Author of the chromosome theory. Linked inheritance

The creator of the chromosome theory (CT) is the scientist Thomas Morgan. CHT is the result of studying heredity at the cellular level.

The essence of the chromosome theory:

Chromosomes are the material carriers of heredity.

The main evidence for this is:

Cytogenetic parallelism

Chromosomal sex determination

sex-linked inheritance

Gene linkage and crossing over

The main provisions of the chromosome theory:

Hereditary inclinations (genes) are localized in chromosomes.

Genes are located on the chromosome in a linear order.

Each gene occupies a specific area (locus). Allelic genes occupy similar loci on homologous chromosomes.

Genes located on the same chromosome are inherited together, linked (Morgan's Law) and form a linkage group. The number of linkage groups is equal to the haploid number of chromosomes (n).

Between homologous chromosomes, an exchange of regions, or recombination, is possible.

The distance between genes is measured in percent of crossing over - morganides.

The frequency of crossing over is inversely proportional to the distance between genes, and the strength of linkage between genes is inversely proportional to the distance between them.

Cytogenetic parallelism

Morgan's graduate student Sutton noticed that the behavior of genes according to Mendel coincides with the behavior of chromosomes: (TABLE - Cytogenetic Parallelism)

Each organism carries 2 hereditary inclinations, only 1 hereditary inclination from a pair enters the gamete. During fertilization in the zygote and further in the body, again 2 hereditary inclinations for each trait.

Chromosomes behave in exactly the same way, which suggests that genes lie on chromosomes and are inherited along with them.

Chromosomal sex determination

In 1917, Allen showed that male and female mosses differ in the number of chromosomes. In the cells of the diploid tissue of the male body, the sex chromosomes are X and Y, in the female X and X. Thus, chromosomes determine such a trait as sex, and therefore can be material carriers of heredity. Later, chromosomal sex determination was also shown for other organisms, including humans. (TABLE)

sex-linked inheritance

Since the sex chromosomes are different in male and female organisms, traits whose genes are located on the X or Y chromosomes will be inherited differently. Such signs are called sex-linked traits.

Features of the inheritance of sex-linked traits

Mendel's 1st law is not respected

Reciprocal crosses give different results

There is a criss-cross (or criss-cross inheritance).

For the first time, inheritance associated with a trait was discovered by Morgan in Drosophila.

(TABLE sex-linked inheritance)

Sex-linked inheritance is explained by the absence of genes on the Y chromosome that are allelic to genes on the X chromosome. The Y chromosome is much smaller than the X chromosome, it currently contains 78 (?) genes, while there are more than 1098 on the X chromosome.

Examples of sex-linked inheritances:

Hemophilia, Duchenne dystrophy, Duncan's syndrome, Alport's syndrome, etc.

There are genes that, on the contrary, are found on the Y chromosome and are absent on the X chromosome; therefore, they are found only in male organisms, and never in female organisms (Holandric inheritance) and are transmitted only to sons from the father.

Gene linkage and crossing over

In genetics, such a phenomenon as "gene attraction" was known: some non-allelic traits were not inherited independently, as they should according to Mendel's III law, but were inherited together, did not give new combinations. Morgan explained this by saying that these genes are on the same chromosome, so they diverge into daughter cells together in one group, as if linked. He called this phenomenon linked inheritance.

Morgan's Coupling Law:

Genes located on the same chromosome are inherited together, linked.

Genes located on the same chromosome form a linkage group. The number of linkage groups is equal to "n" - the haploid number of chromosomes.

Homozygous lines of flies with a gray body color and long wings and flies with a black body and short wings were crossed. The genes for body color and wing length are linked, i.e. lie on the same chromosome.

As a result, in F 2, splitting occurs as in a 3: 1 monohybrid cross.

Crossing over.

In a small percentage of cases in F 2 in Morgan's experiments, flies appeared with new combinations of characters: long wings, black body; the wings are short and the body is grey. Those. the signs "disconnected". Morgan explained this by the fact that chromosomes exchange genes during conjugation in meiosis. As a result, individuals with new combinations of traits are obtained, i.e. as required by Mendel's third law. Morgan called this gene exchange recombination.

Later, cytologists did indeed confirm Morgan's hypothesis by discovering the exchange of chromosome regions in corn and in the salamander. They called this process crossing over.

Crossing over increases the diversity of offspring in a population.

1) Genes are located on chromosomes.

