Normally, a person has no chromosomes. The male chromosome was ordered to live long. Genetic information on the Y chromosome
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Sometimes they give us amazing surprises. For example, do you know what chromosomes are and how they affect?
We propose to look into this issue in order to dot the i’s once and for all.
Looking at family photographs, you may have probably noticed that members of the same family resemble each other: children look like parents, parents look like grandparents. This similarity is passed on from generation to generation through amazing mechanisms.
All living organisms, from single-celled organisms to African elephants, contain chromosomes in the cell nucleus - thin, long threads that can only be seen with an electron microscope.
Chromosomes (ancient Greek χρῶμα - color and σῶμα - body) are nucleoprotein structures in the cell nucleus, in which most of the hereditary information (genes) is concentrated. They are designed to store this information, implement it and transmit it.
How many chromosomes does a person have Also in late XIX centuries, scientists have found that the number of chromosomes in different types
not the same. For example, peas have 14 chromosomes, y have 42, and in humans – 46 (that is, 23 pairs)
. Hence the temptation arises to conclude that the more there are, the more complex the creature that possesses them. However, in reality this is absolutely not the case.
Of the 23 pairs of human chromosomes, 22 pairs are autosomes and one pair are gonosomes (sex chromosomes). The sexes have morphological and structural (gene composition) differences.
In a female organism, a pair of gonosomes contains two X chromosomes (XX-pair), and in a male organism, one X-chromosome and one Y-chromosome (XY-pair).
The gender of the unborn child depends on the composition of the chromosomes of the twenty-third pair (XX or XY). This is determined by fertilization and the fusion of the female and male reproductive cells.
This fact may seem strange, but in terms of the number of chromosomes, humans are inferior to many animals. For example, some unfortunate goat has 60 chromosomes, and a snail has 80. Chromosomes
An interesting fact is that if there is an extra chromosome or if at least one of the 46 is missing, a person experiences a mutation and serious developmental abnormalities (Down's disease, etc.).
The human body is a complex multifaceted system that functions at various levels. So that organs and cells can work in correct mode, certain substances must participate in specific biochemical processes. This requires a solid foundation, that is, the correct transmission of the genetic code. It is the underlying hereditary material that controls the development of the embryo.
However, changes sometimes occur in hereditary information that appear in large associations or concern individual genes. Such errors are called gene mutations. In some cases, this problem relates to the structural units of the cell, that is, to entire chromosomes. Accordingly, in this case the error is called a chromosome mutation.
Each human cell normally contains the same number of chromosomes. They are united by the same genes. The complete set is 23 pairs of chromosomes, but in germ cells there are 2 times fewer of them. This is explained by the fact that during fertilization, the fusion of sperm and egg must represent a complete combination of all the necessary genes. Their distribution does not occur randomly, but in a strictly defined order, and such a linear sequence is absolutely the same for all people.
3 years later, the French scientist J. Lejeune discovered that the disorder in humans mental development and resistance to infections are directly related to It was about the extra 21 chromosome. She is one of the smallest, but she has a lot of genes. The extra chromosome was observed in 1 in 1000 newborns. This chromosomal disease is by far the most studied and is called Down syndrome.
In the same 1959, it was studied and proven that the presence of an extra X chromosome in men leads to Klinefelter's disease, in which a person suffers from mental retardation and infertility.
However, despite the fact that chromosomal abnormalities have been observed and studied for quite a long time, even modern medicine is not able to treat genetic diseases. But methods for diagnosing such mutations have been quite modernized.
Causes of an extra chromosome
The anomaly is the only reason for the appearance of 47 chromosomes instead of the required 46. Medical experts have proven that main reason the appearance of an extra chromosome - the age of the expectant mother. The older the pregnant woman, the greater the likelihood of chromosome nondisjunction. For this reason alone, women are recommended to give birth before the age of 35. If pregnancy occurs after this age, you should undergo an examination.
Factors that contribute to the appearance of an extra chromosome include the level of anomaly that has increased globally, the degree of environmental pollution, and much more.
