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When linking genes, traits are inherited. Linked inheritance of traits. Chromosomal theory of heredity

The Thomas Morgan Act is also known as the Law of Linked Inheritance. According to Morgan Law genes on the same chromosome form a linkage group and are often inherited together. In this case, the strength of adhesion depends on the distance between the genes in the chromosome.

Morgan’s law contradicts Mendel’s third law, according to which genes are inherited independently of each other. The fact is that each of these laws has a different place of application. In one case, for genes contained in one chromosome. In the other, for genes located in different chromosomes.

Linked inheritance is possible only for genes located on the same chromosome. However, it may be disrupted as a result of a process such as crossing over. Crossingover - This is an exchange of equivalent regions between homologous chromosomes. Crossover occurs during prophase I of meiosis. In this phase of cell division, homologous chromosomes conjugate (converge and join).

If a crossingover occurred between homologous chromosomes, then the linkage between the genes is broken, previously linked alleles of different genes end up in different homologous chromosomes. New gene combinations are forming.

Example. In a Drosophila fly, body color is determined by a gene having the alleles A (gray body) and a (black body). The length of the wings is determined by other allelic genes: B (long), b (short). In most cases, gray flies have long wings, and black flies have short wings. This suggests that genes A and B are linked together, i.e., are localized on the same chromosome. In turn, genes a and b are also linked.

When crossing the AABB and aabb genotypes in the first generation, all flies will be gray with long wings (AaBb). This result says nothing about whether genes are linked or not. It will be the same anyway. If the genes are linked, then in one chromosome there will be A and B genes from one parent, in the homologous chromosome - a and b (this chromosome is inherited from the other parent).

If genes A and B were not localized on the same chromosome, but on different non-homologous ones, then with equal probability gene A could appear in the gamete with both gene b and gene B. Then, in the second generation, standard Mendelian phenotype splitting would be observed : 9A-B-: 3A-bb: 3aaB-: 1aabb (instead of dashes, there may be either a dominant or a recessive allele). That is, 6 out of 16 flies would have recombinant traits - a gray body with short wings and a black body with long wings.

However, the number of crossover flies is significantly less, which indicates gene linkage, when the dominant gene A is predominantly inherited together with gene B, the recessive gene a together with the recessive gene b. The presence of crossover organisms suggests that the linkage between A and B, as well as a and b, is not complete.

If, as a result of crossing over, a chromosome containing genes A and b (or a and B) appears, then in the future they will be inherited together, i.e., form a new linkage group.

The percentage of crossing over depends on the degree of distance of genes in one chromosome. The farther the genes are from each other, the less they are linked, that is, there is a greater likelihood of an exchange of a site with a homologous chromosome. Close genes are almost always inherited according to Morgan's law.

Crossover analysis allows you to build genetic maps. The distance between genes is measured in centimorganids (or simply morganides). Moreover, if crossover gametes are 1%, then the distance between the genes is considered equal to 1 morganide. This means that genes are located close enough to each other, and crossing over between them is rare. If the distance between the genes is 25 mb, then the probability of obtaining a crossover organism is 25%, i.e., the genes are located on the same chromosome far enough from each other.

The law of the independent distribution of traits (the third law of Mendel) is violated if the genes that determine the different traits are on the same chromosome. Such genes are usually inherited jointly, i.e., it is observed linked inheritance. The phenomenon of linked inheritance was studied by Thomas Morgan and his collaborators and therefore is called morgan Law.

The law of T. Morgan can be formulated as follows: genes located on the same chromosome form a linking group and are often inherited jointly, while the frequency of joint inheritance depends on the distance between the genes (the closer, the more often).

The reason why linked inheritance is impaired is crossing over, which occurs in meiosis during chromosome conjugation. In this case, homologous chromosomes exchange their sites, and thus previously linked genes can appear in different homologous chromosomes, which leads to an independent distribution of characters.

For example, gene A is linked to gene B (AB), and the recessive alleles of the corresponding genes (ab) are located in the homologous chromosome. If in the process of crossing over homologous chromosomes almost never exchange regions so that one gene transfers to another chromosome and the other remains the same, then such an organism forms gametes of only two types: AB (50%) and ab (50%). If the exchange of the corresponding sites occurs, then a certain percentage of gametes will contain the Ab and aB genes. Usually their percentage is lower than with independent distribution of genes (when A and B are on different chromosomes). If, with an independent distribution of all types of gametes (AB, ab, Ab, aB), there will be 25% each, then in the case of linked inheritance, the gametes Ab and aB will be less. The smaller they are, the closer the genes are located to each other on the chromosome.

