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With the development of the zygote, the sex of a person is predetermined. Sex determination in humans

11.11.2020

Two basic rules for sex determination in mammals.

Classical embryogenetic studies have established two rules for determining sex in mammals. The first of them was formulated in the 60s by Alfred Zhost on the basis of experiments to remove the rudiment of future gonads (gonadal fold) from early rabbit embryos: removal of the ridges before the formation of the gonad led to the development of all embryos as females... It was suggested that the male gonads (testes) secreted an effector (Testosterone) responsible for masculinization of fetuses, and the presence of a second effector, anti-Müllerian hormone (MIS), which directly controls such anatomical transformations, was predicted. The observation results were formulated as a rule: the specialization of developing gonads in the testis or ovary determines the subsequent sexual differentiation of the embryo. Until about 1959, it was assumed that the number of X chromosomes, known to be two in females and one in males, was the most important factor in controlling sex in mammals. However, the discovery of individuals with a single X chromosome developing as females, and individuals with one Y chromosome and multiple X chromosomes as males, forced to abandon such notions. The second rule for determining sex in mammals has been formulated: The Y chromosome carries the genetic information required for sex determination in males... The combination of the above two rules is sometimes called the principle of growth: The chromosomal sex, associated with the presence or absence of the Y chromosome, determines the differentiation of the embryonic gonad, which, in turn, controls the phenotypic sex of the organism. This mechanism of sex determination is called genetic (GSD) and is contrasted with that based on the controlling role of environmental factors (ESD) or the ratio of sex chromosomes and autosomes (CSD).

Physiological basis of the gonadal level of sex determination.

The physiological basis of the sex determination mechanism is the bisexuality of the mammalian embryonic gonads. In such projections, the Muller's duct and the Wolf's channel are simultaneously present - the rudiments of the reproductive tract of females and males, respectively. Primary sex determination begins with the appearance of specialized cell lines in the projections - the Sertoli cell. In the latter, the MIS predicted by Jost is synthesized, which is responsible for the direct or indirect inhibition of the development of the Müllerian duct, the rudiment of the future fallopian tubes and uterus.

Genetic sex determination mechanism.

Human Y chromosome showing the location of the SRY gene

In 1987, David Page and his colleagues, investigating a XX male who inherited 280 kb. a fragment of Y chromosomespecific DNA, and a woman XY with a deletion (lack) capturing this region as a result of the exchange of regions between chromosomes, seemed to find the elusive TDF. It turned out to be present in the Y-chromosome of all true animals of Eutheria and located in an area of \u200b\u200b140 kbp. in 100 kb from the border of the pseudoautosomal region gene ZFY. The homologue ZFY - ZFX is found on the X chromosome, and it avoids the inactivation characteristic of the genes localized in it. Both of these factors encode a protein that forms the structure of the so-called zinc fingers, which has DNA-binding activity, which can be considered a transcription factor. Further detailed analysis of Y chromosomes of specific sequences in individuals with sex reversal limited the search to a 35 kb region. and led to the discovery of a gene considered to be the true equivalent of classic TDF. This gene was named SRY ( Sex determining Region Y gene). Here are some of its characteristics that make us reckon with this assumption. SRY is located in the sex-defining region and contains a conserved domain (HMG box) encoding a protein of 80 amino acid residues. Its activity was noted on the eve of the period of differentiation of the progonade in the testis - the 10-12th day of embryonic development in mice and, at least at this stage, does not depend on the presence of germ cells. Specific point mutations or deletions in the HMG box of this gene in XY women lead to sex reversal. Transfer 14 kb a DNA fragment containing this gene with flanking regions into a fertilized oocyte of a homogametic individual using microinjection (the procedure of transgenesis - gene transfer) led to the appearance of a male with an XX karyotype. True, this animal showed defective spermatogenesis.

Functions of the SRY gene.

