Monozygotic twins.

Twins are two offspring resulting from the same pregnancy, either of the same or opposite sex. In Biology, offspring are the product of Reproduction, a new Organism produced by one or more Parents Collective offspring may be known Pregnancy ( Latin graviditas) is the carrying of one or more offspring known as a Fetus or Embryo, inside the Uterus of a Female An organism's sex is defined by the gametes it produces males produce male gametes (spermatozoa or Sperm) while females produce female gametes (ova or Egg cells; individual

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. A fetus (or foetus or fœtus) is a developing Mammal or other Viviparous Vertebrate, after the Embryonic stage and

Twins are usually, but not necessarily, born in close succession. Childbirth (also called labour, birth, partus or parturition) is the culmination of a Human Pregnancy or 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 because they will be less than full term. ^[1] Since premature births can have health consequences for the babies, twin births are often handled with special precautions. Premature birth (also known as preterm birth) is the Birth of a Baby before the standard period of Pregnancy is completed

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. The world population is the total number of living Humans on Earth at a given time 2% of the world population and 8% of all twins). ^[2] The current rate in the United States is 31 twin births per 1,000 women. ^[3]

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 as shown in the CIA World Factbook ^[4]. The World Factbook ( ISSN; also known as the CIA World Factbook) is an annual publication of the Central Intelligence Agency of the For example, the sex ratio at birth in the US is 1. 05 males/female, while it is 1. 07 males/female in 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. A polar body is a cell structure found inside an Ovum. Both Animal and Plant ova possess it Polar body twinning would result in "half-identical" twins. Semi-identical 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 ^[5]

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 fertilized eggs are implanted in the uterine wall at the same time. For soil improvement see Fertilization (soil. The two eggs form two zygotes, and these twins are therefore also known as dizygotic as well as "biovular" twins. For other meanings see Zygote (disambiguation. A zygote (from Greek ζυγωτός zugōtos "joined" or "yoked" When two eggs are independently fertilized by two different sperm cells, DZ twins result. The term sperm is derived from the Greek word (σπέρμα sperma (meaning "seed" and refers to the male reproductive cells.

Dizygotic twins, like any other siblings, have an extremely small chance of having the exact same chromosome profile. Like any other siblings, 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. An ovum (plural ova) is a Haploid Female reproductive cell or Gamete. 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]

DZ twins are also more common for older mothers, with twinning rates doubling in mothers over the age of 35. ^[7] 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. The City of New York The Upper East Side is a neighborhood in the borough of Manhattan in New York City, between Central Park and the East River. Triplet births have also risen, from 60 in 1995 to 299 in 2004.

Monozygotic twins

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. The placenta is an Ephemeral organ present in placental Vertebrates, such as Eutherial Mammals and Sharks during Gestation The amniotic sac is the sac in which the Fetus develops Some sources consider it to be equivalent to the Amnion.

Monozygotic twins, frequently referred to as identical twins, occur when a single egg is fertilized to form one zygote (monozygotic) which then divides into two separate embryos. An ovum (plural ova) is a Haploid Female reproductive cell or Gamete. For other meanings see Zygote (disambiguation. A zygote (from Greek ζυγωτός zugōtos "joined" or "yoked" An embryo (from Greek:, plural, lit "that which grows" from en- "in" + bryein "to swell be full" is a multicellular Their traits and physical appearances are not exactly the same; although they have nearly identical DNA[1], 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,^[8] regardless of ethnic background. The two embryos develop into fetuses sharing the same womb. A fetus (or foetus or fœtus) is a developing Mammal or other Viviparous Vertebrate, after the Embryonic stage and 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). The placenta is an Ephemeral organ present in placental Vertebrates, such as Eutherial Mammals and Sharks during Gestation For the entertainment company see Chorion (company. For the alien race in Stephen Donaldson 's The Gap Cycle, see Amnion (Gap Cycle. 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. Conjoined twins are whose bodies are joined in utero A rare phenomenon the occurrence is estimated to range from 1 in 50000 births to 1 in 200000 births with a somewhat higher incidence 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. Twin-to-twin transfusion syndrome ( TTTS, also known as Feto-Fetal Transfusion Syndrome (FFTS and Twin Oligohydramnios-Polyhydramnios Sequence (TOPS is a complication with Di/di twins have the lowest mortality risk at about nine percent, although that is still significantly higher than that of singletons. ^[9]