Sex-linked inheritance is especially distinguished when the gene being studied is on the sex (usually X) chromosome. In this case, the inheritance of one trait is studied, and the second is gender. If the inherited trait is sex-linked, then it is inherited in different ways in reciprocal crosses (when the trait is first possessed by the female parent, then the male).

If the mother has the aa genotype, and the father shows a dominant trait (there is definitely one A gene), then in case of sexing, all daughters will have a dominant trait (in any case, they will get the only X chromosome from the father, and all sons will be recessive (From the father, the Y chromosome is obtained, in which there is no corresponding gene, and from the mother, in any case, gene a.) If the trait were not sexually linked, then among both sexes of the children there could be owners of a dominant trait.

When the genes under study are linked in an autosome, this linkage is called autosomal. Linkage is called complete if parental combinations of alleles are not broken from generation to generation. This is very rare. Usually there is incomplete linked inheritance, which violates both the third law of Mendel and Morgan's law (in its abbreviated wording: genes located on the same chromosome are inherited jointly).

Genes on the chromosome are linear. The distance between them is measured in centimorgan (cm). 1 cm corresponds to the presence of 1% crossover gametes. Through various crosses and statistically analyzing descendants, scientists identify linked genes, as well as the distance between them. Based on the data obtained, genetic maps are constructed that reflect the localization of genes in the chromosomes.

Mendel’s third law on independent inheritance is violated if the genes are on the same chromosome. For the first time, the phenomenon of linked genes, i.e. located on the same chromosome, observed geneticist Thomas Morgan. Subsequently, the simultaneous inheritance of two traits was called the law of Morgan.

Chromosomes

To talk briefly and clearly about the law of Thomas Morgan, you should first remember what a chromosome is.

A chromosome is a structure located in the nucleus of a cell that carries hereditary information. It consists of a long chain of DNA, which in turn consists of genes - units of hereditary information. Each gene is responsible for a specific trait. A set of chromosomes is called a karyotype.

Fig. 1. Chromosome.

Mendel examined traits located on different chromosomes. When crossing, different combinations of genes are formed that form the individual's genotype.

Unlike Mendel’s law, Morgan’s law applies to genes on the same chromosome.

Law

The wording of the law is as follows: genes located on the same chromosome close to each other form a group and are inherited linked. The number of linked groups corresponds to the haploid set - half of the complete set of chromosomes. A person has 46 chromosomes, i.e. 23 pairs, respectively 23 clutch groups.

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Fig. 2. The law of Morgan.

The frequency of inheritance depends on the distance between the genes. The closer the genes that form the groups are, the more often linked characters are inherited, i.e. in close proximity, the adhesion is stronger.

Examples of linked inheritance:

the color of corn seeds is linked to the structure of their surface (smooth or wrinkled);
the color of sweet pea flowers is linked to the shape of pollen;
diseases (color blindness, hemophilia) are linked to the X chromosome.

If the genes are not linked, then four types of gametes AaBb are formed - AB, aB, Ab, ab. When crossing hybrids, the phenotype ratio will be 9: 3: 3: 1 (splitting will occur). With linked inheritance, two types of gametes are formed - AB and ab. In this case, the F2 generation will produce offspring with a 3: 1 phenotype.

The law of cohesive inheritance of T. Morgan may be violated. There is an exchange of gene regions between homologous chromosomes, and new gene combinations are formed. This phenomenon is called crossover. Disruption of connections occurs in meiosis during conjugation (I prophase - rapprochement and connection). Homologous chromosomes exchange sites, breaking linked links. In this case, the third law of Mendel is fully respected.

Fig. 3. Crossover.

Morgan's work was as follows:

drosophila flies have linked genes - individuals with a gray body (A) have long wings (B), and individuals with a black body (a) have short wings (b);
when two individuals with the genotype AABB and aabb are crossed, the entire first generation (100%) will be gray with long wings (AaBb);
it was assumed that when analyzing the crossing of AaBb with aabb according to Mendel’s law, the phenotype ratio would be 1: 1: 1: 1 (25% each), i.e. AaBb, Aabb, aaBb, aabb, therefore, genes lie on different chromosomes;
Morgan received two phenotypes, AaBb and aabb, when analyzing crosses. two signs are linked;
according to crossing over, about 7% of the flies were gray with short wings or dark with long wings.

The possibility of crossing over increases if the linked genes are located at a considerable distance from each other. The lower the crossover percentage, the greater the likelihood of chained inheritance.

