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In the case of dizygotic (heterozygous) twins, each of the fertilized eggs (blastocysts), after implantation into the decidua (falling off) membrane of the uterine wall, forms its own aqueous and villous membrane, from which a separate (own) placenta with an independent network of fetal blood vessels subsequently develops for each fetus . In this case, often both placentas remain separate. Even in cases where the edges of both placentas are closely adjacent to each other (as if merging into a single whole), nevertheless, the villous and aqueous membranes of each of the fetal eggs remain separate while they have a common capsular membrane.
Polyzygotic twins (as well as identical twins) are not only twins, but also triplets, quadruplets, and so on up to 9 children. In addition, cases have been recorded when, for example, in triplets two identical (homozygous) twins and one non-identical (heterozygous in relation to the other two) twins were born.
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Notes
Literature
- G. M. Savelyeva, V. I. Kulakov. Obstetrics. - M.: Medicine, 2000. - P. 816. - ISBN 5-225-04549-9
Links
An excerpt describing polyzygotic twins
The next day, the troops gathered in the appointed places in the evening and set out at night. It was an autumn night with black-purple clouds, but no rain. The ground was wet, but there was no mud, and the troops marched without noise, only the occasional clink of artillery could be faintly heard. They forbade talking loudly, smoking pipes, lighting fires; the horses were kept from neighing. The mystery of the enterprise increased its appeal. People walked cheerfully. Some of the columns stopped, put their guns in their trestles and lay down on the cold ground, believing that they had come to the right place; some (most) columns walked all night and, obviously, went to the wrong place.
Count Orlov Denisov with the Cossacks (the most insignificant detachment of all the others) alone ended up in their place and at their time. This detachment stopped at the extreme edge of the forest, on the path from the village of Stromilova to Dmitrovskoye.
Before dawn, Count Orlov, who had dozed off, was awakened. They brought a defector from the French camp. This was a Polish non-commissioned officer of Poniatowski's corps. This non-commissioned officer explained in Polish that he had defected because he had been wronged in his service, that he should have been an officer long ago, that he was braver than everyone else and therefore abandoned them and wanted to punish them. He said that Murat was spending the night a mile away from them and that if they gave him a hundred men as an escort, he would take him alive. Count Orlov Denisov consulted with his comrades. The offer was too flattering to refuse. Everyone volunteered to go, everyone advised me to try. After many disputes and considerations, Major General Grekov with two Cossack regiments decided to go with a non-commissioned officer.
“Well, remember,” Count Orlov Denisov said to the non-commissioned officer, releasing him, “if you lied, I’ll have you hanged like a dog, but the truth is a hundred ducats.”
The non-commissioned officer with a decisive look did not answer these words, sat on horseback and rode off with Grekov, who had quickly gathered. They disappeared into the forest. Count Orlov, shaking from the freshness of the morning that was beginning to break, excited by what he had started on his own responsibility, having seen Grekov off, came out of the forest and began to look around the enemy camp, which was now visible deceptively in the light of the beginning of the morning and the dying fires. To the right of Count Orlov Denisov, along the open slope, our columns should have appeared. Count Orlov looked there; but despite the fact that they would have been noticeable from afar, these columns were not visible. In the French camp, as it seemed to Count Orlov Denisov, and especially according to his very vigilant adjutant, they began to stir.
“Oh, really, it’s late,” said Count Orlov, looking at the camp. Suddenly, as often happens, after the person we trust is no longer in front of his eyes, it suddenly became completely clear and obvious to him that the non-commissioned officer is a deceiver, that he lied and will only ruin the whole attack by the absence of these two regiments, whom he will lead God knows where. Is it possible to snatch the commander-in-chief from such a mass of troops?
“Really, he’s lying, this scoundrel,” said the count.
“We can turn it back,” said one of the retinue, who, like Count Orlov Denisov, felt distrust of the enterprise when he looked at the camp.
- A? Right?..what do you think, or leave it? Or not?
-Would you like to turn it back?
- Turn back, turn back! - Count Orlov suddenly said decisively, looking at his watch, “it will be late, it’s quite light.”
And the adjutant galloped through the forest after Grekov. When Grekov returned, Count Orlov Denisov, excited by this canceled attempt, and by the vain wait for the infantry columns, which still did not show up, and by the proximity of the enemy (all the people of his detachment felt the same), decided to attack.
He commanded in a whisper: “Sit down!” They distributed themselves, crossed themselves...
- With God blessing!
“Hurray!” - there was a rustle through the forest, and, one hundred after another, as if pouring out of a bag, the Cossacks flew cheerfully with their darts at the ready, across the stream to the camp.
