Why can not immediately identify identical twins. What is the difference between twins and twins

Many people live and do not know how twins differ from twins. And, by the way, they live well. But sometimes this knowledge becomes relevant. As a rule, this happens in those cases when people come into your life in one way or another who were born with their brother or sister with a difference of several minutes. It can be friends, lovers or ... future children. In the latter case, interest becomes far from idle, because parents want to know about their future babies, if not all, then as much as possible. So what is the difference between twins and twins? Let's figure it out.

Let's be clear: definitions and formulations

When two children develop in one tummy at once, most people imagine two completely identical babies. And someone else, starting planning a second pregnancy, firmly wishes to have both a girl and a boy at once, and that they be so similar, like a reflection of each other in a mirror. However, you've probably noticed the fact that twins don't always look alike. And sometimes, if you don’t know for sure, it’s generally difficult to say that they are relatives. In order to better understand why this happens, it is necessary to understand, at least at a basic level, how twins differ from twins.

If you ask a person connected with medicine with the question of how twins differ from twins, you will get an exhaustive answer that absolutely all children born during one pregnancy are twins. But all the educated numerals, such as "twins" or "triplets" - refer exclusively to the number of children born as a result of one multiple pregnancy. That is, twins may not be twins, but the children of twins are always twins.

"How so?" - you will be surprised. And the thing is that twins can be born under various conditions, depending on which they are called identical (monozygotic) twins and fraternal (dizygotic) twins.

Identical, similar children of the same sex appear in the rare case when one egg, fertilized by one sperm, after the formation of a zygote, for reasons inexplicable to science, is divided in half. In this case, babies get exactly the same genotype and gender.
But if a woman, as a result of any hormonal changes, produces two mature eggs at once, and both of them are fertilized by two different spermatozoa, then babies with a different genetic set are obtained. Such children may be of different sexes and resemble each other no more than ordinary brothers and sisters. It is they who are popularly called by a simple and capacious word - twins.

By the way, if recently the birth of twins (fraternal twins) has become commonplace, and twins are born in one case out of a hundred, then the appearance of identical twins occurs many times less often - only one case in three to four thousand births.

The mechanism of the appearance of twins

The appearance of two children during one pregnancy is an incident so rare that it is equated to a small miracle. This complex and not fully understood by science phenomenon still excites the minds of many people. Let's look at the principle of how twins differ from twins, and where they come from in general in more detail. At the moment, there are three different ways of forming a multiple pregnancy.

identical twins

The fundamental differences between twins and twins are laid in the first seconds of the formation of a new life. As already mentioned above, identical twins appear from one zygote - merged together one egg and one sperm. After fertilization, this zygote suddenly divides into two (or more) parts, each of which has exactly the same set of genes. After that, some variations in the course of pregnancy are possible:

Each of the resulting babies can have their own placenta and their own fetal bladder;
Babies can share one placenta for two, but each of the children has their own fetal bladder;
Twins develop in one "house" for two - sharing one fetal bladder and one placenta for two.

It depends on how this happened, whether the babies will develop separately from each other or whether they will grow "side by side" throughout the perinatal period. This is also important for complications of pregnancy of various etiologies.

But still, what is it that characterizes and how twins differ from twins? If we systematize the available information, we get the following picture.

All identical twins have an identical appearance, but in a quarter of all cases, the differences are mirrored (one of the children has a mole on the right, the other has exactly the same, but on the left).
Not only the color of the eyes, hair and physique will be the same for such children, but even such trifles as the location of the teeth and the structure of the body.
These twins have exactly the same blood type and fingerprints.
Monozygotic twins are always of the same sex.
Organ and tissue transplants are always successful between identical twins.
These twins often have similar illnesses.
Identical twins can be in a woman of absolutely any age.
Planning identical twins is impossible in principle.

Semi-identical twins

In addition to identical twins and twins, an intermediate option is possible. These are the so-called semi-identical twins, which are also monozygotic. In this case, the most important difference between twins and twins, namely, the same-sex nature of such children, can be refuted.

