with separate heads and necks, a chest that is wider than normal, two arms and two legs. At birth, they had a rudimentary arm attached to a shoulder blade at the back. The arm was removed, leaving the shoulder blade.

Abigail's head tilts laterally outward about 5 degrees to the right, while Brittany's head tilts laterally at about 15 degrees to the left, causing Brittany to appear shorter. At age 12, they underwent surgery at Gillette Children's Specialty Healthcare to correct scoliosis and to expand their chest cavity to prevent future difficulties with breathing.^[1]

Each of the twins manages one side of their conjoined body. The sense of touch of each is restricted to her body half; this shades off at the midsagittal plane such that there is a small amount of overlap at the midline. They are effective in cooperatively using their limbs when both hands or both legs are required. By coordinating their efforts, they are able to walk, run, swim and ride a bicycle normally—all tasks that they learned at a normal speed. Together, they can type on a computer keyboard at a normal speed and drive a car.

Organ distribution[link]

Abigail and Brittany have individual organs in the upper part of their body, while most of the organs located at or below the level of the navel are shared, the exception being the spinal cord.^[2]

• 2 heads
• 2 spines merging at the coccyx and joined at the thorax by sections of ribs. Surgery was employed to correct scoliosis.
• 2 completely separate spinal cords
• 2 arms (originally 3, but rudimentary central arm was surgically removed, leaving central shoulder blade in place)
• 1 broad ribcage with 2 highly fused sternums and traces of bridging ribs. Surgery was employed to expand the pleural cavities.
• 2 breasts
• 2 hearts in a shared circulatory system (nutrition, respiration, medicine taken by either affects both)
• 4 lungs with the medial lungs moderately fused, not involving Brittany's upper right lobe; three pleural cavities
• 1 diaphragm with well-coordinated involuntary breathing, slight central defect
• 2 stomachs
• 2 gallbladders
• 1 liver, enlarged and elongated right lobe
• Y-shaped small intestine, which experiences a slightly spastic double peristalsis at the juncture
• 1 large intestine with one colon
• 3 kidneys: 2 left, 1 right
• 1 bladder
• 1 set of reproductive organs
• 2 separate half-sacrums, which converge distally
• 1 slightly broad pelvis
• 2 legs

Separation[link]

Upon their birth, their parents rejected the option to attempt surgical separation after hearing from doctors that it was not likely that both girls would survive the operation. As the girls grew and learned to walk and develop other skills, the parents confirmed their decision against separation, arguing that the quality of life for the surviving twin or twins living separately would be less than their quality of life as conjoined beings.^[3]

Adulthood[link]

The Hensel twins both successfully passed their drivers license exams, both the written and driving tests. They had to take the tests twice, once for each twin. Abby controls the pedals, radio, heat, defogger, and other devices located to the right of the driver's seat, while Brittany controls the turn signal and lights; together, they control the steering wheel.

They both graduated from high school in 2008. They began college at Bethel University in St. Paul, Minnesota, USA.

In conversation, the twins are clearly distinct persons, with distinct likes and dislikes. Their preferences in food, clothing color, etc., differ. Some of their clothes are altered by their seamstress so that they have two separate necklines in order to emphasize their individuality. They will usually have separate meals, but sometimes will share a single meal for the sake of convenience (e.g., each takes a bite of the same hamburger). Abigail is better at mathematics, and Brittany is better at writing. For tasks such as responding to e-mail, they type and respond as one, anticipating each other's feelings with little verbal communication between them. In such cases as the latter, their choice of grammatical person is to use the first person singular out of habit when they agree, but when their responses do differ, they use their names in the third person singular.

There is some concern about their ability to have continued good health, because only four known sets of conjoined twins who share an undivided torso and two legs have ever survived into adulthood, and most have congenital heart defects or other organ anomalies. None have shown up in the Hensels' case. They have so far had no desire to make themselves available for any medical studies. They intend to make a rather limited number of media appearances in the future, primarily just to appease the world's curiosity and to reduce the number of people who might otherwise be taken aback by their unusual body configuration. They intensely dislike being stared at or photographed by strangers while going about their private lives.^[4] In the interviews made for Discovery Channel in 2006, the girls, then 16, said that they hoped to date, get married, and have children. They also stated that they hoped that by providing some information about themselves, they would be able to lead otherwise fairly typical social lives as together, they continue to make new friends.^[4][5]

