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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 a 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 light detection 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 the help of 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
Another known disease associated with defects in X chromosome genes is hemophilia, a blood clotting disorder. After an injury in the blood healthy person a complex system of reactions is launched, leading to the formation of fibrin protein filaments. 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 what more complex system, those 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 the integrity of muscle cell membranes. 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 the gene that encodes common component 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 It 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, there is mental retardation, 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 regular composition XY sex chromosomes? 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 characterized by even larger numbers of X chromosomes, up to five per genotype. Such pathologies occur in both women and men. If there are excess X chromosomes, all but one of them are inactivated. However, let extra X chromosomes and do not work, the more of them, 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 for 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
Every man has a so-called “Y chromosome” in his body, which makes a man a man. Typically, chromosomes in the nucleus of any cell are arranged in pairs. So for the Y chromosome there is a corresponding pair - the X chromosome. At conception, the future new organism inherits all of its genetic information from parents (half the chromosomes from one parent, half from the other). He can only inherit the X chromosome from his mother. From the father - either X or Y. If two X chromosomes are collected in the fetus, a girl will be born. If X and Y together are a boy (there cannot be two Y chromosomes in one organism). During for long years geneticists believed that nature did not assign any other useful function to the Y chromosome. However, they were wrong.
By winter, geneticists hope to completely decipher the genetic code embedded in the Y chromosome. Decoding the Y chromosome is part of the project to decipher the human genome, which is carried out by an international group of geneticists. Information about genetic map This chromosome is extremely important, since it is in it that the answers to questions about the causes of male infertility lie. However, during the course of the study it became clear that the Y chromosome is far from being as simple as it seemed at first.
For almost a hundred years, geneticists believed that the tiny chromosome (and the Y chromosome is indeed the smallest in human body, noticeably smaller than its pair - the 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. The Y chromosome was the first chromosome discovered using a microscope. But it turned out to be impossible to match the Y chromosome with any hereditary genetic information; for the X chromosome research technologies those times (the study of several generations of families to identify hereditary traits) were quite suitable.
In the mid-20th century, geneticists were “on suspicion” of several very specific genes, which could be contained in the Y chromosome. However, in 1957, at a meeting of the American Society of Human Genetics, all these theories were destroyed. The Y chromosome was officially recognized as a “dummy”, not carrying any important hereditary information. The point of view has become established that “yes, the Y chromosome carries some kind of gene that determines the sex of a person, but no other functions are assigned to it.”
And only now geneticists have begun to understand that the Y chromosome is something unique in the world of genes. It is extremely highly specialized: all the genes contained in it (and there were about two dozen of them) are either responsible for the production of sperm by the male body, or are responsible for “related” processes. And, naturally, the most important gene on this chromosome is SRY, the same gene in the presence of which the human embryo develops along the male path
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, the genome with fewer harmful 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 organism according to 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
- Grützner F, Rens W, Tsend-Ayush E et al. In the platypus a meiotic chain of ten sex chromosomes shares genes with the bird Z and mammal X chromosomes (English) // Nature. - 2004. - Vol. 432. - P. 913–917. - DOI:10.1038/nature03021.
- Warren WC, Hillier LDW, Graves JAM, et al. Genome analysis of the platypus reveals unique signatures of evolution // Nature. - 2008. - Vol. 453. - P. 175–183. - DOI:10.1038/nature06936.
- =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.
- Lahn B.T., Page D.C. Four evolutionary strata on the human X chromosome. (English) // Science (New York, N.Y.). - 1999. - Vol. 286, no. 5441. - P. 964-967. - PMID 10542153.[to correct]
- Graves J.A. Sex chromosome specialization and degeneration in mammals. (English) // Cell. - 2006. - Vol. 124, no. 5 . - P. 901-914. - DOI:10.1016/j.cell.2006.02.024. - PMID 16530039.[to correct]
- Graves J. A., Koina E., Sankovic N. How the gene content of human sex chromosomes evolved. (English) // Current opinion in genetics & development. - 2006. - Vol. 16, no. 3. - P. 219-224. - DOI:10.1016/j.gde.2006.04.007. - PMID 16650758.[to correct]
- Graves J.A. The degenerate Y chromosome--can conversion save it? (English) // Reproduction, fertility, and development. - 2004. - Vol. 16, no. 5 . - P. 527-534. - DOI:10.10371/RD03096. - PMID 15367368.[to correct]
- Hughes J. F., Skaletsky H., Pyntikova T., Minx P. J., Graves T., Rozen S., Wilson R. K., Page D. C. Conservation of Y-linked genes during human evolution revealed by comparative sequencing in chimpanzees. (English) // Nature. - 2005. - Vol. 437, no. 7055. - P. 100-103. - DOI:10.1038/nature04101. - PMID 16136134.[to correct]
- The male chromosome will remain stable for the next millions of years (undefined) . MedNews (February 24, 2012). Retrieved May 16, 2017.