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 ^{[10] [11]}). Aneuploidy is defined as an abnormal number of Chromosomes Syndromes caused by an extra or missing chromosome are among the most widely recognized Genetic disorders Klinefelter's syndrome, 47XXY or XXY syndrome is a condition caused by a Chromosome Aneuploidy. Monozygotic twins look alike, although they do not have the same fingerprints (which are environmental as well as genetic). A fingerprint is an impression of the friction ridges of all or any part of the finger 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. ^[7] 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^[12]) 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. Deoxyribonucleic acid ( DNA) is a Nucleic acid that contains the genetic instructions used in the development and functioning of all known This is called epigenetic modification. In Biology, the term epigenetics refers to changes in Gene expression caused by mechanisms other than changes in the underlying DNA sequence 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. ^[13] However, certain characteristics become more alike as twins age, such as IQ and personality. ^[14][15] 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. The European Society of Human Reproduction and Embryology (ESHRE was created in 1985 by both Professor R ||-||} Lyon, also known as Lyons in English is a city in east-central France. This article is about the country For a topic outline on this subject see List of basic France topics. 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. ^[16]

Demographics

A recent study found that vegan mothers are five times less likely to have twins than those who eat animal products. ^[17]

From 1980–97, the number of twin births in the United States rose 52%. ^[18] 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. Fertility medication is medication that enhances Fertility. For women fertility medication is used to stimulate follicle development of the Ovary. Clomifene ( INN) or clomiphene ( USAN and former BAN) or Clomid or Clomifert is a Selective estrogen receptor modulator (SERM In vitro fertilisation ( IVF) is a process by which It may also be linked to the increase of growth hormones in food. Growth hormone ( GH) is a Peptide hormone that stimulates growth and cell reproduction in humans and other animals ^[17]

Ethnicity

About 1 in 90 human births (1. 1%) results from a twin pregnancy. ^[19] The rate of dizygotic twinning varies greatly among ethnic groups, ranging as high as about 6% for the Yoruba or 10% for Linha Sao Pedro, a tiny Brazilian village. The Yoruba (Yo•row•ba ( Yorùbá in Yoruba Orthography) are one of the largest ethno-linguistic or Ethnic groups in West Africa ^[20] 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". A multiple birth occurs when more than one Fetus is carried to term in a single Pregnancy. In 2001, for the first time ever in the US, the twinning rate exceeded 3% of all births. The United States of America —commonly referred to as the 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. Malaria is a vector -borne Infectious disease caused by Protozoan Parasites It is widespread in tropical and subtropical regions including ^[21] 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 African descent (especially Yoruba or Hausa)^[7]
• She is between the age of 30 and 40 years
• She is greater than average height and weight
• She has had several previous pregnancies. West Africa or Western Africa is the Westernmost Region of the African Continent. The Yoruba (Yo•row•ba ( Yorùbá in Yoruba Orthography) are one of the largest ethno-linguistic or Ethnic groups in West Africa The Hausa are a Sahelian people chiefly located in the West African regions of northern Nigeria and southeastern Niger.
• 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. In vitro fertilisation ( IVF) is a process by which Some other treatments such as the drug Clomid can stimulate a woman to release multiple eggs, allowing the possibility of multiples. Clomifene ( INN) or clomiphene ( USAN and former BAN) or Clomid or Clomifert is a Selective estrogen receptor modulator (SERM 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 yams in their diet because yams contain phytoestrogens, which may stimulate the ovaries. Yam is the common name for some species in the genus Dioscorea (family Dioscoreaceae) Phytoestrogens, sometimes called "dietary estrogens" are a diverse group of naturally occurring non steroidal plant compounds that because of their structural similarity with Most likely this is a myth, as phytoestrogens would tend to suppress twinning by reducing gonadotropin levels. Gonadotropins are Protein Hormones secreted by Gonadotrope cells of the Pituitary gland of Vertebrates Gonadotropin is sometimes