There is an opinion that an extra chromosome occurs if there were similar cases in the family. This is just a myth: studies have shown that parents whose children suffer from a chromosomal disorder have a completely healthy karyotype.
Diagnosis of a child with a chromosomal abnormality
Recognition of a violation of the number of chromosomes, the so-called aneuploidy screening, reveals a deficiency or excess of chromosomes in the embryo. Pregnant women over 35 years of age are advised to undergo a procedure to obtain a sample of amniotic fluid. If a karyotype abnormality is detected, then to the expectant mother it will be necessary to terminate the pregnancy, since the born child will suffer from a serious illness throughout his life in the absence of effective methods treatment.
Chromosome disruption is mainly of maternal origin, so it is necessary to analyze not only the cells of the embryo, but also the substances that are formed during the maturation process. This procedure is called the diagnosis of genetic disorders using polar bodies.
Down syndrome
The scientist who first described Mongolism is Daun. An extra chromosome, a gene disease in the presence of which necessarily develops, has been widely studied. In Mongolism, trisomy 21 occurs. That is, a sick person has 47 chromosomes instead of the required 46. The main symptom is developmental delay.
Children who have an extra chromosome experience serious difficulties in mastering material in school, so they need an alternative teaching method. In addition to mental, there is a deviation in physical development, namely: slanted eyes, flat face, wide lips, flat tongue, shortened or widened limbs and feet, large accumulation of skin in the neck area. Life expectancy reaches 50 years on average.
Patau syndrome
Trisomy also includes Patau syndrome, in which there are 3 copies of chromosome 13. Distinctive feature is a violation of the activity of the central nervous system or its underdevelopment. Patients have multiple developmental defects, possibly including heart defects. More than 90% of people with Patau syndrome die in the first year of life.
Edwards syndrome
This anomaly, like the previous ones, refers to trisomy. In this case we are talking about chromosome 18. characterized by various disorders. Mostly, patients experience bone deformation, an altered shape of the skull, problems with the respiratory system and cardiovascular system. Life expectancy is usually about 3 months, but some babies live up to a year.
Endocrine diseases due to chromosome abnormalities
In addition to the listed chromosomal abnormality syndromes, there are others in which a numerical and structural abnormality is also observed. Such diseases include the following:
- Triploidy is a fairly rare disorder of chromosomes in which their modal number is 69. Pregnancy usually ends early miscarriage, but if the child survives, he lives no more than 5 months, and numerous birth defects are observed.
- Wolf-Hirschhorn syndrome is also one of the rarest chromosomal abnormalities that develops due to deletion of the distal end of the short arm of the chromosome. The critical region for this disorder is 16.3 on chromosome 4p. Characteristic signs- developmental problems, growth delays, seizures and typical facial features
- Prader-Willi syndrome is a very rare disease. With such an abnormality of chromosomes, 7 genes or some parts of them on the 15th paternal chromosome do not function or are completely deleted. Signs: scoliosis, strabismus, delayed physical and intellectual development, fatigue.
How to raise a child with a chromosomal disorder?
Raising a child with congenital chromosomal diseases is not easy. In order to make your life easier, you need to follow some rules. First, you must immediately overcome despair and fear. Secondly, there is no need to waste time looking for the culprit, he simply does not exist. Thirdly, it is important to decide what kind of help the child and family need, and then turn to specialists for medical, psychological and pedagogical help.
In the first year of life, diagnosis is extremely important, since motor function develops during this period. With the help of professionals, the child will quickly acquire motor abilities. It is necessary to objectively examine the baby for vision and hearing pathologies. The child should also be observed by a pediatrician, neuropsychiatrist and endocrinologist.
The carrier of an extra chromosome is usually friendly, which makes his upbringing easier, and he also tries, to the best of his ability, to earn the approval of an adult. The level of development of a special child will depend on how persistently they teach him basic skills. Although sick children lag behind the rest, they require a lot of attention. It is always necessary to encourage a child's independence. Self-service skills should be instilled by your own example, and then the result will not be long in coming.