- Lydia Gradova. Male extinction turned out to be a myth (undefined) . "Morning" (February 23, 2012). Retrieved May 16, 2017.
- Jon Hamilton. Human Male: Still A Work In Progress(English) . NPR (January 13, 2010). Retrieved May 16, 2017.
As the now Prime Minister (then President) of Russia Vladimir Putin said in 2006, “if a grandmother had certain sexual characteristics, she would be a grandfather.” The discussion was about the possibility of Russia adopting sanctions against Iran, but the comparison is not entirely correct. Thanks to advances in genetics, we know that a grandmother differs from a grandfather not only in appearance, but also in the set of sex chromosomes.
In most mammals, sex is determined by them: the male body is the carrier of the X- and Y-chromosomes, and women “make do” with two X-chromosomes. Once this division did not exist, but as a result of evolution about 300 million years ago, chromosomes differentiated. There are variations whereby some men's cells contain two X chromosomes and one Y chromosome, or one X chromosome and two Y chromosomes; Some women's cells contain three or one X chromosome. Occasionally, female XY organisms or male XX organisms are observed, but the vast majority of people still have a standard configuration of sex chromosomes. For example, the phenomenon of hemophilia is associated with this feature. The defective gene that impairs blood clotting is linked to the X chromosome and is recessive. For this reason, women only endure the disease without suffering from it themselves due to the presence of a duplicate gene due to the second X chromosome, but men in a similar situation carry only a defective gene and get sick.
One way or another, the Y chromosome has traditionally been considered weak point male organisms, reducing genetic diversity and hindering evolution.
However latest research showed that fears about the extinction of the male race are greatly exaggerated: the Y chromosome does not even think of stagnating.
On the contrary, its evolution is very active, it changes much faster than other areas genetic code person.
Research published in Nature, showed that a specific part of the human Y chromosome and one of its immediate family- chimpanzees are very different. Over the 6 million years of separate evolution of monkeys and humans, the fragment of the chromosome responsible for the production of germ cells has changed by a third or even half. The rest of the chromosome is actually quite constant.
Scientists' assumptions about the conservatism of the Y chromosome were based on objective factors: being transmitted from father to son without changes (for the X chromosome there are as many as three options - two from the mother and one from the father, all of them can exchange genes), it cannot gain genetic diversity from the outside, changing only due to the loss of genes. According to this theory, in 125 thousand years the Y chromosome will finally die out, which could be the end of all humanity.
However, for 6 million years of separate evolution of humans and chimpanzees, the Y chromosome has been successfully changing and progressing. IN new job, held at the Massachusetts Institute of Technology, talks about the chimpanzee's Y chromosome. The human Y chromosome was deciphered in 2003 by the same group led by Professor David Page.
The results of the new study surprised geneticists: they expected that the sequence of genes on the two chromosomes would be very similar.
For comparison: in total mass The DNA of humans and chimpanzees is different in only 2% of genes, and the Y chromosome differs by more than 30%!
Professor Page compared the process of evolution of the male chromosome to a change in the appearance of a house, the owners of which remain the same. “Despite the fact that the same people live in the house, almost constantly one of the rooms is completely updated and renovated. As a result, after a certain period of time, as a result of “room-by-room” renovation, the entire house changes. However, this trend is not normal for the entire genome,” he noted.
The reason for this unexpected instability of the Y chromosome is not yet precisely clear. Scientists suggest that genetic diversity in it is ensured by instability to mutations. The usual mechanism for “repairing” genes fails on the Y chromosome, opening the way for new mutations. Statistically, a larger number of them become fixed and change the genome.