Media appearances[link]

In April 2006, they appeared in Joined for Life, a TV documentary produced by Advanced Medical and distributed on the Discovery Health Channel. They also appeared on The Oprah Winfrey Show on April 8 and April 29, 1996. In April 1996, the twins were featured on the cover of Life under the caption "One Body, Two Souls", and their daily lifestyle was depicted in the corresponding article entitled "The Hensels' Summer". Life followed up with another story in September 1998. In 2003, an updated story of them at age 11 (filmed in 2001) was published in Time and again in Life. They appeared in a follow-up documentary on The Learning Channel on December 17, 2006, filmed around the time of their 16th birthday, in which they discuss dealing with puberty and getting their driver's licenses. In the summer of 2006, they had a vacation in Texas at the home of a family whose dicephalus twin girls had died at a few hours old.^{[3][6][7][8][9][10]}

Filmography[link]

Documentaries and other television appearances include:

References[link]

External links[link]

• Abby Hensel at the Internet Movie Database
• Britty Hensel at the Internet Movie Database

Aphex Twin
In Turin, Italy in March 2007.
Background information
Birth name	Richard David James
Born	(1971-08-18) 18 August 1971 (age 40) Limerick, Ireland
Origin	Lanner, Cornwall, United Kingdom
Genres	Electronic, ambient, IDM, acid techno
Occupations	Musician, composer, remixer
Instruments	Synthesizers, piano, electronics, harpsichord, organ, percussion, laptop, softsynth, turntables, drum machine, guitar, bass guitar, computer, sequencer, sampler
Years active	1985–present
Labels	Warp Records Sire Records Rephlex Records
Associated acts	Universal Indicator, Mike & Rich

Richard David James (born 18 August 1971), best known under the pseudonym Aphex Twin, is a British electronic musician and composer. He founded the record label Rephlex Records in 1991 with Grant Wilson-Claridge. He was described by The Guardian as "the most inventive and influential figure in contemporary electronic music."^[1]

Aphex Twin has also recorded music under the aliases AFX, Blue Calx, Bradley Strider, Caustic Window, DJ Smojphace, GAK, Martin Tressider, Polygon Window, Power-Pill, Prichard D. Jams, Q-Chastic, Tahnaiya Russell, The Dice Man, Soit-P.P., and speculatively The Tuss.

Aphex Twin has released recordings on Rephlex, Warp, R&S, Sire, Mighty Force, Rabbit City, and Men Records.

History[link]

Early years[link]

Richard David James was born to Welsh parents Lorna and Derek James in St. Munchin's Limerick Regional Maternity Hospital, Ireland. He grew up in Lanner, Cornwall, enjoying, along with two older sisters, a "very happy" childhood during which they, according to James, "were pretty much left to do what [they] wanted."^[2] He "liked growing up there, being cut off from the city and the rest of the world".^[3] James attended Redruth School, located in Redruth, Cornwall.^[4]

According to musician Benjamin Middleton, James started producing music at the age of 12.^[5] As a teenager he was a disc jockey at the Shire Horse Inn in St Ives, with Tom Middleton at the Bowgie Inn in Crantock, and also along the numerous beaches around Cornwall. James studied for a National Diploma in Engineering from 1988 to 1990 (aged 16 to 18) at Cornwall College. Talking about his studies, James has said that "music and electronics went hand in hand."^[6] James graduated from college; according to an engineering lecturer, however, he often had his headphones on during practical lessons, "no doubt thinking through the mixes he'd be working on later".^[7]

Early career: early 1990s[link]

In 1989 James met and befriended Grant Wilson-Claridge when working as a DJ on alternate weeks at the Bowgie pub, near Newquay in Cornwall. Wilson-Claridge was intrigued by James's sets and was surprised to discover that James was playing tapes of his own music.

