Mysteries of the Y chromosome: a fragile creature that will soon disappear. The male Y chromosome is more than a sex switch

How does the process of birth of men and women take place? The X and Y chromosomes are responsible for this. And it all begins when 400 million sperm rush to search for an egg. This is not such a difficult task as it might seem at first glance. In the human body, the egg can be compared to a huge star, towards which small sperm star fighters are rushing from all sides.

Now let's talk about chromosomes. They contain all the information necessary for the creation of man. A total of 46 chromosomes are needed. They can be compared to 46 thick volumes of an encyclopedia. Each person receives 23 chromosomes from their mother, and the remaining 23 from their father. But only 2 are responsible for sex, and one must be the X chromosome.

If you get a set of 2 X chromosomes, you will use the women's restroom for the rest of your life. But if the set consists of X and Y, then in this case you are doomed to go to school for the rest of your days. men's toilet. At the same time, you need to know that the man bears full responsibility for gender, since the Y chromosome is contained only in the sperm, and it is absent in the egg. So the birth of boys or girls is entirely dependent on male genetic material.

A remarkable fact is that to recreate the male sex, the Y chromosome is not needed at all. Only an initial push is needed for the development program to start male body. And it is provided by a special sex determination gene.

X and Y chromosomes are not equal. The first one takes on the main work. And the second only protects the genes associated with it. There are only 100 of them, while the X chromosome carries 1,500 genes.

From each X chromosome, one gene is needed to form the male sex. And for the formation of the female sex, two genes are needed. It's like a pie recipe with one cup of flour. If you take two glasses, then everything will change dramatically.

However, you should know that the female embryo, having two X chromosomes, ignores one of them. This behavior is called inactivation. This is done so that 2 copies of the X chromosomes do not produce twice as many genes as required. This phenomenon is referred to as gene dosage compensation. An inactivated X chromosome will be inactive in all subsequent cells resulting from division.

This shows that the cells of a female embryo form a rather complex mosaic, assembled from inactive and active paternal and maternal X chromosomes. As for the male embryo, no inactivation of the X chromosome occurs in it. This means that women are genetically more complex than men. This is a rather loud and bold statement, but a fact is a fact.

But as for the genes of the X chromosome, of which there are 1,500, many of them are associated with brain activity and determine human thinking. We all know that the chromosome sequence of the human genome was determined in 2005. It was also found that high percent X chromosome genes ensure the generation of a protein that is involved in the formation of the medulla.

Some of the genes are involved in the formation of the brain mental activity. These are verbal skills social behavior, intellectual abilities. Therefore, today scientists consider the X chromosome to be one of the main points of knowledge.

Y-chromosome

In the body of every man there is a so-called Y-chromosome that makes a man a man. Typically, chromosomes in the nucleus of any cell are arranged in pairs. For Y- paired chromosome is X-chromosome. At conception, the future new organism inherits all of its genetic information from its parents (half the chromosomes from one parent, half from the other). From his mother he can only inherit X- chromosome, from the father - either X, or Y. If an egg contains two X- chromosomes, a girl will be born, and if X- And Y- chromosomes - boy.

For almost 100 years, geneticists believed that the tiny chromosome (a Y-the chromosome is really the smallest, noticeably smaller X-chromosome) is simply a “stub”. The first guesses that the chromosome set of men differs from that of women were put forward in the 1920s. Y-chromosome was the first chromosome discovered using a microscope. But to determine the presence of any genes localized in Y- chromosome turned out to be impossible.

In the middle of the 20th century. geneticists have suggested that several very specific genes may be contained in Y- chromosome. However, in 1957, at a meeting of the American Society of Human Genetics, these hypotheses were criticized. Y- the chromosome was officially recognized as a “dummy”, not carrying any important hereditary information. The point of view has been established that “ Y“The chromosome, of course, carries some kind of gene that determines the sex of a person, but no other functions are assigned to it.”

Just 15 years ago Y- the chromosome did not arouse much interest among scientists. Now decryption Y- chromosomes is part of the project to decipher the human genome, which is being carried out international group geneticists. During the study it became clear that Y-the chromosome is far from being as simple as it seemed at first. Information about the genetic map of this chromosome is extremely important because It is here that the answers to questions about the causes of male infertility lie.

Research Y- chromosomes may provide answers to many other questions: Where did man appear? How did the language develop? What makes us different from monkeys? Is the “war of the sexes” really programmed into our genes?