In addition, these mutations are subject to significantly greater selection pressure. This is determined by their function - the production of germ cells. Any beneficial mutations will be fixed with to a greater extent probabilities, since they act directly - increasing the ability of an individual to reproduce. At the same time, ordinary mutations have an indirect effect - increasing the body's resistance to disease or harsh conditions environment, For example. Thus, the benefit of a mutation in a nonspecific DNA section will only be revealed if the organism falls into the corresponding unfavourable conditions. In other cases, mutant and non-mutant organisms will perform similarly. Fertility appears very quickly - already in the second generation. An individual either reproduces as a result of mutation more successfully and leaves numerous offspring, or reproduces noticeably worse and cannot increase the share of its genes in the general population. This mechanism functions more efficiently in chimpanzees, whose females constantly mate with big amount males. As a result, the germ cells enter into direct competition, and “selection” occurs as efficiently as possible. In humans, due to more conservative models of reproduction, the Y chromosome has not evolved so rapidly, geneticists say.
This hypothesis is supported by the fact that the parts of the chromosome involved in sperm production are most different between humans and chimpanzees.
Professor Page's group, in collaboration with the University of Washington Genome Center, continues to work on deciphering the Y chromosome of other mammals. They hope to shed light on the evolution of sex chromosomes and its relationship to population behavior patterns.
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Subtitles
Genes, DNA and chromosomes are what make us unique. They are a set of instructions passed down to you from your father and mother. These instructions are found in your cells. And all living organisms are made up of cells. There are many types of cells - nerve cells, hair cells or skin cells. They all differ in shape and size, but each has certain components. The cell has an outer boundary called the membrane, which contains a fluid called the cytoplasm. The cytoplasm contains the nucleus, in which the chromosomes are located. Each human cell usually has 23 pairs of chromosomes, or 46 in total. 22 pairs of these are called autosomes and are the same in men and women. The 23rd pair are sex chromosomes; they are different in men and women. Women have 2 X chromosomes, men have one X and one Y chromosome. Chromosomes are long molecules of DNA - deoxyribonucleic acid. The shape of DNA resembles a twisted ladder. And it's called a double helix. The steps in the ladder are 4 bases: Adenine - A Thymine - T Guanine - G And Cytosine - C A section of DNA is called a gene. The body reads genes as recipes for making proteins. The length and order of bases in the DNA of genes determines the size and shape of the resulting proteins. The size and shape of a protein determine its function in the body. Proteins make up the cells that form the tissues that make up organs, such as our eyes or skin. Thus, genes determine whether you are a cow, an apple or a person and what you look like - the color of your hair, skin, eyes and everything else.
General information
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 of deleterious 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, suggests that chromosomal sex determination was absent as early as 166 million years ago, with the divergence 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, which is almost 2% of human DNA - in cell nucleus. 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, through the assessment of single-nucleotide polymorphism, to determine direct paternal ancestors.
see also
Sources
- Grützner F, Rens W, Tsend-Ayush E; et al. (2004). “In the platypus a meiotic chain of ten sex chromosomes shares genes with the bird Z and mammal X chromosomes.” Nature. 432 : 913-917. DOI:10.1038/nature03021.
- Warren WC, Hillier LDW, Graves JAM; et al. (2008). “Genome analysis of the platypus reveals unique signatures of evolution” . Nature. 453 : 175-183. DOI:10.1038/nature06936.
- Veyrunes F, Waters PD, Miethke P; et al. (2008). “Bird-like sex chromosomes of platypus imply recent origin of mammal sex chromosomes” . Genome Research. 18 : 965-973. DOI:10.1101/gr.7101908.
- Lahn B, Page D (1999). “Four evolutionary strata on the human X chromosome.” Science. 286 (5441): 964-7. DOI:10.1126/science.286.5441.964. PMID.
- Graves J.A.M. (2006). “Sex chromosome specialization and degeneration in mammals.” Cell. 124 (5): 901-14. DOI:10.1016/j.cell.2006.02.024. PMID.
- Graves J. A. M., Koina E., Sankovic N. (2006). “ How the gene content of human sex chromosomes evolved.” Curr Opin Genet Dev. 16 (3): 219-24. DOI:10.1016/j.gde.2006.04.007. PMID.
- Graves J.A. The degenerate Y chromosome--can conversion save it? (English) // Reproduction, fertility, and development. - 2004. - Vol. 16, no. 5 . - P. 527-534. - DOI:10.10371/RD03096. - PMID 15367368.[to correct ]