It is such a complex series of coincidences that can lead to the birth of semi-identical twins, who, with complete external similarity, can be of different sexes. The similarity of the genotype of such twins is about 75%, because in children the genes of the mother's egg cell and half of the father's genes completely coincide.

If in other cases the key difference between twins and twins, which concerns their gender and appearance, is of fundamental importance, then when semi-identical twins are conceived, we can observe an intermediate option when outwardly very similar children may be (or may not be) of a different sex or different race.

Dizygotic twins or twins

And if we have already realized the main difference between twins and twins, then it is worth considering the other side of the issue. Or rather, what signs distinguish twins, or what is the correct name for such babies - dizygotic fraternal twins.

As you remember, such a pregnancy is obtained when two or more eggs mature in a woman at the same time. They are fertilized by two different spermatozoa. As a result, children are born that are similar to each other in the same way as brothers and sisters born in different periods. Nevertheless, if we summarize the available facts, then twins and twins, the difference between which we are interested in, differ as follows:

In children from twins, on average, about half of the genes coincide, more often about 40-60%.
Each of the embryos is formed by the fusion of different eggs and different sperm.
Both babies have their own fetal bladder and their own placenta.
Children can be either heterosexual or same-sex.
When transplanting organs and tissues, the candidacy of one of the fraternal twins is considered on an equal basis with other relatives, and is not always possible.
The birth of dizygotic twins can be influenced by factors known to science and their appearance can be predicted and even planned, which you can read about here.
It is twins that are obtained as a result of IVF, when a woman is implanted with not one, but several fertilized eggs.
Twins can both resemble each other and radically differ in facial features, ox and eye color, skin color and physique.
Children of their dizygotic twins may have different blood types.

Diagnosis of twins or twins during pregnancy

When the doctor leading your pregnancy pleases you with the news that you are expecting two babies at once, your heart stops, then to beat with renewed vigor. But then curiosity also wakes up - boys, girls or heterosexual? Twins or twins?

Of course, in the first weeks of pregnancy, the doctor will only be able to diagnose a larger uterus compared to a normal pregnancy and the presence of two beating hearts. However, the situation becomes absolutely transparent on ultrasound, where already at 4-5 weeks you can be absolutely sure that the pregnant woman is expecting twins.

But whether they will be identical twins or twins remains a surprise until the very birth, if the babies are not of different sexes and were not the result of IVF. In the latter cases, you can be absolutely sure that the expected babies are twins.

Twins and twins - similarities and differences

Comparing the parameters of how twins differ from twins, one should not forget about their inclinations and character. It is also worth considering the social environment, the environment, the desire of the kids themselves to be like each other, or vice versa, protest and the desire to show their pronounced individuality. These factors greatly influence whether grown-up babies will look like each other.

First of all, for close people and parents, only the medical parameters of how twins differ from twins make sense. This helps to better understand what and why is happening with babies. Whether the children will be like two drops of water, or different, as if they were not relatives, in any case, do not forget that each child is a separate person, with his own goals and desires.

For some unknown reason, every fifth child from a multiple pregnancy is born left-handed. Among ordinary children, left-handers are 2 times less. What increases the likelihood of left-handedness has not been established by science, perhaps this is due to the position of children in the womb.

twin statistics

The number of twins varies among different peoples. Especially a lot of them are born in Africa, and Japanese women give birth to them least often. Moreover, even among a roughly genetically equivalent population in different regions, the number of twins born is different. For example, in the US, the states of Connecticut and New Jersey are the most fertile for twins, there are 25% more twins born there than in the US as a whole. And in Hawaii, they are born less than in the United States as a whole by 30%.

How to explain it?
Perhaps there is still an influence of the general genetic composition of the population in specific regions, nutrition and other factors.

Monozygotic twins are born in 33% of cases, dizygotic of the same sex in 33% of cases, dizygotic of opposite sex in other cases. Boys and girls are born in all groups with a frequency of 50/50.