The domain encoded by the HMG box of the SRY gene specifically binds to DNA, leading to bending of its molecule. Such deformation of the DNA structure, induced by the SRY protein or related molecules (more than 100 proteins with an HMG domain are known), can be mechanically transmitted over a distance and play an important role in the regulation of transcription, replication, and recombination. The region of DNA in which SRY is localized is responsible for encoding two key enzymes involved in male-type differentiation of the primary gonad: aromatase P450, which controls the conversion of testosterone to estradiol, and a factor or hormone that inhibits the development of Miller's ducts, which reverses their development and promotes differentiation testicles. SRY is also involved in the processes of sexual differentiation in close interaction with another gene, named by K. McElreavey et al. (1993) genome Z, the function of which is normally to suppress specific male genes. In the case of the normal male 46XY genotype, the SRY gene produces a protein that inhibits the Z gene and specific male genes are activated. In the case of the normal female 46XX genotype, in which SRY is absent, the Z gene is activated and inhibits a specific male gene, which creates conditions for female development.

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Sex first appears as a purely reproductive (recombination) phenomenon. In the process of evolution, it gradually acquires evolutionary functions as well. At the same time, sex determination naturally goes from gene (in hermaphrodites) to chromosomal (in dioecious forms, apparently starting with fish) and genomic (in bees). At the same time, the level of differentiation increases and there is an increase in the manifestation of sexual dimorphism: in asexual forms it is absent, in hermaphrodites there is sexual dimorphism only at the level of primary sexual characteristics (gametes, gonads), in dioecious monogams, organismic sexual dimorphism (secondary sexual characteristics) appears, in dioecious polygams -population, including sexual dimorphism in the number and dispersion of sexes, and in bees (possibly other social insects), sexual dimorphism of the genome appears (haplo-diploidy).

During the development of the organism (ontogenesis, sex determination can occur at the time of fertilization (gene level), and also be controlled by internal (hormones) and / or external factors. In humans and higher animals, education and training also play an important role.

Genetic sex determination

Chromosomal sex determination:

In animals, plants and humans, the chromosomal mechanism is the initial mechanism that determines sex. According to chromosomal theory, the sex of an organism is determined by the sex chromosomes at the time of fertilization.

XY gender determination:

In one sex, the nuclei of all somatic cells contain a diploid set of autosomes 2A and two identical sex chromosomes (XX). Therefore, all gametes of this sex contain one X chromosome. This is a homogametic floor. In the other sex, each somatic cell, in addition to the diploid set of autosomes 2A, contains two different sex chromosomes X and Y. Therefore, it has two types of gametes: X- and Y-bearing. This is a heterogametic floor. In most species of animals and plants, the female sex is homogametic, and the male is heterogametic. This includes mammals, most insects, many fish, plants, etc. Species with male heterogamety are classified as Drosophila. There are two types of XY gender determination. One of them is like in humans: gender depends on the presence of the Y chromosome (if there is one, the genotype of the male, if not, the female). The second one is like in representatives of the genus Drosophila: sex is determined by the ratio of the number of X chromosomes and the number of autosomes.

ZW gender determination:

In a smaller number of species (birds, some reptiles, fish, butterflies, caddis flies, from strawberry plants), the opposite picture is observed - male sex is homogametic, and female is heterogametic. Species with female heterogamety are classified as Abraxas.

X0 sex determination:

With this mechanism for determining sex, one of the sexes (homogametic) has 2 X chromosomes, while the second (heterogametic) only the 1st. Basically, it is a kind of XY-mechanism, since in this case the sex is determined in the same way as in Drosophila: by the ratio of the number of X-chromosomes and autosomes.

Haplo-diploid (genomic) sex determination:

In insects (bees and other hymenoptera, worms, ticks), females (or females and males) are obtained from fertilized eggs, and only males develop from unfertilized eggs.

Environment sex determination:

With this mechanism of sex determination, the development of an organism into a male or a female is determined by external factors, for example, temperature (in most crocodiles).

Hormonal sex determination:

Sex determination can be represented as a relay race that the chromosomal mechanism passes on to undifferentiated gonads that develop into male or female genital organs. When studying the role of sex chromosomes in the development of gonads, it was shown that the presence or absence of the Y chromosome is decisive in humans. In the absence of the Y chromosome, the gonads differentiate into the ovaries and the woman develops. In the presence of the Y chromosome, the male system develops. Apparently the Y chromosome produces a substance that stimulates testicular differentiation. "It seems that nature's primary plan was to make a woman, and that the addition of a Y chromosome produces a male variation." The next stage of the relay is continued by hormones that determine the process of fetal sexual differentiation and its anatomical development. At birth, the first part of the program ends. After birth, the baton shifts to environmental factors that complete gender formation — usually, but not always according to genetic sex. Determination of sex is a complex multistage process, which in humans depends, in addition to biological, also on psychosocial factors. This can lead to heterosexual, bisexual or homosexual behaviors and lifestyles.