Now geneticists have begun to understand that Y-chromosome is something unique in the world of chromosomes. It is extremely highly specialized: all the genes contained in it (and there were about two dozen of them) are responsible either for the production of sperm by the male body, or for “related” processes. And, naturally, the most important gene on this chromosome is SRY– in the presence of which the human embryo develops along the male path.

Approximately 300 million years ago, it did not exist in nature Y- chromosomes. Most animals had a pair X- chromosomes, and sex was determined by other factors such as temperature (in some reptiles such as crocodiles and turtles, the same egg can still hatch into either a male or female, depending on temperature). Then a mutation occurred in the body of a certain mammal, and the new gene that appeared began to determine the “male type of development” for carriers of this gene.

Gene survived in natural selection, but for this he needed to block the process of replacement with an allelic gene from X-chromosomes. These long-standing events determined the uniqueness Y- chromosomes: it is found only in male organisms. Investigating mutations in Y- chromosome, scientists can estimate how distant men from two ethnic groups are from our own common ancestor. Some of the results obtained in this way were quite surprising.

Last November, a branch of biology called archaeogenetics took a big step forward. Leading scientific journal, Nature Genetics, offered new version family tree humanity, based on hitherto unknown variations, so-called haplotypes Y- chromosomes. These data confirmed that the ancestors modern people migrated from Africa.

It turned out that “genetic Eve,” the progenitor of all humanity, is 84 thousand years older than “genetic Adam,” if age is measured by Y- chromosome. Female equivalent Y- chromosomes, i.e. The genetic information passed only from mother to daughter is known as m-DNA. This is the DNA of mitochondria, which is the source of energy in the cell.
For the past few years, it has been generally accepted that "mitochondrial Eve" lived about 143 thousand years ago, which does not fit with the estimated age of "Adam" of 59 thousand years.

In fact, there is no contradiction here. These data only suggest that the different chromosomes found in human genome, appeared in different time. About 143 thousand years ago, a new variety of m-DNA appeared in the gene pool of our ancestors. It, like any successful mutation, spread more and more widely until it crowded out all other varieties from the gene pool. This is why all women now carry this new, improved version of m-DNA. The same happened with Y- chromosome in men, but it took evolution another 84 thousand years to create a version that could displace all competitors.

It is not yet clear what the success of these new versions was based on: perhaps an increase in the reproductive ability of the offspring of their carriers.

Research Y-chromosomes not only allow us to trace migrations ancient peoples, but they can also tell what part of the genome a man shares with another bearer of the same surname (since both the man’s surname and his Y-chromosomes are inherited through the male line). This technique can also be used to determine the alleged name of the criminal based on traces of his DNA at the crime scene.

Data obtained during the study Y-chromosomes confirm that the “war of the sexes” is programmed in genes. The fact that men and women have different life programs is now common knowledge. While a man can theoretically have an almost unlimited number of natural children, women are limited in this.

Special position Y- chromosome allows the genes located on it to influence only the male individual and “not worry” about how they affect female individuals.

It was found that the genes responsible for the production of sperm proteins mutate very quickly, apparently due to intense competition. Y-chromosome contains a large number of these genes, and researchers are now trying to understand which ones are involved in this competition.

Availability Y- chromosome is a risk factor for the fetus due to the maternal immune response. This may explain some interesting patterns. For example, according to statistics, the more older brothers a man has (namely, brothers, not sisters), the more likely he is to develop homosexual tendencies. One possible explanation for this fact is that in Y- There is a gene on the chromosome responsible for the production of a masculinizing hormone called AMH. This hormone stops the development of glands, which, in its absence, turn into the uterus and ovaries. In addition, AMN causes an immune reaction on the part of the mother’s body, and the antibodies produced in this case prevent the hormone from performing another important function, namely, to direct the development of the fetal brain according to male type.

Isolation is one of the most important features Y- chromosomes. Copying genes is accompanied by errors. When eggs and sperm are formed, parts of the paired chromosomes are swapped, and the damaged areas are discarded. But Y-the chromosome has closed its borders, and this creates “abandoned lands” where repair and renewal of genes does not occur. Therefore, gene structures gradually decline, and once functional genes become useless.

The common picture of DNA copying as something like photocopying fails to convey the true dynamism of the genome. Although nature has tried to ensure the maximum accuracy of this procedure, just one piece of DNA, like an asteroid invading someone else's chromosome, can instantly change the sequence carefully preserved for many thousands of generations. These uninvited guests are called jumping genes, or transposons.