Twin types

The classical division of twins into identical and fraternal does not cover the entire spectrum of possible anomalies in the occurrence of twins. Other types of twins are also possible.

polar twins

These twins have the same genetic material from the mother, and different from the father, that is, this is an intermediate option between identical and dizygotic twins.
How does this type of twins arise?
In many ways, the explanation is theoretical in nature, a special role is attributed to the polar body, which is formed during ovulation and has the same set of genes with it (this phenomenon is not described in classical biology). Normally, this polar cell dies, but in the case of semi-identical twins, a spermatozoon fertilizes it, and as a result, 2 zygotes are obtained, carrying the same set of mother's genes and different from the father. Semi-identical (polar) twins can be of different sexes.

Chimerism

The literature describes only 25 cases of the birth of such twins, in which chimerism was diagnosed.
These twins are obtained when two different eggs, fertilized by different sperm, begin to divide and "mix". Twins have mosaicism, their body contains cells with different genotypes. Outwardly, this may look rather strange, in twins one part of the body may differ significantly from the other, for example, there may be a different skin color.

Superfecundation

This phenomenon occurs when the mother had sexual contact with different men in the cycle when the conception of fraternal twins occurred. As a result, one egg is fertilized by a sperm from one man, and another from another. Children of twins born at the same time end up with completely different fathers.

Usually this goes unnoticed, but not always, imagine that one dad was white and the second was black, in which case the superfecundation will manifest itself very clearly and the awl in the bag will not be hidden.

vanishing twin

Statistically, when performing ultrasound at very early stages of pregnancy, twins are detected many times more often than they are born. One of the twins "disappears". Perhaps this is the reason for the vague longing of many people for something lost, as if for their soulmate.

Monozygotic twins

Monozygotic twins, developed from a single egg, despite the portrait resemblance, have different fingerprints, and their DNA has unique regions that distinguish them from each other. That is, in fact, despite the same set of genes received from the father and mother, these twins have differences at the gene level.

Relationship between twins

Identical twins are so similar that even their brains work in the same way, their encephalogram pattern is the same. They think the same. That is why for the rest of their lives they have a paradoxical feeling of each other, even at a distance they feel each other's state and easily guess thoughts.

At an early age, their own language is formed in twins. With the beginning of the formation of speech at the age of a little over 2 years, they talk to each other using their own dialect, which is not understandable to others.

Gender of twins

Surprisingly, sometimes identical twins are born ... of different sexes. Their identity is determined by genetic testing. The reason for the presence of Klinefelter's syndrome is a genetic anomaly in which the number of sex chromosomes is disturbed. With mosaicism (when not all cells of the body have defective genes), the manifestation of the syndrome can be different, and one of the fetuses develops male genital organs.

Twin births

Usually, twin births occur with an interval of 15-20 minutes, but not always. The longest interval between the birth of two twins described in the literature was 85 days. As a rule, a sick, genetically defective or malformed child is born first, giving the remaining twin the opportunity to further develop and be born at term and healthy.

twins weight

Although, according to statistics, the weight of twins is always less than that of children with a singleton pregnancy, cases of very large babies have been described. For example, a resident of South Carolina in 2008 gave birth to twins with a total weight of 12 kg 200 grams.

Identical triplets

If you have triplets... (not my article)

Watching the development and behavior of twins - what could be more interesting? That's what I thought when I dreamed of having twins. Then I could not even imagine that I was destined by fate to give birth to identical triplets ...

Triplets - what are the "twin" features?

Unfortunately, there is little Russian-language literature on triplets. In our culture, twins enjoy universal sympathy, which prompts journalists to write touching notes about the happy births of triplets. This basically ends the information about triplets. In books about twins, as a rule, the authors talk about twins, about twins. Of course, most of the provisions expressed by various authors about twins can also apply, in some cases, to triplets.
But, nevertheless, the features of development, the behavior of twins also depend on how many twins grow in the family, two or more. For example, twins often have a pronounced fixation on each other. Psychologists say that it is this feature of a pair of twins that has the greatest impact on their development. What can you say about triplets? Where can I find a scientific explanation for the behavior of triplets? In this regard, I found it interesting to say Ursula Grützner-Henning, one of the founders of the first self-help organization in Germany for families who were hit by a natural disaster called “twins”: “ Three is the smallest, but already a group. Two are a couple, couples exist according to other laws, two are constantly “fixated” on each other. When two of the triplets start fighting for some toy, the third one always comes and takes it for himself.". As confirmation of Ursula's words, I can say that I constantly observe the described situation on my babies.
That is, in order to talk about the peculiarities of the upbringing and development of triplets, I think it is necessary to have some idea of these very “laws” according to which triplets exist.
Turning to foreign resources about triplets, I have so far singled out one of the “twin” features of triplets - this is a possible opposition of two from triplets to one. For example, forums about triplets problems discuss topics of two against one in triplets, separation of triplets according to the formulas “2 + 1” or “1 + 1 + 1”