2) Genes in chromosomes are located linearly one after another and do not overlap.

3) Genes located on the same chromosome are called linked and form a linkage group. Since homologous chromosomes include allelic genes responsible for the development of the same traits, both homologous chromosomes are included in the linkage group; thus, the number of linkage groups corresponds to the number of chromosomes in the haploid set. Within each linkage group, due to crossing over, gene recombination occurs.

4) Morgan's Law - "Genes located on the same chromosome are inherited together."

Complete linkage of genes. If the genes are located in the chromosome directly next to each other, then crossing over between them is almost unbelievable. They are almost always inherited together, and a 1:1 split is observed in test crosses.

Incomplete linkage of genes. If the genes in the chromosomes are located at some distance from each other, then the frequency of crossing over between them increases and, therefore, crossover chromosomes appear that carry new combinations of genes: Ab and aB

Their number is directly proportional to the distance between the genes. With incomplete linkage, a certain number of crossover forms appear in the offspring, and their number depends on the distance between the genes. The percentage of crossover forms indicates the distance between genes located on the same chromosome.

Interactions of non-allelic genes

Complementarity is a phenomenon in which the gene of one allelic pair contributes to the manifestation of the genes of another allelic pair.

1) Sweet peas have gene A, which determines the synthesis of a colorless pigment precursor - propigment. Gene B determines the synthesis of an enzyme, under the action of which a pigment is formed from the propigment. Sweet pea flowers with the genotype aaBB and Aabb are white: in the first case there is an enzyme, but there is no pigment, in the second there is a pigment. but there is no enzyme that converts propigment to pigment:

2) Neoplasm of a trait - inheritance of the shape of the crest in chickens of some breeds. As a result of various combinations of genes, four variants of the crest shape arise:

Pic. Rooster comb shape: A - simple (aabb); B - pea-shaped (aaBB or aaBB); B - nut-shaped (AABB or AaB); G - rose-shaped (AAB or Aabb)

Epistasis is a phenomenon in which a gene of one allelic pair prevents the manifestation of genes from another allelic pair, for example, the development of fruit color in a pumpkin. Pumpkin fruits will be colored only if the dominant gene B from another allelic pair is absent in the plant genotype. This gene suppresses color development in pumpkin fruits, and its recessive allele b does not prevent color development (Aabb - yellow fruits; aabb - green fruits; AABB and aaBB - white fruits).

Polymerism is a phenomenon in which the degree of expression of a trait depends on the action of several different pairs of allelic genes, and the more dominant genes of each pair in the genotype, the more pronounced the trait. In wheat, the red color of grains is determined by two genes: a1, a2;. Non-allelic genes are designated here by one letter A (a) because they determine the development of one trait. With the genotype A1A1A2A2, the color of the grains is the most intense, with the genotype a1a1a2a2 they are white. Depending on the number of dominant genes in the genotype, all transitions between intense red and white color can be obtained:

Rice. 26. Inheritance of the color of wheat grains (polymeria)

Chromosomal theory of heredity. Chromosomal maps of a person.

T.Morgan's chromosome theory.

Maps of human chromosomes.

T.Morgan's chromosome theory.

Observing a large number of flies, T. Morgan revealed many mutations that were associated with changes in various traits: eye color, wing shape, body color, etc.

When studying the inheritance of these mutations, it turned out that many of them are inherited, linked to the floor.

Such genes were easy to isolate, because they were passed from maternal individuals only to male offspring, and through them only to their female offspring.

In humans, traits inherited through the Y chromosome can only be in males, and those inherited through the X chromosome can be in individuals of both one and the other sex.

In this case, a female individual can be homozygous or heterozygous for genes located on the X chromosome, and recessive genes can appear in her only in the homozygous state.

A male individual has only one X chromosome, therefore all genes localized in it, including recessive ones, appear in the phenotype. Pathological conditions such as hemophilia (slow blood clotting, causing increased bleeding), color blindness (vision anomaly in which a person confuses colors, most often red with green), are inherited in a person linked to sex.

The study of sex-linked inheritance has stimulated the study of linkages between other genes.

As an example, experiments on Drosophila can be cited.

Drosophila has a mutation that causes a black body color. The gene that causes it is recessive with respect to the gray gene characteristic of the wild type. The mutation that causes vestigial wings is also recessive to the gene that results in the development of normal wings. A series of crosses showed that the gene for black body color and the gene for rudimentary wings were passed on together, as if both of these traits were caused by the same gene.