Sex is a combination of morphological, physiological, biochemical and other characteristics of an organism that determine the reproduction of one's own kind. When studying the sets of chromosomes of males and females, attention was drawn to the fact that in female organisms all chromosomes form pairs, and in males, in addition to paired (homologous) chromosomes, there are two unpaired ones. Later it was found that these unpaired chromosomes precisely determine the sex of the organism. Most of the unpaired chromosomes, which are contained in the female karyotype in a double set, and in the male karyotype in a single set, is called the X chromosome. The smaller of the unpaired chromosomes, which is only found in males, is called the Y chromosome. Paired chromosomes that are the same in the male and female body are called autosomes (A), and the X and Y chromosomes are called sex chromosomes. A diploid set in humans contains 23 pairs or 46 chromosomes: 22 pairs of autosomes and one pair of sex chromosomes. In the female body, these are two X chromosomes, and in the male, the X and Y chromosomes. The set of chromosomes of a woman can be represented by the entry: 44A + 2X, and men - 44A + XY.

A sex that has two identical sex chromosomes (XX) is called homogametic, since it forms only one type of gamete containing an X chromosome. Sex, determined by different sex chromosomes (XY), is called heterogametic, since it forms two types of gametes: containing X- and Y-chromosomes, respectively. The sex of the future human body is determined at the time of fertilization and depends on which of the sperm will fertilize the egg. When an egg is fertilized by a sperm containing an X chromosome, there will be two X chromosomes in the zygote, and a female body will develop from it. When an egg is fertilized by a sperm with a Y-chromosome, the zygote will contain X- and Y-chromosomes and it will give rise to a male body. It is easy to see that the formation of spermatozoa with X and Y chromosomes is equally probable and, therefore, the mechanism of gametogenesis determines not only sex, but also the approximate numerical equality of the sexes in each generation.

In all mammals, humans and the Drosophila fly, the female sex is homogametic, and the male is heterogametic.

Homogametic sex gives one type of gametes, and heterogametic (hemizygous) - two types of gametes.

In birds and some insects, such as butterflies, the male sex is homogametic (ZZ), and the females are heterogametic (ZW).

In some animals (bees, ants and wasps) there is a special type of sex determination, called haplodiploid. These individuals do not have sex chromosomes. Females and worker bees develop from fertilized eggs and are diploid, while males from unfertilized eggs are haploid. During spermatogenesis, the number of chromosomes is not reduced. If a diploid larva is fed with royal jelly, then a female is formed from it. When feeding a diploid larva with honey, it is a worker bee.

In humans and in the animals described above, sex is inherited at the time of gamete fusion (this syngamic or chromosomal sex determination).

However, in some multicellular animals, sex determination occurs before the onset of cleavage, without regard to fertilization (such sex determination is called progamous). In some roundworms, females develop from a large egg, and males from a small one. Many roundworms are examples of this sex determination.

Epigamous the variant of sex determination occurs at the larval stage and depends on the action of the environment. For example, females or males are formed from the degree of heating of eggs in a turtle's clutch. In the Bonneli worm, the female can reproduce by parthenogenesis. If the larva settles on the female's proboscis, then a male is formed from it (under the influence of the female's hormones), and if she does not meet the female, then the larva becomes a female.

In some animals, sex may change several times during their life, depending on environmental conditions. For example, if a male dies in the cardinals' harem fish (in nature), then the most active female begins to function as a male. The same is observed in some amphibians and bivalve molluscs. That is, if the year is supposed to be successful for the development of juveniles, then some males in the population of these animals turn into females. Thus, the emerging juveniles have a greater chance of survival, and the population boundaries expand. During a lean year, some females in the population turn into males; There will be fewer juveniles in the population, but the intraspecific struggle at the juvenile level (the most severe struggle) will be smoothed out.

