Chromosomal sex determination mechanism. Lecture (2 hours). Topic: Genetics of sex. Sex chromosome pathology
Sex determination
Environmental sex determination
With this sex determination mechanism, the development of an organism into a male or female is determined by external factors, such as temperature (in most crocodiles).
Hormonal sex determination
Sex determination can be thought of as a relay race that the chromosomal mechanism passes on to undifferentiated gonads that develop into male or female reproductive 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 a woman develops. In the presence of the Y chromosome, it develops male system. Apparently, the Y chromosome produces a substance that stimulates testicular differentiation. "It appears that nature's master plan was to make a female, and that the addition of a Y chromosome produces a male variation." The next stage of the relay race is continued by hormones that determine the process of sexual differentiation of the fetus and its anatomical development. At birth, the first part of the program ends. After birth, the baton passes to environmental factors that complete sex formation—usually, but not always, according to genetic sex. Sex determination is a complex multi-stage process, which in humans depends, in addition to biological, also on psychosocial factors. This can lead to transsexuality, heterosexual, bisexual or homosexual behaviors and lifestyles.
see also
Notes
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See what "Gender determination" is in other dictionaries:
Sex determination- * designated sex * sex determination ...
SEX DETERMINATION An organism's sex is determined by its genetic basis. Sex depends on the combination of sex chromosomes. Mammals that have a pair of sex chromosomes have two types of chromosomes X and Y. All eggs have one X ... ... Scientific and technical encyclopedic dictionary
Occurs by the XY mechanism (see also Sex determination). In this case, the heterogametic sex is male, homogametic female. Sex determination is divided into three stages: chromosomal, gonadal and phenotypic. Contents 1 Two basic rules ... ... Wikipedia
Primary sex determination- * first sex determination * primary sex determination sex determination by primary sexual characteristics (see) ... Genetics. encyclopedic Dictionary
Autosexing autosexing, indirect sex determination. Use of sex-linked core genes (phenotypically clearly identifiable) or visible mutations to external definition sex in immature organisms, for example, chickens, ... ... Molecular biology and genetics. Dictionary.
Autosexing indirect sex determination- Autosexing, indirect sex determination * autosexing, explicit sexing * autosexing the use of sex-linked core genes that are clearly defined phenotypically, or visually manifested mutations for external determination ... ... Genetics. encyclopedic Dictionary
Sex determination is epigamous- * semi epigamous * epigamic sex determination sex determination during zygote development, after fertilization, observed, for example, in the marine worm Bonellia viridis: larvae of the worm, swimming freely, develop into females, and ... ... Genetics. encyclopedic Dictionary
Gender definition eusingamous* eusyngamic sex determination sex determination, depending on whether the egg is fertilized or not. It is observed in Hymenoptera insects (bees, riders, etc.), in which fertilization depends on the uterus and ... ... Genetics. encyclopedic Dictionary
Sex definition phenotypic- * semi-significant phenotypic * pheno typic sex determination type of sex determination (), when in the formation of female and male gametes, genital organs and sex in general, it is not genetic stimulants that are decisive, but the corresponding external factors ... ... Genetics. encyclopedic Dictionary
Gender determination type X0- * sex determination of X0 type chromosomal sex determination mechanism, in which the homogametic sex (XX) carries two, and the heterogametic (X0) one X chromosome. Homogametic sex in the process of meiosis forms gametes of only one ... ... Genetics. encyclopedic Dictionary
Books
- Methods for studying hunting and protected animals in the field. Tutorial. Vulture UMO on classical university education, Mashkin Viktor Ivanovich. The manual provides intravital methods for studying game animals in field conditions: accounting of the number and immobilization of animals, trapping and the construction of shelters, monitoring the state ...
Sex Genetics
Sex is characterized by a complex of traits determined by genes located on chromosomes. In the cells of the human body, chromosomes form paired diploid sets. In species with dioecious individuals, the chromosome complex of males and females is not the same and differs in one pair of chromosomes (sex chromosomes). The identical chromosomes of this pair were called the X (x) -chromosome, unpaired, absent in the other sex - Y (y) -chromosome; the rest, for which there are no differences, are autosomes (A).
The cells of a woman contain two identical sex chromosomes, which are designated XX, in men they are represented by two unpaired chromosomes X and Y. Thus, the set of chromosomes of a man and a woman differs in only one chromosome: the chromosome set of a woman contains 44 autosomes + XX, men - 44 autosomes + XY.
During the division and maturation of germ cells in humans, gametes with a haploid number of chromosomes are formed: eggs, as a rule, contain 22 + X chromosomes. Thus, in women, only one type of gamete is formed (gametes with an X chromosome). In males, gametes contain 22 + X or 22 + Y chromosomes, and two types of gametes are formed (an X-chromosome gamete and a Y-chromosome gamete). If, during fertilization, a sperm with an X chromosome enters the egg, a female embryo is formed, and with a Y chromosome, a male.
Therefore, the determination of the sex of a person depends on the presence of X- or Y-chromosomes in the male germ cells - spermatozoa that fertilize the egg.
There are four main types of chromosomal sex determination:
1. The male sex is heterogametic; 50% of gametes carry an X-, 50% -Y - chromosome, for example, humans, mammals, Diptera, beetles, bugs (Slide 4).
2. The male sex is heterogametic; 50% of gametes carry X-, 50% - do not have a sex chromosome, for example, grasshoppers, kangaroos (Slide 7).
3. The female sex is heterogametic; 50% of the gametes carry the X-, 50% of the gametes carry the Y-chromosome, for example, birds, reptiles, tailed amphibians, silkworms (Slide 7).
4. The female sex is heterogametic; 50% of gametes carry X-, 50% do not have a sex chromosome, for example, a mole.
The inheritance of traits whose genes are located on the sex chromosomes is called inheritance, bonded to the floor.