The reason for this result was that the genes responsible for the two traits are located on the same chromosome. This phenomenon is called complete linkage of genes. There are many genes on each chromosome that are inherited together, and such genes are called a linkage group.

Thus, the law of independent inheritance and combination of traits, established by G. Mendel, is valid only when the genes that determine a particular trait are located on different chromosomes (different linkage groups).

However, genes on the same chromosome are not perfectly linked.

Linked genes, crossing over.

Cause incomplete clutch is an crossing over. The fact is that during meiosis, during the conjugation of chromosomes, they cross over, and homologous chromosomes exchange homologous regions. This phenomenon is called crossover. It can occur anywhere on homologous X chromosomes, even multiple locations on the same pair of chromosomes. Moreover, the farther apart the loci are located on the same chromosome, the more often one should expect crossover and exchange of sites between them.

Figure 17 Crossing over: a - process diagram; b - variants of crossing over between homologous chromosomes

Maps of human chromosomes.

Each gene linkage group contains hundreds or even thousands of genes.

In the experiments of A. Sturtevant in 1919, it was shown that the genes inside the chromosome are arranged in a linear order.

This was proven by analysis of incomplete linkage in a gene system belonging to the same linkage group.

The study of the relationship between three genes during crossing over revealed that if the frequency of crossover between genes A and B is equal to M, and between genes A and C, the frequency of exchanges is equal to N, then the frequency of crossover between genes B and C will be M + N, or M - N, depending on the sequence in which the genes are located: ABC or DIA. And this pattern applies to all genes of this linkage group. An explanation for this is possible only with a linear arrangement of genes in the chromosome.

These experiments were the basis for the creation of genetic maps of the chromosomes of many organisms, including humans.

The unit of the genetic or chromosomal map is the centimorganide (cM). This is a measure of the distance between two loci, equal to the length of the chromosome segment, within which the probability of crossing over is 1%.

Methods for studying gene linkage groups, such as: genetic analysis of somatic hybrid cells, the study of morphological variants and chromosome anomalies, hybridization of nucleic acids on cytological preparations, analysis of the amino acid sequence of proteins, and others, which made it possible to describe all 25 linkage groups in humans.

One of the main goals of the study of the human genome is to build an accurate and detailed map of each chromosome. A genetic map shows the relative location of genes and other genetic markers on a chromosome, as well as the relative distance between them.

A genetic marker for mapping could potentially be any inherited trait, be it eye color or the length of DNA fragments. The main thing in this case is the presence of easily detectable interindividual differences in the considered markers. Chromosome maps, like geographic maps, can be built on a different scale, i.e. with different levels of resolution.

The smallest map is the pattern of differential staining of chromosomes. The maximum possible resolution level is one nucleotide. Therefore, the largest map of any chromosome is the complete nucleotide sequence. The size of the human genome is approximately 3,164.7 m.p.

To date, small-scale genetic maps have been built for all human chromosomes with a distance between adjacent markers of 7–10 million base pairs or 7–10 Mb (megabase, 1 Mb = 1 million base pairs).

Modern data on human genetic maps contain information on more than 50,000 markers. This means that they are, on average, tens of thousands of base pairs apart, with several genes in between.

For many sites, of course, there are more detailed maps, but still most of the genes have not yet been identified and not localized.

By 2005, more than 22,000 genes have been identified and about 11,000 genes have been mapped on individual chromosomes, about 6,000 genes have been localized, of which 1,000 are disease-determining genes.

The discovery of an unusually large number of genes on chromosome 19 (more than 1400) was unexpected, which exceeds the number of genes (800) known on the largest human chromosome 1.

Figure 18 Pathological anatomy of chromosome 3

Mitochondrial DNA is a small circular molecule 16,569 base pairs long. Unlike the DNA of the nuclear genome, it is not associated with proteins, but exists in a “pure” form.

Figure 19 Structure of the mitochondrial genome

Mitochondrial genes lack introns, and intergenic gaps are very small. This small molecule contains 13 protein-coding genes and 22 transfer RNA genes. Mitochondrial DNA has been fully sequenced and all structural genes have been identified on it. Mitochondrial genes have a much higher copy number than chromosomal ones (several thousand per cell).

Hereditary properties of blood.

The mechanism of inheritance of blood groups of the ABO system and the Rh system.

One locus could have either a dominant or a recessive gene. However, often a trait is determined not by two, but by several genes.

