The subject of genetic research is the phenomena of heredity and variability. American scientist T-X. Morgan created the chromosome theory of heredity, proving that each biological species can be characterized by a certain karyotype, which contains such types of chromosomes as somatic and sex. The latter are represented by a separate pair, differing in male and female individuals. In this article, we will study the structure of female and male chromosomes and how they differ from each other.
What is a karyotype?
Each cell containing a nucleus is characterized by a certain number of chromosomes. It is called a karyotype. In various biological species, the presence of structural units of heredity is strictly specific, for example, the human karyotype is 46 chromosomes, chimpanzees - 48, crayfish - 112. Their structure, size, shape differ in individuals,relating to various systematic taxa.
The number of chromosomes in a body cell is called the diploid set. It is characteristic of somatic organs and tissues. If, as a result of mutations, the karyotype changes (for example, in patients with Klinefelter's syndrome, the number of chromosomes is 47, 48), then such individuals have reduced fertility and in most cases are infertile. Another hereditary disease associated with sex chromosomes is Turner-Shereshevsky syndrome. It occurs in women who have not 46, but 45 chromosomes in the karyotype. This means that in a sexual pair there are not two x chromosomes, but only one. Phenotypically, this manifests itself in the underdevelopment of the gonads, mild secondary sexual characteristics and infertility.
Somatic and sex chromosomes
They differ both in shape and in the set of genes that make up their composition. The male chromosomes of humans and mammals are part of the XY heterogametic sex pair, which ensures the development of both primary and secondary male sexual characteristics.
In male birds, the sexual pair contains two identical ZZ male chromosomes and is called homogametic. Unlike the chromosomes that determine the sex of an organism, the karyotype contains hereditary structures that are identical in both male and female. They are called autosomes. There are 22 pairs in the human karyotype. The sex male and female chromosomes form the 23rd pair, so the karyotype of a man can be represented as a general formula: 22 pairs of autosomes + XY, and women - 22 pairs of autosomes +XX.
Meiosis
The formation of germ cells - gametes, at the fusion of which a zygote is formed, occurs in the sex glands: testes and ovaries. Meiosis is carried out in their tissues - 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 type: 22 autosomes + X, and spermatogenesis ensures the maturation of two types of hometes: 22 autosomes + X or 22 autosomes + Y. In humans, the sex of the unborn child is determined at the time of nuclear fusion egg and sperm and depends on the karyotype of the sperm.
Chromosomal mechanism and sex determination
We have already considered at what point a person's sex is determined - at the time of fertilization, and it depends on the chromosome set of the sperm. In other animals, representatives of different sexes differ in the number of chromosomes. For example, in marine worms, insects, grasshoppers, in the diploid set of males, there is only one chromosome from the sexual pair, and in females, both. So, the haploid set of chromosomes of the male marine worm atsirocanthus 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 eggs.
What affects sexual dimorphism?
Besides chromosomal, there are other ways to determine sex. In some invertebrates - rotifers, polychaete worms - sex is determined even before the fusion of gametes - fertilization, as a result of which male and female chromosomes formhomologous couples. Females of the marine polychaete - dinophyllus in the process of ovogenesis form eggs of two types. The first - small, depleted in yolk - males develop from them. Others - large, with a huge supply of nutrients - serve for the development of females. In honey bees - insects of the Hymenoptera series - females produce two types of eggs: diploid and haploid. From unfertilized eggs, males develop - drones, and from fertilized - females, which are worker bees.
Hormones and their effect on sex formation
In humans, male glands - the testes - produce sex hormones of the testosterone series. 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, features of the figure, body hair, voice timbre, and the structure of the larynx. In a woman's body, the ovaries produce not only sex cells, but also hormones, being glands of mixed secretion. Sex hormones, such as estradiol, progesterone, estrogen, promote the development of external and internal genital organs, female-type body hair, regulate the menstrual cycle and the course of pregnancy.
In some vertebrates, fish, annelids and amphibians, biologically active substances produced by the gonads strongly influence the development of primary and secondary sexual characteristics, and specieschromosomes do not have such a big impact on the formation of sex. For example, larvae of marine polychaetes - bonellias - under the influence of female sex hormones stop their growth (sizes 1-3 mm) and become dwarf males. They live in the genital tract of females, which have a body length of up to 1 meter. In cleaner fish, males keep harems of several females. Female individuals, in addition to the ovaries, have the rudiments of the testes. As soon as the male dies, one of the harem females takes over his function (male gonads that produce sex hormones begin to actively develop in her body).
Gender regulation
In human genetics, it is carried out by two rules: the first determines the dependence of the development of the rudimentary gonads on the secretion of testosterone and the hormone MIS. The second rule indicates the exclusive role played by the Y chromosome. The male sex and all the anatomical and physiological characteristics corresponding to it develop under the influence of genes located on the Y chromosome. The interrelation and dependence of both rules in human genetics is called the principle of growth: in an embryo that is bisexual (that is, having the rudiments of female glands - the Müllerian duct and male gonads - the Wolffian channel), the differentiation of the embryonic gonad depends on the presence or absence of the Y-chromosome in the karyotype.
Genetic information on the Y chromosome
Research by genetic scientists, in particular T-X. Morgan, it was found that in humans and mammals the gene composition of the X and Y chromosomes is not the same. Male chromosomes in humans do not have some alleles,present on the X chromosome. However, their gene pool contains the SRY gene, which controls spermatogenesis, leading to the formation of a male. Hereditary disorders of this gene in the embryo leads to the development of a genetic disease - Swire's syndrome. As a result, a female individual developing from such an embryo contains a sexual pair in the XY karyotype or only a portion of the Y chromosome containing the gene locus. It activates the development of the gonads. Affected women do not differentiate secondary sexual characteristics and are infertile.
Y-chromosome and hereditary diseases
As noted earlier, the male chromosome differs from the X chromosome both in size (it is smaller) and in shape (looks like a hook). It also has a specific set of genes. So, a mutation of one of the genes of the Y chromosome is phenotypically manifested by the appearance of a bunch of hard hair on the earlobe. This sign is characteristic only for men. There is such a hereditary disease caused by a chromosomal mutation as Klinefelter's syndrome. A sick man has extra female or male chromosomes in the karyotype: XXY or XXYU.
The main diagnostic features are pathological growth of the mammary glands, osteoporosis, infertility. The disease is quite common: for every 500 newborn boys, there is 1 sick.
Summarizing, 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 zygotesex X- and Y-chromosomes.