Children with chromosomal diseases are endowed with special talents that need to be discovered. This could be music lessons or drawing. It is important to develop the baby’s speech, play active games that develop motor skills, read, and also teach him routine and neatness. If you show your child all your tenderness, care, attentiveness and affection, he will respond in kind.
Can it be cured?
To date, it is impossible to cure chromosomal diseases; Each proposed method is experimental, and their clinical effectiveness has not been proven. Systematic medical and educational assistance helps to achieve success in development, socialization and acquisition of skills.
A sick child should be observed by specialists at all times, since medicine has reached the level at which it is able to provide necessary equipment And different kinds therapy. Teachers will use modern approaches in the education and rehabilitation of the child.
B chromosomes have not yet been discovered in humans. But sometimes an additional set of chromosomes appears in cells - then they talk about polyploidy, and if their number is not a multiple of 23 - about aneuploidy. Polyploidy occurs in certain types of cells and contributes to their increased work, while aneuploidy usually indicates disturbances in the functioning of the cell and often leads to its death.
We must share honestly
Most often, an incorrect number of chromosomes is a consequence of unsuccessful cell division. In somatic cells, after DNA duplication, the maternal chromosome and its copy are linked together by cohesin proteins. Then kinetochore protein complexes sit on their central parts, to which microtubules are later attached. When dividing along microtubules, kinetochores move to different poles of the cell and pull chromosomes with them. If the crosslinks between copies of a chromosome are destroyed ahead of time, then microtubules from the same pole can attach to them, and then one of the daughter cells will receive an extra chromosome, and the second will remain deprived.
Meiosis also often goes wrong. The problem is that the structure of linked two pairs of homologous chromosomes can twist in space or separate in the wrong places. The result will again be an uneven distribution of chromosomes. Sometimes the reproductive cell manages to track this so as not to pass the defect on to inheritance. The extra chromosomes are often misfolded or broken, which triggers the death program. For example, among spermatozoa there is such selection for quality. But the eggs are not so lucky. All of them are formed in humans even before birth, prepare for division, and then freeze. The chromosomes have already been duplicated, tetrads have been formed, and division has been delayed. They live in this form until the reproductive period. Then the eggs mature in turn, divide for the first time and freeze again. The second division occurs immediately after fertilization. And at this stage it is already difficult to control the quality of division. And the risks are greater, because the four chromosomes in the egg remain cross-linked for decades. During this time, damage accumulates in cohesins, and chromosomes can spontaneously separate. Therefore, the older the woman, the greater the likelihood of incorrect chromosome segregation in the egg.
Aneuploidy in germ cells inevitably leads to aneuploidy of the embryo. If a healthy egg with 23 chromosomes is fertilized by a sperm with extra or missing chromosomes (or vice versa), the number of chromosomes in the zygote will obviously be different from 46. But even if the sex cells are healthy, this does not guarantee healthy development. In the first days after fertilization, embryonic cells actively divide in order to quickly gain cell mass. Apparently, during rapid divisions there is no time to check the correctness of chromosome segregation, so aneuploid cells can arise. And if an error occurs, then further fate embryo depends on the division in which this happened. If the balance is disturbed already in the first division of the zygote, then the entire organism will grow aneuploid. If the problem arose later, then the outcome is determined by the ratio of healthy and abnormal cells.
Some of the latter may continue to die, and we will never know about their existence. Or he can take part in the development of the organism, and then it will turn out mosaic- different cells will carry different genetic material. Mosaicism causes a lot of trouble for prenatal diagnosticians. For example, if there is a risk of having a child with Down syndrome, sometimes one or more cells of the embryo are removed (at a stage when this should not pose a danger) and the chromosomes in them are counted. But if the embryo is mosaic, then this method becomes not particularly effective.
Third wheel
All cases of aneuploidy are logically divided into two groups: deficiency and excess of chromosomes. The problems that arise with a deficiency are quite expected: minus one chromosome means minus hundreds of genes.