G. Mendel traced the inheritance of seven pairs of traits in peas. Many researchers, repeating the experiments of Mendel, confirmed the laws that he discovered. It was recognized that these laws are universal. However, in 1906, English geneticists V. Betson and R. Pennet, crossing plants of sweet peas and analyzing the inheritance of pollen form and flower color, found that these characters do not give an independent distribution in the offspring. Descendants always repeated signs of parental forms. It became clear that not all genes are characterized by independent distribution in the offspring and free combination.

Each organism has a huge number of signs, and the number of chromosomes is small. Therefore, each chromosome carries not one gene, but a whole group of genes responsible for the development of different characters.

The study of the inheritance of traits whose genes are localized on the same chromosome was carried out by the outstanding American geneticist T. Morgan. If Mendel conducted his experiments on peas, then for Morgan the fruit fly of Drosophila became the main object. The fly every two weeks at a temperature of 25 ° C gives numerous offspring. The male and the female are clearly distinguishable externally - the male has a smaller and darker abdomen.

In addition, they have only 8 chromosomes in a diploid set and differences in numerous ways can multiply in test tubes on a cheap nutrient medium.

Crossing a fly with a gray body and normal wings with a fly with a dark body color and rudimentary wings, Morgan received hybrids with a gray body and normal wings in the first generation (the gene that determines the gray color of the abdomen dominates the dark color, and the gene that determines development of normal wings - over the underdeveloped gene) (Fig. 327). When analyzing the crossing of a female F 1 with a male who had recessive characters, it was theoretically expected to produce offspring with combinations of these characters in a ratio of 1: 1: 1: 1. However, in the offspring, individuals with signs of parental forms (41.5% gray long-winged and 41.5% black with embryonic wings) clearly predominated and only a small part of the flies had recombined characters (8.5% black long-winged and 8.5% gray with embryonic wings).

Analyzing the results, Morgan came to the conclusion that the genes that determine the development of the gray color of the body and long wings are localized on one chromosome, and the genes that cause the development of the black color of the body and embryo wings in the other. Morgan called the phenomenon of joint inheritance of traits clutch. The material basis for gene linkage is the chromosome. Genes located on the same chromosome are inherited together and form one clutch group. Since homologous chromosomes have the same set of genes, the number of linkage groups is equal to the haploid set of chromosomes (for example, a person has 46 chromosomes, or 23 pairs of homologous chromosomes, respectively, the number of linkage groups in human somatic cells is 23). The phenomenon of joint inheritance of genes localized on the same chromosome is called linked inheritance. The linked inheritance of genes located on the same chromosome is called Morgan's law.

Let us return to our example of crossing flies of Drosophila. If the body color genes and the shape of the wings are localized on the same chromosome, then with this crossing two groups of individuals should be obtained, repeating the signs of the parental forms, since the mother body should form gametes of only two types - AB and av and father’s - one type - av. Therefore, in the offspring two groups of individuals with a genotype should be formed AABB and aavv. However, individuals appear in the offspring (albeit in a small amount) with recombined characters, that is, having a genotype Aavv and aaBb. What are the reasons for the appearance of such individuals? To explain this fact, it is necessary to recall the mechanism of the formation of germ cells - meiosis. In the prophase of the first meiotic division, homologous chromosomes are conjugated, and at this moment an exchange of sites may occur between them. As a result of crossing-over, in some cells there is an exchange of chromosome regions between genes Aand IN, gametes appear Avand aB, and, as a result, four groups of phenotypes are formed in the offspring, as in the free combination of genes. But since crossing over does not occur in all gametes, the numerical ratio of phenotypes does not correspond to a ratio of 1: 1: 1: 1.

Depending on the features of the formation of gametes, distinguish:

non-crossover gametes - gametes with chromosomes formed without crossing over:

crossover gametes - gametes with chromosomes undergoing crossingover:

Accordingly, they distinguish:

Genes in chromosomes have different adhesion forces. Gene coupling can be:

© completeif recombination is impossible between genes belonging to the same linking group (in Drosophila males, full linking of genes is possible, although in the vast majority of other species crossing-over proceeds similarly in both males and females);

The probability of occurrence of cross between genes depends on their location on the chromosome: the farther the genes are from each other, the higher the probability of cross between them. For a unit of distance between genes located on the same chromosome, 1% crossing-over is taken. Its value depends on the strength of adhesion between the genes and corresponds to the percentage of recombinant individuals of the total number of offspring obtained by crossing. For example, in the analyzing crossing considered above, 17% of individuals with recombined traits were obtained. Consequently, the distance between the genes of the gray color of the body and the long wings (as well as the black color of the body and the embryo wings) is 17%. In honor of T. Morgan, the unit of distance between genes is named morgida.

