The subject of genetic research is the phenomena of heredity and variability. American scientist TX. 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 sexual. The latter are represented by a separate pair, differing in male and female. In this article we will study what structure the female and male chromosomes have and how they differ from each other.
What is a karyotype?
Each cell containing the nucleus is characterized by a certain number of chromosomes. It is called the karyotype. In various biological species, the presence of structural units of heredity is strictly specific, for example, the human karyotype is 46 chromosomes, in chimpanzees - 48, crayfish - 112. Their structure, size, shape differ in individuals belonging to different systematic taxa.
The number of chromosomes in a body cell is called a diploid set. It is characteristic of somatic organs and tissues. If the karyotype changes as a result of mutations (for example, the number of chromosomes 47, 48 in patients with Klinefelter syndrome), 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 the sexual pair there are not two x-chromosomes, but only one. Phenotypically, this is manifested in the underdevelopment of the sex glands, poorly expressed secondary sexual characteristics and infertility.
Somatic and sex chromosomes
They differ both in form and in the set of genes that make up their composition. Male chromosomes of humans and mammals are included in the heterogametic sex pair XU, which provides 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 chromosomes that determine the sex of the body, 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. Sexual male and female chromosomes form 23 pairs, so the karyotype of a man can be represented as a general formula: 22 pairs of autosomes + XU, and women - 22 pairs of autosomes + XX.
Meiosis
The formation of germ cells - gametes, at the merger of which a zygote is formed, occurs in the sex glands: the testes and ovaries. In their tissues, meiosis is carried out - 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 oocytes of only one type: 22 autosomes + X, and spermatogenesis provides the maturation of the throat of two types: 22 autosomes + X or 22 autosomes + U. In humans, the sex of the unborn child is determined at the time of fusion of the nuclei of the egg and sperm and depends from the sperm karyotype.
Chromosomal mechanism and sex determination
We have already considered at what moment 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 of the sea worm of acyrocanthus can be expressed by the formulas: 5 chromosomes + 0 or 5 chromosomes + x, and females have only one set of 5 chromosomes + x in their eggs.
What affects sexual dimorphism?
In addition to the chromosomal, there are also other methods for determining sex. In some invertebrates - rotifers, polychaete worms - the sex is determined even before the gametes merge - fertilization, as a result of which the male and female chromosomes form homologous pairs. Females of marine polychaete - dinofilus in the process of ovogenesis form two types of ovule. 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 — drones — develop, and from fertilized ones, females, which are working bees.
Hormones and their effects on sex formation
In humans, the male glands - the testes - 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 physiological features. Under the influence of testosterone, secondary sexual characteristics are formed - the structure of the skeleton, body features, body hair, timbre of the voice, and the structure of the larynx. In a woman’s body, the ovaries produce not only germ cells, but also hormones, being glands of mixed secretion. Sex hormones, such as estradiol, progesterone, estrogen, contribute to the development of external and internal genital organs, body hair of a female type, regulate the menstrual cycle and pregnancy.
In some vertebrates, fish,
annelids and amphibians, the biologically active substances produced by the gonads strongly affect the development of primary and secondary sexual characteristics, while the types of chromosomes do not have such a large effect on sex formation. For example, larvae of marine polychaetes - bonella - 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 contain harems of several females. Females, except for the ovaries, have the beginnings of the testes. As soon as the male dies, one of the harem females takes on its function (male gonads that produce sex hormones begin to actively develop in her body).
Floor regulation
In human genetics, it is implemented by two rules: the first determines the dependence of the development of the embryonic sex glands on the secretion of testosterone and the hormone MIS. The second rule indicates the exceptional role that the Y chromosome plays. The male sex and all the corresponding anatomical and physiological signs develop under the influence of genes located in the U chromosome. The relationship and dependence of both rules in human genetics is called the growth principle: in an embryo that is bisexual (that is, having the rudiments of the female glands - the Müller duct and male gonads - the Wolf channel), the differentiation of the embryonic sex gland depends on the presence or absence of the U chromosome in the karyotype.
Genetic information on the Y chromosome
The research of geneticists, in particular the TX. Morgan, it was found that in humans and mammals the gene composition of the X and Y chromosomes varies. 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 male formation. Hereditary disorders of this gene in the embryo lead to the development of a genetic disease - Swire 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 U chromosome containing a gene locus. It activates the development of gonads. In sick women, secondary sexual characteristics do not differentiate, and they are infertile.
U-chromosome and hereditary diseases
As noted earlier, the male chromosome differs from the X chromosome in both size (it is smaller) and shape (it looks like a hook). A set of genes is also specific for her. Thus, a mutation of one of the genes of the U chromosome is phenotypically manifested by the appearance of a bundle of hard hair on the earlobe. This symptom is characteristic only for men. A hereditary disease caused by a chromosomal mutation, such as Klinefelter's syndrome , is known. A sick man has extra female or male chromosomes in the karyotype: or .
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.
Summing up, 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 the sex X and Y chromosomes.