If the homologous chromosome works normally, then the cell can get away with only an insufficient amount of the proteins encoded there. But if some of the genes remaining on the homologous chromosome do not work, then the corresponding proteins will not appear in the cell at all.
In the case of an excess of chromosomes, everything is not so obvious. There are more genes, but here - alas - more does not mean better.
Firstly, excess genetic material increases the load on the nucleus: an additional strand of DNA must be placed in the nucleus and served by information reading systems.
Scientists have discovered that in people with Down syndrome, whose cells carry an extra 21st chromosome, the functioning of genes located on other chromosomes is mainly disrupted. Apparently, an excess of DNA in the nucleus leads to the fact that there are not enough proteins to support the functioning of chromosomes for everyone.
Secondly, the balance in the amount of cellular proteins is disrupted. For example, if activator proteins and inhibitor proteins are responsible for some process in a cell, and their ratio usually depends on external signals, then an additional dose of one or the other will cause the cell to stop responding adequately to the external signal. Finally, an aneuploid cell has an increased chance of dying. When DNA is duplicated before division, errors inevitably occur, and the cellular repair system proteins recognize them, repair them, and start doubling again. If there are too many chromosomes, then there are not enough proteins, errors accumulate and apoptosis is triggered - programmed cell death. But even if the cell does not die and divides, then the result of such division will also most likely be aneuploids.
You will live
If even within one cell aneuploidy is fraught with malfunctions and death, then it is not surprising that it is not easy for an entire aneuploid organism to survive. On this moment Only three autosomes are known - the 13th, 18th and 21st, trisomy for which (that is, an extra, third chromosome in cells) is somehow compatible with life. This is likely due to the fact that they are the smallest and carry the fewest genes. At the same time, children with trisomy on the 13th (Patau syndrome) and 18th (Edwards syndrome) chromosomes live at best up to 10 years, and more often live less than a year. And only trisomy on the smallest chromosome in the genome, the 21st chromosome, known as Down syndrome, allows you to live up to 60 years.
People with general polyploidy are very rare. Normally, polyploid cells (carrying not two, but from four to 128 sets of chromosomes) can be found in the human body, for example, in the liver or red bone marrow. These are usually large cells with enhanced protein synthesis that do not require active division.
An additional set of chromosomes complicates the task of their distribution among daughter cells, so polyploid embryos, as a rule, do not survive. Nevertheless, about 10 cases have been described in which children with 92 chromosomes (tetraploids) were born and lived from several hours to several years. However, as in the case of other chromosomal abnormalities, they lagged behind in development, including mental development. However, many people with genetic abnormalities come to the aid of mosaicism. If the anomaly has already developed during the fragmentation of the embryo, then a certain number of cells may remain healthy. In such cases, the severity of symptoms decreases and life expectancy increases.
Gender injustices
However, there are also chromosomes, the increase in the number of which is compatible with human life or even goes unnoticed. And these, surprisingly, are sex chromosomes. The reason for this is gender injustice: approximately half of the people in our population (girls) have twice as many X chromosomes as others (boys). At the same time, the X chromosomes not only serve to determine sex, but also carry more than 800 genes (that is, twice as many as the extra 21st chromosome, which causes a lot of trouble for the body). But girls come to the aid of a natural mechanism for eliminating inequality: one of the X chromosomes is inactivated, twists and turns into a Barr body. In most cases, the choice occurs randomly, and in some cells the result is that the maternal X chromosome is active, while in others the paternal one is active. Thus, all girls turn out to be mosaic, because different copies of genes work in different cells. A classic example of such mosaicism is tortoiseshell cats: on their X chromosome there is a gene responsible for melanin (a pigment that determines, among other things, coat color). Different copies work in different cells, so the coloring is spotty and is not inherited, since inactivation occurs randomly.