The result of T. Morgan's research was the creation of the chromosome theory of heredity:

Each gene has a specific place (locus) in the chromosome; allelic genes are located at identical loci of homologous chromosomes;

© genes localized on the same chromosome are inherited together, forming a linkage group; the number of linkage groups is equal to the haploid set of chromosomes and is constant for each type of organism;

¨ is a function of the distance between genes: the greater the distance, the greater the crossover value (direct relationship);

¨ depends on the strength of adhesion between the genes: the stronger the genes are linked, the lower the crossover value (inverse relationship);

Each species has a set of chromosomes that is characteristic only for it — a karyotype.

40.4. Genetics of gender

As you know, most animals and dioecious plants are dioecious organisms, and inside the species the number of males is approximately equal to the number of females.

Sex can be considered as one of the signs of the body. Inheritance of body traits is usually determined by genes. The mechanism for determining sex has a different character - chromosomal (Fig. 328).

Gender is most often determined at the time of fertilization. In humans, the female sex is homogametic, that is, all the eggs have an X chromosome. The male organism is heterogametic, that is, it forms two types of gametes - 50% of the gametes carry the X chromosome and 50% the Y chromosome. If

if a zygote is formed that carries two X-chromosomes, then a female organism will be formed from it, if the X-chromosome and Y-chromosome are male.

A sex ratio close to 1: 1 cleavage corresponds to cleavage in analyzing crosses. Since the female body has two identical sex chromosomes, it can be considered as homozygous, male, forming two types of gametes - as heterozygous.

The above diagram shows how the formation in equal amounts of two groups of individuals that differ in the set of sex chromosomes occurs.

There are four main types of chromosome sex determination (Fig. 329):

40.5. Inheritance of traits
sex-linked

Genetic studies have found that sex chromosomes are not only responsible for determining the sex of the body - they, like autosomes, contain genes that control the development of certain signs.

Inheritance of traits whose genes are localized in X- or Y-chromosomes, called sex-linked inheritance.

The study of the inheritance of genes localized in the sex chromosomes was carried out by T. Morgan.

In Drosophila, red eye color dominates over white. Conducting reciprocal crossing, T. Morgan received very interesting results. When crossing red-eyed females with white-eyed males, in the first generation all offspring turned out to be red-eyed. If you cross hybrid F 1 hybrids, then in the second generation all the females turn out to be red-eyed, and the males split - 50% of the white-eyed and 50% of the red-eyed. If you cross white-eyed females and red-eyed males, then in the first generation all the females turn out to be red-eyed, and the males are white-eyed. In F 2, half of the females and males are red-eyed, half are white-eyed.

T. Morgan could explain the results of the observed cleavage by eye color only by assuming that the gene responsible for eye color is localized on the X chromosome, and the Y chromosome does not contain such genes.

Thus, thanks to the crosses, a very important conclusion was made: the eye color gene is linked to the floor, that is, it is located on the X chromosome.

In humans, a man receives an X chromosome from his mother. Human sex chromosomes have small homologous regions carrying the same genes (for example, the gene for general color blindness), these are conjugation regions (Fig. 330). But most genes linked to the X chromosome are absent on the Y chromosome, therefore these genes (even recessive) will appear phenotypically, since they are represented in the singular in the genotype. These genes are called hemizygous.

The human X chromosome contains a number of genes whose recessive alleles determine the development of severe abnormalities (hemophilia, color blindness). These anomalies are more common in men (since they are heterogametic), although a woman is more often the carrier of these anomalies.

In most organisms, only the X chromosome is genetically active, while the Y chromosome is almost inert, since it does not contain genes that determine the characteristics of the organism. In humans, only some genes that are not vital are located on the Y chromosome (for example, hypertrichosis - increased hairiness of the auricle). Genes localized in the Y chromosome are inherited in a special way - only from father to son.

Full adhesion to the floor is observed only if the Y chromosome is genetically inert. If on the Y chromosome there are genes allelic to the genes of the X chromosome, the nature of inheritance of characters is different. For example, if the mother has recessive genes, and the father is dominant, then all descendants of the first generation will be heterozygous with a dominant manifestation of the trait. In the next generation, the usual 3: 1 splitting will be obtained, and only girls will be with recessive symptoms. This type of inheritance is called partially coupled to the floor. In this way, some traits of a person are inherited (general color blindness, skin cancer).