26. Genotype as an integral system. Interaction of genes, multiple action of genes.
Genotype as complete system
properties of genes. Based acquaintance With examples of inheritance of traits in mono- and dihybrid crosses, one might get the impression that the genotype of an organism is made up of the sum of individual, independently acting genes, each of which determines the development of only its own trait or property. Such an idea of a direct and unambiguous relationship between a gene and a trait most often does not correspond to reality. In fact, there are a huge number of traits and properties of living organisms that are determined by two or more pairs of genes, and vice versa, one gene often controls many traits. In addition, the action of a gene can be changed by the proximity of other genes and environmental conditions. Thus, it is not individual genes that act in ontogeny, but the entire genotype as an integral system with complex connections and interactions between its components. This system is dynamic: the appearance of new alleles or genes as a result of mutations, the formation of new chromosomes and even new genomes leads to a noticeable change in the genotype over time.
The nature of the manifestation of the action of a gene in the composition of the genotype as a system can change in different situations and under influence various factors. This can be easily seen if we consider the properties of genes and the features of their manifestation in traits:
The gene is discrete in its action, that is, it is isolated in its activity from other genes.
The gene is specific in its manifestation, that is, it is responsible for a strictly defined feature or property of the organism.
A gene can act gradually, i.e., increase the degree of manifestation of a trait with an increase in the number of dominant alleles (gene dose).
One gene can influence development different signs- this is a multiple, or pleiotropic, action of the gene.
Different genes can have the same effect on the development of the same trait (often quantitative traits) - these are multiple genes, or polygenes.
A gene can interact with other genes, resulting in new traits. Such interaction is carried out indirectly - through the products of their reactions synthesized under their control.
The action of a gene can be modified by changing its location in the chromosome (position effect) or by the influence of various environmental factors.
Interactions of allelic genes. The phenomenon when several genes (alleles) are responsible for one trait is called gene interaction. If these are alleles of the same gene, then such interactions are called allelic, and in the case of alleles of different genes - non-allelic.
The following main types of allelic interactions are distinguished: dominance, incomplete dominance, overdominance and codominance.
domination - the type of interaction of two alleles of one gene, when one of them completely excludes the manifestation of the action of the other. Such a phenomenon is possible under the following conditions: 1) the dominant allele in the heterozygous state provides the synthesis of products sufficient for the manifestation of a trait of the same quality as in the state of the dominant homozygote in the parental form; 2) the recessive allele is completely inactive, or the products of its activity do not interact with the products of the activity of the dominant allele.
Examples of such an interaction of allelic genes can be the dominance of purple coloring pea flowers over white, smooth seed over wrinkled, dark hair over light, brown eyes over blue in humans, etc.
incomplete dominance, or intermediate nature of inheritance, observed in the case when the phenotype of the hybrid (heterozygous) differs from the phenotype of both parental homozygotes, i.e., the expression of the trait is intermediate, with a greater or lesser deviation towards one or the other parent. The mechanism of this phenomenon is that the recessive allele is inactive, and the degree of activity of the dominant allele is insufficient to provide the desired level of manifestation of the dominant trait.
An example of incomplete dominance is inheritance coloring flowers in plants of the night beauty (Fig. 3.5). As can be seen from the diagram, homozygous plants have either red (AA) either white (ah) flowers, and heterozygous (ah)- pink. When crossing a plant with red flowers and a plant with white flowers in F 1, all plants have pink flowers, i.e. intermediate nature of inheritance. When crossing hybrids With pink flowers in F 2 there is a coincidence of splitting by phenotype and genotype, since the dominant homozygote (AA) different from heterozygous (Ah). So, in the example under consideration with plants of the night beauty, splitting in F 2 according to the color of the flowers, usually the following is 1 red (AA): 2 pink (Ah): 1 white (aa).
Rice. 3. 5. Inheritance of flower color with incomplete dominance in a nocturnal beauty.
Incomplete dominance proved to be widespread. It is observed in the inheritance of curly hair in humans, the color of cattle, the color of plumage in chickens, and many other morphological and physiological characteristics in plants, animals and humans.
overdominance- more strong manifestation trait in a heterozygous individual (ah) than any of the homozygotes (AA and aa). It is assumed that this phenomenon underlies heterosis (see § 3.7).
Codemining- participation of both alleles in determining the trait in a heterozygous individual. A striking and well-studied example of coding is the inheritance of the IV blood group in humans (group AB).
The erythrocytes of people in this group have two types of antigens: antigen BUT(determined by the genome /\ existing in one of the chromosomes) and antigen AT(determined by the gene / a, localized in another homologous chromosome). Only in this case both alleles show their effect - 1 BUT (in homozygous controls blood type II, group A) and I B(in the homozygous state controls the III blood group, group B). alleles 1 BUT and I B work in a heterozygote as if independently of each other.
Inheritance example groups blood illustrates and manifestation multiple allelism: a gene can be represented by three different alleles, and there are genes that have dozens of alleles. All alleles of one gene are named a series of multiple alleles, of which each diploid organism can have any two alleles (and only). All of the listed variants of allelic interactions are possible between these alleles.
The phenomenon of multiple allelism is common in nature. Extensive series of multiple alleles are known that determine the type of compatibility during fertilization in fungi, pollination in seed plants, determining the color of animal fur, etc.
Non-allelic Gene Interactions Non-allelic gene interactions have been described in many plants and animals. They lead to the appearance in the offspring of a diheterozygote of an unusual splitting according to the phenotype: 9:3:4; 9:6:1; 13:3; 12:3:1; 15:1 i.e. modifications of the general Mendelian formula 9:3:3:1. Cases of interaction of two, three or more non-allelic genes are known. Among them, the following main types can be distinguished: complementarity, epistasis and polymerization.