Three or more genes that can be at the same locus (occupy the same place on homologous chromosomes) are called multiple alleles.

In the genotype of one individual, there can be no more than two genes from this set, however, in the gene pool of a population, the corresponding locus can be represented by a large number of alleles.

An example is the inheritance of the blood group.

Gene I A encodes the synthesis of a specific agglutinogen A protein in erythrocytes, gene I B - agglutinogen B, gene I O does not encode any protein and is recessive with respect to I A and I B ; I A and I B do not dominate each other. Thus, the genotype I O I O determines the blood type 0 (first); I A I A and I A I O - group A (second); I B I B and I B I O - group B (third); I A I B - group AB (fourth).

If one of the parents has blood type 0, then (with the exception of unlikely situations requiring additional examinations) he cannot have a child with blood type AB.

Causes and mechanism of occurrence of complications in blood transfusion associated with improperly selected donor blood.

According to the definition of immunogenetics, a blood group is a phenomenon of a combination of erythrocyte antigens and antibodies in plasma.

The blood group is determined by a combination of alleles. Currently, more than 30 types of alleles that determine blood groups are known. When transfusion takes into account those groups that can cause complications. These are the blood groups of the ABO system, Rh-factor, C, Kell. Antibodies are stored in the donated blood of these groups. In other known groups, antibodies in donated blood are rapidly destroyed.

On fig. 20 a) shows the blood groups of the ABO system, where the antibodies corresponding to group B antigens are blue, group A is red. The figure shows that plasma of group A has antibodies to group B, group B has antibodies to group A, group AB has no antibodies, group O has antibodies to groups A and B.

During hemotransfusion (blood transfusion), plasma is transfused, since the erythrocytes of each person carry a huge amount of antigens specific to that person on the membrane surface. Once in the blood of the recipient, they cause severe immune reactions.

Figure 20 Covi groups of the ABO system; a) a combination of antigens on erythrocytes and antibodies in plasma, b) hemolysis of recipient erythrocytes with antibodies from donor blood.

If a recipient with group B is transfused with blood (plasma) of group B, the antibodies in the plasma will immediately interact with erythrocyte antigens, followed by lysis of erythrocytes (Fig. 20 b). The same mechanism of occurrence of complications in blood transfusion associated with improperly selected donor blood.

Practical lesson

Solving problems modeling crossbreeding, sex-linked inheritance, inheritance of blood groups according to the ABO system and the Rh system

§ 5. T. G. Morgan and his chromosome theory

Thomas Gent Morgan was born in 1866 in Kentucky (USA). After graduating from the university at twenty, Morgan was awarded the title of Doctor of Science at twenty-four, and at twenty-five he became a professor.

Since 1890, Morgan has been engaged in experimental embryology. In the first decade of the 20th century, he was fond of questions of heredity.

It sounds paradoxical, but at the beginning of his activity Morgan was an ardent opponent of Mendel's teachings and was going to refute his laws on animal objects - rabbits. However, the Columbia University trustees found the experience too costly. So Morgan began his research on a cheaper object - the Drosophila fruit fly, and then not only did not come to the denial of Mendel's laws, but also became a worthy successor to his teachings.

A researcher in experiments with Drosophila creates chromosome theory of heredity- the largest discovery, occupying, by expression N. K. Koltsova, "the same place in biology as the molecular theory in chemistry and the theory of atomic structures in physics."

In 1909-1911. Morgan and his equally illustrious students A. Sturtevant, G. Moeller, C. Bridges showed that Mendel's third law requires significant additions: hereditary inclinations are not always inherited independently; sometimes they are transmitted in whole groups - linked to each other. Such groups located on the corresponding chromosome can move to another homologous chromosome during conjugation of chromosomes during meiosis (prophase I).

The full chromosome theory was formulated T. G. Morgan in the period from 1911 to 1926. This theory owes its appearance and further development not only to Morgan and his school, but also to the work of a significant number of scientists, both foreign and domestic, among which, first of all, one should name N. K. Koltsova and A. S. Serebrovsky (1872-1940).