As a result of inactivation, only one X chromosome is always active in human cells. This mechanism allows you to avoid serious troubles with X-trisomy (XXX girls) and Shereshevsky-Turner syndrome (XO girls) or Klinefelter (XXY boys). About one in 400 children is born this way, but vital functions in these cases are usually not significantly impaired, and even infertility does not always occur. It is more difficult for those who have more than three chromosomes. This usually means that the chromosomes did not separate twice during the formation of sex cells. Cases of tetrasomy (ХХХХ, ХХYY, ХХХY, XYYY) and pentasomy (XXXXX, XXXXY, XXXYY, XXYYY, XYYYY) are rare, some of them have been described only a few times in the history of medicine. All of these options are compatible with life, and people often live to an advanced age, with abnormalities manifested in abnormal skeletal development, genital defects, and decreased mental abilities. Typically, the additional Y chromosome itself does not significantly affect the functioning of the body. Many men with the XYY genotype do not even know about their peculiarity. This is due to the fact that the Y chromosome is much smaller than the X and carries almost no genes that affect viability.
Sex chromosomes also have one more interesting feature. Many mutations of genes located on autosomes lead to abnormalities in the functioning of many tissues and organs. At the same time, most gene mutations on sex chromosomes manifest themselves only in impaired mental activity. It turns out that sex chromosomes largely control brain development. Based on this, some scientists hypothesize that they are responsible for the differences (however, not fully confirmed) between the mental abilities of men and women.
Who benefits from being wrong?
Despite the fact that medicine has been familiar with chromosomal abnormalities for a long time, Lately aneuploidy continues to attract scientific attention. It turned out that more than 80% of tumor cells contain an unusual number of chromosomes. On the one hand, the reason for this may be the fact that proteins that control the quality of division can slow it down. In tumor cells, these same control proteins often mutate, so restrictions on division are lifted and chromosome checking does not work. On the other hand, scientists believe that this may serve as a factor in the selection of tumors for survival. According to this model, tumor cells first become polyploid, and then, as a result of division errors, they lose different chromosomes or parts thereof. This results in a whole population of cells with great variety chromosomal abnormalities. Most are not viable, but some may succeed by chance, for example if they accidentally gain extra copies of genes that trigger division or lose genes that suppress it. However, if the accumulation of errors during division is further stimulated, the cells will not survive. The action of taxol, a common cancer drug, is based on this principle: it causes systemic chromosome nondisjunction in tumor cells, which should trigger their programmed death.
It turns out that each of us may be a carrier of extra chromosomes, according to at least in individual cells. However modern science continues to develop strategies to deal with these unwanted passengers. One of them suggests using proteins responsible for the X chromosome and targeting, for example, the extra 21st chromosome of people with Down syndrome. It is reported that this mechanism was activated in cell cultures. So, perhaps, in the foreseeable future, dangerous extra chromosomes will be tamed and rendered harmless.
Idiogram of the 2nd human chromosome. The 2nd human chromosome is one of 23 human chromosomes and the second largest, one of 22 human autosomes. The chromosome contains more than 242 million base pairs... Wikipedia
Idiogram of the 22nd human chromosome The 22nd human chromosome is one of 23 human chromosomes, one of 22 autosomes and one of 5 acrocentric human chromosomes. The chromosome contains o... Wikipedia
Idiogram of the 11th human chromosome. The 11th human chromosome is one of 23 pairs of human chromosomes. The chromosome contains almost 139 million base pairs... Wikipedia
Idiogram of human chromosome 12. Human chromosome 12 is one of 23 human chromosomes. The chromosome contains almost 134 million base pairs... Wikipedia
Idiogram of the 21st human chromosome The 21st human chromosome is one of 23 human chromosomes (in the haploid set), one of 22 autosomes and one of 5 acrocentric human chromosomes. The chromosome contains about 48 million base pairs, which ... Wikipedia
Idiogram of the 7th human chromosome. The 7th human chromosome is one of 23 human chromosomes. The chromosome contains more than 158 million base pairs, which is from 5 to 5.5% ... Wikipedia
Idiogram of the 1st human chromosome. The 1st human chromosome is the largest of the 23 human chromosomes, one of the 22 human autosomes. The chromosome contains about 248 million base pairs... Wikipedia