Complementary or additional, Such an interaction of non-allelic dominant genes is called, as a result of which a trait appears that is absent in both parents. For example, when two varieties of sweet peas with white flowers are crossed, offspring with purple flowers are produced. If we designate the genotype of one variety AAbb, and the other - aaBB, then
First generation hybrid with two dominant genes (BUT and AT) received the biochemical basis for the production of the purple pigment anthocyanin, while singly neither gene BUT, neither gene B provided the synthesis of this pigment. Anthocyanin synthesis is a complex chain of sequential biochemical reactions controlled by several non-allelic genes, and only in the presence of at least two dominant genes (A-B-) a purple color develops. In other cases (aaB- and A-bb) the flowers of the plant are white (the “-” sign in the genotype formula indicates that this place can be occupied by both a dominant and a recessive allele).
During self-pollination of sweet pea plants from F 1 in F 2 splitting into purple- and white-flowered forms was observed in a ratio close to 9:7. Purple flowers have been found in 9/1 6 plants, white - in 7/16. The Punnett lattice clearly shows the reason for this phenomenon (Fig. 3.6).
epistasis- this is a type of gene interaction in which the alleles of one gene suppress the expression of the allelic pair of another gene. genes, suppressing the action of other genes are called epistatic, inhibitors or suppressors. The suppressed gene is called hypostatic.
According to the change in the number and ratio of phenotype and chesk classes during dihybrid splitting in F 2 consider several types of epistatic interactions: dominant epistasis (A>B or B>A) with splitting 12:3:1; recessive epistasis (a>B or b >A), which is expressed in the 9:3:4 split, etc.
Polymerism manifests itself in the fact that one sign is formed under influence several genes with the same phenotypic expression. Such genes are called polymeric. In this case, the principle of the unambiguous action of genes on the development of a trait is adopted. For example, when crossing shepherd's purse plants with triangular and oval fruits (pods), plants with triangular fruits are formed in F 1. When they self-pollinate in F 2 there is a splitting into plants with triangular and oval pods in a ratio of 15:1. This is because there are two genes that act uniquely. In these cases, they are designated the same - BUT 1 and A 2 .
Rice. 3.6 . Inheritance of flower color in sweet peas
Then all genotypes (BUT 1 ,-BUT 2 ,-, BUT 1 -a 2 a 2 , a 1 a 1 A 2 -) will have the same phenotype - triangular pods, and only plants a 1 a 1 a 2 a 2 will differ - form oval pods. This is the case noncumulative polymer.
Polymeric genes can also act according to the type cumulative polymer. The more similar genes in the genotype of the organism, the stronger the manifestation of this trait, i.e. with an increase in the dose of the gene (BUT 1 BUT 2 BUT 3 etc.) its action is summed up or cumulated. For example, the color intensity of the endosperm of wheat grains is proportional to the number of dominant alleles of different genes in a trihybrid cross. The most colored grains were BUT 1 BUT 1 BUT 2 BUT 2 BUT 3 ,BUT 3 a grains a 1 a 1 a 2 a 2 a 3 a 3 did not have pigment.
According to the type of cumulative polymer, many traits are inherited: milk production, egg production, weight and other traits of farm animals; many important parameters of physical strength, health and mental abilities of a person; the length of the spike in cereals; sugar content in sugar beet roots or lipids in sunflower seeds etc.
Thus, numerous observations indicate that the manifestation of most of the traits is the result of the influence of a complex of interacting genes and environmental conditions on the formation of each specific trait.
Gene Interaction
The relationship between genes and traits is quite complex. In the body, not always one gene determines only one trait and, conversely, one trait is determined by only one gene. More often, one gene can contribute to the manifestation of several traits at once, and vice versa. An organism's genotype cannot be viewed as a simple sum of independent genes, each of which functions independently of the others. Phenotypic manifestations of a particular trait are the result of the interaction of many genes.
Multiple action of genes (pleiotropy) - the processes of the influence of one gene on the formation of several traits.
For example, in humans, the gene that determines red hair color causes lighter skin and the appearance of freckles.
Sometimes genes that determine morphological traits affect physiological functions, reducing viability and fertility, or turn out to be lethal. So, the gene that causes the blue color in the mink reduces its fertility. The dominant gene for gray color in astrakhan sheep in the homozygous state is detailed, since such lambs have an underdeveloped stomach and they die when switching to grass feeding.
Complementary interaction of genes. Several genes can influence the development of one trait. The interaction of several non-allelic genes, leading to the development of one trait, is called complementary. For example, chickens have four forms of crest, the manifestation of any of them is associated with the interaction of two pairs of non-allelic genes. The pink comb is due to the action of the dominant gene of one allele, the pea-shaped comb is due to the dominant gene of the other allele. In hybrids, in the presence of two dominant non-allelic genes, a nut-shaped comb is formed, and in the absence of all dominant genes, i.e. in a recessive homozygous for two non-allelic genes, a simple crest is formed.
The result of the interaction of genes is the color of the coat in dogs, mice, horses, the shape of a pumpkin, the color of sweet pea flowers.
Polymerism is such an interaction of non-allelic genes, when the degree of development of a trait depends on the total number of dominant genes. According to this principle, the color of grains of oats, wheat, skin color in humans is inherited. For example, blacks have 4 dominant genes in two pairs of non-allelic genes, and none in people with white skin, all genes are recessive. Combinations different quantity dominant and recessive genes lead to the formation of mulattoes with different intensity of skin color: from dark to light.
There are two main groups of gene interaction: the interaction between allelic genes and the interaction between non-allelic genes. However, it should be understood that this is not the physical interaction of the genes themselves, but the interaction of primary and secondary products that will determine one or another trait. In the cytoplasm, there is an interaction between proteins - enzymes, the synthesis of which is determined by genes, or between substances that are formed under the influence of these enzymes.
The following types of interaction are possible:
1) for the formation of a certain trait, the interaction of two enzymes is necessary, the synthesis of which is determined by two non-allelic genes;
2) an enzyme that was synthesized with the participation of one gene completely suppresses or inactivates the action of the enzyme that was formed by another non-allelic gene;
3) two enzymes, the formation of which is controlled by two non-allemic genes that affect one trait or one process so that they joint action leads to the emergence and strengthening of the manifestation of the symptom.