The birth of a child is an unusual, enchanting moment. The whole life of a young mother begins to revolve around a small miracle that has come into the world. And if a woman is given two (or more) babies, then her happiness increases.
The birth of twins brings with it excitement, upcoming caregiving chores, sleepless nights, but the moment of the birth of children is filled with happiness.
Types of twins
If a woman is diagnosed, she will give birth to twins. Children can be either absolutely identical or not very similar at all, and even of different sexes. It all depends on the egg. Depending on this, twins are divided into:
- identical (monozygotic);
- non-identical (dizygotic).
Identical twins
The birth of two absolutely identical children remains a mystery to humanity. Why at the initial stage the egg begins to divide and create identical embryos remains unknown.
Monozygotic twins are the result of fertilization of one egg by one sperm. The resulting diploid cell begins to divide, resulting in the formation of independent embryos. Most often there are two, but there may be more. The similarity of future babies from such a pregnancy is determined by the time of separation of the embryos. If it occurs in the first 5 days, then the embryos each have their own placenta and amniotic sac. In this case, there will be minor differences in the appearance of future babies. If separation occurs after the fifth day, then the embryos have one common placenta and their similarity to each other becomes absolute.
Fun fact: monozygotic twins have identical fingerprints in most cases.
The reasons why a zygote divides into two or more identical embryos have not yet been studied. The probability of having identical babies is 3 in 1000 and does not depend on heredity.
Fraternal twins
The conception of such twins results from the fertilization of two eggs by different sperm. During intrauterine development, each embryo has its own placenta and develops independently of the other. Dizygotic twins can be either same-sex or opposite-sex. They will have the same external resemblance as brothers and sisters born from a normal singleton pregnancy.
Fraternal twins can be conceived within hours, or perhaps even days, of each other.
Features of the development of dizygotic twins
The following facts are known about born fraternal twins:
- their genes coincide by about 40-50%;
- each embryo has its own placenta and its own amniotic fluid;
- born babies can be either same-sex or different-sex;
- children may have different blood types;
- the likelihood of their occurrence increases in women over 35 years of age;
- as a result of the IVF procedure, twins most often appear, since the woman is given several
Is it possible to plan the birth of twins?
The birth of a child is an unusual, enchanting feeling that arises in a young mother who has given birth to a child. And birth contributes even more to the feeling of happiness. However, it is impossible to schedule their appearance.
When an egg is fertilized, information is immediately stored both about the future baby and about what kind of pregnancy it will be - singleton or multiple, so it is impossible to influence this process naturally. You can only find out the factors that increase the likelihood of twins.
These factors include:
- pregnancy at the age of 35-39 years;
- the presence of twins in a generation (the appearance of identical twins is not associated with hereditary factors. To date, it has not been studied why and under what conditions the egg begins to divide);
- short menstrual cycle (20-21 days) - with this cycle the likelihood of several eggs maturing increases;
- infertility treatment. In the treatment of infertility, hormonal drugs are used that stimulate ovulation, which can result in multiple pregnancies;
- artificial insemination - in vitro, often leads to the birth of twins or even more babies.
Differences between twins and twins
We can summarize how identical twins differ from fraternal twins. Let's consider the characteristic features that babies born during a multiple pregnancy will have.
Identical twins:
- Children have an identical similarity, but in some moments it can be mirrored. For example, a mirror image of existing moles.
- In addition to external similarity, the same physique, hair structure, teeth arrangement, voice timbre, and even thoughts are quite similar.
- They have the same blood type and most often similar fingerprints.
- Always same-sex, they are also called natural clones.
- It is impossible to plan the birth of monozygotic twins.
Fraternal twins:
- There are both same-sex and heterosexual.
- May have different blood types.
- External similarity is superficial.
As you can see, there are no similarities between identical and fraternal twins, only one name - “twins”. At the same time, it is common for people to call identical babies twins, and fraternal babies - twins, triplets, etc. Differences between them begin to appear from the moment of conception.
Conclusion
The appearance of two or more babies as a result of one pregnancy brings with it increased attention from people around them to children. At the same time, fraternal twins look like an ordinary brother and sister (2 sisters or 2 brothers), so they do not attract close attention.
Identical twins always arouse interest in society. Most likely, this is due to the rarity of such a phenomenon. Therefore, seeing two absolutely identical people, a passerby will definitely fix his gaze on them.