James' first release was the 12-inch EP Analogue Bubblebath on Mighty Force Records in 1991. It was first released under the moniker Aphex Twin, later changed to AFX. The track "En Trance to Exit" was made in collaboration with Tom Middleton, also known as Schizophrenia.^[8] The EP got on the playlist of Kiss FM, an influential London radio station, which helped the release to become a success.^[9]

In 1991, James and Wilson-Claridge founded Rephlex Records to promote "innovation in the dynamics of Acid — a much-loved and -misunderstood genre of house music forgotten by some and indeed new to others, especially in Britain."^[10]

Between 1991 and 1993, James released two Analogue Bubblebath EPs as AFX, and an EP under Bradley Strider, Bradley's Beat. James moved to London to take an electronics course at Kingston Polytechnic, but at the time admitted to David Toop that his "electronics studies were already slipping away as a career in the techno business took precedence." After quitting his course, James remained in London and released a number of albums and EPs on Warp Records and other labels under many aliases including AFX, Polygon Window, and Power-Pill. A number of James' tracks (released under the aliases Blue Calx, The Dice Man, and others) appeared on various compilations. Local legend has it that James lived on the roundabout in Elephant and Castle, South London during his early years in the capital - in fact he lived in a nearby disused bank.^[2][11]

Gaining success: 1992–1995[link]

	"Ageispolis" noicon Sample of "Ageispolis" from the album Selected Ambient Works 85-92.
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	"Ventolin (Video Version)" noicon Sample of "Ventolin (Video Version)" from the album ...I Care Because You Do.
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The first full-length Aphex Twin album, Selected Ambient Works 85–92, was released in 1992 on R&S Records. It received high ratings and praise from critics. John Bush of Allmusic described it as a "watershed of ambient music".^[12] In 2002, Rolling Stone wrote of the album: "Aphex Twin expanded way beyond the ambient music of Brian Eno by fusing lush soundscapes with oceanic beats and bass lines."^[13] Pitchfork Media's review called it, "among the most interesting music ever created with a keyboard and a computer."^[14] Critics also noted that the songs were recorded on cassette and that the sound quality was "relatively poor".

In 1992, he also released the EPs Xylem Tube EP and Digeridoo (first played by DJ Colin Faver on London's Kiss FM) as Aphex Twin, as Power-Pill the Pac-Man EP based on the arcade game Pac-Man, and two of his four Caustic Window EPs. "Digeridoo" reached #55 in the UK Singles Chart, and was later described as foreshadowing drum and bass by Rolling Stone.^[15] "Digeridoo" was recorded initially for the benefit of FIZZ-BOMB (at the Shire Horse, St Ives, Cornwall). These early releases came out on Rephlex Records, Mighty Force of Exeter, and R&S Records of Belgium.^[16]

In 1993, James released Analogue Bubblebath 3. He also released a single titled "On"; his second Bradley Strider EP, Bradley's Robot; two more Caustic Window EPs; and his first releases on Warp Records, Surfing on Sine Waves and "Quoth" under the alias Polygon Window.

Warp Records pressed and released a follow-up to Selected Ambient Works 85-92, Selected Ambient Works Volume II in 1994. The sound was much less beat and melody-driven than the previous volume. All of the track names were described with pie chart symbols, each of which was meant to be paired with a corresponding image in the album jacket, with exception for one song, which was named "Blue Calx". To decipher song titles, listeners had to compare the length of each track with the size of each pie chart symbol. For example, the first title, which is often labelled cliffs, is realised by pairing the first symbol with the first image, which is that of a rocky cliffside.^[17] James claimed in The Wire magazine and other media that these songs were inspired by lucid dreams and synaesthesia. Other releases are a fourth Analogue Bubblebath; GAK, derived from early demos sent to Warp Records; and Classics, a compilation album that includes "Digeridoo" and the Xylem Tube EP.

For his 1995 release, ...I Care Because You Do, James used an image of his face for the album cover, a motif that would continue on many of his later records. The album was a compilation of songs composed between 1990 and 1994 and represented a mish-mash of Aphex Twin's various music styles. This was James's last record of the 1990s to use mostly analogue synthesizers. He commissioned Western classical-music composer Philip Glass to create an orchestral version of one of the songs from this album, "Icct Hedral", which appeared on the Donkey Rhubarb EP.^[18]

Jungle, DSP, and laptops: 1995–1999[link]

In 1995 (primarily with Hangable Auto Bulb, a near anagram of Analogue Bubblebath), James began releasing more material composed on computers, combining a jungle sound with nostalgic childhood themes and computer-generated acid lines.

