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Can identical twins have different genders (as in biological sex)?
I thought they should have absolutely the same genes. If they can, what caused it? I'd assume it MAY be because of errors in cellular division at conception.
Identical twins occur when a single egg is fertilized to form one zygote, which then divides into two separate embryos. Thus, identical twins are always the same sex unless there has been a mutation during development.
However, two identical twins expressing different sexual phenotypes does happen in some extremely rare cases due to environmental factors, deactivation of different X chromosomes in females, or aneuploidy. This is normally because an XXY Klinefelter syndrome zygote has split unevenly.
You are correct though, in that identical twins usually have hundreds of genetic differences early in fetal development which can be attributed to mutations in the DNA. But usually these differences do not result in twins with different sexual phenotypes, though it is possible.
Identical twins' biological sexes - Biology
one of two offspring produced in the same pregnancy twins occur about once in every 86 births.
Dizygotic or fraternal twins develop from two separate ova fertilized at the same time. They may be of the same sex or of opposite sexes, and are no more similar than any other two children of the same parents. Called also binovular, dichorial, dissimilar, and unlike twins.
Monozygotic or identical twins develop from a single ovum that divides after fertilization. (See illustration at monozygotic .) Because they share the same set of chromosomes, they are always of the same sex, and are remarkably similar in hair color, finger and palm prints, teeth, and other respects. Monozygotic twins have exactly the same blood type and can accept tissue or organ transplants from each other. Called also enzygotic, monochorial, mono-ovular, similar, or true twins.
Approximately one third of all twins are identical and two thirds are fraternal. It is not clearly understood exactly what causes a single ovum to divide shortly after conception and thereby produce identical twins, although it seems to be a chance occurrence. The reasons for the production and fertilization of two separate ova that result in fraternal twins are not well understood either, but it is thought that a tendency toward fraternal twins runs in families and is transmitted through the genes of the mother. Women are more likely to have fraternal twins in their later childbearing years, between the ages of 30 and 38 years, than earlier. Older age in the father also seems to be a factor with fraternal twins.
Can a Boy and a Girl Be Identical Twins?
Rarely, a boy and girl may be identical twins. This condition, called Turner Syndrome, afflicts very few pairs of twins. The gender difference occurs when an egg and a sperm pair that begins as two males, loses a copy of its dominant Y chromosome when it splits.
A twin boy and girl may be identical, according to a study found on NCBI, although this situation is very rare. Usually identical twins share the same gender: they are either a pair or boys or girls. In rare instances, however, identical twins form from an egg and sperm that begin as males (sharing XY chromosomes) then change to become a male and female pair. This occurs when one half of the split fertilized egg loses a copy of its genetically encoded Y chromosome. This error generally happens early on the pregnancy. The babies that form include one defined male, who has the proper XY chromosomes required to create male reproductive systems and traits, and a female with XO chromosomes. Although categorized as female, the girl in the dual-sex pair exhibits outward traits of the female gender but carries only one copy of the X chromosome, rather than the XX chromosome used by biologists to classify females as such. Furthermore, the female child often lacks the hormones needed to grow properly and reproduce as a result, she will be much shorter than the male and lack proper ovarian development. The official term for this condition is Turner Syndrome. It is quite rare and affects very few pairs of twins.
The human twin birth rate in the United States rose 76% from 1980 through 2009, from 9.4 to 16.7 twin sets (18.8 to 33.3 twins) per 1,000 births.  The Yoruba people have the highest rate of twinning in the world, at 45–50 twin sets (90–100 twins) per 1,000 live births,    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.   In Central Africa, there are 18–30 twin sets (or 36–60 twins) per 1,000 live births.  In North America, South Asia (India, Pakistan, Bangladesh, Nepal), and Southeast Asia, the lowest rates are found only 6 to 9 twin sets per 1,000 live births. North America and Europe have intermediate rates of 9 to 16 twin sets per 1,000 live births. 
Multiple pregnancies are much less likely to carry to full term than single births, with twin pregnancies lasting on average 37 weeks, three weeks less than full term.  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.  Other factors that increase the odds of having fraternal twins include maternal age, fertility drugs and other fertility treatments, nutrition, and prior births.  Some women intentionally turn to fertility drugs in order to conceive twins. 
The vast majority of twins are either dizygotic (fraternal) or monozygotic (identical). Less common variants are discussed further down the article.