40.6. Holistic genotype,
historically established system of genes.

Studying the laws of inheritance, G. Mendel proceeded from the assumption that one gene is responsible for the development of only one trait. For example, the gene responsible for the development of the color of pea seeds does not affect the shape of the seeds. Moreover, these genes are located on different chromosomes, and their inheritance is independent of each other. Therefore, it may seem that the genotype is a simple collection of body genes. However, Mendel himself in a number of experiments faced with the phenomena of inheritance, which could not be explained using the laws discovered by him. So, while studying the inheritance of the color of the seed peel, Mendel discovered that the gene that causes the formation of a brown seed peel also contributes to the development of pigment in other parts of the plant. Plants with brown seed peel had purple flowers, and plants with white seed peel had white flowers. In other experiments, crossing white and purple beans, he received in the second generation a whole series of shades - from purple to white. Mendel came to the conclusion that the inheritance of the purple color does not depend on one, but on several genes, each of which gives an intermediate color. We can say that Mendel not only established the laws of independent inheritance of pairs of alleles, but also laid the foundations for the study of the interaction of genes.

After the rediscovery of the laws of inheritance of characters, numerous experiments confirmed the correctness of the laws established by Mendel. At the same time, facts gradually accumulated, showing that the numerical ratios obtained by Mendel during the splitting of the hybrid generation were not always observed. This indicated that the relationship between genes and traits is more complex. It turned out that:

There are several types of interaction of allelic genes:

© Coding, in this case both signs appear in hybrids. For example, coding occurs in people with 4 blood types. The first blood group in people with alleles i O i O, the second - with alleles I A I A or I A í 0; the third is I B I B or I B í 0; the fourth group has alleles I A I B.

Many examples are known when genes affect the nature of the manifestation of a particular non-allelic gene or the very possibility of manifestation of this gene.

Complementary called genes, which, when combined together in the genotype in a homozygous or heterozygous state, determine a new phenotypic manifestation of the trait.

A classic example of complementary gene interaction is the inheritance of the crest shape in chickens (Fig. 331). When crossing chickens with a pink-like and pea-shaped comb, the entire first generation has a walnut-shaped comb. When crossing the hybrids of the first generation, the offspring observed splitting in the shape of the ridge: 9 nut-shaped: 3 pink-like: 3 pea-shaped: 1 leaf-shaped. Genetic analysis showed

that hens with a pink crest have a genotype A_bbpea-shaped - aaB_with walnut - A_B_ and with leaf - aabb, that is, the development of the pink ridge occurs if the genotype has only one dominant gene - A, pea-shaped - the presence of only a gene INa combination of genes A b causes the appearance of a nut-like crest, and the combination of recessive alleles of these genes is leaf-shaped.

With the complementary interaction of genes in dihybrid crosses, cleavage of offspring other than Mendelian is obtained: 9: 7, 9: 3: 4, 13: 3, 12: 3: 1, 15: 1, 10: 3: 3, 9: 6: 1. However, all of them are modifications of the general Mendelian formula 9: 3: 3: 1.

White plumage is determined by several different genes, for example, in white leggorn genes CCIIand the white plymouthrocks ccii (Fig. 332). Dominant Allele Gene FROM determines the synthesis of the pigment precursor (a chromogen that provides the color of the pen), and its recessive allele from - lack of chromogen. Gene I is a gene suppressor FROM, and the allele i does not suppress its action. Thus, the white color in chickens is determined not by the presence of special genes that determine the development of this color, but by the presence of a gene that inhibits its development.

When crossing, for example, leggorn ( CCII) with plymouth rods ( ccii), all offspring of F 1 has a white color, which is determined by the presence of a suppressor gene in their genotype ( CCIi) If the hybrids F 1 are crossed among themselves, then in the second generation there is a splitting in color with respect to 13/16 white: 3/16 colored. The part of the offspring, in the genotype of which there is a color gene and its suppressor is absent, is colored ( C_ii).

Crossing white and purple beans, Mendel encountered a polymer phenomenon. Polymer call the unambiguous effect of two, three or more non-allelic genes on the development of

the existence of one and the same sign. These genes are called polymer, or multiple, and are denoted by one letter with the corresponding index, for example, A 1, A 2, a 1, a 2.

Polymeric genes control most of the quantitative characteristics of organisms: plant height, seed weight, oil content, sugar content in sugar beet root crops, milk production of cows, egg production, body weight, etc.