Interaction of allelic genes
Genes that occupy identical (homologous) loci at homologous chromosomes ah, are called allelic. Each organism has two allelic genes.
Such forms of interaction between allelic genes are known: complete dominance, incomplete dominance, codominance and overdominance.
The main form of interaction is complete dominance, which was first described by G. Mendel. Its essence lies in the fact that in a heterozygous organism, the manifestation of one of the alleles dominates the manifestation of the other. With complete dominance, splitting by genotype 1:2:1 does not coincide with splitting by phenotype - 3:1. AT medical practice out of two thousand monogenic hereditary diseases, almost half of them have a dominant manifestation of pathological genes over normal ones. In heterozygotes, the pathological allele manifests itself in most cases as signs of the disease (dominant phenotype).
Incomplete dominance is a form of interaction in which in a heterozygous organism (Aa) the dominant gene (A) does not completely suppress the recessive gene (a), as a result of which an intermediate between parental trait appears. Here, the splitting by genotype and phenotype coincides and is 1:2:1
When co-dominated in heterozygous organisms each of the allelic genes causes the formation of a product dependent on it, that is, the products of both alleles turn out to be. A classic example of such a manifestation is the blood group system, in particular the ABO system, when human erythrocytes carry on the surface antigens controlled by both alleles. This form of manifestation is called co-dominance.
Overdominance - when the dominant gene in the heterozygous state is more pronounced than in the homozygous state. Thus, Drosophila with the AA genotype has a normal life expectancy; Aa - elongated trivatism of life; aa - lethal outcome.
Multiple alelism
Each organism has only two allelic genes. At the same time, the number of alleles in nature can often be more than two, if some locus can be in different states. In such cases, one speaks of multiple alleles or multiple allelomorphism.
Multiple alleles are denoted by one letter with different indices, for example: A, A1, A3 ... Allelic genes are localized in the same regions of homologous chromosomes. Since the karyotype always contains two homologous chromosomes, even with multiple alleles, each organism can simultaneously have only two identical or different alleles. Only one of them enters the germ cell (together with the difference between homologous chromosomes). For multiple alleles characteristic influence all alleles for the same trait. The difference between them is only in the degree of development of the trait.
The second feature is that somatic cells or cells of diploid organisms contain a maximum of two of several alleles, since they are located in the same locus of the chromosome.
Another feature is inherent in multiple alleles. By the nature of dominance, allelomorphic traits are placed in a sequential row: more often, a normal, unchanged trait dominates the others, the second gene of the row is recessive relative to the first, but dominates the following, etc. One example of the manifestation of multiple alleles in humans is the blood group of the ABO system.
Multiple allelism is of great biological and practical importance, since it enhances combinative variability, especially genotypic variability.
Interaction of non-alel genes
Many cases are known when a trait or properties are determined by two or more non-allelic genes that interact with each other. Although here the interaction is also conditional, because it is not genes that interact, but the products controlled by them. In this case, there is a deviation from the Mendelian patterns of splitting.
There are four main types of gene interaction: complementarity, epistasis, polymerization, and modifying action (pleiotropy).
Complementarity is a type of interaction of non-allelic genes, when one dominant gene complements the action of another non-allelic dominant gene, and together they determine a new trait that is absent from the parents. Moreover, the corresponding trait develops only in the presence of both non-allelic genes. For example, sulfur coat color in mice is controlled by two genes (A and B). Gene A determines pigment synthesis, however, both homozygotes (AA) and heterozygotes (Aa) are albinos. Another B gene provides pigment accumulations mainly at the base and at the ends of the hair. Crossing of diheterozygotes (AaBb x AaBb) leads to the splitting of hybrids in a ratio of 9:3:4. Numerical ratios for complementary interactions can be as 9:7; 9:6:1 (modified Mendelian splitting).
An example of a complementary interaction of genes in humans can be the synthesis of a protective protein - interferon. Its formation in the body is associated with the complementary interaction of two non-allelic genes located on different chromosomes.
Epistasis is an interaction of non-allelic genes in which one gene suppresses the action of another non-allelic gene. Both dominant and recessive genes (A> B, a> B, B> A, B> A) can cause oppression, and depending on this, dominant and recessive epistasis is distinguished. The suppressor gene is called an inhibitor or suppressor. Inhibitor genes generally do not determine the development of a certain trait, but only suppress the action of another gene.
The gene whose effect is suppressed is called hypostatic. With epistatic interaction of genes, the splitting by phenotype in F2 is 13:3; 12:3:1 or 9:3:4, etc. The color of pumpkin fruits, the color of horses are determined by this type of interaction.
Gender of organisms, a set of morphological and physiological features of the organism, providing sexual reproduction, the essence of which ultimately boils down to fertilization . At the same time, male and female sex cells - gametes merge into a zygote , from which a new organism develops. In the zygote, 2 haploid (single) sets of chromosomes of the maternal and paternal gametes are combined. In the germ cells of a new organism, haploid sets of already recombined paternal and maternal chromosomes are formed (as a result of the exchange of sections of homologous parental chromosomes - crossing over - and their random divergence in daughter cells during meiosis) . Therefore, in a bisexual population, many genetically different individuals constantly arise, which creates favorable conditions for natural selection more adapted forms. This is the main advantage of sexual reproduction over asexual reproduction. Sexual reproduction predominates in animals and higher plants; it is also found in many microorganisms (conjugation in bacteria is accompanied by a partial exchange of hereditary material - DNA strands). The sexual process in unicellular organisms does not require significant differentiation of P. (one and the same cell can be both a cell of the body and a sex cell). In multicellular diploid organisms, special haploid sex cells arose: large and inactive or immobile in the female, small and usually mobile in the male. In most plants and only in some animals, both types of gametes are produced by one individual. , in most animals - different individuals, which in connection with this are strictly divided, respectively, into females and males. In addition to producing cells of different sexes, males and females differ in a number of morphological and physiological signs, as well as sexual behavior, which ensure the fusion of germ cells.