According to the chromosome theory, transmission of hereditary information is associated with chromosomes, in which linearly, at a certain locus (from lat. locus- place), genes lie. Since the chromosomes are paired, each gene on one chromosome corresponds to a paired gene on the other chromosome (homolog) lying in the same locus. These genes can be the same (in homozygotes) or different (in heterozygotes). Various forms of genes that arise by mutation from the original are called alleles, or allelomorphs(from Greek allo - different, morph - form). Alleles affect the manifestation of a trait in different ways. If a gene exists in more than two allelic states, then such alleles in populations* form a series of so-called multiple alleles. Each individual in a population can contain any two (but no more) alleles in its genotype, and each gamete can contain only one allele, respectively. At the same time, individuals with any alleles of this series can be in the population. Hemoglobin alleles are an example of multiple alleles (see Chapter I, § 5).

* (A population (from Latin popularus - population) is a group of individuals of the same species, united by mutual crossing, to some extent isolated from other groups of individuals of this species.)

The degree of dominance in a series of alleles can increase from the extreme recessive gene to the extreme dominant. Many examples of this type can be cited. So, in rabbits, the recessive gene series multiple alleles is the c gene that determines the development of albinism*. The c h gene of Himalayan (ermine) coloration (pink eyes, white body, dark tips of the nose, ears, tail and limbs) will be dominant in relation to this gene; over this gene, as well as over the c gene, the gene of light gray color (chinchilla) c ch dominates. An even more dominant stage is the agouti gene - c a (dominates over genes c, c h and c ch). The most dominant of the entire series, the black color gene C dominates over all the "lower steps of alleles" - genes c, c h, c ch, c a.

* (Lack of pigment (see chapter VII, § 5).)

Dominance, like the recessiveness of alleles, is not an absolute, but their relative property. The degree of dominance and recessiveness can be different. The same trait can be inherited in a dominant or recessive manner.

So, for example, the fold above the inner corner of the eye (epicanthus) is dominantly inherited in Mongoloids, and recessively in Negroids (Bushmen, Hottentots).

As a rule, newly emerging alleles are recessive, on the contrary, alleles of old varieties of plants or animal breeds (even more wild species) are dominant.

Each pair of chromosomes is characterized by a certain set of genes that make up the linkage group. That is why groups of different traits are sometimes inherited together with each other.

Since the somatic cells of Drosophila contain four pairs of chromosomes (2n = 8), and the sex cells contain half as many (1n = 4), the fruit fly has four groups clutch; similarly, in humans, the number of linkage groups is equal to the number of chromosomes of the haploid set (23).

For a number of organisms (Drosophila, corn) and some human chromosomes *, chromosomal or genetic maps have been compiled, which are a schematic arrangement of genes in chromosomes.

* (To date, it has been possible to establish the exact localization of human genes (if we take into account the total number of genes) only in isolated and relatively rare cases, for example, for traits linked to sex chromosomes.)

As an example, let us give a chromosome map of a part of the Drosophila X chromosome (Fig. 24). With greater or lesser accuracy, this map reflects the sequence of genes and the distance between them. It was possible to determine the distance between the genes using genetic and cytological analyzes of the crossing over, which occurs during the conjugation of homologous chromosomes during the zygonema of the prophase I of meiosis (see Chapter II, § 7).

The movement of genes from one chromosome to another occurs with a certain frequency, which is inversely proportional to the distance between genes: the shorter the distance, the higher crossover percentage(the unit of distance between genes is named after Morgan morganida and is equal to the minimum distance in the chromosome that can be measured by crossing over). Crossover is shown in Fig. 25.

At present, the close linkage of some gene loci is known, and the percentage of crossover has been calculated for them. Linked genes determine, for example, the expression Rh factor and genes of the MN-system of blood (on the inheritance of blood properties, see Chapter VII, § 3). In some families, it was possible to trace the linkage of the Rh factor with ovalocytosis(the presence of approximately 80-90% of oval-shaped erythrocytes - the anomaly proceeds, as a rule, without clinical manifestations), which give about 3% of the crossover. Up to 9% of crossover is observed between the genes that control the manifestations of ABO blood groups and the Lu factor. It is known that the gene that affects the anomaly of the structure of the nails and knee is also linked to the loci of the ABO system; the percentage of crossover between them is about 10. The linkage groups (and, consequently, the chromosome maps) of the human X and Y chromosomes are much better studied (see Chapter VII, § 6). It is known, for example, that the genes that determine the development of color blindness(color blindness) and hemophilia(bleeding); the percentage of overlap between them is 10.