James's early adoption of software synthesizers predated the later popularity of using computers to make music. In the mid-to-late 1990s, his music gradually gained more mainstream appeal, as he released Richard D. James Album and Expert Knob Twiddlers (a collaboration with fellow dance producer µ-ziq) in 1996, "Come to Daddy" in 1997 (#36 on UK charts) and "Windowlicker" in 1999 (#16 on UK charts). Two pop songs that heavily use Digital Signal Processing (DSP) techniques, "Come to Daddy" and "Windowlicker" were shown on MTV and became cover features for music magazines such as NME. The videos for both singles were directed by British artist Chris Cunningham and caused controversy on release because of their disturbing images and themes.

In 1998, Aphex Twin's track "[rhubarb]" (from Selected Ambient Works Volume II) appeared as part of the BBC's digital widescreen test transmission, which was on loop for many months as the service was introduced across the UK.^[19]

Prepared piano, laptops, and more DSP: 2000–2003[link]

In 2001 Aphex Twin released drukqs, a two-CD album that featured computer controlled piano songs influenced by Erik Satie and John Cage. Many of the tracks' names are written in the Cornish language (i.e., 'jynweythek' translatable as 'machinemusic'). Also included were abrasive, fast, and meticulously programmed computer-made songs. Rolling Stone described the piano songs as "aimlessly pretty".^[20] Some reviewers concluded that drukqs was released as a contract breaker with Warp Records—a credible guess, as James's next big release was released on his own Rephlex label. James claimed to interviewers he had accidentally left an MP3 player labelled "Aphex Twin—unreleased tracks" on a plane, containing a large set of new songs, and rushed the album release to preempt an Internet leak.^[21] He also released a short EP titled 2 Remixes By AFX the same year as Drukqs. It featured two remixes, the originals being from 808 State and DJ Pierre. In addition, there is one untitled third track that consists of an SSTV image, which consists of mostly of high-pitched sounds, which can be decoded to a viewable image by proper software such as MultiMode (for Macintosh) or MMSSTV (for Windows).

Synthesizers and drum machines: 2004–present[link]

In late 2004, rumours of James's return to an acid techno-based sound were realised with the Analord series, an 11-part series of EPs with 42 total tracks, averaging 2–4 tracks per EP. The series was created by playing and sequencing analogue and digital electronic music equipment such as synthesizers and drum machines. The series was recorded on magnetic tape and then later pressed on vinyl.

James was meticulous about the whole process of recording, mastering, and pressing. James has said Rephlex Records was strict on quality control, trying out various pressing-plant companies until they felt it sounded perfect. To James's ears, vinyl or tape is better than digital, as no two copies are the same. However, label co-owner Grant Wilson-Claridge convinced James to release a CD compilation, Chosen Lords, which included 10 selected tracks from the Analord series. For the Analord series James used his collection of vintage synthesizer and drum machines, some of which were quite rare by that time. Some of the record inserts have pictures of rare synthesizers like the Synton Fenix, and the notoriously difficult-to-program Roland MC-4 sequencer, as well as the legendary Roland TB-303.

In 2007 media reports indicated that Aphex Twin was recording under a new alias called The Tuss. Rephlex Records has denied that Aphex Twin is The Tuss, but Aphex Twin fans and the media have ignored Rephlex's denial and The Tuss is treated as yet another Aphex Twin project;^[22][23] further evidence being provided by the fact that all Tuss tracks are published in the BMI Repertoire under the name JAMES RICHARD DAVID.^[24]

Some of Aphex Twin's music was featured in the award winning documentary about the atrocities of shark finning, Sharkwater.

In March 2009 Steve Beckett of Warp Records announced a new album for a possible release but the album never materialised.^[25] However, 20 more tracks were added to the Analord series in December 2009, only available through download from the Rephlex Records website.

In an October 2010 interview with British magazine Another Man, James stated that he had completed 6 albums, one of which was a remake of the unreleased Melodies from Mars, originally produced around the time of Richard D. James Album.^[26] In June 2011, he spoke to Spanish magazine EL PAIS. When asked about the 6 albums, he answered 'More than 10 or 11 are already compiled, and many more songs are orphans.' He also revealed that a new album 'will show in a while' and that the reason there has been so long since his last album was that he was waiting for a divorce from his wife.^[27]

Background details[link]

	"Jynweythek" noicon Sample of "Jynweythek" from the album Drukqs.
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	"Vordhosbn" noicon Sample of "Vordhosbn" from the album Drukqs.
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	"Fenix Funk 5" noicon Sample of "Fenix Funk 5" from the EP Analord 10.
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	"VBS.Redlof. B" noicon Sample of "VBS.Redlof. B" from the EP Analord 11.
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The Aphex Twin name[link]

The name Aphex is derived from Aphex Systems Limited, a brand of audio signal processing equipment (itself derived from "analogue effects").^{[dubious – discuss]} It is used with permission, as acknowledged on the back sleeves of Richard D. James Album and drukqs. He has explained in interviews that the Twin portion of his moniker is used in memory of his brother, also named Richard James, who died at birth in 1968.