Sex determination
All organisms, including dioecious ones, are genetically bisexual (bisexual); their zygotes receive genetic information that potentially makes it possible to develop male and female traits. In bisexual plants and some hermaphroditic animals, female and male reproductive organs and germ cells develop from genetically identical cells under the influence of internal conditions (in relation to individual cells, they can be considered as external). The mechanism of switching cells to the development of female in one case, male in the other reproductive organs not fully disclosed. AT rare cases in dioecious species, potentially bisexual zygotes develop into females or males under the influence external conditions. For example, in the marine annelids bonellia, the larva, settling on the proboscis of the female, develops into a male, and at the bottom of the sea - into a female. The plant Arisaema japonica develops from large, nutrient-rich tubers into plants with female flowers, and from small tubers, into plants with male flowers. Sex determination under the influence of external conditions is called phenotypic, or modification.
Genetic sex determination is more widespread. In this case, the zygote during fertilization also receives potential opportunities for the development of characteristics of both sexes. However, under the influence genetic factors in one half of the zygotes, the tendency to develop the male sex overpowers, and in the other, the female. A special chromosomal mechanism ensures the transfer of female genes to one half of the offspring, and male genes to the other. At the beginning of the 20th century it was found that in males of some insect species in diploid (with a double set of chromosomes) cells, along with pairs of homologous chromosomes, there is one unpaired chromosome. The female has two such chromosomes. In male insects of other species, all chromosomes are paired, but in one of the pairs they are morphologically dissimilar. These chromosomes, involved in sex determination, were called sexual and the rest - autosomes. Sex chromosomes have been found in many dioecious organisms. The male sex chromosome, which is repeated in females, was called the X chromosome, and not repeated, the Y chromosome. The combination of the sex chromosomes of the male is denoted by the formula X0 or XY, and the females - XX. Males with one sex chromosome produce an equal amount of gametes with the X chromosome and gametes lacking it, that is, with only one haploid set of autosomes (A); females are gametes with only the X chromosome. After a random fusion of male and female gametes, half of the resulting zygotes will have two X chromosomes (XX), and the other half will have only one X chromosome. The first will become females, the second - males.
Males with different sex chromosomes produce an equal number of gametes with an X chromosome and gametes with a Y chromosome. The female gametes of this species are genetically identical - they all carry one X chromosome. As a result, half of the eggs will be fertilized by sperm with a Y chromosome, and the other half with an X chromosome. The first zygotes with the XY structure will develop into males, the second - with XX - into females. Males with one X chromosome or with two different (XY) chromosomes have a heterogametic sex, females with XX chromosomes have a homogametic sex. In many animals, on the contrary, females have a heterogametic sex. Their sex chromosomes are denoted by the letters Z and W or XY, and the sex chromosomes of homogametic males are ZZ or XX. In mammals, nematodes, molluscs, echinoderms, and most arthropods, the male sex is heterogametic. In insects and fish, heterogamety is observed in both males and females. Female heterogamety is characteristic of birds, reptiles and some amphibians.
The bisexual potency inherent in the zygote is due to genes localized in autosomes and manifested only under the control of other genes that realize sex. It is these genes that open the way in one case to the genes that promote the formation of the female sex, in the other - to the genes that determine the development of the male sex. With genetic sex determination according to the type X0, XX, the female sex implementers are localized in the X chromosomes, and the male ones - in autosomes. When one dose of female P.'s implementers, localized in one X-chromosome, is combined with a diploid set of male P.'s implementers, localized in autosomes, the male sex develops. sex and thus determine the female gender. In humans, the Y chromosome plays a sex-determining role. In abnormal cases, it is combined with 2, 3 and even 4 X chromosomes with a normal set of autosomes. Although this leads to pathological abnormalities, however, all individuals with such sets of chromosomes are male. The sex-determining role of Y-chromosomes has been noted in many animal species, and among plants - in meadow drowse. In Drosophila, the Y chromosome contains almost no genes, that is, it is hereditarily inert; female implementers. are localized in X-chromosome, male P.'s implementers - in autosomes. The development of sex is controlled by the ratio of X chromosomes to the set of autosomes (X: A), conventionally taken as a unit in the female (2X: 2A = 1): this ratio in the male is 0.5 (X: 2A = 0.5). An increase in this ratio (sex index) above one leads to the excessive development of female sexual characteristics (“superfemales”), while a decrease below 0.5 contributes to the appearance of males with more pronounced male characteristics("supermales"). Individuals with a sexual index of 0.67 and 0.75 have an intermediate development of the characteristics of both sexes and are called intersexes. The phenomenon of intersexuality demonstrates the bisexual potency of hereditary information transmitted to all descendants.
The mechanism of genetic control over the development of sexual characteristics can be intra- and intercellular. The intracellular determination of P. is not associated with the formation of sex hormones (for example, in insects), and the action of the genes that determine P. is limited to the cells in which these genes function. At the same time, parts of the body with female and male characteristics can develop normally in one organism, without affecting each other. sex hormones , which, penetrating into all cells of the body, cause phenotypic development characteristics of the respective sex. There are progamous, syngamous and epigamous sex determination. Progam sex determination occurs before the fertilization of the egg, for example, the differentiation of eggs into fast-growing and slow-growing ones. The former become large, and after fertilization, females develop from them, the latter are smaller and give males, although both types of eggs are genetically the same. Syngamous sex determination occurs at the time of fertilization, but on different stages this process. In some species with male heterogamety and physiological polyspermy (fertilization of an egg by several spermatozoa), sex is determined at the time of fusion of germ cell nuclei (karyogamy). If a male nucleus with a Y chromosome fuses with the nucleus of an egg, a male will develop, if with an X chromosome, a female will develop. In female heterogamety, the sex of the offspring depends on which of the sex chromosomes enters the nucleus of the egg during meiosis. If the Z chromosome is in the nucleus, a male will develop, if the W chromosome is female. T. o., in this case the sex of the zygote is established before karyogamy. Epigamous sex determination is observed in heterosexual species with phenotypic sex determination, when the direction of development towards the male or female sex is determined by the influence of external conditions after fertilization.