The correctness of Morgan's hypothesis was confirmed at the beginning of the century by Kurt Stern (cytological studies) and Morgan's collaborators Theophilus Painter (cytologist) and Calvin Bridges (geneticist) on the giant chromosomes of the salivary glands of Drosophila larvae (similar to the giant chromosomes of other Diptera). On fig. 26 shows part of the giant chromosome of the salivary gland of the chironomus larva (bloodworm).

When studying giant chromosomes with a conventional light microscope, the transverse striation is clearly visible, formed by the alternation of light and darker stripes of disks - chromomeres; they are formed by highly spiralized, densely adjacent areas.

The formation of such giant chromosomes is called polythenia, i.e., the reduplication of chromosomes without increasing their number. At the same time, the reduplicated chromatids remain side by side, tightly adjoining each other.

If a chromosome, consisting of a pair of chromatids, doubles consecutively nine times, then the number of strands (chromonemes) in such a polytene chromosome will be 1024. Due to the partial despiralization of chromonemes, the length of such a chromosome increases compared to the usual one by 150-200 times.

In 1925, Sturtevant showed the presence unequal crossover: in one of the homologous chromosomes there may be two identical loci, in which, for example, genes that affect the shape of the Drosophila eye - Bar are located, and in the other - not a single locus. This is how flies with a pronounced sign of narrow striped eyes (gene ultra bar)(see fig. 31).

In addition to cytological evidence of the correctness of the chromosome theory, genetic experiments were carried out - crossing different races of Drosophila. So, among the many linked genes in the fruit fly, there are two recessive genes: the gene for black body color ( black) and the gene for rudimentary wings ( vestigial).

Let's call them genes a and b. They correspond to two dominant alleles: the gene for the gray body and normally developed wings (A and B). When crossing purebred flies aabb and AABB, the entire first generation of hybrids will have the genotype AaBb. Theoretically speaking, the following results should be expected in the second generation (F 2).

However, in a small but constant percentage of cases, unusual offspring from unusual gametes were encountered. About 18% of such gametes were observed in each crossing (9% Ab and 9% aB).

The occurrence of such exceptions is well explained by the crossover process. Thus, genetic studies have also made it possible to establish that clutch disorder is crossing over, leading to an increase in shape variability, is statistically constant.

In conclusion, we note that a number of provisions of classical genetics have undergone a number of changes today.

We have repeatedly used the terms "dominant" and "recessive" genes (alleles) and traits. However, recent studies have shown that so-called recessive genes may in fact not be recessive at all. It is more correct to say that recessive genes give a very weak visible or invisible manifestation in the phenotype. But in the latter case, recessive alleles, outwardly invisible in the phenotype, can be detected using special biochemical techniques. In addition, the same gene under certain environmental conditions can behave as dominant, under others - as recessive.

Since the development of all organisms occurs depending on and under the influence of the external environment, the manifestation of the genotype in a certain phenotype is also influenced by environmental factors (temperature, food, humidity and gas composition of the atmosphere, its pressure, the presence of forms pathogenic for a given organism, the chemical composition of water, soil, etc., but for a person and a phenomenon of a social order). The phenotype never shows all the genotypic possibilities. Therefore, under different conditions, the phenotypic manifestations of similar genotypes can differ greatly from each other. Thus, both the genotype and the environment are involved (to a greater or lesser extent) in the manifestation of a trait.

And fertilization. These observations formed the basis for the assumption that genes are located on chromosomes. However, experimental proof of the localization of specific genes in specific chromosomes was obtained only in the year by the American geneticist T. Morgan, who in subsequent years (-) substantiated the chromosome theory of heredity. According to this theory, the transmission of hereditary information is associated with chromosomes, in which genes are localized linearly, in a certain sequence. Thus, it is the chromosomes that are the material basis of heredity.

The formation of the chromosome theory was facilitated by the data obtained in the study of the genetics of sex, when differences were established in the set of chromosomes in organisms of different sexes.

Sex Genetics

A similar method of sex determination (XY-type) is inherent in all mammals, including humans, whose cells contain 44 autosomes and two X chromosomes in women or XY chromosomes in men.

Thus, XY-type sex determination, or the type of Drosophila and man, - the most common way to determine gender, characteristic of most vertebrates and some invertebrates. The X0 type is found in most orthopterans, bugs, beetles, spiders, which do not have a Y chromosome at all, so the male has the X0 genotype, and the female has XX.