Not all animals can change sex throughout their life. Sex reassignment is possible only in those animals that have external fertilization and a similar structure of the gonads (sex glands) in females and males

In all mammals, humans and the Drosophila fly, the female sex is homogametic, and the male is heterogametic.

Homogametic sex gives one type of gametes, and heterogametic (hemizygous) - two types of gametes.

In birds and some insects, such as butterflies, the male sex is homogametic (ZZ), and the females are heterogametic (ZW).

In humans and in the animals described above, sex is inherited at the time of gamete fusion (this is a syngamic or chromosomal sex determination).

However, in some multicellular animals, sex determination occurs before cleavage begins, without regard to fertilization (this sex determination is called progamnom). In some roundworms, females develop from a large egg, and males from a small one. Many roundworms are examples of this sex determination.

The epigamous variant of sex determination occurs at the larval stage and depends on the action of the environment. For example, from the degree of heating of the eggs in the clutch of a turtle, females or males are formed. In the Bonneli worm, the female can reproduce by parthenogenesis. If the larva settles on the female's proboscis, then a male is formed from it (under the influence of the female's hormones), and if she does not meet the female, then the larva becomes a female.

Not all animals can change sex throughout their life. Sex change is possible only in those animals that have external fertilization and a similar structure of the gonads (sex glands) in females and males.

Most animals are dioecious organisms. Sex can be considered as a set of features and structures that provide a way of reproduction of offspring and the transmission of hereditary information. Sex is most often determined at the time of fertilization, that is, the karyotype of the zygote plays the main role in determining the sex. The karyotype of each organism contains chromosomes that are the same in both sexes - autosomes, and chromosomes by which the female and male sex differ from each other - sex chromosomes. In humans, the "female" sex chromosomes are two X-chromosomes. When gametes are formed, each ovum receives one of the X chromosomes. The sex in which the gametes of the same type are formed, bearing the X-chromosome, is called homogametic. In humans, the female sex is homogametic. The "male" sex chromosomes in humans are the X chromosome and the Y chromosome. When gametes are formed, half of the spermatozoa receive an X chromosome, the other half - a Y chromosome. The sex in which gametes of different types are formed is called heterogametic. In humans, the male sex is heterogametic. If a zygote is formed carrying two X-chromosomes, then a female organism will form from it, if the X-chromosome and Y-chromosome are male.

In animals, the following can be distinguished four types of chromosome sex determination.

The female sex is homogametic (XX), the male is heterogametic (XY) (mammals, in particular, humans, Drosophila).

Genetic scheme for chromosomal sex determination in humans:

R ♀46, XX × ♂46, XY
Gamete types 23, X 23, X 23, Y
F 46, XX
females, 50% 46, XY
males, 50%
Genetic scheme of chromosomal sex determination in Drosophila:

R ♀8, XX × ♂8, XY
Gamete types 4, X 4, X 4, Y
F 8, XX
females, 50% 8, XY
males, 50%
The female sex is homogametic (XX), the male is heterogametic (X0) (Orthoptera).

Genetic scheme for chromosomal sex determination in Desert Locust:

R ♀24, XX × ♂23, X0
Gamete types 12, X 12, X 11, 0
F 24, XX
females, 50% 23, X0
males, 50%
The female sex is heterogametic (XY), the male sex is homogametic (XX) (birds, reptiles).

The genetic scheme for chromosomal sex determination in a pigeon:

R ♀80, XY × ♂80, XX
Gamete types 40, X 40, Y 40, X
F 80, XY
females, 50% 80, XX
males, 50%
The female sex is heterogametic (X0), the male sex is homogametic (XX) (some species of insects).

Genetic scheme for chromosomal sex determination in moths:

R ♀61, X0 × ♂62, XX
Gamete types 31, X 30, Y 31, X
F 61, X0
females, 50% 62, XX
males, 50%

Inheritance of sex-linked traits

It has been established that the sex chromosomes contain genes that are responsible not only for the development of sexual characteristics, but also for the formation of non-sexual characteristics (blood clotting, the color of tooth enamel, sensitivity to red and green colors, etc.). The inheritance of non-sexual traits, the genes of which are localized in the X - or Y-chromosomes, is called sex-linked inheritance.