Fraternal twins can be any of the following:
- Female–female twins: Sometimes called sororal twins (25%).
- Male–male twins: Sometimes called fraternal twins (25%).
- Female-male twins: This is the most common pairing (50%), simply by virtue of it encompassing both "female-male" (25%) and "male-female" (25%) twins.
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,  while it is 1.07 males/female in Italy.  However, males are also more susceptible than females to die in utero, and since the death rate in utero is higher for twins, it leads to female twins being more common than male twins. 
Zygosity is the degree of identity in the genome of twins.
|Common name||Scientific name||Zygosity||Development||Occurrence||Identification||Health||Other|
|Superfecundation||x||x||Eggs are fertilized during different acts of intercourse||x||x||x||Usage is practically equivalent with heteropaternal superfecundation, which occurs when two different males father fraternal twins, because though superfecundation by the same father is thought to be a common occurrence, it can only be proven to have occurred with multiple fathers.|
|Superfetation||x||x||A woman gets pregnant again while already pregnant, resulting in multiple fetuses at differing developmental stages||x||x||x||x|
|Parasitic twin||x||x||x||x||x||By definition only healthy fully formed fetus||x|
|Vanishing twin||Twin resorption, twin embolisation syndrome||x||x||Up to 1 of every 8 multifetus pregnancies||x||By definition only healthy fully formed fetus||Chimerism, mosaicism|
|Conjoined twin||x||x||x||x||x||Ranges from normal to compromised||x|
Dizygotic (fraternal) twins Edit
Dizygotic (DZ) or fraternal twins (also referred to as "non-identical twins", "dissimilar twins", "biovular twins", and, informally in the case 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. Fraternal twins are, essentially, two ordinary siblings who happen to be born at the same time, since they arise from two separate eggs fertilized by two separate sperm, just like ordinary siblings.
This is the most common type of twin. 
Dizygotic twins, like any other siblings, will practically always have different sequences on each chromosome, due to chromosomal crossover during meiosis. Dizygotic twins share only 50 percent of each other's genes, which resemble amongst siblings that are conceived and born at different times. Like any other siblings, dizygotic twins may look similar, particularly as they are the same age. However, dizygotic twins may also look very different from each other (for example, be of opposite sexes).
Studies show that there is a genetic proclivity for dizygotic twinning. However, it is only the mother who 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. 
Dizygotic twins are also more common for older mothers, with twinning rates doubling in mothers over the age of 35.  With the advent of technologies and techniques to assist women in getting pregnant, the rate of fraternals has increased markedly.
Monozygotic (identical) twins Edit
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. The chances of having identical twins is relatively rare — around 3 or 4 in every 1,000 births 
Regarding spontaneous or natural monozygotic twinning, a recent theory proposes that monozygotic twins are probably formed when a blastocyst contains two inner cell masses (ICM), each of which will lead to a separate fetus, rather than by the embryo splitting while hatching from the zona pellucida (the gelatinous protective coating around the blastocyst). 
Monozygotic twins may also be created artificially by embryo splitting. It can be used as an expansion of in vitro fertilization (IVF) to increase the number of available embryos for embryo transfer. 
Monozygotic twinning occurs in birthing at a rate of about 3 in every 1000 deliveries worldwide (about 0.3% of the world population). 
The likelihood of a single fertilization resulting in monozygotic twins is uniformly distributed in all populations around the world.  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  and up to over 20 in some Central African countries.  The exact cause for the splitting of a zygote or embryo is unknown.
IVF techniques are more likely to create dizygotic twins. For IVF deliveries, there are nearly 21 pairs of twins for every 1,000. 
Genetic and epigenetic similarity Edit
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 a pair of monozygotic twins reproduces with another pair or with the same person), rather than first cousins. Identical twins do not have the same fingerprints however, because even within the confines of the womb, the fetuses touch different parts of their environment, giving rise to small variations in their corresponding prints and thus making them unique. 
Monozygotic twins always have the same genotype. 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 syndrome zygote splitting unevenly.   
Monozygotic twins, although genetically very similar, are not genetically exactly the same. The DNA in white blood cells of 66 pairs of monozygotic twins was analyzed for 506,786 single-nucleotide polymorphisms known to occur in human populations. Polymorphisms appeared in 2 of the 33 million comparisons, leading the researchers to extrapolate that the blood cells of monozygotic twins may have on the order of one DNA-sequence difference for every 12 million nucleotides, which would imply hundreds of differences across the entire genome.  The mutations producing the differences detected in this study would have occurred during embryonic cell-division (after the point of fertilization). If they occur early in fetal development, they will be present in a very large proportion of body cells.