In all birds, most butterflies, and some reptiles, males are the homogametic sex, while females are heterogametic (XY type or XO type). The sex chromosomes in these species are denoted by the letters Z and W, in order to highlight this way of determining sex; while the set of male chromosomes is denoted by the symbol ZZ, and females - by the symbol ZW or Z0.

Evidence that the sex chromosomes determine the sex of an organism was obtained by studying the nondisjunction of the sex chromosomes in Drosophila. If both sex chromosomes fall into one of the gametes, and none into the other, then when such gametes merge with normal ones, individuals with a set of sex chromosomes XXX, XO, XXY, etc. can be obtained. It turned out that in Drosophila, individuals with a set of XO are males , and with a set of XXY - females (in humans - vice versa). Individuals with the XXX set have hypertrophied female traits (superfemales). (Individuals with all of these chromosomal aberrations are sterile in Drosophila.) Later it was proved that in Drosophila, sex is determined by the ratio (balance) between the number of X chromosomes and the number of sets of autosomes.

Inheritance of sex-linked traits

In the case when the genes that control the formation of a particular trait are localized in autosomes, inheritance occurs regardless of which of the parents (mother or father) is the carrier of the studied trait. If the genes are located on the sex chromosomes, the nature of the inheritance of traits changes dramatically. For example, in Drosophila, genes located on the X chromosome, as a rule, do not have alleles on the Y chromosome. For this reason, recessive genes on the X chromosome of the heterogametic sex almost always appear in the singular.

Traits whose genes are located on the sex chromosomes are called sex-linked traits. The phenomenon of sex-linked inheritance was discovered by T. Morgan in Drosophila.

X- and Y-chromosomes in humans have a homologous (pseudoautosomal) region, where genes are localized, the inheritance of which does not differ from the inheritance of autosomal genes.

In addition to homologous regions, X- and Y-chromosomes have non-homologous regions. The non-homologous region of the Y chromosome, in addition to the genes that determine the male sex, contains the genes for webbing between the toes and hairy ears in humans. Pathological traits linked to a non-homologous region of the Y chromosome are transmitted to all sons, since they receive the Y chromosome from their father.

The non-homologous region of the X chromosome contains a number of genes important for the life of organisms. Since in the heterogametic sex (XY) the X chromosome is represented in the singular, the traits determined by the genes of the non-homologous section of the X chromosome will appear even if they are recessive. This state of the genes is called hemizygous. An example of this kind of X-linked recessive traits in humans is hemophilia, Duchenne muscular dystrophy, optic nerve atrophy, color blindness (color blindness), etc.

Hemophilia is an inherited disease in which the blood loses its ability to clot. A wound, even a scratch or bruise, can cause profuse external or internal bleeding, which often ends in death. This disease occurs, with rare exceptions, only in men. Both of the most common forms of hemophilia (hemophilia A and hemophilia B) have been found to be caused by recessive genes located on the X chromosome. Women heterozygous for these genes (carriers) have normal or slightly reduced blood clotting.

The phenotypic manifestation of hemophilia in girls will be observed if the girl's mother is a carrier of the hemophilia gene, and the father is a hemophiliac. A similar pattern of inheritance is also characteristic of other recessive, sex-linked traits.

Linked inheritance

The independent combination of traits (Mendel's third law) is carried out on the condition that the genes that determine these traits are in different pairs of homologous chromosomes. Therefore, in each organism, the number of genes that can independently combine in meiosis is limited by the number of chromosomes. However, in an organism, the number of genes significantly exceeds the number of chromosomes. For example, in corn before the era of molecular biology, more than 500 genes were studied, in the Drosophila fly - more than 1 thousand, and in humans - about 2 thousand genes, while they have 10, 4 and 23 pairs of chromosomes, respectively. The fact that the number of genes in higher organisms is several thousand was already clear to W. Setton at the beginning of the 20th century. This gave reason to assume that many genes are localized in each chromosome. Genes located on the same chromosome form a linkage group and are inherited together.

T. Morgan proposed to call the joint inheritance of genes linked inheritance. The number of linkage groups corresponds to the haploid number of chromosomes, since the linkage group consists of two homologous chromosomes in which the same genes are localized. (In individuals of the heterogametic sex, for example, male mammals, there are actually one more linkage groups, since the X and Y chromosomes contain different genes and represent two different linkage groups. Thus, women have 23 linkage groups, and in men - 24).

