In most organisms (with rare exceptions), 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 may be inherent only to males or females of a certain species. Such signs seem to be inextricably linked with one of the sexes, "linked" to the floor. What is 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.
Boy or girl?
Before starting to answer the question of what sex-linked inheritance is, it is necessary to understand how the female or male organism is formed. Since the readers of this article belong to the species Homo sapiens, they will probably be most interested in learning about how this process is carried out in humans. It is necessary to understand what the genetics of sex is: the inheritance of sex-linked traits proceeds according to a rather specific mechanism, and it is necessary to understand the basic laws of formationorganisms of different sexes.
So, every human cell has 46 chromosomes. Of these, 22 pairs are identical in representatives of different biological sexes. These chromosomes are called autosomes. And only one pair - the sex chromosomes - in the representatives of the stronger and fairer sex have some differences. In women, the sex chromosomes are identical: they have two X (X) - chromosomes. In men, the sex chromosomes have a different structure: one of them is the X chromosome, the second is the Y (y) 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.
Similarly, sex is not inherited in all species. In some birds, the male does not carry the Y chromosome in his genome at all: only the maternal X chromosome passes to them. There are also species in which the presence of a Y chromosome is necessary for the development of the female body, while males, on the contrary, are carriers of two X chromosomes.
Homogamety and heterogamety
For what reasons do some zygotes "get" two X chromosomes, while others get a Y chromosome? This is due to the fact that during the maturation of germ cells, that is, meiosis, all eggs receive 22 “normal” chromosomes and one sex X chromosome. That is, in the female body, the gametes are the same. Spermatozoa, on the other hand, exist in two types: exactly half of them carry the sex X chromosome, while the others "receive" the Y chromosome.
Sex, forming identical sex gameteschromosomes is called homogametic. If the gametes are different, then it is heterogametic. Humans have heterogametic males, while females are homogametic.
Which sperm will fertilize an egg depends on chance. Thus, with a probability of 50/50, the zygote will receive two X chromosomes, or an X and a Y chromosome. 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 a Y chromosome in their genome or are carriers of an incomplete set of genes, that is, there is only one sex chromosome in their cells. However, these are the rarest cases.
Why can a trait be sex-linked?
Now that readers know what the genetics of sex is, sex-linked inheritance will be a more understandable mechanism. Chromosomes are unique carriers of information: at conception, each human organism becomes the owner of a "library" of 46 volumes, in which all the characteristics and features of its organism are described in detail. The amount of information contained in each human cell reaches 1.5 gigabytes! Moreover, each chromosome contains a certain part of it: eye and hair color, finger dexterity, short-term memory, a tendency to be overweight … Chromosomes have a narrow specialization: some are responsible for metabolism, others for eye color or the speed of nervous processes … However, by and large genes contain information about protein molecules -enzymes that determine the functioning of the human body.
Sex chromosomes also encode part of hereditary information. This means that the traits that are “recorded” in these chromosomes can only be transmitted to representatives of the same biological sex: this is the sex-linked inheritance of genes. Thus, an important conclusion can be drawn. 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 traits has a number of features: they will manifest themselves differently in heterosexual organisms.
First studies
Geneticists have noted that the inheritance of the color of the eye facets in Drosophila is 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 eyes, then in the first generation of offspring an equal number of females with red and males with white eyes is obtained. If you get offspring from a male with white eyes and a red-eyed female, then you get an equal number of red-eyed females and males. Thus, in males, a recessive trait in the phenotype reveals itself more often than in females. This led to the conclusion that the eye color gene is located on the X chromosome, which means that the inheritance of eye color in Drosophila is the inheritance of sex-linked traits.
Specificity of sex-linked inheritance
Sex-linked inheritance has certain characteristics. They are connected with the fact that the Y chromosome carries fewer genes than the X chromosome. This means that for many genes that are located on the X chromosome, which ended up 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 one gene. An 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 allele - only if it is presented in the homozygous state. To illustrate, we can cite the inheritance of eye color in humans. The color depends on the amount of melanin pigment in the iris. If there is little melanin, the eyes will be light, if there is a lot - dark. At the same time, the dominant allele is responsible for the dark color of the eyes: 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, which 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 its name implies, dominates the recessive one: if there is onethe gene that regulates the color of the iris, the dominant, recessive allele cannot manifest itself phenotypically.