Another cause of difference between monozygotic twins is epigenetic modification, caused by differing environmental influences throughout their lives. Epigenetics refers to the level of activity of any particular gene. A gene may become switched on, switched off, or could become partially switched on or off in an individual. This epigenetic modification is triggered by environmental events. Monozygotic twins can have markedly different epigenetic profiles. A study of 80 pairs of 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.  However, certain characteristics become more alike as twins age, such as IQ and personality.   
In January 2021, new research from a team of researchers in Iceland was published in the journal Nature Genetics suggesting that identical twins may not be quite as identical as previously thought. The four-year study of monozygotic (identical) twins and their extended families revealed that these twins have genetic differences that begin in the early stages of embryonic development. 
Polar body and semi-identical twins Edit
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 probably a polar body twin. The authors were unable to predict whether a healthy fetus could result from a polar body twinning. 
In 2003, a study argued that many cases of triploidy arise from sesquizygotic (semi-identical) twinning. 
Sesquizygotic twin type Edit
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 (the number of placentae) 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. Non-conjoined monozygotic twins form up to day 14 of embryonic development, but when twinning occurs after 14 days, the twins will likely be conjoined.  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.
DiDi twins have the lowest mortality risk at about 9 percent, although that is still significantly higher than that of singletons. 
Monochorionic twins generally have two amniotic sacs (called Monochorionic-Diamniotic "MoDi"), which occurs in 60–70% of the pregnancies with monozygotic twins,  and in 0.3% of all pregnancies.  Monochorionic-Diamniotic twins are almost always monozygotic, with a few exceptions where the blastocysts have fused. 
Monochorionic twins share the same placenta, and thus have a risk of twin-to-twin transfusion syndrome.
Monoamniotic twins are always monozygotic. 
The survival rate for monoamniotic twins is somewhere between 50%  to 60%. 
Monoamniotic twins, as with diamniotic monochorionic twins, have a risk of twin-to-twin transfusion syndrome. Also, the two umbilical cords have an increased chance of being tangled around the babies. Because of this, there is an increased chance that the newborns may be miscarried or suffer from cerebral palsy due to lack of oxygen.
When the division of the developing zygote into 2 embryos occurs, 99% of the time it is within 8 days of fertilization.
Mortality is highest for conjoined twins due to the many complications resulting from shared organs.
Dichorionic-diamniotic twins at 8 weeks and 5 days since co-incubation as part of IVF. The twin at left in the image is shown in the sagittal plane with the head pointing towards upper left. The twin at right in the image is shown in the coronal plane with the head pointing rightwards.
Abdominal ultrasonography of monoamniotic twins at a gestational age of 15 weeks. There is no sign of any membrane between the fetuses. A coronal plane is shown of the twin at left, and a sagittal plane of parts of the upper thorax and head is shown of the twin at right.
A 2006 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." 
From 1980 to 1997, the number of twin births in the United States rose 52%.  This rise can at least partly be attributed to the increasing popularity of fertility drugs and procedures such as IVF, which result in multiple births more frequently than unassisted fertilizations do. It may also be linked to the increase of growth hormones in food. 
About 1 in 90 human births (1.1%) results from a twin pregnancy.  The rate of dizygotic twinning varies greatly among ethnic groups, ranging as high as about 45 per 1000 births (4.5%) for the Yoruba to 10% for Linha São Pedro, a tiny Brazilian settlement which belongs to the city of Cândido Godói.  In Cândido Godói, one in five pregnancies has resulted in twins.  The Argentine historian Jorge Camarasa has put forward the theory that experiments of the Nazi doctor Josef Mengele could be responsible for the high ratio of twins in the area. 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.  A high twinning rate has also been observed in other places of the world, including:
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 monozygotic. 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.  
Twins are more common in African Americans. 
Predisposing factors Edit
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. In the United States it has been estimated that by 2011 36% of twin births resulted from conception by assisted reproductive technology. 
The risk of twin birth 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. Ovarian hyperstimulation without IVF has a very high risk of multiple birth. Reversal of anovulation with clomifene (trade names including Clomid) has a relatively less but yet significant risk of multiple pregnancy.