The mode of inheritance of linked genes differs from the inheritance of genes located in different pairs of homologous chromosomes. So, if, with independent combination, a diheterozygous individual forms four types of gametes (AB, Ab, aB and ab) in equal quantities, then with linked inheritance (in the absence of crossing over), the same diheterozygote forms only two types of gametes: (AB and ab) also in equal amounts. The latter repeat the combination of genes in the parent's chromosome.

It was found, however, that in addition to ordinary (non-crossover) gametes, other (crossover) gametes also arise with new gene combinations - Ab and aB, which differ from the combinations of genes in the parent's chromosomes. The reason for the occurrence of such gametes is the exchange of sections of homologous chromosomes, or crossing over.

Crossing over occurs in prophase I of meiosis during conjugation of homologous chromosomes. At this time, parts of two chromosomes can cross over and exchange their parts. As a result, qualitatively new chromosomes arise, containing sections (genes) of both maternal and paternal chromosomes. Individuals that are obtained from such gametes with a new combination of alleles are called crossing-over or recombinant.

The frequency (percentage) of crossover between two genes located on the same chromosome is proportional to the distance between them. Crossing over between two genes occurs less frequently the closer they are to each other. As the distance between genes increases, the likelihood that crossing over will separate them on two different homologous chromosomes increases more and more.

The distance between genes characterizes the strength of their linkage. There are genes with a high percentage of linkage and those where linkage is almost not detected. However, with linked inheritance, the maximum crossover frequency does not exceed 50%. If it is higher, then there is a free combination between pairs of alleles, indistinguishable from independent inheritance.

The biological significance of crossing over is extremely high, since genetic recombination allows you to create new combinations of genes that did not previously exist and thereby increase hereditary variability, which provides ample opportunities for the organism to adapt to various environmental conditions. A person specifically conducts hybridization in order to obtain the necessary combinations for use in breeding work.

The concept of a genetic map

T. Morgan and his collaborators C. Bridges, A. G. Sturtevant and G. J. Meller experimentally showed that knowledge of the phenomena of linkage and crossing over allows not only to establish the linkage group of genes, but also to build genetic maps of chromosomes, which indicate the order of arrangement genes on a chromosome and the relative distances between them.

A genetic map of chromosomes is a diagram of the mutual arrangement of genes that are in the same linkage group. Such maps are compiled for each pair of homologous chromosomes.

The possibility of such mapping is based on the constancy of the percentage of crossing over between certain genes. Genetic maps of chromosomes have been compiled for many types of organisms: insects (drosophila, mosquito, cockroach, etc.), fungi (yeast, aspergillus), bacteria and viruses.

The presence of a genetic map indicates a high degree of study of a particular type of organism and is of great scientific interest. Such an organism is an excellent object for further experimental work, which has not only scientific but also practical significance. In particular, knowledge of genetic maps makes it possible to plan work on obtaining organisms with certain combinations of traits, which is now widely used in breeding practice. Thus, the creation of strains of microorganisms capable of synthesizing proteins, hormones and other complex organic substances necessary for pharmacology and agriculture is possible only on the basis of genetic engineering methods, which, in turn, are based on knowledge of the genetic maps of the corresponding microorganisms.

Human genetic maps may also prove useful in health care and medicine. Knowledge about the localization of a gene in a particular chromosome is used in the diagnosis of a number of severe human hereditary diseases. Already now there is an opportunity for gene therapy, that is, to correct the structure or function of genes.

The main provisions of the chromosome theory of heredity

Analysis of the phenomena of linked inheritance, crossing over, comparison of genetic and cytological maps allow us to formulate the main provisions of the chromosome theory of heredity:

• Genes are located on chromosomes. Moreover, different chromosomes contain an unequal number of genes. In addition, the set of genes for each of the non-homologous chromosomes is unique.
• Allelic genes occupy the same loci on homologous chromosomes.
• Genes are located on the chromosome in a linear sequence.
• The genes of one chromosome form a linkage group, that is, they are inherited predominantly linked (jointly), due to which the linked inheritance of some traits occurs. The number of linkage groups is equal to the haploid number of chromosomes of a given species (in the homogametic sex) or more by 1 (in the heterogametic sex).
• Linkage is broken as a result of crossing over, the frequency of which is directly proportional to the distance between genes in the chromosome (therefore, the strength of linkage is inversely related to the distance between genes).
• Each biological species is characterized by a certain set of chromosomes - karyotype.

Sources

• N. A. Lemeza L. V. Kamlyuk N. D. Lisov "Biology manual for applicants to universities"

Notes

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