In most organisms (with rare exceptions), future affiliation with a particular biological sex is determined at the moment fertilization occurs, and depends on which sex chromosomes are in the zygote. However, there are some signs that may be inherent only to males or females of a certain type. Such signs seem to be inextricably linked with one of the sexes, “linked” to the floor. What is gender-related inheritance, and for what reasons only men or only women suffer from certain diseases? We will try to answer this difficult question.
Boy or girl?
Before proceeding to answer the question of what gender-related inheritance is, it is necessary to understand how the formation of the female or male body occurs. Since the readers of this article refer 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 genetics of sex is: the inheritance of traits linked to sex proceeds according to a rather specific mechanism, and one must understand the basic laws of the formation of organisms of different sexes.
So, in any human cell there are 46 chromosomes. Of these, representatives of different biological sex have 22 identical pairs. 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, sex chromosomes have a different structure: one of them is the X chromosome, the second - the U (game) - chromosome. By the way, the Y chromosome carries a small number of genes. Gender-linked inheritance depends on genes that are located on the sex chromosomes.
Similarly, inheritance of the sex is far from being carried out by all species. In some birds, the male does not carry the U chromosome in his genome at all: 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 U chromosome, while males, on the contrary, are carriers of two X chromosomes.
Homogametosity and heterogamenity
For what reasons do one Zygotes get two X chromosomes, while others get the U 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 sexual X chromosome. That is, in the female body the gametes are the same. Sperm cells, however, exist of two types: exactly half of them carry the sex X chromosome, while others “receive” the U chromosome.
Sex, forming chromosomes identical in sex to gamete, is called “homogametic”. If the gametes are different, it is heterogametic. In men, men are heterogeneous, while women are homogeneous.
Which sperm fertilizes an egg 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 U-chromosome in the genome or are carriers of an incomplete set of genes, that is, in their cells there is only one sex chromosome. However, these are rare cases.
Why can a sign be “linked” to the floor?
Now that readers know what gender genetics are, gender- related inheritance will be a more understandable mechanism. Chromosomes are unique carriers of information: at conception, every human body becomes the owner of a "library" of 46 volumes, which describe in detail all the characteristics and characteristics of its body. The amount of information that is 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, inclination to fullness ... Chromosomes have a narrow specialization: some are responsible for metabolism, others for eye color or the speed of nervous processes .. However, by and large, the genes contain information about protein molecules - enzymes that determine the characteristics of the functioning of the human body.
Sexual chromosomes also encode part of the hereditary information. This means that traits that are “recorded” on these chromosomes can only be transmitted to representatives of one biological sex: this is genetically linked sex. Thus, an important conclusion can be drawn. If the gene is located on one of the sex chromosomes, then biologists talk about such a thing as the inheritance of traits linked to the floor. Moreover, the inheritance of such characters has a number of features: they will manifest themselves differently in heterogeneous organisms.
First research
Genetics noted that the inheritance of the color of the eye facets in Drosophila is in direct proportion to the sex of individuals that are owners of different alleles. The gene that determines the development of red eyes is dominant over the white-eye gene. If the male has red eyes, and the female has white, 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, 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 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.
Gender-specific inheritance
Sex-linked inheritance has certain characteristics. They are associated with the fact that the U chromosome carries fewer genes than the X chromosome. So, for many genes that are located on the X chromosome, which appeared in the male body, there are no alleles on the U 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 manifest itself in the phenotype.
It is worth explaining the above. Allele is a variation of one gene. Allele can be of two main types: dominant and recessive. Moreover, the dominant allele in the genotype necessarily manifests itself phenotypically, and the recessive one only if it is presented in a homozygous state. To illustrate, we can cite the inheritance of eye color in humans. The color depends on the amount of pigment melanin in the iris. If there is little melanin, the eyes will be light; if there is a lot, they will be dark. In this case, the dominant allele is responsible for the dark color of the eyes: if it is in the genome, then the child will be brown-eyed. It does not matter if both genes are dominant or one of them is recessive, that is, whether the child is homo- or heterozygous for this feature. But blue eyes are a relatively "young" mutation that is regulated by the recessive allele. All light-eyed people are carriers of two recessive alleles, that is, they are homozygous for the color of their eyes. The dominant gene, as its name implies, dominates the recessive one: if the child’s genome has one 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 U-chromosome differs from the X-chromosome in its size and shape: it really does not resemble the cross, like the other chromosomes, but the letter U. Therefore, some of the genes that are located on it will show themselves even if there is only one recessive copy: this inherits sex-related inheritance from other types of inheritance.
Recessive X-linked diseases
Gender-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 chromosome X syndrome, hydrocephalus and many other diseases.
Sex-linked inheritance in humans is as follows. If the pathological gene is located in one of the X chromosomes of a woman, then half of her daughters and half of her 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 the 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 U chromosome. This factor is important to consider when deciding on gender-linked inheritance.
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 the 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.
There are always twice as many women with dominant X-linked disease as men. A sick woman with a 50% probability of transmitting the disease to all her children, and a sick man - exclusively to daughters.
Sometimes sex-related inheritance can be observed with rather rare diseases that are fatal to male fetuses, therefore, women are more likely to have spontaneous abortions.
One can endlessly cite gender-related inheritance. Let us dwell on a disease such as pigment incontinence, or Bloch-Sulzberg melanoblastosis. Pigment incontinence is found only in women: the gene is lethal for the male fetus. On the body of girls suffering from this disease, rashes appear in the form of vesicles. After the rashes pass, characteristic pigmentation in the form of splashes and swirls remain on the skin. 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 in only one gender. As an example of what gender-related inheritance is, hemophilia is given in almost every textbook. Hemophilia is a disease in which blood does not clot. Hemophilia is devoted to many tasks related to sex-linked inheritance: 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. Tsarevich Alexei, the heir to Tsar Nicholas II, suffered from it.
Even a slight cut can cause severe blood loss. However, not only bleeding associated with a violation of the integrity of the skin is dangerous. Intestinal, intracranial and throat bleeding, as well as bleeding in the joints, pose a serious threat to the life of patients. At the same time, hemophilia is incurable: patients are forced to take substitution drugs all their lives, which are not very effective.
Do women have hemophilia?
Many mistakenly believe that women never suffer from hemophilia. However, this is not so: the fair sex also suffers from this terrible disease. True, this is extremely rare. More often, women are carriers of the gene that determines this disease.
A female carrier in one of the sex chromosomes has a recessive gene that determines blood clotting. 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, to whom the recessive gene will pass, he will manifest hemophilia. But for a girl to develop hemophilia, it is necessary for a woman who carries the gene to marry a man suffering from hemophilia (but even under such circumstances, the probability of having such a girl will be 25%). Such cases are practically not noted: firstly, the hemophilia gene is quite rare, and secondly, people suffering from this dangerous disease rarely live to a reproductive age.
Importance of research on inheritance of sex-linked traits
A study of how inheritance of sex-linked traits occurs is of great importance to humanity. This is primarily due to the fact that many diseases are inherited from the floor, including life-threatening diseases. Perhaps in the future it will be possible to create innovative means of treating such diseases, based on the effect 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 rapid diagnostic methods for sex-linked diseases. This is especially important for diseases that are associated with metabolic disorders: the sooner therapy is started, the greater success will be achieved.
Genetics occupies a critical place in human life. Indeed, this particular science makes it possible to explain how human genes function, including those that carry pathological signs. There is a group of characters whose inheritance is due to sex chromosomes. What is gender-related inheritance? The definition says that this is a transmission of traits for which genes located on the sex chromosomes are responsible.