A 15-year German study  of 8,220 vaginally delivered twins (that is, 4,110 pregnancies) in Hesse yielded a mean delivery time interval of 13.5 minutes.  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.
There have also been cases in which twins are born a number of days apart. Possibly the worldwide record for the duration of the time gap between the first and the second delivery was the birth of twins 97 days apart in Cologne, Germany, the first of which was born on November 17, 2018. 
Vanishing twins Edit
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 fetus has died very early in the pregnancy and has not been detected or recorded.  Early obstetric ultrasonography exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes in the uterus. There are several reasons for the "vanishing" fetus, including it being embodied or absorbed by the other fetus, placenta or the mother. This is known as vanishing twin syndrome. Also, in an unknown proportion of cases, two zygotes may fuse soon after fertilization, resulting in a single chimeric embryo, and, later, fetus.
Conjoined twins Edit
Conjoined twins (or the once-commonly used term "siamese") are monozygotic twins whose bodies are joined together during pregnancy. This occurs when the zygote starts to split after day 12  following fertilization and fails to separate completely. 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.
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. In one case DNA tests determined that a woman, mystifyingly, was not the mother of two of her three children she was found to be a chimera, and the two children were conceived from eggs derived from cells of their mother's twin. 
Parasitic twins Edit
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 treated medically.
Partial molar twins Edit
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 Edit
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 Edit
It is very common for twins to be born at a low birth weight. More than half of twins are born weighing less than 5.5 pounds (2.5 kg), while the average birth weight of a healthy baby should be around 6–8 pounds (3–4 kg).  This is largely due to the fact that twins are typically born premature. Premature birth and low birth weights, especially when under 3.5 pounds (1.6 kg), can increase the risk of several health-related issues, such as vision and hearing loss, mental disabilities, and cerebral palsy.  There is an increased possibility of potential complications as the birth weight of the baby decreases.
Twin-to-twin transfusion syndrome Edit
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.
Stillbirths occurs when a fetus dies after 20 weeks of gestation. There are two types of stillbirth, including intrauterine death and intrapartum death. Intrauterine death occurs when a baby dies during late pregnancy. Intrapartum death, which is more common, occurs when a baby dies while the mother is giving birth. The cause of stillbirth is often unknown, but the rate of babies who are stillborn is higher in twins and multiple births. Caesareans or inductions are advised after 38 weeks of pregnancy for twins, because the risk of stillbirth increases after this time. 
Heterotopic pregnancy Edit
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. [ citation needed ]
For otherwise healthy twin pregnancies where both twins are head down a trial of vaginal delivery is recommended at between 37 and 38 weeks.   Vaginal delivery in this case does not worsen the outcome for the infant as compared with Caesarean section.  There is controversy on the best method of delivery where the first twin is head first and the second is not.  When the first twin is not head down a caesarean section is often recommended.  It is estimated that 75% of twin pregnancies in the United States were delivered by caesarean section in 2008.  In comparison, the rate of caesarean section for all pregnancies in the general population varies between 14% and 40%.  In twins that share the same placenta, delivery may be considered at 36 weeks.  For twins who are born early, there is insufficient evidence for or against placing preterm stable twins in the same cot or incubator (co-bedding). 
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. Classical twin studies are now being supplemented with molecular genetic studies which identify individual genes.
Bi-paternal twins Edit
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. [ citation needed ]
Dizygotic twins from biracial couples can sometimes be mixed twins, which exhibit differing ethnic and racial features. One such pairing was born in London in 1993 to a white mother and Caribbean father. 
Monozygotic twins of different sexes Edit
Among monozygotic twins, in extremely rare cases, twins have been born with different sexes (one male, one female).  When monozygotic twins are born with different sexes it is because of chromosomal defects. The probability of this is so small that multiples having different sexes is universally accepted as a sound basis for in utero clinical determination that the multiples are not monozygotic.
Another abnormality that can result in monozygotic twins of different sexes is if the egg is fertilized by a male sperm but during cell division only the X chromosome is duplicated. This results in one normal male (XY) and one female with Turner syndrome (45,X).  In these cases, although the twins did form from the same fertilized egg, it is incorrect to refer to them as genetically identical, since they have different karyotypes.