Artwork[link]

James's own face, grinning or distorted in some way, is a common theme in his album covers, his music videos, and the songs themselves. The cover of I Care Because You Do features a painting of James, while the cover of Richard D. James Album presents a close-up photograph. His face is transplanted onto other peoples' bodies in the music videos of "Come to Daddy" and "Windowlicker". Near the end of the second track of the "Windowlicker" single (commonly referred to as "Equation"), a photo of James' face is embedded as a steganogram which is revealed when run through spectral analysis.^[28] Another embedded image of James, along with collaborator Tom Jenkinson, appears in the third track of 2 Remixes by AFX, "Bonus High Frequency Sounds", encoded in SSTV format, with text relating to the release.

James has used his own photography for some of his releases, such as the elaborate album sleeve for Selected Ambient Works Volume II.

Braindance[link]

Richard's own Rephlex Records label, which he co-owns with Grant Wilson-Claridge, coined the term Braindance in 1991 to describe Aphex Twin's music.^[22][29][30] Rephlex Records' official definition of Aphex Twin and his followers' music is quoted as follows: "Braindance is the genre that encompasses the best elements of all genres, e.g traditional, classical, electronic music, popular, modern, industrial, ambient, hip-hop, electro, house, techno, breakbeat, hardcore, ragga, garage, drum and bass, etc."^[31]

Braindance applies to forward-thinking electronic music that can appeal to the mind as well as the desire to dance and party. Examples including Ed-DMX's Breakin' records label, µ-ziq's Planet-mu label, the Aphex Twin EP Come to Daddy and Astrobotnia Parts 1, 2 & 3.^[32]

Influences[link]

At age 17, Richard D. James mentioned these influences: "Phonic Boy, Computer World, Mental Telepathy, Industrial Inc., Tomita, Tangerine Dream". Mixmaster Morris mentions on the "I Luv AFX" BBC Radio 1 Breezeblock session that James' preferred moniker while working as a DJ in Cornwall was Phonic Boy on Dope. More recently, he has said that he gets inspiration from "everyday sounds that can be emulated/reconstructed electronically, quality techno, especially from Europe, which overshadows the current hardcore pop crap." When asked about what is next for electronic music, he said "acid-techno, ambient-techno." Avant-garde music has been an influence on James, including artists such as Kraftwerk, Can, Tangerine Dream, Tod Dockstader, Xenakis, Piero Umiliani, Bernard Parmegiani, Karlheinz Stockhausen,^[33] John Cage, and the French composer Erik Satie for his piano works and his ideas for furniture music.

In acknowledgment of another influence, James released Music from the BBC Radiophonic Workshop, a compilation of music recorded by the pioneers of BBC Radiophonic Workshop, which included artists such as Delia Derbyshire,^[34] on his own label, Rephlex Records.

Aphex Twin has said, "I don't really like rock & roll." Despite this, he has mentioned being a fan of Led Zeppelin, citing their use of breakbeats as an influence, as well as Pink Floyd for their psychedelic music.^[35]

Many songs include sounds from and references to the ZX Spectrum. For instance, "Carn Marth" from Richard D. James Album includes the tape-loading noise of the game Sabre Wulf.