Dependence of signs on gender
Signs that are limited and controlled by sex depend on sex. Sex-limited traits due to sexual differentiation can appear only in one of the sexes (milk or egg production is characteristic only of the female sex), although the polymer genes for these traits are localized in the autosomes of both sexes. Sex-controlled traits appear either in both sexes (with varying degrees of severity), or (more often) only in one of the sexes (more powerful development of horns in rams, beards in goats), although both equally contain the genes of these signs. Their dissimilar development is due to a significant difference physiological processes in organisms of different sexes.
Genes that determine sex-linked traits are localized on both paired and unpaired sex chromosomes and are therefore inherited differently than traits determined by paired genes localized on the autosomes of both sexes. If the genes are localized in the unpaired Y-chromosome of a heterogametic male, then the traits determined by them are inherited only by sons, and if the genes are localized in the chromosome of a heterogametic female, only by daughters. Inherited t. the characters are called hollandic. This type of inheritance is found in some species of fish and insects. In other animal species, it has not been proven with complete certainty. When genes are localized in homologous X- or Z-chromosomes, the traits caused by them are transmitted sex-linked according to a type called crosswise inheritance, when recessive trait mothers will appear in sons, and dominant in daughters (T. X. Morgan), which is found in many animal species (for example, the tricolor of cats, striped plumage and its growth rate in chickens). Many sex-linked mutations have been found in Drosophila and the silkworm.
Letali, the genes that cause death during the development of the organism, can also be linked to P.. If a homogametic parent is heterozygous for flying, localized in one of the homologous sex chromosomes (X or Z), then half of its heterogametic descendants will die, having received a detail whose destructive effect in the genotype will not be opposed by the normal allele. With female heterogamety, half of the daughters die from lethals, and with male heterogamety, half of the sons. Sometimes mutant genes in the X and Z chromosomes only partially reduce the viability of the offspring or cause various diseases, most often manifested in the heterogametic sex. More than 50 sex-linked mutations have been found in humans. for the most part to disrupt the normal functioning of the body.
sex ratio
With the phenotypic definition of P., it depends on the number of developing organisms that fall under the influence external factors that determine one gender or another. In genetic sex determination, the sex ratio in most species tends to be very close to 100♀:100♂ (100 females: 100 males). However, even with this definition of sex, there are deviations. So, in some species of mammals with male heterogamety, statistically significantly more male offspring are born by 1-2%.
Floor regulation
A significant shift in the ratio of organisms towards one of the sexes has both theoretical and practical significance, since one of the sexes is usually more productive. Methods of sex regulation, reduced to 4 main directions, are used depending on the type of sex determination and the biological and economic characteristics of the species.
Phenotypic sex reassignment. If the action of sex genes is realized through hormones, sexual characteristics change when the genital organs of one sex are transplanted to another or when hormones of the opposite sex, as well as certain amino acids, are introduced into the body. The degree of phenotypic changes in sex depends on the characteristics of the type and dose of the administered drug. However, only in rare cases (in some fish and amphibians) individuals with a phenotypically redefined sex produce gametes opposite to their genotypic sex. In the next generation, if the action of hormones ceases, the genetic mechanism of sex determination again comes into force.
Management of the genetic mechanism of sex determination or an artificial combination of sex chromosomes in the egg. A directional change in the sex ratio was achieved in experiments with the silkworm, in which the sex is strictly determined by the combination of sex chromosomes (ZW - ♀ ; ZZ - ♂). Unfertilized eggs after warming up develop parthenogenetically due to the diploid nucleus, which has not completed the reduction division. All cells of the parthenogenetic embryo retain the maternal structure, in particular with respect to the ZW sex chromosomes, and, consequently, develop only into females (B. L. Astaurov). By exposure to ionizing radiation and heating, it was possible to suppress the female nucleus in a freshly laid inseminated egg and switch development to the male principle. The diploid nucleus of the male zygote is formed by the fusion of two male nuclei and therefore has the structure of the male P. ZZ. Caterpillars are always male from such zygotes (H. Hashimoto; B. L. Astaurov). These methods for the first time at page - x. species of silkworm solved the problem of arbitrary regulation of sex. In mammals, scientists are trying to divide by morphological and physiological features X- and Y-spermatozoa for the purpose of subsequent insemination with one category of spermatozoa. However, this method has not yet been able to reliably shift the sex ratio.
Early sex recognition is used to sort hatched chicks into males and females by sex-linked plumage color, as well as for "super early" sorting by silkworm sex. Under the influence of ionizing irradiation, an autosome with a dominant gene responsible for the dark color of silkworm eggs was transplanted into the sex W-chromosome in a silkworm. The linkage of chromosomes is persistently inherited. Those eggs into which the W-chromosome with the transplanted dominant gene enters acquire dark color and develop into females, while the male eggs, having not received the dominant gene, remain unpigmented. Photovoltaic automatons separate the different-colored eggs into the sexes at high speed. Bred in this way (V. A. Strunnikov and L. M. Gulamova), sex-labeled breeds of silkworms are found practical use in Soviet sericulture. In the 60s. 20th century in the experiments of the English scientists R. Edwards and
R. Gardner recorded the birth of offspring of only one sex in mammals. In rabbits, early embryos were removed from the mother's body, their sex was determined by the cytological method, and then the embryos of the unwanted sex were discarded, and the embryos of the desired sex were returned to the uterus. About 20% of the returned embryos took root and developed into rabbits of the sex predicted by scientists.