Semi-identical twins Edit
Monozygotic twins can develop differently, due to their genes being differently activated.  More unusual are "semi-identical twins", also known as "sesquizygotic". As of 2019 [update] , only two cases have been reported.   These "half-identical twins" are hypothesized to occur when an unfertilized egg cleaves into two identical attached ova, both of which are viable for fertilization. Both ova are then fertilized, each by a different sperm, and the coalesced zygotes undergo further cell duplications developing as a chimeric blastomere. If this blastomere then undergoes a twinning event, two embryos will be formed, with different paternal genetic information and identical maternal genetic information.
This results in a set of twins with identical gene sequence from the mother's side, but different sequences from the father's side. Cells in each fetus carry chromosomes from either sperm, resulting in chimeras. This form had been speculated until only recently being recorded in Western medicine.   
In 2007, a study reported a case of a pair of living twins, which shared an identical set of maternal chromosomes, while each having a distinct set of paternal chromosomes, albeit from the same man, and thus they most likely share half of their father's genetic makeup. The twins were both found to be chimeras. One was an intersex XX, and one a XY male. The exact mechanism of fertilization could not be determined but the study stated that it was unlikely to be a case of polar body twinning.  
A second case of sesquizygotic twins was reported in 2019. In this case they shared 100% of their maternal chromosomes and 78% of their paternal genomic information.  
Mirror image twins Edit
Mirror image twins result when a fertilized egg splits later in the embryonic stage than normal timing, around day 9–12. This type of twinning could exhibit characteristics with reversed asymmetry, such as opposite dominant handedness, dental structure, or even organs (situs inversus).  If the split occurs later than this time period, the twins risk being conjoined. There is no DNA-based zygosity test that can determine if twins are indeed mirror image.  The term "mirror image" is used because the twins, when facing each other, appear as matching reflections. 
There have been many studies highlighting the development of language in twins compared to single-born children. These studies have converged on the notion that there is a greater rate of delay in language development in twins compared to their single-born counterparts.  The reasons for this phenomenon are still in question however, cryptophasia was thought to be the major cause.  Idioglossia is defined as a private language that is usually invented by young children, specifically twins. Another term to describe what some people call "twin talk" is cryptophasia where a language is developed by twins that only they can understand. The increased focused communication between two twins may isolate them from the social environment surrounding them. Idioglossia has been found to be a rare occurrence and the attention of scientists has shifted away from this idea. However, there are researchers and scientists that say cryptophasia or idioglossia is not a rare phenomenon. Current research is looking into the impacts of a richer social environment for these twins to stimulate their development of language. 
Twins are common in many mammal species, including cats, sheep, ferrets, giant pandas, dolphins, dogs, deer, marmosets, tamarins, and elephants. The incidence of twinning among cattle is about 1–4%, and research is underway 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.
Pervasive misrepresentation of twin evidence for a genetic component of gender dysphoria
Major voices in the contemporary anti-trans movement, including American College of Pediatricians president Dr. Michelle Cretella and Heritage Foundation research fellow Ryan T. Anderson, have recently put forth questionable claims about studies of gender dysphoria in twins. Last year, Michelle Cretella argued:
But in the largest study of twin transgender adults, published by Dr. Milton Diamond in 2013, only 28 percent of the identical twins both identified as transgender. Seventy-two percent of the time, they differed. (Diamond’s study reported 20 percent identifying as transgender, but his actual data demonstrate a 28 percent figure, as I note here in footnote 19.)
That 28 percent of identical twins both identified as transgender suggests a minimal biological predisposition, which means transgenderism will not manifest itself without outside nonbiological factors also impacting the individual during his lifetime.
And in his noxiously titled book “When Harry Became Sally”, Ryan T. Anderson claims:
There is no scientific evidence that a transgender identity is biologically determined. In fact, there is some evidence—though by no means conclusive—that other factors must be involved. Professor Paul Hruz cites studies on identical twins, with the same genetic complements and the same prenatal environment, who developed differing gender identities. If transgender identity were innate and independent of nurture, then two children who shared a womb and have identical genetic material would both be transgender, or neither would be. But that isn’t what the research indicates.
For example, a study by Milton Diamond published in the International Journal of Transgenderism looked at transgender individuals who have an identical twin, and found that the twin also identified as transgender in 20 percent of the cases. That figure suggests some measure of biological predisposition, but far from biological determination. Dr. Quentin Van Meter noted in court testimony that the concordance rate would be close to 100 percent if genes and/or prenatal hormones were the primary causes of transgender identities. Clearly, life experiences play a large role. Neither twin studies nor brain imaging supports the hypothesis that a transgender identity is innate and fixed, or that a person could really be “a man trapped in a woman’s body” or “a woman trapped in a man’s body.”
