Sex-linked inheritance (definition)

In the greater part of organisms (with rare exceptions), the future belonging to a particular biological sex is determined at the time when fertilization occurs, and depends on which sex chromosomes fall into the zygote. In this case, there are some signs that can be inherent only to males or females of a certain type. Such signs seem to be inextricably linked to one of the sexes, "linked" to the sex. What is sex-linked inheritance and for what reasons do only men or exclusively women suffer from certain diseases? We will try to answer this difficult question.

A boy or a girl?

Before we begin to answer the question of what sex-linked inheritance is, it is necessary to understand how the formation of a female or a male organism takes place. Since the readers of this article refer to the form of Homo sapiens, they will most likely find it most interesting to learn how this process is carried out by people. It is necessary to understand what gender genetics is: the inheritance of characters linked to the sex proceeds according to a rather specific mechanism, and it is necessary to understand the basic laws of the formation of organisms of different sexes.

So, in any human cell there are 46 chromosomes. Of these, 22 pairs are identical in the representatives of different biological sex. Such chromosomes are called "autosomes". And only one pair - sex chromosomes - in 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, the sex chromosomes have a different structure: one of them is an X chromosome, the second is a Y (igrok) - a chromosome. By the way, the Y chromosome carries an insignificant number of genes. The sex-linked type of inheritance depends on the genes that are dislocated on the sex chromosomes.

Similarly, sex inheritance is not carried out in 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 development of the female body requires the presence of the Y chromosome, while the males, on the other hand, are carriers of two X chromosomes.

Homogamety and heterogamety

For what reasons do two X chromosomes "get" one zygote and the other receive the Y chromosome? This is due to the fact that during the maturation of germ cells, that is meiosis, all eggs receive 22 "ordinary" chromosomes and one sex X chromosome. That is, in the female body gametes are the same. Spermatozoa exist in two types: exactly half of them carry the sex X chromosome, while others "get" the Y chromosome.

The sex that forms identical chromosomes on the sex gamete is called "homogametic". If the gametes are different, it is heterogametic. Men have heterogametic men, women are homogamous.

Which sperm will impregnate an egg depends on chance. Thus, with a probability of 50/50, the zygote will receive two X-chromosomes or X- and Y-chromosomes. Naturally, in the first case the girl will develop, in the second - the boy. Of course, there may be some exceptions: under certain circumstances, girls have a 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, these are the rarest cases.

Why the sign can be "linked" to the floor?

Now that readers know what gender genetics are, inheritance linked to the sex will be a more understandable mechanism. Chromosomes are unique media: at conception each human organism becomes the owner of a "library" of 46 volumes, in which all the characteristics and characteristics of his organism are described in detail. The amount of information that is contained in every human cell reaches 1.5 gigabytes! Moreover, each chromosome contains a certain part of it: eye and hair color, finger dexterity, short-term memory volume, propensity to fullness ... Chromosomes have a narrow specialization: some are responsible for metabolism, others for eye color or speed of nervous processes. However, by and large, the genes contain information about protein molecules - enzymes, which determine the features of the functioning of the human body.

Part of the hereditary information is encoded by the sex chromosomes. So, the signs that are "recorded" in these chromosomes can be transmitted only to representatives of one biological sex: this is the inheritance of genes linked to sex. Thus, one can draw an important conclusion. If the gene is located on one of the sex chromosomes, then biologists talk about a phenomenon such as the inheritance of traits linked to the sex. In this case, the inheritance of such features has a number of features: they will differently manifest themselves in heterozygous organisms.

First studies

Genetics noted that the inheritance of the color of the ocular facets in Drosophila is directly dependent on the sex of the individuals who own the different alleles. The gene that determines the development of red eyes is dominant over the "white-skin" gene. If the male has red eyes and the female has white eyes, then in the first generation of the offspring, an equal number of females with reds and males with white eyes is obtained. If you get offspring from a male with white eyes and a red-eyed female, you will 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 eye coloring gene is located on the X chromosome, which means that the inheritance of eye color in Drosophila is an inheritance of characters linked to the sex.

Specificity of sex-linked inheritance

Gender-linked inheritance has certain characteristics. They are related to the fact that the Y chromosome carries a smaller number of genes than the X chromosome. So, for many genes that are located on the X chromosome, which 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 it will necessarily appear in the phenotype.

