In most of the organisms (for rarewith the exception of) the future belonging to one or another biological sex is determined at the moment when fertilization occurs, and depends on which sex chromosomes are in the zygote. At the same time there are some signs that can be inherent only to males or females of a certain type. Such signs are as if inextricably linked with one of the sexes, “linked” with the floor. What is the sex-linked inheritance and for what reasons do only men or only women suffer from certain diseases? We will try to answer this difficult question.
A boy or a girl?
Before you answer the question thatsuch a sex-linked inheritance, it is necessary to understand how the formation of the female or male body occurs. Since the readers of this article refer to the mind of Homo sapiens, they will most likely find it most interesting to learn how this process is carried out in humans. It is necessary to understand what gender genetics is: the inheritance of sex-linked traits proceeds according to a rather specific mechanism, and the basic laws of the formation of organisms of different sexes must be understood.
Итак, в любой человеческой клетке имеется по 46 chromosomes. Of these, representatives of different biological sexes are identical to 22 pairs. Such chromosomes are called autosomes. And only one pair - the sex chromosomes - in the representatives of the strong and fair sex have some differences. In women, the sex chromosomes are identical: they have two X (X) - chromosomes. In men, sex chromosomes have a different structure: one of them is the X chromosome, the second - Y (play) - the chromosome. By the way, the Y chromosome carries a small number of genes. The sex-linked type of inheritance depends on the genes that are located on the sex chromosomes.
Подобным образом наследование пола осуществляется far from all species. In some birds, the male does not carry the Y chromosome in its genome: only the maternal X chromosome passes to them. There are also species in which the Y chromosome is necessary for the development of the female body, while the males, on the contrary, are carriers of two X chromosomes.
Homogame and heterogame
For some reason, some zygotes "get"two X chromosomes, while others get the Y chromosome? This is due to the fact that during the maturation of the germ cells, that is, meiosis, all 22 eggs receive 22 "normal" chromosomes and one sex X chromosome. That is, in the female body gametes are the same. There are two types of spermatozoa: exactly half of them carry the sex X chromosome, while others “receive” the Y chromosome.
The floor that forms chromosomes identical in the sexual gamete is called “homogametic.” If the gametes are different - it is heterogametic. In humans, men are heterogametic, women are homogametic.
What kind of sperm will fertilizeovum depends on chance. Thus, with a 50/50 probability, the zygote will receive two X chromosomes or X and Y chromosomes. Naturally, in the first case a girl will develop, in the second - a boy. Of course, there may be some exceptions: under certain circumstances, girls have the Y chromosome in the genome or are carriers of an incomplete set of genes, that is, there is only one sex chromosome in their cells. However, this is the rarest of cases.
Why can a sign be “linked” to the floor?
Now that readers know what genetics aresex, sex-linked inheritance will be a clearer mechanism. Chromosomes are unique information carriers: at conception, each human body becomes the owner of a “library” of 46 volumes in which all the characteristics and characteristics of its organism are described in great detail. The amount of information that is contained in each cell of a person reaches 1.5 gigabytes! Moreover, each chromosome contains a certain part of it: the color of the eyes and hair, finger dexterity, the volume of short-term memory, the tendency to be complete ... Chromosomes have a narrow specialization: some are responsible for the metabolism, others are for the color of the eyes or the speed of the nervous processes. However, by and large, the genes contain information about protein molecules - enzymes, which determine the characteristics of the functioning of the human body.
Some of the hereditary information is encoded andsex chromosomes. This means that the signs that are “recorded” in these chromosomes can be transmitted only to members of the same biological sex: this is the genetically linked inheritance of genes. Thus, an important conclusion can be made. If the gene is located on one of the sex chromosomes, then biologists talk about such a phenomenon as the inheritance of sex-linked traits. At the same time, the inheritance of such features has a number of features: they will manifest themselves in different ways in opposite-sex organisms.
First studies
Genetics have noted that the inheritance of eye colorDrosophila facets are directly dependent on the sex of individuals that are owners of different alleles. The gene that determines the development of red eyes is dominant over the white-eyed gene. If the male has red eyes, and the female has white, then in the first generation of the offspring an equal number of females with red and males with white eyes are obtained. If you get offspring from a male with white eyes and a red-eyed female, you get an equal number of red-eyed females and males. Thus, in males the recessive trait in the phenotype reveals itself more often than in females. This led to the conclusion that the gene for eye color is located on the X chromosome, which means that the inheritance of the eye color in Drosophila is the inheritance of sex-linked traits.
Specificity of sex-linked inheritance
Sex-linked inheritance hascertain features. They are connected with the fact that the Y chromosome carries fewer genes than the X chromosome. So, for many genes that are located on the X chromosome that is in the male body, there are no alleles on the Y chromosome. Therefore, any recessive gene that appears on the X chromosome may not have an allelic gene on the Y chromosome, which means that it will necessarily appear in the phenotype.