In almost all animals with genetic sex determination, a change in the sex ratio can be the result of the death of half of the embryos of the heterogametic sex under the action of sex-linked parts. However, for many pages - x. animals, such an approach to sex regulation is not economically justified. The exception is the silkworm. In the USSR, a genetically special breed of silkworm was bred by the radiation method (V. A. Strunnikov), in which in both Z-chromosomes of males there is always one non-homologous lethal to each other (balanced lethals). If these males are crossed with females of ordinary breeds, at the egg stage, one half of the females will die from the first, and the other from the second flying. The male eggs hatch into normal caterpillars. This method allows you to get only one more productive male sex from the silkworm in unlimited quantities.
Determining the final sex in a person begins with determining the genetic (chromosomal) sex; this is the most milestone, but he does not yet determine the sex definitively; known series pathological conditions in which, despite the chromosomal sex, further development gender occurs in the opposite direction.
McClung (1902) was the first to establish the relationship between chromosomes and sex; he found one additional chromosome pair in insect cells and concluded that it determines the sex of the male. They did not agree with his assumption for a long time. However, a few years later, Stevens (1905) and independently Wilson (1905), who also studied insect cells, found one particular chromosome pair in individual first-order spermatocytes (Fig. 1); at present, we know that the chromosome pair described by them corresponds to XY - a pair of sex chromosomes. The same authors described that in the process of the reduction division of spermatocytes, one of the chromosomes penetrates into one, and the second into the other. daughter cell. Thus, they found that two spermatocytes are formed, one of which contains X, and the second contains a Y chromosome. They concluded that XX is female and XY is male. This concept received general recognition only after 20 years.
Rice. one. normal process spermatogenesis.
Rice. 2. Normal process of ovogenesis.
Thanks to the studies of Tjio and Levan (1956), Ford and Hamerton (1956), it became known that human cells contain 46, and not 48, chromosomes, as previously thought. Of the 46 chromosomes, 22 pairs are autosomes and one pair are sex chromosomes. In the cells of the female individual there is a combination of XX, and in the cells of the male - a combination of XY. When the reduction division ends in the process of gametogenesis, one chromosome passes into each cell; thus, each egg cell contains one X chromosome, while half of the spermatozoa contain one X, and the other half contain one Y chromosome (Fig. 2).
In some species of insects, the chromosome set differs from the chromosome set described above, which is characteristic of humans and most vertebrates. In other species of insects and vertebrates, spermatozoa contain an X or O chromosome. If a sperm containing the X chromosome enters the egg, the XX combination is formed, which is characteristic of the female; if the egg is fertilized by a sperm carrying the O-chromosome, a combination of XO occurs, which is characteristic of the male.
In birds and some species of butterflies, the situation is reversed: their eggs contain two types of chromosomes, and spermatozoa only one. Fertilization in these animal species occurs as follows: the cells of females contain XX or XO chromosomes, and the cells of males contain XX chromosomes; if a spermatozoon fertilizes an egg carrying the X chromosome, the XX combination characteristic of the male is formed in the zygote; If a spermatozoon fertilizes an egg containing a Y chromosome, a XY combination of sex chromosomes is formed in the zygote, which is characteristic of a female individual. It should be noted that in animals in which the sex of the offspring is determined by the female gamete, the sex chromosomes of females are usually denoted by the letters ZW, and the sex chromosomes of males by the letters ZZ. The water frog (rana esculenta) has an XX and XY chromosome set, while other frog species have a ZW and ZZ chromosome set.
In connection with the study of the hereditary transmission of sex in humans, a very significant question arose: is the female sex determined by the presence of two X chromosomes or the absence of a Y chromosome, or, conversely, is the male sex determined by the presence of only one X chromosome or the presence of a Y chromosome? For a long time The prevailing view was that the male sex is determined by the presence of only one X chromosome. Studies over the past 10 years, and especially the study of cases of Klinefelter syndrome, have convincingly shown that the male is determined due to the presence of the Y chromosome and therefore gonad the male (testicle) produces androgenic hormone. The presence of the X chromosome is unlikely to affect sex determination. In the chapter on hermaphroditism, it will be stated that patients with Klinefelter syndrome have an XXY chromosome set and that these patients have a male phenotype. The search for the reasons for the previously existing misconception about the definition of sex would take us very far; suffice it to recall that all authors previously proceeded from the results of a study of the chromosomes of the fruit fly (Drosophila), in which the species number and set of chromosomes are different from those in humans.
The process of transmission of sex by inheritance in humans is shown schematically in Fig. 1, 2 and 3.
Rice. 3. Determination of chromosomal sex.
Chromatin genetic sex. Sex differences are determined during fertilization by different chromosomal content of gametes. Fusion of two heterosomal portions of the X chromosomes (female subject) results in a chromatin mass defined as a globular agglomeration located under the nuclear envelope of the desquamated epithelium of the vaginal and buccal mucosa. In mature neutrophils, this accumulation is located in the form of a "drumstick". Men do not have these nuclear formations, since the Y chromosome is small, and the XY combination is small.
A regular blood smear is stained but Giemsa - Romanovsky.
Count the number " drumsticks» in mature neutrophils. These nuclear outgrowths protrude towards the periphery of the cell. The size of each of them is 1.5 microns, the head is rounded. In each cell there is no more than one "drumstick". They should be distinguished from granular, club-shaped and stab outgrowths in neutrophils. Such outgrowths, although more common in women, do not play a role in establishing genetic sex. In males, the number of "drum sticks" ranges from 0 to 4 per 500 neutrophils. In females, they are at least 6 per 500 leukocytes.
Determination of genetic sex by desquamated epithelium of the oral mucosa.
Technique. Scraping is done with a dry sterile spatula made of glass, wood or metal inner wall cheeks. The material is placed on a slide and covered with a cover slip. Fix with a solution for 1-2 hours equal parts 95% ethyl alcohol and sulphate ether. Then stain with the following reagents:
min,
70% ethanol.............2
50% ethyl alcohol............2
Distilled water..............2
Cresyl violet 1% water solution. . . . 5
95% ethyl alcohol...............5
95% ethyl alcohol ........... .5
Absolute ethyl alcohol ........... 5
Xylene......................5
Xylene.....................5
Canadian balsam ................ 5
Sex chromatin from a scraping of the epithelium of the mucous surface of the cheeks consists of glomerular formations, densely stained and located peripherally, under the nuclear membrane itself.