Cretella and Anderson’s argument misleadingly portrays the evidence from twin studies and selectively omits relevant information. Their assertion that biological predisposition to gender dysphoria is negligible because identical twins are not universally both dysphoric or non-dysphoric gives the false impression that a finding of concordance in 20% or more of identical twins is somehow spurious or meaningless. It is not.
What Cretella and Anderson neglected to inform their readers is that concordance for gender dysphoria among dizygotic (fraternal) twins is far more rare. A 2012 review of the literature found that of the reported cases of identical twins where at least one was gender dysphoric, 39.1% of twin pairs were concordant for gender dysphoria (Heylens et al., 2012). However, out of all the reported cases of fraternal twins where at least one was gender dysphoric, none were concordant for gender dysphoria. In other words, genetically identical twins were vastly more likely to be concordant for gender dysphoria than those who were not genetically identical. That is not a negligible biological contribution to gender identity – it is a significant one.
Additionally, Cretella and Anderson’s assertion that a lack of concordance in some identical twins points to a lack of a biological determination for gender dysphoria is faulty. There are numerous possible biological influences on gender identity other than the genetic code itself, as Heylens et al. (2012) explain:
The higher concordance for GID in MZ than in DZ twins is consistent with a genetic influence on its genesis although shared and nonshared environmental factors cannot be ruled out. … The discordance for GID in five (62.5%) out of eight of the described MZ female twin pairs might have several explanations. For example, differential prenatal and postnatal environmental factors might both contribute to discordance between MZ twins. …
In addition, several epigenetic differences may underlie phenotypic discordance between the MZ twin pairs. In female MZ twins, skewed X inactivation in the affected twin might be an illustration of an epigenetic difference explaining phenotypic discordance. A study by Fraga et al. demonstrated that epigenetic differences increase during the lifetime of MZ twins, which may help to understand phenotypic differences. Genetic differences between concordant and discordant MZ twins are illustrated in a recent study by Bruder et al., showing copy number variations (CNVs) in their genomes. These findings suggested that CNV analysis in phenotypically discordant MZ twins may provide a powerful tool for identifying disease predisposition loci.
On the basis of these and other twin and family studies, Polderman et al. (2018) have proposed a polygenic threshold model for gender identity, concluding that there is “significant and consistent evidence for the role of innate genetic factors in the development of both cisgender and transgender identities, a negligible role for shared environmental factors, and a small potential role for unique environmental factors.” Cretella and Anderson do a disservice to their readers when they neglect this evidence in favor of their own motivated oversimplifications of “biology”. ■
Physical differences between identical and fraternal twins
One of the issues for those who want to differentiate between identical and fraternal twins are the physical similarities or differences between the siblings. Identical twins, as the name states, are identical, meaning they are indistinguishable. This is due to them sharing 100% of the genetic material. If they have any visible difference, it’s due to an external factor, such as hairstyle or scars.
Fraternal twins look alike, just like two ordinary siblings. They may have differences in height, skin tone, body build, etc. In certain cases, fraternal twins can be very similar, given that they share about 50% of the genes. However, they’re never identical.
And then there’s the golden rule: if the siblings’ genders are different (one of them is a girl and the other one a boy), then they’re definitely fraternal twins. Identical twins always share the same gender.
Separated at birth
The idea of twin telepathy has been around for well over a century. The 1844 Alexandre Dumas novella, "The Corsican Brothers," tells the story of two once-conjoined brothers who were separated at birth yet even as adults continue to share not only thoughts but physical sensations. As one twin describes, "However far apart we are now we still have one and the same body, so that whatever impression, physical or mental, one of us perceives has its after-effects on the other."
Dumas drew upon a considerable body of folklore associated with twins — not all of it positive. In many places if a cow gives birth to twin calves, they are sold immediately, as they are considered a bad omen. Centuries ago in West Africa — where women bear twins at a rate four times higher than the rest of the world — among the Yoruba people of Nigeria the arrival of twins was cause for alarm. Sometimes the mother and babies were banished from their village or even put to death, though modern-day twins are revered.
In their book "The Encyclopedia of Superstitions," (Metro Books 1961) folklorists E. and M.A. Radford note that "It is a very common belief that twins, especially identical twins, are united by a strong bond of sympathy that each knows when danger or misfortune threatens the other, even when they are separated. In the same mysterious way, any special state of happiness or wellbeing in one of the pair is reflected in the feelings of the other. It is often said that if one twin dies, the other will not live long thereafter."