It is worth explaining the above.An allele is a variation of a single gene. The allele can be of two main types: dominant and recessive. In this case, the dominant allele in the genotype necessarily manifests itself phenotypically, and the recessive - only if it is presented in a homozygous state. To illustrate, we can cite the inheritance of human eye color. Color depends on the amount of melanin in the iris. If there is little melanin, the eyes will be bright, if many are dark. At the same time, the dominant allele is responsible for the dark eye color: if it is present in the genome, then the child will be brown-eyed. It does not matter whether both genes are dominant or one of them is recessive, that is, whether the child is homo-or heterozygous for this trait. But blue eyes are a relatively “young” mutation that is regulated by a recessive allele. All light-eyed people are carriers of two recessive alleles, that is, they are homozygous for eye color. The dominant gene, as is clear from its name, dominates the recessive one: if the genome of a child has one gene regulating the color of the iris, the dominant, recessive allele cannot manifest itself phenotypically.
However, the sex chromosomes in this case arethe exception. The male Y chromosome differs from the X chromosome in its size and shape: it really does not resemble a cross, like the other chromosomes, but the letter U. Therefore, some of the genes that are located on it will manifest themselves even if there is only one recessive copy: by this, sex-linked inheritance is different from other types of inheritance.
Recessive X-linked diseases
Наследование, сцепленное с полом, имеет огромное important for medical genetics, because at the moment there are about three hundred recessive genes that are localized on the X chromosome and cause hereditary diseases. Such diseases include hemophilia, Duchenne myopathy, ichthyosis, fragile chromosome X syndrome, hydrocephalus and many other diseases.
Sex-linked inheritance in humansas follows. If the pathological gene is located in one of the X chromosomes of a woman, then half of the daughters and half of the sons will receive it. At the same time, girls, in whose genome there was a defective chromosome, will become carriers of the disease: the mutant gene will not be affected by the phenotype, since the daughter receives a normal X chromosome from the father. But the boy will suffer from a disease that he "inherited" from his mother, because there is no allelic dominant gene in the Y chromosome. This factor is important to consider when solving problems on sex-linked inheritance.
Inheritance of X-linked recessive diseasesis rather complicated. For example, among relatives of a patient, a similar disease is usually found in the maternal uncle and in cousins who are born from mother’s sisters.
Dominate X-linked diseases
These diseases can develop in both sexes. Examples of floor-linked inheritance such as hypophosphatemic rickets and dark tooth enamel can be given.
Sufferers of dominant X-linked diseasethere are always two times more women than men. A sick woman with a probability of 50% transfers the disease to all her children, and a sick man only to her daughters.
Sometimes sex-linked inheritance can occur with fairly rare diseases that are lethal to male fetuses, so women are more likely to have spontaneous abortions.
You can endlessly give examples of inheritance,coupled to the floor. Let us dwell on such a disease as pigment incontinence, or Bloch-Sulzberg melanoblastosis. Pigment incontinence is found only in women: the gene is lethal to the male fetus. On the body of girls suffering from this disease, there are rashes in the form of bubbles. After the rashes, the skin remains a characteristic pigmentation in the form of splashes and turbulence. 80% of patients have other disorders: malformations of the brain and internal organs, eyes and bone system.
Hemophilia
There are a number of hereditary diseaseswhich are found only in one sex. Hemophilia is cited in almost every textbook as an example of what sex-related inheritance is. Hemophilia is a disease in which the blood does not clot. Hemophilia is devoted to many tasks on sex-linked inheritance: we can say that this example is almost textbook. Despite the fact that hemophilia is quite rare, almost everyone knows about its existence. And this is not surprising: in many respects it predetermined the course of Russian history. She suffered Tsarev Nicholas II's heir, Tsarevich Alexei.
Даже незначительный порез при этом может стать cause severe blood loss. However, not only bleeding associated with the violation of the integrity of the skin is dangerous. Intraintestinal, intracranial and throat bleeding, as well as bleeding into the joints, pose a serious threat to the lives of patients. At the same time, hemophilia is incurable: patients are forced all their life to take replacement drugs that are not very effective.
Do women have hemophilia?
Many mistakenly believe that women neversuffer from hemophilia. However, this is not so: the fair sex also suffer from this terrible disease. True, this happens extremely rarely. More often than not, women are carriers of the gene that predetermines this disease.
In a female bearer in one of the genitalchromosomes there is a recessive gene, which determines blood clotting. If the genotype has a dominant allele of this gene, the disease will not manifest. However, if a woman gives birth to a boy to whom the recessive gene passes, he will manifest hemophilia. But for hemophilia to develop in a girl, it is necessary that the woman carrying the gene marry a man suffering from hemophilia (but even in such circumstances, the probability of having such a girl will be equal to 25%). Such cases are practically not observed: firstly, the hemophilia gene is quite rare, and secondly, people suffering from this dangerous disease rarely live to reproductive age.
The value of the study of inheritance of sex-linked traits
Research on howinheritance of sexually coupled traits is of paramount importance to humanity. This is primarily due to the fact that many diseases, including those that are dangerous to human life, are inherited from the sex-linked one. Perhaps in the future it will be possible to create innovative means of treating such diseases, based on the impact directly on the human genome, which is the carrier of the pathological allele. In addition, such studies play a large role in the development of methods for the rapid diagnosis of sex-linked diseases. This is especially important for diseases that are associated with metabolic disorders: the earlier the therapy is started, the more success will be achieved.
Genetics is the most important place in lifeperson After all, it is this science that makes it possible to explain how human genes function, including those that carry pathological signs. There is a group of signs, the inheritance of which is due to sex chromosomes. What is sex-linked inheritance? The definition says that it is the transfer of traits for which genes located in sex chromosomes are responsible.