In females, they are found in every fifth cell, while in males, their number is 0-4 per hundred cells.
To establish the genetic sex, at least 25 cells must be examined, and only cells with large, round, light-colored nuclei, no wrinkles on the body of the nucleus, not covered by neighboring cells, and no accumulations of microflora that darken the structure of the nucleus are taken into account.
Determination of genetic sex by desquamated epithelium of the vaginal mucosa. A vaginal smear is prepared and stained. In genetically female subjects, the sex chromatin is densely stained globular formations that are located under the shell of the nucleus. There are no such nuclear formations in genetic males.
Most simple method is the determination of sex chromatin in a buccal scraping, vaginal smears require at least a rudimentary vagina, and blood smears are more laborious due to the duration of the neutrophil count, especially in genetic males.
The determination of the genetic sex allows you to establish the presence of a discrepancy between the genital organs and the genetic sex, as well as violations, on the one hand, of the somatic sex, and on the other, of the genetic sex or genital organs.
1. What chromosomes are called sex chromosomes?
Answer. Sex chromosomes are a pair of chromosomes that differ between males and females of the same species. In one of the sexes, these are, as a rule, two identical large chromosomes (X chromosomes, genotype XX); the other has one X chromosome and one smaller Y chromosome (XY genotype). In some species, the male sex is formed in the absence of one sex chromosome (X0 genotype).
2. What organisms are called hermaphrodites?
Answer. A hermaphrodite is an organism that has male and female gonads that form sex cells in one individual. Such hermaphroditism occurs in flat and annelids. This is true hermaphroditism. A variation of it can be the hermaphroditism of mollusks, the gonad of which, depending on age and conditions of existence, periodically produces either male or female gametes. In the case of false hermaphroditism, one individual develops external genitalia and secondary features both sexes, and the gonads are of the same sex (male or female).
3. What diseases are called hereditary?
Answer. hereditary diseases- these are diseases caused by a change in the genotype (i.e. mutations). They are not always passed on from parents to children. Many hereditary diseases cannot be inherited (passed down from generation to generation), as they reduce the viability of the patient or cause infertility. May occur in children of healthy parents as a result of a new mutation. For example, healthy, as a rule, parents have a child with Down syndrome. On the other hand, some endemic diseases are observed in parents and children. The impression is made of inheritance, but the diseases are not hereditary (for example, endemic goiter).
Questions after §45
1. What types of chromosomes do you know?
Answer. Chromosomes are divided into sex and non-sex (autosomes). Sex chromosomes are a pair of chromosomes that differ between males and females of the same species. In one of the sexes, these are, as a rule, two identical large chromosomes (X chromosomes, genotype XX); the other has one X chromosome and one smaller Y chromosome (XY genotype). In some species, the male sex is formed in the absence of one sex chromosome (X0 genotype). Autosomes are pairs of chromosomes that are identical in individuals of the same biological species belonging to different sexes. The number of pairs of autosomes is equal to the number of pairs of chromosomes in the genotype minus one (one pair of sex chromosomes). So, in humans, 22 pairs of autosomes, in Drosophila - 3 pairs. All autosomes of each biological species are given sequence numbers according to their size (the first is the largest; the last is the shortest and therefore carries the fewest genes)
2. What is homogametic and heterogametic sex?
Answer. Homogametic is the sex that forms gametes of the same type on the sex chromosomes (XX genotype). The heterogametic sex in the process of gametogenesis forms gametes of two types according to the sex chromosomes (genotype XY or X0). In humans, the female is homogametic, the male is heterogametic (XY genotype)
3. How is sex inherited in mammals?
Answer. In men's and female organisms all pairs of chromosomes, except for one, are the same and are called autosomes, and one pair of chromosomes, called sex chromosomes, differs in males and females. Males and females have different sex chromosomes: females have two X chromosomes, and males have X and Y. The sex of the future individual is determined during fertilization. If the sperm contains the X chromosome, then a female (XX) will develop from the fertilized egg, and if the sperm contains the sex Y chromosome, then the male (XY).
4. What other variants of chromosomal and non-chromosomal sex determination in living organisms do you know? Give specific examples.
Answer. In birds and reptiles, males are homogametic (XX) and females are heterogametic (XY). In some insects, males in the chromosome set have only one sex chromosome(X0), while females are homogametic (XX).
Bees and ants do not have sex chromosomes, and females have a diploid set of chromosomes in the cells of the body, and males that develop parthenogenetically (from unfertilized eggs) have a haploid set of chromosomes. Naturally, in this case, the development of spermatozoa in males proceeds without meiosis, since it is impossible to reduce the number of chromosomes of a less than haploid set.
Crocodiles have no sex chromosomes. The sex of the embryo developing in the egg depends on the temperature environment: at high temperatures more females develop, and in the event that it is cool, more males.
5. Is the male or female sex in humans heterogametic?
Answer. Humans are heterogametic male body(XY).
6. Are there differences in the number of chromosomes between the queen and workers of the honey bee?
Answer. Each cell of a honey bee (wombs and working individuals) has 32 chromosomes in the nuclei of cells.
However, honey bees have significant differences from the generally accepted mechanism for passing on the heredity of parents to their descendants and determining the sex of individuals. If in all farm animals the sex of an individual is determined by certain sex chromosomes, then in honey bees the sex of an individual is determined differently: when eggs are fertilized, females (wombs and worker bees) are formed from them, and males (drones) develop from an unfertilized uterus egg.
Thus, female individuals of the family are diploid organisms (have 32 chromosomes), and male individuals (drones) are haploid (have 16 chromosomes in their cells).