The vast majority of genes never leave their original chromosome. In contrast, jumping genes are “genome wanderers.” Sometimes they “jump” from one chromosome and “land” in a random place on another. They can insert themselves into the middle of a gene, causing chaos, or they can “moor” at the edge, slightly modifying its function. Aliens are usually “expelled” from ordinary chromosomes due to endless mixing of genes, but once on Y-chromosome they remain in it for millions of years. Sometimes, quite by accident, it allows them to do something wonderful. "Jumping emigrants" could turn Y-chromosome into the start button that starts evolution. The first of these Y- there were immigrants DAZ, discovered by D. Page (USA).

At the time when D. Page began to study Y-chromosome, all that was known about it was that it contained a gene SRY, which at the right moment triggers the development of male organs in the embryo. It is now known that Y-chromosome contains more than twenty genes (compare with 2 thousand genes in X-chromosome). Most of these genes are involved in sperm production or help the cell synthesize proteins. Gene DAZ probably arrived in Y- chromosome about 20 or 40 million years ago, approximately when the first primates appeared (perhaps the reason for their appearance was DAZ). The absence of this gene in a man’s body leads to a decrease or complete absence of spermatogenesis. According to statistics, one in six couples have problems conceiving a child, and for 20% of them the key factor is male sperm.

Currently, ectopic fertilization technology partially solves this problem. But bypassing the laws of nature is not in vain. Infertility, as paradoxical as it may sound, becomes hereditary.

Recently, British researchers made a bold assumption: the critical factor in the emergence of speech in humans was precisely a certain “jumping gene” that invaded Y-chromosome.

Gene DAZ by enhancing spermatogenesis allowed primates to flourish, but what gene was the impetus for the separation of humans from the primate lineage? A direct way to find it is with the human and chimpanzee genomes. A more elegant way is to imagine what the consequences of such mutations would be and where these mutations might be found.

This is exactly what was done at Oxford. At first, researchers assumed that there was a certain gene that had such an influence on brain development what has become possible speech. Moreover, it was suggested that this gene takes a different form in men and women.

At a conference in London in 1999, another research group announced that Y-gene detected on chromosome PCDH, whose activity most likely affects the functioning of the human brain, but not primates. This makes it a good candidate for a speech gene. Primates have it X-version ( PCDHX), but at some point in evolution it jumped to Y- chromosome.

Scientists have been able to trace the connection Y-versions of this gene ( PCDHY) with two turning points in human evolution. The first of these occurred about 3 million years ago, when the size of the human brain increased and the first tools appeared. But that is not all. A piece of DNA carrying PCDHY, transformed again, splitting into two parts, so that the resulting segments turned over in their places. According to scientists, this happened 120–200 thousand years ago, i.e. just at the time when great changes took place in the manufacture of tools.

Human African ancestors developed the ability to transmit information using symbols. Circumstantial evidence is all well and good, but how does this gene actually function? On this moment there are more questions than answers here, but the available data do not contradict the theory about the connection of this gene with the appearance of speech. It is likely one of a family of genes known as cadhedrins. They synthesize proteins that make up the membrane of nerve cells and are thus involved in the transmission of information. Genes PCDHX/Y active in some areas of the human fetal brain.

But behind all these discoveries lies one big mystery. Y- the chromosome can be thought of as a model of a capitalist economy. The winners, the genes that give an advantage, take everything because they don't mix with genes from other chromosomes. Outsiders, because they usually affect fertility, going bankrupt almost instantly. That is, the genes that survive here must do something truly valuable for the organism.

More likely, Y-chromosome has lost most of its genes during evolution, but all the remaining genes thrive. They must perform some elusive function, incomprehensible to us. Probably, to clarify this function, it is necessary to study the connection of genetic markers that allow us to trace a person’s ancestry with his abilities. The idea is dangerous in terms of ethical correctness, but it will provide an opportunity Y- chromosome will surprise us more than once.

Origin of Y chromosomes

It is believed that the X and Y chromosomes originated from a pair of identical chromosomes, called autosomes, when early mammals developed allelic diversity, the presence of the so-called "sex locus" allele leading to the development of a male organism. The chromosomes carrying this allele became the y chromosomes, and the second chromosome in this pair became the X chromosome. Over time, genes that are beneficial for males and harmful (or have no effect) for females either developed on the y chromosome or were acquired through the process of translocation.