What will change in your family if you have triplets?

A family where triplets are born instantly acquires the status of a large family. It's no secret what "stamp" is formed in our society in relation to such families. According to many, the appearance of triplets in the family in our time is an unaffordable luxury. As a result, parents of triplets are more likely to hear words of sympathy from those around them. Someone will sincerely rejoice for happy parents, someone will envy, they say, they suffered at once, in the worst case, they can say, they say, why breed poverty. Whatever opinions people may express, in my opinion, for those who have not experienced this in their own lives, it is rather difficult to understand the feelings experienced by the parents of triplets.
It always seemed to me that a large family is something special, because dad and mom there have their own view of the family and family relationships. Now I understand that in addition to all the traditional features of a large family, where all the children are single, in a family with triplets, there are still more features.
For example, there will be no, or almost no, transfer of things “by inheritance”, from older to younger. And parents will have to take care of three children of the same age at once. Therefore, get ready to look for original solutions for many everyday tasks, there is nowhere to take the proven solutions.
Get ready for constant heightened attention, often simple curiosity. Like it or not, but for neighbors, passers-by, the press, you become an interesting object, since the birth of triplets is, according to statistics, a rather rare case. Surely you will very often hear the exclamations of curious passers-by that your triplets are the first triplets that they had a chance to see in their lives. Perhaps your family will even become a local celebrity.
Most likely, you will quickly learn to answer standard questions and not notice differently expressed surprise. One of my frequently asked questions: How are you doing? It is quite understandable why it sounds more often than others. After all, it is still possible to imagine how to cope with twins, since a woman has two arms, two breasts, finally. But in order to cope with triplets, sometimes it takes incredible ingenuity not only of the mother, but also a special interaction of all family members. Another standard question: Hard, perhaps? It's just that everyone wants to assess how much the load is more than with the birth of one child, is it really three times. This issue is especially of concern to parents who are very tired of raising one child. Of course, it’s hard, I usually answer. But it is still difficult to estimate how much more difficult it is. For example, in the first year of life, one of my babies ate very poorly, and since he was also the most mobile of the three, it was harder to manage him, sometimes I felt that this baby was worth the other two to me.
Another frequently asked question: How does the state help you, but they gave you an apartment? So we have to explain to people that in previous years, the state allocated an apartment and a nanny for babies to those who gave birth to triplets, now one can only dream of such a miracle. That is, the concern declared by our state for motherhood and childhood, concern for the demographic situation in Russia, in this case, is a fiction. Now the housing issue is left to the discretion of the local administration. In most cases, the administration finds an opportunity to help a new large family (which local newspapers and various online publications are in a hurry to report), but it also happens the other way around. My family was lucky, some time after the birth of the babies, the administration of the Mytishchi district adopted a decision to allocate a three-room apartment to my family in a municipal building under construction. The kids are about two years old, and the construction of this house has not yet been completed.

Is the attention flattering or annoying?