Complications of twin pregnancy

Vanishing twins

Main article: 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. A vanishing twin is a Fetus in a multi-gestation pregnancy which dies In utero and is then partially or completely reabsorbed by the mother or twin. ^[22] Early obstetric ultrasonography exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes. Obstetric sonography (ultrasonography is the application of Medical ultrasonography to Obstetrics, in which ultrasound is used to visualize the Embryo This is known as vanishing twin syndrome. A vanishing twin is a Fetus in a multi-gestation pregnancy which dies In utero and is then partially or completely reabsorbed by the mother or twin.

Chang and Eng Bunker, born in Siam (now Thailand) in 1811, were the origin of the term "Siamese twins". The Kingdom of Thailand (ˈtaɪlænd ราชอาณาจักรไทย, râːtɕʰa-ʔaːnaːtɕɑ̀k-tʰɑj

Conjoined twins

Main article: Conjoined twin

Conjoined twins (or "Siamese twins") are monozygotic twins whose bodies are joined together at birth. Conjoined twins are whose bodies are joined in utero A rare phenomenon the occurrence is estimated to range from 1 in 50000 births to 1 in 200000 births with a somewhat higher incidence 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. The brain is the center of the Nervous system in animals All Vertebrates and the majority of Invertebrates have a brain The heart is a muscular organ in all Vertebrates responsible for pumping Blood through the Blood vessels by repeated rhythmic The liver is a vital organ in the human body and is present in Vertebrates and some other animals

Chimerism

Main article: 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. In Zoology, a chimera is an animal that has two or more different populations of genetically distinct cells that originated in different Zygotes if the 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. In Medicine ( Genetics) a mosaic or mosaicism denotes the presence of two populations of cells with different Genotypes in one A chimera may be a hermaphrodite, composed of cells from a male twin and a female twin. A hermaphrodite is an organism having both male and female reproductive organs

Parasitic twins

Main article: Parasitic twin

Sometimes one twin fetus will fail to develop completely and continue to cause problems for its surviving twin. A parasitic twin (also known as an asymmetrical or unequal conjoined twin) is the result of the processes that produce Vanishing twins and Conjoined One fetus acts as a parasite towards the other. Parasitism is a type of symbiotic relationship between Organisms of different Species. 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. Polyploidy occurs in cells and Organisms when there are more than two homologous sets of Chromosomes. Uniparental disomy ( UPD) occurs when a person receives two copies of a Chromosome, or part of a chromosome from one parent and no copies from the other parent

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. Miscarriage or spontaneous abortion is the natural or spontaneous end of a Pregnancy at a stage where the embryo or fetus is incapable of surviving generally defined This occurrence is similar to the vanishing twin syndrome, but typically occurs later than the vanishing twin syndrome. A vanishing twin is a Fetus in a multi-gestation pregnancy which dies In utero and is then partially or completely reabsorbed by the mother or twin.

Low birth weight

Twins typically suffer from the lower birth weights and greater likelihood of prematurity that is more commonly associated with the higher multiple pregnancies. Birth weight is the weight of a baby at its birth. It has direct links with the Gestational age at which the child was born and can be estimated Premature birth (also known as preterm birth) is the Birth of a Baby before the standard period of Pregnancy is completed Throughout their lives twins tend to be smaller than singletons on average.

Twin-to-twin transfusion syndrome

Main article: Twin-to-twin transfusion syndrome

Monozygotic twins who share a placenta can develop twin-to-twin transfusion syndrome. Twin-to-twin transfusion syndrome ( TTTS, also known as Feto-Fetal Transfusion Syndrome (FFTS and Twin Oligohydramnios-Polyhydramnios Sequence (TOPS is a complication with 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. Anemia ( AmE) or anæmia/anaemia ( BrE) (from the Ancient Greek grc-Latn anaîmia, meaning “without blood” is defined as a qualitative Polycythemia (or polycythaemia or erythrocytosis) is a condition in which there is a net increase in the total number of Blood cells, primarily Red The lives of both twins are endangered by this condition.