Until recently, it was believed that the X and Y chromosomes appeared about 300 million years ago. However, recent research, particularly the platypus genome, suggests that chromosomal sex determination was absent as early as 166 million years ago, with the separation of monotremes from other mammals. This reassessment of the age of the chromosomal sex determination system is based on studies showing that sequences on the X chromosome of marsupials and placental mammals are present in the autosomes of the platypus and birds. The older estimate was based on erroneous reports of the presence of these sequences on the platypus X chromosome.

What is the Y chromosome http://ru.wikipedia.org/wiki/Y-chromosome. WIKIPEDIA. Y chromosome.

The Y chromosome is the sex chromosome of humans and most other mammals, which is found only in males and is linked to the X chromosome. All the necessary genetic information for males is encoded inside the Y chromosome. All normal men have 44 chromosomes in each cell, as well as one X and one Y chromosome. Each cell is any normal woman consists of 44 chromosomes and two X chromosomes. The Y chromosome is much smaller than the X chromosome and plays a lesser role in heredity.

Men produce 2 types of sperm: type 22 + X and type 22 + Y. These X chromosomes and Y chromosomes differ in composition and resistance to external stimuli. The presence of a Y chromosome means a boy will be born, and its absence means a girl will be born. This is why the father's sperm determines the sex of the child. Monarchs of the past who divorced their wives (or cut off their heads) and did not have boys simply did not know genetics.

Human Y chromosome

In humans, the Y chromosome consists of 58 million nitrogen base pairs and carries approximately 2% of the DNA material of a human cell. The chromosome contains 86 genes that encode 23 proteins. Traits inherited through the Y chromosome are called holandric. The human Y chromosome is unable to recombine with the X chromosome, except for small pseudoautosomal regions at the telomeres (which make up about 5% of the chromosome length). These are relict areas of ancient homology between the X and Y chromosomes. The main part of the Y chromosome that is not subject to recombination is called NRY (non-recombining region of the Y chromosome). This part of the Y chromosome allows one to determine direct paternal ancestors by assessing single nucleotide polymorphisms.

Thus, the human Y chromosome is genetically almost empty (except for the genes for hairy ears and webbing between the toes). In other species it may contain many active genes. For example, in Drosophila, many genes are known that are localized inside the Y chromosome V.A. Geodakyan, journal "Genetics", 1998, vol. 34, no. 8, p. 1171-1184.

The cells of most mammals contain two sex chromosomes: a Y chromosome and an X chromosome in males, two X chromosomes in females. In some mammals, such as the platypus, sex is determined not by one, but by five pairs of sex chromosomes. At the same time, the sex chromosomes of the platypus are more similar to the Z chromosome of birds, and the SRY gene is probably not involved in its sexual differentiation.

Origin and evolution

Before the appearance of the Y chromosome

Recombination inhibition

Ineffective selection

If genetic recombination is possible, the genome of the offspring will differ from the parent. In particular, a genome with fewer deleterious mutations can be obtained from parental genomes with a large number harmful mutations.

If recombination is impossible, then if a certain mutation appears, it can be expected that it will appear in future generations, since the process of reverse mutation is unlikely. For this reason, in the absence of recombination, the number of harmful mutations increases over time. This mechanism is called a Möller ratchet.

Part of the Y chromosome (95% in humans) is incapable of recombination. It is believed that this is one of the reasons why she is susceptible to gene damage.

Y chromosome age

Until recently, it was believed that the X and Y chromosomes appeared about 300 million years ago. However, recent research, particularly sequencing of the platypus genome, shows that chromosomal sex determination was absent as early as 166 million years ago, with the separation of monotremes from other mammals. This re-evaluation of the age of the chromosomal sex determination system is based on studies showing that sequences on the X chromosome of marsupials and placental mammals are present in the autosomes of the platypus and birds. The older estimate was based on erroneous reports of the presence of these sequences on the platypus X chromosome.

Human Y chromosome

In humans, the Y chromosome consists of more than 59 million base pairs, representing almost 2% of the human genome. The chromosome contains just over 86 genes, which encode 23 proteins. The most significant gene on the Y chromosome is the SRY gene, which serves as a genetic “switch” for the development of the body according to the male type. Traits inherited through the Y chromosome are called holandric.

The human Y chromosome is unable to recombine with the X chromosome, except for small pseudoautosomal regions at the telomeres (which make up about 5% of the chromosome length). These are relict areas of ancient homology between the X and Y chromosomes. The main part of the Y chromosome that is not subject to recombination is called NRY. non-recombining region of the Y chromosome) . This part of the Y chromosome allows one to determine direct paternal ancestors by assessing single nucleotide polymorphisms.