From my personal experience, I can say that increased attention does not always bring pleasure. Since this is far from only tenderness and admiration. After all, it never occurs to anyone to approach the first completely unfamiliar woman with a baby in her arms on the street and ask publicly: How did you manage to get pregnant like that? Do you have a normal baby, without deviations? And they come to me with such questions all the time! Moreover, they are waiting for an immediate answer, and they demand respect for themselves if I start to get angry. So what if I have three babies sitting in a stroller at once, are there many families with three children?
Increased attention sometimes leads to comical situations. More often due to the fact that triplets are first mistaken for twins, without noticing one of the twins. Here, apparently, the stereotype works: twins mean two. At the moment when the third child is discovered, people are so shocked that they completely forget about the sense of tact and decency. Someone suddenly loses the power of speech, someone, on the contrary, begins to laugh out loud or ask stupid questions. What about three of them? Are they twins or triplets? Are these twins or triplets?
Very often people on the street give my triplets names. Here are some of them:

When we are in a wheelchair

Dragon
Three tankers
cinema hall
Crew
Three heroes
Funny company
Team

When we run

Three Musketeers
Brownian motion

tricycle stroller
Stroller for three people

A whole series of questions arise from the fact that people are interested in the sex of children. For example:
- Do you have boys?
- Yes!
- And what, all three boys?
- ?!
- Do you have both boys and girls?
- Boys!
- And what, there is no girl?
- ?!
………………….............................
The classic question asked of parents of twins is: How do you distinguish between them? In the case of triplets, there are also some nuances here. Many argue that two are exactly the same, and the third is different! Moreover, the role of the third is not played by the same child every time.
Those who see my children often enough, let's say, every day, try to learn to distinguish between them, focusing mainly on finding a child different from the other two. As a rule, there are rarely people who really distinguish between my children. But for me, my triplets are three completely different children, and the older they get, the more this difference is felt. This difference is a reflection of the characters that are "written" on their faces.

Gemini horoscopes

The fates of twins are often closely related or similar. It is no secret that many twins, even when separated from birth, can have the same habits, get into similar situations, suffer from the same diseases and even die at the same time, there are many such examples in history. Thinking about what influences the fate of the twins, led me, in the end, to a question from the field of Astrology.
With my question, I turned to one of the forums of the site talk.ru.
Here is my question:
I wonder if there is any peculiarity in compiling horoscopes for twin people (both identical and fraternal). For example, my children (identical twins) were born just one minute apart. How different will their horoscopes be from each other? What determines this difference besides the time interval at birth and the time of birth?
I received this response:
The personality of a person is largely determined by the so-called Ascendant, and it changes by one degree every 4 minutes ... (24 hours / 360 degrees). One degree is the generally accepted unit of measurement, and if the interval of a minute is sufficient for the Ascendant to move into another degree, then the differences will be more noticeable. If all children fall into one degree, then they will be very similar. Differences in terms of the horoscope will be very insignificant - astrology is not able to track changes within one degree. But! In addition to the horoscope, there is also the free will of a person, and it is of decisive importance. If in childhood it does not manifest itself, and the fate of children will be very similar, then the further, the more it will diverge, for the reason that free will will increasingly direct their life path along an individual line. They may choose from one or a very similar list, but each may have a specific choice. This will still be superimposed by the accumulating life experience, which will increasingly be unlike one another. In addition, if their lines intersected at a given point, this does not mean that they coincided before, in past incarnations ... probably, everyone has their own individual developments behind them, unlike others. Then, after this point of intersection, the lines will begin to diverge, and the farther the more.
That is, already initially the fate of the twins to some extent may be similar. After all, most twins are born with small intervals of time.
On the other hand, since “free will is of decisive importance”, then the more individuality is emphasized in the twins, the less they are dependent on each other, the greater the difference in their destinies ...

Lyubanya, November 2000

Cases when not one, but two or more babies settle in the mother's stomach are not so common - 1.5-2.5% of all pregnancies. How does it work? How is a multiple pregnancy different? Can twins be scheduled? Our expert - Anna Romanovna PASTUKHOVA, an obstetrician-gynecologist at the Toast of Family Medicine Centers, answers all questions.

What is the chance of getting pregnant with twins

First, some statistics. As we have already mentioned, this amazing phenomenon occurs in 1.5-2.5% of pregnancies in Europe. It is believed that the appearance of twins, triplets and twins is genetically determined - more often they are born in families where one or both parents were born as a result of multiple pregnancy. However, there are other reasons, including .