Human twin studies

Main article: Twin study

Twin studies are studies that assess monozygotic twins for medical, genetic, or psychological characteristics to try to isolate genetic influence from epigenetic and environmental influence. Twin studies are one of a family of designs in Behavior genetics which aid the study of individual differences by highlighting the role of environmental and genetic Medicine is the art and science of healing It encompasses a range of Health care practices evolved to maintain and restore Human Health by the Genetics (from Ancient Greek grc-Latn genetikos, “genitive” and that from grc-Latn genesis, “origin” a discipline of Biology, is Psychology (from Greek grc ψῡχή psȳkhē, "breath life soul" and grc -λογία -logia) is an Academic and See also Nature The natural environment, commonly referred to simply as the environment, is a terminology that is comprised of all living and 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. Human nature is the concept that there are a set of logical characteristics including ways of thinking feeling and acting that all 'normal' human beings have in common

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 obstetricians or midwives may go their entire careers without encountering a single case. Obstetrics (from the Latin obstare, "to stand by" is the surgical speciality dealing with the care of a woman and her offspring during Pregnancy Midwifery is a Health care profession where providers give Prenatal care to expecting Mothers attend the birth of the Infant

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). Superfecundation is the Fertilization of two or more ova from the same cycle by sperm from separate acts of sexual intercourse Superfetation (also spelt superfoetation, based on a false Etymology &mdash see Fetus) is the formation of a fetus while another fetus is already present 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. The terms multiracial and mixed-race describe people whose ancestries come from different races. Mixed twins are Fraternal twins born to Multiracial families which differ in Human skin color and other racial and Ethnic 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^[23]) 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. A karyotype is the characteristic Chromosome complement of a Eukaryote Species.

Twin calves of the Hereford breed. Hereford Cattle are a widely used breed in Temperate areas mainly for beef production

Semi-identical twins

Monozygotic twins can develop differently, due to different genes being activated. Twins are Offspring resulting from the same Pregnancy, either of the same or opposite Sex. ^[24] 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. A blastomere is a type of cell produced by division of the egg after fertilization. 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. In Zoology, a chimera is an animal that has two or more different populations of genetically distinct cells that originated in different Zygotes if the This form had been speculated until only recently being recorded in western medicine^{[25] [26] [27]}.

Animal twins

Twins are common in many animal species, such as cats, sheep, ferrets and deer. WikipediaManual of Style (spelling, articles should conform to one overall spelling style of English typically the one most linked to the article topic (if it is geographic The ferret is a domestic Mammal of the type Mustela putorius furo. 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. Cattle, colloquially referred to as cows, are domesticated Ungulates a member of the Subfamily Bovinae of the family A species of armadillo (Dasypus novemcinctus) has identical twins (usually four babies) as its regular reproduction and not as exceptional cases. The Nine-banded Long-nosed Armadillo or just Nine-banded Armadillo, Dasypus novemcinctus (also known as the poor man's Pig or poverty pig is a

See also

• Evil twin
• Incest between twins
• List of multiple births
• List of twins
• Look-alike

Notes

References

1. Nieuwint A, Van Zalen-Sprock R, Hummel P, Pals G, Van Vugt J, Van Der Harten H, Heins Y, Madan K. Nature is a prominent Scientific journal, first published on 4 November 1869 (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. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. PMID 10073913.  
2. 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.  
3. Girela E, Lorente JA, Alvarez JC, Rodrigo MD, Lorent M, Villaneuva E (1997). "Indisputable double paternity in dizygous twins". Fertility and Sterility 67 (6): 1159-61. doi:10.1016/S0015-0282(97)81456-2. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. PMID 9176461.  

Further reading

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. Full text