It is worth explaining the above. Allele is a kind 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 recessive - only if it is represented in the homozygous state. For illustration, you can inherit the color of the eyes in humans. The color depends on the amount in the iris of the melanin pigment. If the melanin is small, the eyes will be bright, if a lot - dark. In this case, the dark eye color corresponds to the dominant allele: if it is present in the genome, the child will be brown-eyed. It does not matter whether the two genes are dominant or one of them is recessive, that is, whether the child is homozygous or heterozygous for this. But the blue eyes - 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 the color of the eyes. The dominant gene, as its name suggests, dominates the recessive one: if in the genome of the child one gene regulating the color of the iris, the dominant, recessive allele can not manifest itself phenotypically.

However, the 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 Y. Therefore, some of the genes that are located on it will show themselves even if there is only one recessive Copy: this inheritance, linked to the sex, is different from other types of inheritance.

Recessive X-linked diseases

Inheritance, linked to the sex, is of great importance for medical genetics, because at the moment there are about three hundred recessive genes that are localized in the X chromosome and cause hereditary diseases. Such diseases include hemophilia, Duchenne myopathy, ichthyosis, fragile chromosome X syndrome, hydrocephalus and many other diseases.

Sex inherited from the sex in man is carried out as follows. If the pathological gene is located in one of the X chromosomes of a woman, half of her daughters and half of her sons will receive it. At the same time, girls whose genome is a defective chromosome will become carriers of the disease: the phenotype does not affect the mutant gene, as 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 in the Y chromosome. This factor is important to consider, solving the problem of sex-linked inheritance.

The inheritance of X-linked recessive diseases is rather complicated. For example, among relatives of the patient, 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 representatives of both sexes. You can give examples of inheritance, linked to the sex, like hypophosphatemic rickets and dark enamel of teeth.

Suffering a dominant X-linked disease, women are always twice as many as men. A sick woman with a probability of 50% transmits the disease to all of her children, and a sick man - exclusively to daughters.

Sometimes sex-linked inheritance Can be observed 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, linked to the sex. Let us dwell on such a disease as pigment incontinence, or Bloch-Sulzberg melanoblastosis. Incontinence of pigment is found only in women: the gene is flying for a male fetus. On the body of girls suffering from this disease, there are rashes in the form of vesicles. After the rashes pass, the skin remains characteristic pigmentation in the form of splashes and vortices. 80% of patients have other disorders: malformations of the brain and internal organs, eyes and bone system.

Hemophilia

There are a number of hereditary diseases that occur only in one sex. As an example of what inheritance is, linked to sex, hemophilia is given almost in every textbook. Hemophilia is a disease in which blood does not coagulate. Hemophilia is devoted to many tasks on sex-linked inheritance: we can say that this example is almost a textbook. Despite the fact that hemophilia is rare enough, almost everyone knows about its existence. And this is not surprising: it largely predetermined the course of Russian history. She suffered the heir of Tsar Nicholas II Tsarevich Alexei.

Even a small cut can cause severe blood loss. However, it is dangerous not only bleeding related to the violation of the integrity of the skin. Serious threat to the life of patients are intramuscular, intracranial and throat bleeding, as well as bleeding in the joints. At the same time, hemophilia is incurable: patients are forced to take substitutive drugs all their lives, which are not highly effective.

Do women have hemophilia?

Many mistakenly believe that women never suffer from hemophilia. However, this is not true: the fair sex is also sick with this terrible disease. True, this happens very rarely. Much more often, women are carriers of the gene that predisposes this disease.

A female bearer in one of the sex chromosomes has a recessive gene, which predetermines blood coagulability. If the genotype contains a dominant allele of this gene, the disease will not manifest itself. However, in the event that a woman is born a boy, to whom the recessive gene will pass, he will develop hemophilia. But that the hemophilia developed in the girl, it is necessary that the female carrier of the gene marries a man suffering from hemophilia (but even in such circumstances the probability of the birth of such a girl will be equal to 25%). Such cases are almost not observed: firstly, the gene of hemophilia is quite rare, and secondly, people suffering from this dangerous disease, extremely rarely live to reproductive age.

The value of the inheritance study of sex-linked traits

The study of how the inheritance of sex-linked characteristics takes place is of great importance for mankind. This is due primarily to the fact that linked to the sex inherited many diseases, including those dangerous to human life. Perhaps in the future it will be possible to create innovative means of treating such diseases, based on the effect directly on the genome of the person who is the carrier of the pathological allele. In addition, such studies play an important role in developing methods for 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 occupies the most 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 signs, the inheritance of which is due to sex chromosomes. What is sex-linked inheritance? The definition says that it is the transfer of features for which genes located in sex chromosomes are responsible.