It is impossible to live without women in the world, but…
A truly female X chromosome is most harmful to men
A person has 23 pairs of chromosomes, and the chromosomes of one pair carry the same sets of genes. The exception is a pair of sex chromosomes. In men, one of them 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 and contains more genes. Both sex chromosomes of women are X, and they differ from each other as well as the chromosomes inside the other 22 pairs. Each woman has two X chromosomes, and each 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 decided by chance for each cell, so that one X chromosome works in part of the cells of the woman's body, and the other in the remaining ones.
As a consequence of this mosaic, women rarely show diseases associated with damage to the X chromosomes. Even if a woman turns out to have an X chromosome with a gene defect, the other chromosome of the pair, working in half of the cells, saves the day and prevents the disease from manifesting. In order for the disease associated with damage to the X chromosome to "play out" at full power, a woman should get as many as two copies of this chromosome with a defect of 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 pair to compensate for the damage, and the disease will show itself.
Unfortunately for men, the X chromosome carries many vital genes, so its breakdown is fraught with sad consequences. Color blindness, hemophilia, Duchenne myopathy, brittle X-chromosome syndrome, X-linked immunodeficiency are only the most well–known genetic diseases that affect almost exclusively men.
Color blindnessIt 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 malfunction of one of the pigments associated with the recognition of light of a certain color. There are three such pigments in total, and they are sensitive to waves of red, green and blue. Any complex color can be represented as a combination of these three. In each cone cell, which are located in the retina and are responsible for color recognition, there is only one type of pigment. For unknown reasons, problems with the work of the pigments with which we distinguish between red and green colors are more common than defects in the pigment necessary to correctly recognize the blue color.
The genes located on the X chromosome are responsible for the synthesis of pigments. If a man got a chromosome with a defective gene that determines the recognition of, for example, red, then only this defective X chromosome will be active in all the cones of his retina - he simply does not have another one. Therefore, such a man will not have cones that can correctly recognize the red color. The retina of a woman 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 that carries the normal gene will be active. The perception of color in such a woman will be slightly altered, but still she will be able to distinguish all the colors that people usually distinguish.
HemophiliaAnother well-known disease associated with defects in the X chromosome genes is hemophilia, a blood clotting disorder.
After an injury, a complex system of reactions is triggered in the blood of a healthy person, leading to the formation of fibrin protein strands. Due to the accumulation of these threads, the blood becomes thicker at the site of injury and clogs the wound. If any of the stages of the process is disrupted, the blood does not clot at all or does it too slowly, so that the patient may die from blood loss even after tooth extraction. In addition, patients with hemophilia suffer from spontaneous internal hemorrhages due to the vulnerability of the vessel walls.
The cascade of reactions that eventually leads to the formation of fibrin filaments and blood thickening is very complex, and the more complex the system, the more places where it can break. There are three types of hemophilia associated with defects in three genes encoding proteins participating in the cascade. Two of these genes are located on the X chromosome, so one man out of 5,000 suffers from hemophilia, and only 60 cases of women's diseases have been recorded in history.
Duchene 's myopathyAnother important gene located on the X chromosome is the dystrophin protein gene, which is necessary to maintain the integrity of muscle cell membranes.
With Duchenne myopathy, the work of this gene is disrupted, and dystrophin is not formed. Men who got an X chromosome with such a damaged gene develop progressive muscle weakness, as a result of which boys with such a 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 mosaic, the protein is missing in only half of the body cells. Therefore, women carriers of the defective dystrophin gene suffer only mild muscle weakness, and then not always.
X-linked severe immunodeficiencyPatients with severe immunodeficiency are forced to live in a completely sterile environment, 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 non-functioning receptors, the immune system develops incorrectly from the very beginning, its cells are small, poorly functioning and cannot coordinate their actions. Fortunately, this serious disease is rare: one boy out of 100,000 suffers from it. In girls, the appearance of this disease can be considered almost incredible.
Brittle X chromosome syndromeAnother important gene located on the X chromosome is the FMR1 gene, which is necessary for the normal development of the nervous system.
The work of this gene can be disrupted due to a pathological process in which the number of repetitive DNA fragments increases in the gene. The fact is that exact copying of a repeating number of units is always a difficulty. Imagine that we need to carefully rewrite a long number in which there are many identical digits in a row – it's easy to make a mistake and write a few digits more or less. It's the same in DNA. During cell division, when DNA doubles, the number of repeats may 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 "brittle" section may appear, which is easily torn during cell division. The FMR1 gene is located next to the "brittle" site, and its work is disrupted. As a result of this pathology, mental retardation occurs, which manifests itself in men with a "brittle" X-chromosome more clearly than in women.
Is it always better to have two X chromosomes than one?It seems that having two X chromosomes is more profitable than one: there is less risk of diseases due to unsuccessful genes.
What about males with the following sex chromosome composition: XXY? Can they be expected to have an advantage over males with the usual composition of XY sex chromosomes? It turns out that the composition of chromosomes XXY is not a blessing, but quite the opposite. Men with such a set of chromosomes suffer from Klinefelter syndrome, in which there are many pathologies, but there are no advantages.
Moreover, there are known diseases that are characterized by even larger numbers of X chromosomes, up to five per genotype. Such pathologies occur in both women and men. In the presence of redundant X chromosomes, all but one of them are inactivated. However, even if the extra X chromosomes do not work, the more of them, the more severe the disease. Interestingly, intelligence suffers especially from the presence of excess X chromosomes - each extra chromosome of this type leads to a decrease in IQ on average by about 15 points. It turns out that having a backup version of the X chromosome is good, but not always (men do not get better from an additional X chromosome). Having a lot of spare variants of this sex chromosome is not profitable 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 a violation occurs during development, the more diverse and numerous its manifestations will be. Therefore, extra chromosomes can have time to "harm" quite fundamentally, so that pathologies will manifest themselves in completely different spheres.
Secondly, some genes on inactivated X chromosomes somehow avoid disconnection. 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 disturbed. 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 "hedge" due to an additional copy of the chromosome, which can reduce the severity of the disease. However, such a "spare" is good only 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 get more risk from their conception. Alas.
Portal "Eternal youth" http://vechnayamolodost.ru12.11.2014