At the same time, you need to know that multiple pregnancies are observed under special control, since the body experiences a greater load than during a normal pregnancy. The expectant mother should be attentive to her health. Therefore, in order to give birth to healthy twins, you need to listen to the doctor’s recommendations, then the moment of meeting your babies will be filled only with positive things.
Have you ever heard of monozygotic twins or dizygotic twins? We propose to figure out what these are: monozygotic and dizygotic twins (twins). You won't see precise scientific formulations here. This article is intended for the average person who does not have precise knowledge in the field of genetics, and especially does not know the terminology. It is for those for whom monozygote or dizygote or heterozygote are simply incomprehensible scientific words. And what do twins have to do with it? However, such people often use terms and concepts from these “obscure” areas of knowledge in everyday speech. Next, we provide a scientific, conceptually correct interpretation of the terms used: monozygotic twins (twins) and hetero or dizygotic twins (twins).
Monozygotic twins and dizygotic twins begin with ZYGOTE!
Before talking about any 
monozygotic and dizygotic twins, or God forbid about heterozygotic twins, let's find out in general what a zygote means. How do hetero, di and monozygotes differ from each other? Maybe it's not so scary? To do this, let's get acquainted with the terminology. We will not speak in scientific language, but in as clear and simple a language as possible.
Zygote - the beginning of twins
Zygote - comes from the Greek word meaning “double” - a cell that contains a double complete set of chromosomes, it is formed during the process of fertilization. Fertilization refers to the fusion of male (sperm) and female (egg) cells. The zygote is capable of giving rise to other cells in the process of its fragmentation. Simply put, with the appearance of the zygote, fetal development, pregnancy and the birth of a baby or, in our case, twins begin.
Monozygotic twins (homozytic twins)
Monozygous (homozygous) can be called organisms that have the same genes, i.e. they are similar to each other or identical. In biological terms, the result of the fertilization of one female cell (egg) by one sperm (male cell). Then monozygotic twins are identical or completely similar twins of children, resulting from the same maternal cell.
Dizygotic twins (heterozygous twins)
Dizygous (heterozygous) can be called organisms that have copies of common genes that are slightly different from each other. If explained in terms used in genetics and biology, it turns out that fertilization of two or more eggs is called heterozygous, which leads to the development of two or more fetuses. Since there are words about two or more, the word dizygotic is often used. (Di means two).
The birth of twins occurs much more often than having more children at one time. Thus, for us, dizygotic twins are practically the same thing as heterozygous twins and just mean twins of children who are not completely similar to each other.
Semi-identical twins (monozygotic polar twins) are a rare case!
Semi-identical twins or monozygotic polar (semi-identical) are an intermediate type between monozygotic and dizygotic twins. In a general sense, organisms. This is an exceptional case. Semi-identical twins (twins) share half of the genes from their mother and half of the different genes from their father. This is why they are called semi-identical in biological terms.
The process of forming semi-identical or, in other words, monozygotic polar twins is very complex. Usually, together with the female egg, a polar body is formed and develops. This is such a small cell that appears along with the egg, and then usually dies, and the egg continues to live.
Sometimes, for unknown reasons, the polar body begins to split, although usually this behavior is not characteristic of it. The polar body can increase in size and begins to feed well, and then, contrary to the usual case, it does not die off. Then it begins to behave like a second egg. It turns out that “as if” two separate eggs with the same set of genes begin to exist. Such an unusual polar body and egg can be fertilized without problems by two different sperm.
Thus, semi-identical twins are half monozygotic (maternal) and the other half (father) dizygotic. Twins on their mother's side have the same genes, i.e. They are absolutely similar to each other on their mother’s side, like monozygotic twins. On the father's side, the genes are different, because the egg and the polar body were fertilized by different spermatosides, like in dizygotic twins (there are two different spermatosides for two different eggs). In general, this is a fairly rare case of twins. In order to find out “this” on specific children, it is necessary to do their genetic examination, with all the ensuing difficulties.
Dizygotic and monozygotic twins and EGGS
Here we all say eggs - eggs. How are dizygotic and monozygotic twins related to eggs? Here we are not talking about eggs and chickens, but about the terminology accepted in biology. By the way, one male reproductive cell cannot fertilize more than one female, so from the male side everything looks simple. And we owe the birth of more than one child at a time to women with their eggs. Therefore, let us bring some clarity.
An egg, sometimes scientifically called an oocyte, or oocyte, is a female cell that is characterized by Oogamy. This term “oogamy” refers to the sexual process of fusion of distinctly different germ cells. This means that an egg is a female reproductive cell that, during the sexual process, is capable of merging with a different male reproductive cell - a sperm. By the way, if you are interested, you can look up separately what identical twins and fraternal twins are.