T. Morgan studied the inheritance of genes localized in sex chromosomes.

In Drosophila, red eye color dominates over white. Reciprocal crossing - two crosses, which are characterized by a mutually opposite combination of the analyzed trait and sex in the forms taking part in this crossing. For example, if in the first crossing the female had a dominant trait, and the male had a recessive trait, then in the second crossing the female should have a recessive trait, and the male should have a dominant trait. Through reciprocal crossing, T. Morgan received the following results. When crossing red-eyed females with white-eyed males in the first generation, all offspring turned out to be red-eyed. If F 1 hybrids are crossed among themselves, then in the second generation all females are red-eyed, and among males - half are white-eyed and half are red-eyed. If you cross between white-eyed females and red-eyed males, then in the first generation all females are red-eyed, and males are white-eyed. In F 2, half of females and males are red-eyed, half are white-eyed.

T. Morgan was able to explain the obtained results of the observed cleavage by eye color only by assuming that the gene responsible for eye color is localized in the X chromosome (X A is the red eye color, X a is the white eye color), and the Y chromosome of such does not contain genes.

R	♀X A X A red-eyed	×	♂X a Y white-eyed
Gamete types	X A		X a Y
F 1	X A X a ♀ red-eyed 50%		X A Y ♂ red-eyed 50%

R	♀X A X a red-eyed	×	♂X A Y red-eyed
Gamete types	X A X a		X A Y
F 2	X A X A X A X a ♀ red-eyed 50%		X A Y ♂ red-eyed 25%	X a Y ♂ white-eyed 25%

R	♀X a X a white-eyed	×	♂X A Y red-eyed
Gamete types	X a		X A Y
F 1	X A X a ♀ red-eyed 50%		X a Y ♂ white-eyed 50%

R	♀X A X a red-eyed		×	♂X a Y white-eyed
Gamete types	X A X a			X a Y
F 2	X A X A ♀ red-eyed 25%	X a X a ♀ white-eyed 25%		X A Y ♂ red-eyed 25%	X a Y ♂ white-eyed 25%

Scheme of human sex chromosomes and genes linked to them:
1 - X chromosome; 2 - Y-chromosome.

In humans, a man receives the X chromosome from his mother, and the Y chromosome from his father. A woman receives one X chromosome from her mother, another X chromosome from her father. X -chromosome - middle submetacentric, Y -chromosome - small acrocentric; The X chromosome and the Y chromosome have not only different sizes and structures, but also for the most part carry different sets of genes. Depending on the gene composition in the sex chromosomes of a person, the following regions can be distinguished: 1) a non-homologous region of the X chromosome (with genes present only in the X chromosome); 2) a homologous region of the X chromosome and the Y chromosome (with genes present both in the X chromosome and in the Y chromosome); 3) a non-homologous region of the Y chromosome (with genes present only in the Y chromosome). Depending on the localization of the gene, the following types of inheritance are distinguished.

Inheritance type	Localization of genes	Examples of
X-linked recessive		Hemophilia, various forms of color blindness (protanopia, deuteronopia), absence of sweat glands, some forms of muscular dystrophy, etc.
X-linked dominant	Non-homologous region of the X chromosome	Brown color of tooth enamel, vitamin D resistant rickets, etc.
X-Y - linked (partially linked to the floor)	Homologous region of X and Y chromosomes	Alport's syndrome, general color blindness
Y -linked	Non-homologous region of the Y chromosome	Webbed toes, hypertrichosis of the edge of the auricle

Most of the genes linked to the X chromosome are absent in the Y chromosome; therefore, these genes (even recessive ones) will manifest themselves phenotypically, since they are singular in the genotype. Such genes are called hemizygous. The human X chromosome contains a number of genes, the recessive alleles of which determine the development of severe anomalies (hemophilia, color blindness, etc.). These abnormalities are more common in men (as they are hemizygous), although women are more likely to carry the genes for these abnormalities. For example, if X A is normal blood clotting, X a is hemophilia, and if a woman is a carrier of the hemophilia gene, then phenotypically healthy parents may have a hemophilic son:

♀X A X a "Gene interaction"

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