Idiogram of the 3rd human chromosome. The 3rd human chromosome is one of 23 human chromosomes, one of 22 human autosomes. The chromosome contains almost 200 million base pairs... Wikipedia
Idiogram of the 9th human chromosome. The 9th human chromosome is one of the chromosomes of the human genome. Contains about 145 million base pairs, making up 4% to 4.5% of all cellular DNA material. According to different estimates... Wikipedia
Idiogram of human chromosome 13. Human chromosome 13 is one of 23 human chromosomes. The chromosome contains more than 115 million base pairs, which is from 3.5 to 4% of the total material ... Wikipedia
Idiogram of human chromosome 14. Human chromosome 14 is one of 23 human chromosomes. The chromosome contains approximately 107 million base pairs, which is from 3 to 3.5% of the total material ... Wikipedia
Books
- Telomere effect. A Revolutionary Approach to Living Younger, Healthier, Longer, Elizabeth Helen Blackburn, Elissa Epel. What is this book about? For life to continue, the cells of the body must continuously divide, creating their exact copies - young and full of energy. They, in turn, also begin to share. So…
Item genetic research- phenomena of heredity and variability. American scientist T-X. Morgan created the chromosomal theory of heredity, which proves that each biological species can be characterized by a specific karyotype, which contains such types of chromosomes as somatic and sex chromosomes. The latter are represented by a separate pair, distinguished by male and female individuals. In this article we will study what structure female and male chromosomes have and how they differ from each other.
What is a karyotype?
Each cell containing a nucleus is characterized by a certain number of chromosomes. It is called a karyotype. Various biological species the presence of structural units of heredity is strictly specific, for example, the human karyotype is 46 chromosomes, chimpanzees have 48, crayfish- 112. Their structure, size, shape differ in individuals belonging to different systematic taxa.
The number of chromosomes in a body cell is called the diploid set. It is characteristic of somatic organs and tissues. If as a result of mutations the karyotype changes (for example, in patients with Klinefelter syndrome the number of chromosomes is 47, 48), then such individuals have reduced fertility and in most cases are infertile. Other hereditary disease associated with sex chromosomes - Turner-Shereshevsky syndrome. It occurs in women who have 45 rather than 46 chromosomes in their karyotype. This means that in a sexual pair there are not two X chromosomes, but only one. Phenotypically, this manifests itself in underdevelopment of the gonads, weakly expressed secondary sexual characteristics and infertility.
Somatic and sex chromosomes
They differ both in shape and in the set of genes that make up them. The male chromosomes of humans and mammals are included in the heterogametic sexual pair XY, which ensures the development of both primary and secondary male sexual characteristics.
In male birds, the sexual pair contains two identical ZZ male chromosomes and is called homogametic. Unlike chromosomes that determine the sex of an organism, the karyotype contains hereditary structures that are identical in both males and females. They are called autosomes. There are 22 pairs of them in the human karyotype. Sexual male and female chromosomes form 23 pairs, so a man’s karyotype can be represented as a general formula: 22 pairs of autosomes + XY, and women - 22 pairs of autosomes + XX.
Meiosis
The formation of germ cells - gametes, the fusion of which forms a zygote, occurs in the sex glands: testes and ovaries. In their tissues, meiosis occurs - the process of cell division leading to the formation of gametes containing a haploid set of chromosomes.
Oogenesis in the ovaries leads to the maturation of eggs of only one type: 22 autosomes + X, and spermatogenesis ensures the maturation of two types of gomets: 22 autosomes + X or 22 autosomes + Y. In humans, the sex of the unborn child is determined at the moment of fusion of the nuclei of the egg and sperm and depends from the karyotype of the sperm.
Chromosomal mechanism and sex determination
We have already looked at the moment at which sex is determined in a person - at the moment of fertilization, and it depends on the chromosomal set of the sperm. In other animals, representatives of different sexes differ in the number of chromosomes. For example, in marine worms, insects, and grasshoppers, in the diploid set of males there is only one chromosome from the sexual pair, and in females - both. So, the haploid set of chromosomes of a male sea worm Acirocanthus can be expressed by the formulas: 5 chromosomes + 0 or 5 chromosomes + x, and females have only one set of 5 chromosomes + x in their eggs.