Indeed, when twins die at the same time it can give the appearance of some sort of supernatural or psychic connection linking not only their mental states but physical health. For example in 2017, 97-year-old twin sisters Martha Williams and Jean Haley died outside their Rhode Island home after they both appeared to have tripped and fallen in the cold, dying from exposure. Three years earlier in 2014 identical twins Helen Mae Cook and Clara Mae Mitchel, aged 83, died within a day of each other (though the circumstances were different: one died suddenly of a heart attack and the other after a long struggle with Alzheimer's disease).
As remarkable as these stories are, they are the notable exceptions. After all, they make the news and become noticed precisely because of their rarity and extraordinary coincidence. The deaths of twins who die months, years, or decades apart are much more common and generally not newsworthy.
Of course given their advanced ages their coincident deaths are inherently more likely it's far more common to hear of 83-year-old twins dying at the same time than, for example, 23-year-old twins. That identical twins might die at approximately the same stage of life at higher rates than non-twins is to be expected since genetics plays a role in many life-threatening illnesses including heart disease. Another non-psychic explanation for the timing of twin deaths is stress the death of a close family member is one of the most stressful events in a person's life, and the shock and stress of losing such an intimate sibling could trigger a potentially fatal heart attack in the remaining twin.
What is the biological reason for the birth of identical twins ?
Generally, a fertilized ovum (zygote) develops in to a single individual. But, occasionally, at the time of cleavage of a zygote, the two cells are separated, each producing an individual. Both the individuals are produced at one birth. They are known as identical twins or monozygotic twins because both are identical in all respects. In this case both the offspring arise from the same zygote, which after some cell divisions divides mitotic-ally into two individuals having the same set of chromosomes in each individual. In this respect the identical twins may be called clones.
Genetically identical organisms descended from a single common ancestral cell by mitosis are called clones. The same situation can be observed in identical triplets and multiples. In human beings such births are not common. Since the fertilization takes place only once, the identical twins belong to the same sex. They can be boys or girls and have the same genetic make-up. When identical twins fall to separate completely from each other, they are called Siamese twins.
In some cases two ova are released from the ovary instead of one. Both the ova are fertilized separately by two different sperms at nearly the same time and form two zygotes. As the two zygotes receive different sets of chromosomes, they are genetically different and show variations. They can either be both boys or girls, or a boy and a girl. Such twins are known as non-identical, or dizygotic or fraternal twins. The only common thing in both of them is that they develop simultaneously in the uterus.
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Identifying fraternal and identical twins
Often, identical twins can be identified in the womb, as they often share a placenta. After birth, it can be deduced that twins are fraternal if they are of different genders (a boy and a girl) or if they have different blood types. However, the only way to reliably conclude that they are identical twins is through genetic testing (a twin zygosity DNA test).
Quite paradoxically, identical twins may not always look exactly alike even though they share the same genetic make up. Environmental factors may cause their appearance to evolve differently as early as inside the mother's womb. Here's a video of twin children of an interracial couple, one of whom is black and the other white:
It’s not as simple as X and Y
Apart from transgender issues, other conditions make it clear that defining male and female is not so simple. For instance, there are people with XY chromosomes — which makes them genetically male — who look, act and feel like women because their bodies cannot react to male hormones.
In other cases, some women with a condition that exposed them to high levels of testosterone before birth identify as male — but many more with the same condition do not.
Some of the most compelling evidence for the idea of gender identity being hard-wired into the brain comes from medical reports on people who were born in the 1950s and 1960s with birth defects involving their genitals. Doctors thought the humane solution, to spare such children from being ostracized, was to perform surgery to make them one sex or the other.
Since it is easier for surgeons to make a vagina than a penis, most of these babies were made female. Their parents were advised to raise them as girls and never to tell them about their condition at birth. The general belief was that their upbringing — a triumph of nurture over nature — would make them truly female.
The idea was a failure. As they matured, many had a clear sense that they were male. According to a study of 16 of them, more than half wound up identifying as male.
“Considering the fact that you can brainwash some people about just about anything, failing with so many is catastrophic,” Dr. Safer said in an email.
Of all the information on gender identity, he said that to medical experts, the studies on those cases provide the strongest evidence that gender identity has deep biological roots.