Male chromosomes. Y-chromosome on what influences and for what answers?

The subject of genetic research is the phenomenon of heredity and variability. American scientist T-X. Morgan created a chromosome theory of heredity, proving that each species can be characterized by a certain karyotype that contains such kinds of chromosomes as somatic and sexual. The latter are represented by a separate pair, differing in the male and female individuals. In this article, we will study what structure the female and male chromosomes have and how they differ among themselves.

What is karyotype?

Each cell containing the nucleus is characterized by a certain number of chromosomes. It received the name karyotype. In different biological species, the presence of structural units of heredity is strictly specific, for example, the human karyotype is 46 chromosomes, in chimpanzees - 48, and in river crayfish - 112. Their structure, size, shape is different in individuals belonging to different systematic taxa. The number of chromosomes in a cell of a body is called a diploid set. It is characteristic for somatic organs and tissues. If the karyotype changes as a result of mutations (for example, in patients with Klinefelter's syndrome the number of chromosomes is 47, 48), then such individuals have a reduced fertility and in most cases are infertile. Another hereditary disease associated with sex chromosomes is the Turner-Shereshevsky syndrome. It occurs in women who have in the karyotype not 46 but 45 chromosomes. This means that in the sex pair there are not two x-chromosomes, but only one. Phenotypically, this manifests itself in the underdevelopment of the sexual glands, weakly expressed secondary sexual characters and infertility.

Somatic and sex chromosomes

They differ in both the form and the set of genes that make up their composition. Male chromosomes of humans and mammals enter the heterogametic sex pair XY, which provides development of both primary and secondary male sexual characteristics. In male birds, the sex pair contains two identical ZZ male chromosomes and is called homogametic. Unlike chromosomes, which determine the sex of the organism, there are hereditary structures in the karyotype that are identical in both males and females. They are called autosomes. In the human karyotype, there are 22 pairs. Sex male and female chromosomes form 23 pairs, so the male karyotype can be represented as a general formula: 22 pairs of autosomes + XY, and women - 22 pairs of autosomes + XX.

Meiosis

The formation of gametes, gametes, with the fusion of which a zygote is formed, occurs in the genital 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. Ovogenesis in the ovaries leads to the maturation of eggs of only one species: 22 autosomes + X, and spermatogenesis ensures the maturation of homometes of two types: 22 autosomes + X or 22 autosomes + U. In humans, the sex of the unborn child is determined at the time of the fusion of the nuclei of the egg and sperm From the karyotype of the sperm.

Chromosome mechanism and sex determination

We have already examined at what point the sex is determined in humans - at the time of fertilization, and it depends on the chromosome set of the sperm. In other animals, representatives of different sex differ in the number of chromosomes. For example, in maritime worms, insects, grasshoppers in a diploid set of males there is only one chromosome from the sex pair, and in females - both. Thus, the haploid set of chromosomes of the male aquatic acanthocanthus can be expressed by the formulas: 5 chromosomes + 0 or 5 chromosomes + x, and females have only one set of 5 chromosomes + x in the ovules.

What influences sexual dimorphism?

In addition to chromosomal, there are other ways of determining sex. In some invertebrates - rotifers, polychaete worms - the sex is determined even before the fusion of gamete fertilization, as a result of which male and female chromosomes form homologous pairs. Female sea polychaetes - dinofilus in the process of ovogenesis form the ovule of two species. The first - small, impoverished yolk, - of them develop males. Others - large, with a huge supply of nutrients - serve to develop females. In honey bees - insects of the Hymenoptera series - females produce two types of eggs: diploid and haploid. Of unfertilized eggs, males develop - drones, and from fertilized eggs - females that are working bees.

Hormones and their effect on the formation of sex

In man, the male glands - the testicles - produce sex hormones of a number of testosterone. They affect both the development of primary sexual characteristics (the anatomical structure of the external and internal genital organs) and the features of physiology. Under the influence of testosterone, secondary sexual characteristics are formed - the structure of the skeleton, the features of the figure, the body hair, the timbre of the voice, and the structure of the larynx. In the body of a woman, the ovaries produce not only sex cells, but also hormones, which are glands of mixed secretion. Sex hormones such as estradiol, progesterone, estrogen, promote the development of external and internal genital organs, body hair on a female type, regulate the menstrual cycle and the course of pregnancy. In some vertebrates, fish, ringworms and amphibians, biologically active substances produced by gonads strongly influence the development of primary and secondary sexual characteristics, and the chromosome species do not exert such a great influence on the formation of sex. For example, larvae of marine polychaetes - bonellia - under the influence of female sex hormones cease their growth (1-3 mm in size) and become dwarf males. They live in the genital tract of females, which have a body length of up to 1 meter. In fish-cleaners males contain harems from several females. Female individuals, except the ovaries, have the embryonic stem. As soon as the male dies, one of the harem females takes on its function (in its body, the male gonads that develop the sex hormones begin to actively develop).

Regulation of the floor

In human genetics, it is carried out by two rules: the first determines the dependence of the development of the rudimentary sex glands on the secretion of testosterone and the hormone MIS. The second rule indicates the exceptional role played by the Y chromosome. Male gender and all the corresponding anatomical and physiological signs develop under the influence of genes located in the Y chromosome. The relationship and dependence of both rules in human genetics is called the principle of growth: in an embryo that is bisexual (that is, the rudiments of the female glands - the Mullerian duct and the male gonad - the Wolff canal), the differentiation of the embryonic sex gland depends on the presence or absence of the Y chromosome in the karyotype.

Genetic information in the Y chromosome

Studies of genetic scientists, in particular T-X. Morgan, it was found that the gene composition of X- and Y-chromosomes is not the same in humans and mammals. Male chromosomes in humans do not have some alleles present in the X chromosome. However, their gene pool contains the SRY gene, which controls spermatogenesis, which leads to the formation of the male sex. Hereditary disorders of this gene in the embryo lead to the development of a genetic disease - Suair syndrome. As a result, a female that develops from such an embryo contains a sex pair in the karyotype of XY or only a segment of the Y chromosome containing a gene locus. He activates the development of gonads. In patients, women do not differentiate secondary sexual characteristics, and they are sterile.

U-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). Also for her specific set of genes. Thus, the mutation of one of the genes of the Y chromosome is phenotypically manifested by the appearance of a beam of hard hair on the earlobe. This characteristic is characteristic only of men. A hereditary disease caused by a chromosomal mutation is known as Klinefelter's syndrome. The sick man has in karyotypes superfluous female or man's chromosomes: ХХУ or ХХУУ. The main diagnostic signs are pathological growth of the mammary glands, osteoporosis, 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 time of fertilization, due to a certain combination in the zygote of sex X- and Y-chromosomes.