The birth of twins occurs with a frequency of geometric progression, obstetricians even have a special formula: for 87 births there is one twin, for 87 twins - one triplet, for 87 triplets - one quadruple, etc. True, today this formula needs to be corrected - multiple pregnancies have become more common.

Identical or identical twins are less common, with an average of 1 in 250 births.

Most often, multiple pregnancies are observed in Africa, least often in Asia.

Types of multiple pregnancy

Let's start with a well-known fact. There are two types of twins: identical or monozygotic and fraternal (they are also called dizygotic).

Monozygotic twins result from the division of one egg fertilized by one sperm. It is they who surprise everyone with their striking resemblance.

Fraternal twins result from the fertilization of several eggs by several sperm. They can be of the same or different sexes, often differ in character and are not always similar in appearance.

Expert comment

A multiple pregnancy is a pregnancy when more than one fetus develops in the mother's body, but several at once. In medicine, identical or fraternal twins (triplets and so on) are distinguished. Identical twins are always the same sex, have the same blood type, and are very similar in appearance. Fraternal twins have phenotypic differences (eye color, facial features), can be either the same or different blood type. Fraternal twins may be of different sexes.

How are identical twins made?

To be honest, we have no idea! The reasons why, instead of a single baby from one egg and one sperm, two (three, four) appear are not really known - a miracle, and nothing more. The mechanism of the appearance of monozygotic twins, however, is described in sufficient detail. Everything starts as if nothing had happened: one egg, one sperm, meeting, fertilization... Then the resulting zygote begins to divide, grow and, at some point, under the influence of factors that are not entirely clear, it suddenly splits into two or more parts, each of which takes on a life of its own.

Expert comment

The development of identical twins occurs when one egg is fertilized by one sperm cell, in the process of dividing the fetal egg into two, three or four parts at different stages of its development. In this case, each part develops as a separate organism. It is assumed that such separation can occur when the implantation (attachment) of the fetal egg is delayed due to oxygen deficiency. Also, the occurrence of identical twins is associated with the fertilization of an egg that had two or more nuclei.

Do the twins interfere with each other?

It is very important at what point the splitting of the egg occurs, since the further intrauterine development of the twins depends on this.

There are 3 options for the development of monozygotic twins

Separation of the fetal egg occurred in the first five days after fertilization. In this case, both future babies begin to develop completely independently, each "grows" its own placenta and . Until recently, it was believed that such a development was characteristic only of fraternal twins and identical twins in such cases were erroneously identified as dizygotic (maybe this happened to the Olsen sisters?)

The zygote split approximately 5-8 days after fertilization. By this time, the outer fetal membrane has already begun to form: remains common in babies, however, the inner fetal membrane (amnion) develops in each of them. If babies have a common placenta, then one child can suppress the other - their competition begins with conception.

Separation occurred on day 8 and later (1% of all monozygotic twins). At this time, the fetal bladder is already beginning to form, so the separated embryos will grow in the common amnion and common fetal waters. This situation is considered not the most favorable, there is a risk that the kids each other.

The life of fraternal twins in the womb is organized with great comfort: each of them develops their own personal placenta (sometimes they grow together with walls), which means that babies do not need to seriously compete for a place “under the sun”.

So, if the embryos have different placentas, then most likely they are fraternal, and if the babies “settled” with a common placenta, then these are identical twins who will be born similar as ... twins.

Expert comment

There are the following types of multiple pregnancy:

Monochorionic monoamniotic twins when there is a common placenta and there is no septum between the twins. It's always identical twins

Monochorionic diamniotic twins, when there is a common placenta and septum, i.e. each fetus has its own amniotic sac

Dichorionic diamniotic twins, when each fetus has its own placenta and its own amniotic bladder.

How are fraternal twins made?

Non-identical twins develop from two or more fertilized eggs. How does this happen? The most common reason for this is the so-called "multiple ovulations", when several mature eggs come out of one or both ovaries at once. The reasons for this phenomenon may be different: genetic predisposition, artificial ovarian stimulation, the first cycle after the abolition of hormonal contraceptives.

Do you know that....

The difference in the time of conception of fraternal twins can be from several hours to several days.