Subsequent evolution

Sex ratio 1:1

Fisher's principle shows why in almost all species that use sexual reproduction, the sex ratio is 1:1, meaning that in the case of humans, 50% of the offspring will receive a Y chromosome and 50% will not. W. D. Hamilton gave the following basic explanation in his 1967 article "Extraordinary Sex Ratios":

see also

Sources

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3. =Veyrunes F, Waters PD, Miethke P, et al. Bird-like sex chromosomes of platypus imply recent origin of mammal sex chromosomes (English) // Genome Research. - 2008. - Vol. 18 . - P. 965–973. - DOI:10.1101/gr.7101908.
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Image from unc.edu

Every woman is not just a mystery, but a mosaic consisting of cells with different sets of active chromosomes. Humans have 23 pairs of chromosomes, and the chromosomes of each pair carry the same sets of genes. The exception is a pair of sex chromosomes. In men, one is called X and the other is called Y, and they differ significantly in their sets of genes. The X chromosome is much larger than the Y chromosome and contains more genes. Both female sex chromosomes are X, and they differ from each other just like the chromosomes within the other 22 pairs. Every woman has two X chromosomes, and every man has only one, and so that they are equally active in women and men, the body regulates their work. To do this, in all cells of a woman’s body, one of the X chromosomes is inactivated. Which of the two sex chromosomes will be disabled is determined by chance for each cell, so that in some of the cells of a woman’s body one X chromosome works, and in the remaining cells the other works.

As a result of this mosaic pattern, women rarely develop diseases associated with damage to the X chromosome. Even if a woman has an X chromosome with a defect in some gene, the other chromosome of the pair, working in half of the cells, saves the situation and prevents the disease from manifesting itself. For a disease associated with damage to the X chromosome to “play out” to its full potential, a woman must receive as many as two copies of this chromosome with a defect in the same gene. This is an unlikely event. At the same time, if a man receives a defective X chromosome (it comes from the mother), she will not have a mate to compensate for the damage, and the disease will show itself.

The X chromosome, unfortunately for men, carries many vital genes, so its breakdown is fraught with dire consequences. Color blindness, hemophilia, Duchenne myopathy, fragile X syndrome, X-linked immunodeficiency are just the most well-known genetic diseases that affect almost exclusively men.

Color blindness

It is a common misconception that only men can be colorblind. This is not true, however, colorblind women are much less common. Only 0.4 percent of women and about 5 percent of men have difficulty distinguishing certain colors. Color blindness is the loss or impairment of one of the pigments associated with recognizing light of a certain color. There are three such pigments in total, and they are sensitive to waves of red, green and of blue color. Any complex color can be thought of as a combination of these three. Each cone cell, which is found in the retina and is responsible for color recognition, contains only one type of pigment. For reasons still unknown, problems with the functioning of the pigments with which we distinguish between red and green colors, are more common than defects in the pigment needed to correctly recognize the color blue.

Genes located on the X chromosome are responsible for the synthesis of pigments. If a man received a chromosome with a defective gene that determines the recognition of, for example, the color red, then only this defective X chromosome will be active in all the cones of his retina - he simply does not have another. Therefore, such a man will not have cones that can correctly recognize the color red. A woman’s retina has a mosaic structure, and even if one of the X chromosomes carries a damaged gene, this chromosome will be active only in part of the cones responsible for recognizing the corresponding color. In other cones, the second chromosome will be active, which carries the normal gene. Such a woman's color perception will be slightly altered, but she will still be able to distinguish all the colors that people usually distinguish.

Hemophilia

Other known disease, associated with defects in X chromosome genes, is hemophilia, a blood clotting disorder. After an injury, a complex system of reactions is launched in the blood of a healthy person, leading to the formation of fibrin protein strands. Due to the accumulation of these threads, the blood at the site of injury becomes thicker and clogs the wound. If any stage of the process is disrupted, the blood does not clot at all or does so too slowly, so that the patient may die from blood loss even after the tooth is removed. In addition, patients with hemophilia suffer from spontaneous internal hemorrhages due to the vulnerability of the vessel walls.

The cascade of reactions that ultimately leads to the formation of fibrin threads and blood thickening is very complex, and the more complex the system, the more more places where it might break. There are three known types of hemophilia associated with defects in three genes encoding proteins that participate in the cascade. Two of these genes are located on the X chromosome, so one man in 5,000 suffers from hemophilia, and only 60 cases of the disease have been recorded in women throughout history.