This is how the female egg is large and immobile, the male sperm is small and mobile, and when fused, an embryo and fetus are formed. Pregnancy occurs and if its course is favorable, expect monozygotic or dizygotic twins.
Monozygotic twins and dizygotic twins - about CHILDREN
Now we can talk about specific concepts tied directly to our realities and life in general: monozygotic twins and dizygotic twins. After all, what parent is interested in hearing about some kind of zygotes, hetero and mono, and eggs. It's always interesting about twin children. So.
Dizygotic twins (heterozygous twins)
Dizygotic twins (heterozygous twins) are twin children who can have different sexes and always have a different set of genes. Yes, they are similar to each other, but no more than brother to brother (sister to sister), or brother to sister. This means that they can and will always be different and not identical. Scientifically, they are dizygotic twins.
Monozygotic twins
Monozygotic twins are children who are absolutely similar to each other, of the same sex and with the same set of genes. They are identical to each other in genes. Note that in 25 cases out of 100, i.e. with a 25% probability, they may have a mirror appearance, for example, one has an ear protruding on the left, and another on the right, or a mole on one on the left, on the other on the right. Remember the reflection in the mirror, then everything will become clearer. Scientifically, they are mirror monozygotic twins, or simply mirror twins.
Monozygotic polar (semi-identical) twins
Monozygotic polar (semi-identical) twins are extremely rare in humans; they can be of the same or different sexes, like royal twins. Only a genetic examination can say that it is she. Such twins always and necessarily have the same half of the genes from the mother, and a different half of the genes from the father. Scientifically speaking, they are semi-identical twins or monozygotic polar twins.
Dizygotic twins and monozygotic twins - in terms related to PREGNANCY
Probably, after you have learned what dizygotic and monozygotic twins are, it will be interesting to know how this relates to pregnancy. In medicine, completely different terminology is used to refer to them. I wonder which one - see monochorionic diamniotic twins or monochorionic monoamniotic twins or diamniotic diamniotic twins - WHAT IS THIS or WHO IS THIS - this is a separate article on this topic. If you already know, then see what happens next.
Dizygotic twins (dizygotic twins) are clearly dichorionic diamniotic twins. Different placentas and different houses (amnion sacs) for babies. Dizygotic are always fraternal twins.
Monozygotic twins (monozygotic twins) - may be:
- monochorionic monoamniotic - one placenta and one house for two. The probability of occurrence is approximately 1% of all monozygotes.
- monochorionic diamniotic - one placenta and one house with a partition, i.e. two different apartments. The probability of occurrence is 70-80% of all monozygotic.
- dichorionic diamniotic - analogue of dizygotic twins. The probability of occurrence is 20-30% of all monozygotic.
Monozygotic - always identical twins
In the wild, many animals often give birth to many babies at the same time, but in humans, for every 250 births of one child, there is one case of twins being born. Scientists distinguish between monozygotic and twins, and colloquially they are called fraternal or twins and fraternal twins.
Identical twins
Identicals develop from a single fertilized egg, which splits into two or more parts that begin to develop separately. Thus, all embryos receive the same set of genes and develop approximately the same. Such children always have the same gender and are very similar to each other, sometimes even their parents cannot distinguish between them. Although sometimes they have mirrored features: for example, one has a mole on the right side, another on the left, one is right-handed, and the other is left-handed. They have similar character traits, almost identical temperaments, predispositions to diseases and often even fates.
There are cases where identical twins, separated in childhood and unaware of each other, lived almost identical lives.
Cases of the birth of monozygotic twins are several times more rare than fraternal twins, since for a normal pregnancy the division of the egg into several parts is uncharacteristic.
Fraternal twins
Fraternal twins have different sets of genes because different sperm and eggs were involved in their conception. In some cases, in women, two or more eggs mature in one menstrual cycle; if all of them are fertilized, a multiple pregnancy will begin, and two or more children will be born. These are quite common cases, in some nationalities they occur very often - it is believed that multiple births are of a genetic nature.
Fraternal twins, also called fraternal twins, have different genotypes, so children can be of different sexes and generally differ from each other, like ordinary brothers and sisters. It is known that with age, a woman’s likelihood of multiple pregnancy increases.
In countries where it is customary to give birth after the age of thirty, for example, in the USA or European countries, there are more cases of birth of fraternal twins.
There is another type of twins, which is called transitional between the two described - polar or semi-identical. This occurs when two different sperm fertilize the egg and the polar body formed with it (usually it dies). As a result, the embryos end up with a half different set of genes, the same from the mother.