The polymer phenomenon was discovered in 1908 when studying the color of the grain in Nelson-Ele wheat (Fig. 333). He suggested that the inheritance of color in wheat kernels is due to two or three pairs of polymer genes. When crossing red and white wheat in F 1, an intermediate inheritance of the trait was observed: all hybrids of the first generation had a light red grain. In F 2 there was a splitting in relation to 63 red-grain per 1 white-grain. Moreover, red grain kernels had different color intensities - from dark red to light red. Based on observations, Nelson-Ele determined that the sign of coloration of the grains determines three pairs of polymer genes.

In humans, the type of polymer is inherited, for example, the color of the skin.

Pleiotropy called the multiple action of genes. The pleiotropic effect of genes has a biochemical nature: a single protein-enzyme, formed under the control of one gene, determines not only the development of this trait, but also affects the secondary biosynthesis reactions of various other traits and properties, causing their change.

The pleiotropic effect of genes was first discovered by G. Mendel, who discovered that plants with purple flowers always had red spots in the axils of the leaves, and the seed coat was gray or brown. That is, the development of these characters is determined by the action of one hereditary factor (gene).

A person has a recessive hereditary disease, sickle cell anemia. The primary defect of this disease is the replacement of one of the amino acids in the hemoglobin molecule, which leads to a change in the shape of red blood cells. At the same time, deep disturbances arise in the cardiovascular, nervous, digestive, excretory systems. This leads to the fact that homozygous for this disease dies in childhood.

Pleiotropia is widespread. A study of the action of genes has shown that many, if not all, genes obviously have a pleiotropic effect.

Thus, the expression “gene determines the development of the trait” is largely arbitrary, since the effect of the gene depends on other genes - on the genotypic environment. The manifestation of the action of genes is also affected by environmental conditions. Therefore, the genotype is a system of interacting genes.

Human genetics

Each major stage in the development of genetics was associated with the use of certain objects for genetic research. Gene theory and the main patterns of inheritance of traits were established in experiments with peas, the Drosophila fly was used to substantiate the chromosome theory of heredity, and viruses and bacteria were used to establish molecular genetics. Currently, the main object of genetic research is becoming a person.

For genetic research, a person is a very inconvenient object, since a person has:

However, despite these difficulties, human genetics is fairly well understood. This was made possible through the use of a variety of research methods.

Genealogical method. The use of this method is possible only when direct relatives are known - the ancestors of the owner of the hereditary trait (proband) along the maternal and paternal lines in a number of generations, or the descendants of a proband also in several generations. When compiling pedigrees in genetics, a certain notation system is used (Fig. 334). After the genealogy is compiled, its analysis is carried out with

the purpose of establishing the nature of inheritance of the studied trait.

Thanks to the genealogical method, it was found that a person has all types of inheritance of traits known to other organisms, and the types of inheritance of some specific traits are determined. So, according to the autosomal dominant type, polydactyly (increased number of fingers) is inherited (Fig. 335), the ability to curl the tongue into a tube (Fig. 336), brachidactyly (shortly due to the absence of two phalanges on the fingers), freckles, early baldness, fused fingers, cleft lip, cleft palate, cataract of the eyes, fragility of bones and many others. Albinism, red hair, susceptibility to poliomyelitis, diabetes mellitus, congenital deafness and other signs are inherited as autosomal recessive.

A number of signs are inherited sex-linked: X-linked inheritance - hemophilia, color blindness; Y-linked - hypertrichosis (increased auricle), membranes between the fingers. There are a number of genes, loca-

lysed in homologous regions of the X- and Y-chromosomes, for example, total color blindness.

By establishing the type of inheritance of features, the value of the method is not limited. The use of the genealogical method has shown that in kinship, compared with unrelated, the likelihood of deformities, stillbirths, and early mortality in offspring increases significantly. In related marriages, recessive genes often turn into a homozygous state, as a result of which these or other anomalies develop. A striking example of this is the inheritance of hemophilia in the royal houses of Europe.

A large role in the study of human heredity and the influence of environmental conditions on the formation of characters plays twin method.

Twins called simultaneously born children. They are monozygous (identical) and dizygotic (opposite) (Fig. 337) .

Monozygotic twins develop from one zygote, which at the stage of fragmentation was divided into two (or more) parts. Therefore, such twins are genetically identical and always of the same sex. Monozygotic twins are characterized by a high degree of similarity ( concordance) for many reasons.

Dizygotic twins develop from simultaneously ovulating and fertilized by different sperm eggs. Therefore, they are hereditarily different and may be of the same or different sexes. Unlike monozygous, dizygotic twins are often characterized discordance - dissimilarity in many ways. Data on the concordance of twins for some signs are given in the table.

Table 9.