Duchenne myopathy

Another important gene located on the X chromosome is the dystrophin protein gene, which is necessary to maintain membrane integrity muscle cells. In Duchenne myopathy, the function of this gene is disrupted and dystrophin is not produced. Men who have inherited an X chromosome with such a damaged gene develop progressive muscle weakness, as a result of which boys with this disease cannot walk independently by the age of 12. As a rule, patients die at the age of about 20 years due to respiratory disorders associated with muscle weakness. In girls who received an X chromosome with a faulty dystrophin gene, due to mosaicism, the protein is missing in only half of the body cells. Therefore, women who carry the defective dystrophin gene suffer only from mild muscle weakness, and even then not always.

X-linked severe immunodeficiency

Patients with severe immunodeficiencies are forced to live in completely sterile environments because they are extremely vulnerable to infectious diseases. X-linked severe immunodeficiency occurs due to a mutation in a gene that encodes a common component of several receptors necessary for the interaction of cells of the immune system. As is obvious from the name of the disease, this gene is also located on the X chromosome. Due to dysfunctional receptors the immune system from the very beginning it develops incorrectly, its cells are few in number, function poorly and cannot coordinate their actions. Fortunately, this serious disease is rare: it affects one boy in 100,000. In girls, the occurrence of this disease can be considered almost impossible.

Fragile syndromeX chromosomes

Another important gene located on the X chromosome is the FMR1 gene, which is necessary for normal development nervous system. The functioning of this gene can be disrupted due to a pathological process in which the number of repeating DNA fragments in the gene increases. The point is that exactly copying a repeating number of units is always difficult. Let's imagine that we need to carefully rewrite a long number that contains many identical numbers in a row - it’s easy to make a mistake and write a few numbers more or less. It's exactly the same in DNA. During cell division, when DNA is doubled, the number of repeats can randomly change. It is precisely because of the increase in the number of repeats in a short fragment of DNA on the X chromosome that a “fragile” region can appear that easily breaks during cell division. The FMR1 gene is located next to the “fragile” area, and its work is disrupted. As a result of this pathology, mental retardation occurs, which manifests itself more clearly in men with a “fragile” X chromosome than in women.

Is it always better to have two?X chromosomes than one?

It seems that having two X chromosomes is more beneficial than one: there is less risk of diseases due to bad genes. What about males who have the following sex chromosome composition: XXY? Can we expect them to have an advantage over males with the usual XY sex chromosome composition? It turns out that the composition of XXY chromosomes is not a blessing, but quite the opposite. Men with this set of chromosomes suffer from Klinefelter syndrome, in which many pathologies are observed, but there are no benefits.

Moreover, there are known diseases that are also characterized by large quantities X chromosomes, up to five per genotype. Such pathologies occur in both women and men. If there are excess X chromosomes, all but one are inactivated. However, even if the extra X chromosomes do not work, the more there are, the more severe the disease. Interestingly, intelligence especially suffers from the presence of excess X chromosomes - each extra chromosome of this type leads to a decrease in IQ by an average of about 15 points. It turns out that having a spare X chromosome is good, but not always (an additional X chromosome does not make men any better). Having many spare variants of this sex chromosome is not beneficial for either women or men.

Why are additional inactive X chromosomes harmful, and why does each extra chromosome aggravate the severity of the disease? Firstly, the extra X chromosomes are not turned off immediately, but only after the first 16 days of embryo development. And the earlier during development a disorder occurs, the more diverse and numerous its manifestations will be. Therefore, extra chromosomes can have time to “damage” quite fundamentally, so that pathologies will manifest themselves in completely different areas.

Second, some genes on inactivated X chromosomes somehow escape being turned off. Although the X and Y chromosomes are very different, they still form a pair and have a small number of identical genes. If there are too many sex chromosomes, and these genes remain active on all of them, the gene balance in the cells is disrupted. Therefore, the more extra chromosomes, the more severe the disease.

The X chromosome carries many vital genes, and it is not surprising that its defects have extremely unpleasant manifestations. Women are naturally given the opportunity to “insure themselves” by having an extra copy of the chromosome, which can reduce the severity of the disease. However, such a “reserve” is only good in the singular, and all additional X chromosomes lead to the development of severe pathologies. Well, men who do not have a second X chromosome are at greater risk from the very beginning of their conception. Alas.

Yulia Kondratenko