In very rare cases, a new pregnancy is added to an existing pregnancy. As a rule, after fertilization and implantation of the fetal egg, ovulation is immediately blocked. If, due to hormonal imbalance, this mechanism does not work immediately, a younger brother or sister is added to the older child in utero.

Natalia Danilovich

We all know that a boy and a girl (opposite-sex twins) can only be fraternal twins. They can never be identical. And in the vast majority of cases, in truth, a boy and a girl are only twins. And yet there are very, very, very rare cases where parents of twins can actually say, "My boy and girl are opposite-sex identical twins." And you know, there are three different explanations for this event!

Often the question "Do you have twins or twins?" for the parents of a royal couple of babies in all blue and pink, it leads to ambiguous thoughts that there are just a little crazy around, even when they are interested in this issue with completely good intentions.

Let's sort everything in order and remember the main points from biology from school.

Monozygotic or identical twins are obtained as a result of the splitting at the very beginning of pregnancy of one fertilized egg into two zygotes. Twins have almost a very similar set of chromosomes.

Dizygotic twins or in the common people twins are obtained from two different fertilized eggs. Twins have completely different chromosomes, just like ordinary single-born brothers and sisters. Except that common pregnancy and childbirth.

Identical (monozygous) twins are always of the same sex because they are descended from a single zygote that may contain either male (XY) or female (XX) sex chromosomes.

That's why...

In the absolute majority of cases, a boy and a girl can only be twins (opposite-sex fraternal twins).

Nevertheless....

1. It happens, it's in the chromosomes

Identical opposite-sex twins
For some unexplained reason, in very extremely rare cases (about 1 in 2500 births), in the case of the birth of male fetuses (XY), one of the twins loses the Y chromosome and continues to develop as a girl with the chromosome only (XO).

This extremely rare phenomenon unfortunately refers to chromosomal disorders and is called Turner's syndrome. A twin girl with Turner syndrome will have distinguishing features such as short stature, low-set ears, bone development, delayed or absent puberty, and other possible features.

Because Turner syndrome is a chromosomal disorder, there is no cure for them. However, there is still a cure. Growth hormone can help a child grow bigger. In vitro fertilization can help a woman with Turner Syndrome get pregnant. Hormone therapy (estrogens) can help with the onset of puberty.

Scientists are also familiar with cases of the appearance of identical identical twins caused by Klinfelter's syndrome. With it, the baby has one extra X chromosome, less often there are several extra X chromosomes: XXY; XYY; XXXY; XYYY; XXYY; XXXXY; XXXYY. The most common is Klinefelter syndrome (XXY). Normally, the normal set of sex chromosomes in men is described as XY.

Parents of identical, opposite-sex twins with such characteristics may need to constantly delve into the above lesson from school biology. Explain to everyone around the chromosomal differences between men and women. Walking through the cactus garden out of wonder at their story of their twins. Not to mention meetings with various doctors, procedures, tests and prescriptions.

These situations are so extremely rare (less than 10 cases can be found). Therefore, it can be safely assumed that in 99.9% of cases, the boy and girl will still be opposite-sex fraternal twins.

2. Another type of twin

Semi-identical twins
Everything is more interesting here. There is such an unusual way for the appearance of twins, when the egg is fertilized by several sperm or the egg divides before fertilization, which can lead to the conception of male and female children.

In this case, the twins will be formally identical, because their genotypes will already be different. This kind of twins in the present is called semi-identical twins. They will share 75% of their genetic markers.

This is a very interesting option, various studies have been carried out on it, which prove that the subsequent division of a double-fertilized egg is possible, and the birth of healthy children. And with a possible gender difference, neither a girl nor a boy (if we consider the norm, and not chromosomal abnormalities from among the sex chromosomes) will not suffer from anything.

Interesting? There is one more exception when identical twins can be of different sexes ....

3. Sex reassignment surgery for one of the twins...

Thus, you see, to become parents of identical opposite-sex twins - a boy and a girl is not so completely impossible and simply unbelievable. But this probability is not always worth the extra pleasure from unnecessary communication with doctors, physical features on the part of your beloved children, your emotions and excessive attention to you.