Concordance of some human attributes

As can be seen from the table, the degree of corcondance of monozygotic twins is significantly higher in all of the above signs than in dizygotic twins, but it is not absolute. As a rule, the discordance of identical twins arises as a result of violations of the intrauterine development of one of them or under the influence of the external environment, if it was different.

Thanks to the twin method, a hereditary predisposition of a person to a number of diseases was clarified: schizophrenia, mental retardation, epilepsy, diabetes mellitus and others.

Observations of identical twins provide material for elucidating the role of heredity and environment in the development of characters. Moreover, the external environment is understood not only as physical environmental factors, but also

social conditions.

Cytogenetic method based on the study of human chromosomes in normal and pathological conditions. Normally, a human karyotype includes 46 chromosomes — 22 pairs of autosomes and two sex chromosomes. Using this method allowed us to identify a group of diseases associated with either a change in the number of chromosomes or with changes in their structure. Such diseases are called chromosomal. These include: Klinefelter syndrome, Shereshevsky-Turner syndrome, Trisomy X, Down syndrome, Patau syndrome, Edwards syndrome and others.

Patients with Klinefelter syndrome (47, HHU) are always men. They are characterized by underdevelopment of the sex glands, degeneration of the seminiferous tubules, often mental retardation, high growth (due to disproportionately long legs).

Shereshevsky-Turner syndrome (45, X0) is observed in women. It manifests itself in a slowdown in puberty, underdevelopment of the sex glands, amenorrhea (absence of menstruation), and infertility. Women with Shereshevsky-Turner syndrome are short, the body is disproportionately - the upper body is more developed, the shoulders are wide, the pelvis is narrow - the lower extremities are shortened, the neck is short with folds, the "Mongoloid" eye section and a number of other signs.

Down Syndrome is one of the most common chromosomal diseases. It develops as a result of trisomy on the 21st chromosome (47, 21,21,21). The disease is easily diagnosed, as it has a number of characteristic signs: shortened limbs, a small skull, a flat, wide nose, narrow eye slits with an oblique incision, the presence of an upper eyelid fold, and mental retardation. Often observed and violations of the structure of internal organs.

Chromosomal diseases also arise as a result of changes in the chromosomes themselves. So, a deletion of the 5th chromosome leads to the development of the cat scream syndrome. In children with this syndrome, the structure of the larynx is disturbed, and in early childhood they have a peculiar "mewing" timbre of the voice. In addition, there is a retardation of psychomotor development and dementia. Deletion of chromosome 21 leads to the emergence of one form of bleeding.

Most often, chromosomal diseases are the result of mutations that occur in the germ cells of one of the parents.

Biochemical method allows you to detect metabolic disorders caused by changes in genes and, as a consequence, changes in the activity of various enzymes. Hereditary metabolic diseases are divided into diseases of carbohydrate metabolism (diabetes mellitus), the exchange of amino acids, lipids, minerals, etc.

Phenylketonuria refers to diseases of amino acid metabolism. The conversion of the essential amino acid phenylalanine to tyrosine is blocked, while phenylalanine is converted to phenylpyruvic acid, which is excreted in the urine. The disease leads to the rapid development of dementia in children. Early diagnosis and diet can stop the development of the disease.

Human genetics is one of the most intensively developing branches of science. It is the theoretical basis of medicine, reveals the biological foundations of hereditary diseases. Knowing the genetic nature of the disease allows you to make an accurate diagnosis on time and carry out the necessary treatment.

Population Genetics

Population - this is a collection of individuals of the same species, living for a long time in a certain territory, freely crossing with each other, having a common origin, a certain genetic structure and to some extent isolated from other such collections of individuals of this species. A population is not only a unit of a species, a form of its existence, but also a unit of evolution. Microevolutionary processes that end with speciation are based on genetic transformations in populations.

The study of the genetic structure and dynamics of populations is engaged in a special section of genetics - population genetics.

From a genetic point of view, a population is an open system, and a species is a closed system. In general form, the process of speciation is reduced to the transformation of a genetically open system into a genetically closed one.

Each population has a specific gene pool and genetic structure. Gene pool Populations are the totality of genotypes of all individuals in a population. Under genetic structure populations understand the ratio of different genotypes and alleles in it.

One of the basic concepts of population genetics is the frequency of the genotype and the frequency of the allele. Under genotype frequency (or allele) understand its share related to the total number of genotypes (or alleles) in a population. The frequency of the genotype, or allele, is expressed either as a percentage or in fractions of a unit (if the total number of genotypes or alleles of a population is taken as 100% or 1). So, if the gene has two allelic forms and the proportion of the recessive allele a is ¾ (or 75%), then the proportion of the dominant allele A will be equal to ¼ (or 25%) of the total number of alleles of a given gene in a population.