What influences sexual dimorphism?
In addition to chromosomal, there are other ways to determine sex. In some invertebrates - rotifers - sex is determined even before the fusion of gametes - fertilization, as a result of which male and female chromosomes form homologous pairs. Females of the marine polychaete Dinophyllus produce two types of eggs during oogenesis. The first ones are small, depleted in yolk, and males develop from them. Others - large, with a huge supply of nutrients - serve for the development of females. In honeybees - insects of the Hymenoptera series - females produce two types of eggs: diploid and haploid. From unfertilized eggs, males develop - drones, and from fertilized eggs - females, who are worker bees.
Hormones and their effect on gender formation
In humans, male glands - the testes - produce sex hormones such as testosterone. They influence both development (anatomical structure of the external and internal genital organs) and physiological features. Under the influence of testosterone, secondary sexual characteristics are formed - skeletal structure, figure features, body hair, timbre of voice. In a woman’s body, the ovaries produce not only sex cells, but also hormones, being Sex hormones, such as estradiol, progesterone, estrogen, contribute to the development of external and internal genital organs, body hair growth female type, regulate menstrual cycle and the course of pregnancy.
In some vertebrates, fish, and amphibians, biologically active substances produced by the gonads strongly influence the development of primary and secondary sexual characteristics, but the types of chromosomes do not have such a great impact on the formation of sex. For example, the larvae of marine polychaetes - Bonellias - under the influence of female sex hormones stop their growth (size 1-3 mm) and become dwarf males. They live in the genital tract of females, which have a body length of up to 1 meter. In cleaner fish, males maintain harems of several females. Females, in addition to the ovaries, have the rudiments of the testes. As soon as the male dies, one of the harem females takes over his function (male gonads that produce sex hormones begin to actively develop in her body).
Sex regulation
It is carried out by two rules: the first determines the dependence of the development of the rudimentary gonads on the secretion of testosterone and the hormone MIS. The second rule indicates the exceptional role played by the Y chromosome. The male sex and all the anatomical and physiological characteristics corresponding to it develop under the influence of genes located on the Y chromosome. The interrelation and dependence of both rules in human genetics is called the principle of growth: in an embryo that is bisexual (that is, having the rudiments of the female glands - the Müllerian duct and the male gonads - the Wolffian canal), the differentiation of the embryonic gonad depends on the presence or absence of the Y chromosome in the karyotype.
Genetic information on the Y chromosome
Research by genetic scientists in particular T-X. Morgan, it was found that in humans and mammals the gene composition of the X and Y chromosomes is not the same. Human male chromosomes lack some of the alleles present on the X chromosome. However, their gene pool contains the SRY gene, which controls spermatogenesis, leading to the formation of the male sex. Hereditary disturbances of this gene in the embryo lead to the development of a genetic disease - Swire's syndrome. As a result, the female individual developing from such an embryo contains in the XY karyotype a sexual pair or only a section of the Y chromosome containing the gene locus. It activates the development of gonads. In sick women, secondary sexual characteristics are not differentiated and they are infertile.
Y chromosome and hereditary diseases
As noted earlier, the male chromosome differs from the X chromosome both in size (it is smaller) and in shape (it looks like a hook). The set of genes is also specific to it. Thus, a mutation in one of the genes on the Y chromosome is phenotypically manifested by the appearance of a tuft of coarse hair on the earlobe. This sign is typical only for men. There is a known hereditary disease called Klinefelter syndrome. A sick man has extra female or male chromosomes in his karyotype: XXY or XXYY.
The main diagnostic signs are pathological growth of the mammary glands, osteoporosis, and infertility. The disease is quite common: for every 500 newborn boys, there is 1 patient.
To summarize, we note that in humans, as in other mammals, the sex of the future organism is determined at the moment of fertilization, due to a certain combination of sex X and Y chromosomes in the zygote.