Influence on others[link]

The London Sinfonietta has performed arrangements of Aphex Twin tracks.^[36] In 2005, the orchestra Alarm Will Sound released Acoustica: Alarm Will Sound Performs Aphex Twin. The album consists of acoustic arrangements of some of James's electronic tracks. He has also had an influence on rock bands like Radiohead.^[37] However, he has dismissed the notion of going on tour with them: "I wouldn't play with them since I don't like them."^[35] Despite his previous comments, he premiered new music alongside Radiohead guitarist/composer Jonny Greenwood in a 2011 collaboration with Polish composer Krzysztof Penderecki.^[38] The animator David Firth has much of his work influenced and soundtracked by Aphex Twin.^[39]

Intelligent Dance Music[link]

The term 'intelligent dance music', or IDM, was coined in August 1993 by the IDM mailing list based at hyperreal.org, as a term to describe the sound pioneered by the Warp Records Artificial Intelligence Series. The series featured Aphex Twin tracks (under a different pseudonym), as well as early productions from labelmates such as Autechre and LFO. The usage of this term spread in the USA and on internet messageboards, but the term is still a source of controversy and derision amongst the artists and fans, including Aphex Twin. As of October 2011 the list is still active.

I just think it's really funny to have terms like that. It's basically saying, 'this is intelligent and everything else is stupid.' It's really nasty to everyone else's music. (laughs) It makes me laugh, things like that. I don't use names. I just say that I like something or I don't.

—Aphex Twin^[40]

Aphex Twin refers to his own breakbeat tracks as breakbeat, jungle, or drum and bass interchangeably. Other genres that he indulges in include acid, ambient, ambient techno, noise, and many more.

Equipment[link]

During an interview with Future Music, Richard said he likes using Ableton Live but prefers Liveslice for beat editing/stretching. Asked what equipment did use, he responded "Raveolution 309, the Raven Max, MC-909 limited edition, Quasimidi Van Helden, MAM Freebass 383, Roland DJ-70, E-15, SP-808, Akai S3200, Behringer MX602A and all the Behringer effects that copy other things."^[44]

Aphex Twin's press[link]

James described himself in The Guardian as follows: "I'm just some irritating, lying, ginger kid from Cornwall who should have been locked up in some youth detention centre. I just managed to escape and blag it into music."^[1]

James said he composed ambient techno music at the age of 13; he claims to have "over 100 hours" of unreleased music, and to have made his own software to compose with, including algorithmic processes that automatically generate beats and melodies; in addition he claims to experience synaesthesia, and that he is able to incorporate lucid dreaming into the process of making music.^[45]

He lives in southeast London in a converted bank, which was formerly the Bank of Cyprus and then HSBC. Contrary to popular opinion, however, he does not own the silver structure in the centre of the roundabout at Elephant and Castle. This is, in fact, the Michael Faraday Memorial, containing a power transformer for the Northern Line, which James jokingly claimed to be buying in an interview with The Face magazine in 2001.^[2] Some of these rumours are hard to confirm as he has been known to spread mistruths in the prankster tradition, making such claims as only sleeping two to three hours a night.^[46]

Stockhausen vs. The Technocrats[link]

In November 1995, The Wire published an article titled "Advice to Clever Children". In the process of producing the interview, a package of tapes containing music from several artists, including Aphex Twin, was sent to Karlheinz Stockhausen.

Stockhausen commented:

I heard the piece Aphex Twin of Richard James (sic) carefully: I think it would be very helpful if he listens to my work "Song of the Youth," which is electronic music, and a young boy's voice singing with himself. Because he would then immediately stop with all these post-African repetitions, and he would look for changing tempi and changing rhythms, and he would not allow to repeat any rhythm if it varied to some extent and if it did not have a direction in its sequence of variations.^[47]

Aphex Twin, a fan of Stockhausen, responded:

"I thought he should listen to a couple of tracks of mine: "Digeridoo", then he'd stop making abstract, random patterns you can't dance to".^[47]

ZX81 competition[link]

Richard claims to have produced sound on a Sinclair ZX81 (a machine with no sound hardware) at the age of 11:

When I was 11, I won 50 pounds in a competition for writing this program that made sound on a ZX81. You couldn't make sound on a ZX81, but I played around with machine code and found some codes that retuned the TV signal so that it made this really weird noise when you turned the volume up.^[2]

Discography[link]

Studio albums (as Aphex Twin)

• Selected Ambient Works 85–92 (1992)
• Selected Ambient Works Volume II (1994)
• ...I Care Because You Do (1995)
• Richard D. James Album (1996)
• Drukqs (2001)

References[link]

External links[link]

Wikimedia Commons has media related to: Aphex Twin

• Aphex Twin at Warp Records
• Aphex Twin at the Open Directory Project
• Aphex Twin discography at Discogs.