However, sex chromosomes are an exception in this case. The male Y chromosome differs from the X chromosome in its size and shape: it really does not resemble a cross, like the rest of the 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: this is how sex-linked inheritance differs from other types of inheritance.
Recessive X-linked diseases
Sex-linked inheritance is of great importance 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 X syndrome, hydrocephalus and many other diseases.
Sex-linked inheritance in humans is as follows. If the pathological gene is located on one of the woman's X chromosomes, then half of the daughters and half of the sons will receive it. At the same time, girls whose genome contains a defective chromosome will become carriers of the disease: the mutant gene will not affect the phenotype, since the daughter receives a normal X chromosome from her father. But the boy will suffer from a disease that he “inherited” from his mother, because there is no allelic dominant gene on the Y chromosome. It is important to take this factor into account when solving problems onsex-linked inheritance.
The inheritance of X-linked recessive diseases is rather complicated. For example, among the patient's relatives, a similar disease is usually found in the maternal uncle and in cousins who are born from the mother's sisters.
Dominant X-linked diseases
These diseases can develop in both sexes. Examples of sex-linked inheritance include hypophosphatemic rickets and dark tooth enamel.
Suffering from a dominant X-linked disease, women are always twice as many as men. A sick woman has a 50% chance of transmitting the disease to all her children, and a sick man only to his daughters.
Sometimes sex-linked inheritance can occur in rare enough diseases that are fatal to male fetuses, so women are more likely to have spontaneous abortions.
One could give endless examples of sex-linked inheritance. Let us dwell on such a disease as pigment incontinence, or Bloch-Sulzberg melanoblastosis. Pigment incontinence occurs only in women: the gene is lethal to the male fetus. On the body of girls suffering from this disease, rashes appear in the form of bubbles. After the rash passes, characteristic pigmentation in the form of splashes and swirls remains on the skin. 80% of patients also have other disorders: malformations of the brain and internal organs, eyes andskeletal system.
Hemophilia
There are a number of hereditary diseases that occur only in one sex. As an example of what sex-linked inheritance is, hemophilia is given in almost every textbook. Hemophilia is a disease in which the blood does not clot. Many tasks on sex-linked inheritance are devoted to hemophilia: we can say that this example is almost a textbook. Despite the fact that hemophilia is quite rare, almost everyone knows about its existence. And this is not surprising: it largely predetermined the course of Russian history. The heir to Tsar Nicholas II, Tsarevich Alexei, suffered from it.
Even a minor cut can cause severe blood loss. However, not only bleeding associated with a violation of the integrity of the skin is dangerous. A serious threat to the life of patients is intra-intestinal, intracranial and throat bleeding, as well as bleeding into the joints. At the same time, hemophilia is incurable: patients are forced to take replacement drugs all their lives, which are not very effective.
Do women get hemophilia?
Many mistakenly believe that women never suffer from hemophilia. However, this is not so: the fair sex also suffer from this terrible disease. True, this happens very rarely. Much more often, women are carriers of the gene that predetermines this disease.
A carrier woman in one ofsex chromosomes have a recessive gene that predetermines blood incoagulability. If the genotype has a dominant allele of this gene, the disease will not manifest itself. However, if a woman gives birth to a boy who passes on the recessive gene, he will develop hemophilia. But in order for a girl to develop hemophilia, a woman carrying the gene must marry a man with hemophilia (but even under such circumstances, the probability of having such a girl will be 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 importance of research into the inheritance of sex-linked traits
The study of how sex-linked traits are inherited is of great importance to humanity. This is primarily due to the fact that many diseases are inherited sex-linked, including those dangerous to human life. Perhaps in the future it will be possible to create innovative treatments for such diseases based on the impact directly on the genome of a person who is a carrier of a pathological allele. In addition, such studies play an important 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 therapy is started, the more success can be achieved.
Geneticsoccupies an important place in human life. 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 traits, the inheritance of which is due to the sex chromosomes. What is sex-linked inheritance? The definition says that this is the transfer of traits for which genes located on the sex chromosomes are responsible.