A great influence on the genetic structure of populations is exerted by the method of reproduction. For example, populations of self-pollinating and cross-pollinating plants differ significantly from each other.

The first study of the genetic structure of a population was undertaken by W. Johannsen in 1903. Populations of self-pollinating plants were chosen as objects of study. Studying for several generations the mass of seeds in beans, he found that in self-pollinators the population consists of genotypically heterogeneous groups, the so-called clean linesrepresented by homozygous individuals. Moreover, from generation to generation in such a population, an equal ratio of homozygous dominant and homozygous recessive genotypes remains. Their frequency in each generation increases, while the frequency of heterozygous genotypes will decrease. Thus, in populations of self-pollinating plants, homozygotization, or decomposition into lines with different genotypes, is observed.

Most plants and animals in populations reproduce sexually with free crossing, which ensures the equally probable occurrence of gametes. The equiprobable occurrence of gametes in free crossing is called panmixia, and such a population - panmictic.

Hardy and Weinberg, summarizing the data on the frequency of genotypes resulting from the equally probable occurrence of gametes, derived a formula for the frequency of genotypes in the panmictic population:

P 2 + 2pq + q 2 \u003d 1.

AA + 2Aa + aa \u003d 1

However, this law is subject to the following conditions:

In real populations, the fulfillment of these conditions is impossible, therefore, the law is valid only for the ideal population. Despite this, the Hardy-Weinberg law is the basis for the analysis of some genetic phenomena occurring in natural populations. For example, if it is known that phenylketonuria occurs at a frequency of 1: 10000 and is inherited in an autosomal recessive manner, one can calculate the frequency of occurrence of heterozygotes and homozygotes for a dominant feature. Phenylketonuria patients have a genotype q 2 (aa) \u003d 0.0001. From here q = 0,01. p = 1 - 0.01 \u003d 0.99. The frequency of occurrence of heterozygotes is equal to 2pq is equal to 2 x 0.99 x 0.01 ≈ 0.02 or about 2%. The frequency of homozygotes for dominant and recessive traits: AA = p 2 = 0,99 2 ≈ 98%, aa = 0,01%.

A change in the balance of genotypes and alleles in the panmictic population occurs under the influence of constantly acting factors, which include:

Thanks to these phenomena, an elementary evolutionary phenomenon arises - a change in the genetic composition of a population, which is the initial stage of the speciation process.

Variability

Genetics studies not only heredity, but also the variability of organisms. Volatility called the ability of living organisms to acquire new signs and properties. Due to variability, organisms can adapt to changing environmental conditions.

There are two types of variability:

¨ combinative - arising as a result of recombination of chromosomes during sexual reproduction and sections of chromosomes in the process of crossing over;

¨ mutational - arising as a result of a sudden change in the state of genes;

Mutational variability

Hereditary changes in genetic material are now called mutations. Mutations - sudden changes in genetic material, leading to a change in certain signs of organisms.

The term "mutation" was first introduced into science by the Dutch geneticist G. de Vries. Carrying out experiments with evening primrose (ornamental plant), he accidentally discovered specimens that differ in a number of signs from the rest (large growth, smooth, narrow and long leaves, red veins of leaves and a wide red stripe on the cup of a flower, etc.). Moreover, during seed propagation, plants from generation to generation persistently retained these signs. As a result of a generalization of his observations, de Vries created a mutational theory, the main provisions of which have not lost their significance to this day:

Mutations do not form continuous series, do not group around the middle type (as with modification variability), they are qualitative changes;

Mutations are individual, that is, they arise in individual individuals.

The process of occurrence of mutations is called mutagenesisorganisms that have mutations - mutants, and environmental factors causing the appearance of mutations - mutagenic.

The ability to mutate is one of the properties of a gene. Each individual mutation is caused by some reason, usually associated with changes in the external environment.

There are several classifications of mutations:

¨ Generative - arising in germ cells . They do not affect the signs of this organism, but appear only in the next generation.

¨ Somatic -arising in somatic cells . These mutations appear in this organism and are not transmitted to the offspring during sexual reproduction (black spot against the background of brown coat color in astrakhan sheep). Somatic mutations can only be saved by asexual reproduction (primarily vegetative).

¨ Useful - increasing the viability of individuals.

¨ Harmful:

§ lethal - causing death of individuals;

§ semi-lethal - reducing the viability of the individual (in men, the recessive hemophilia gene is semi-lethal in nature, and homozygous women are not viable).

¨ Neutral -not affecting the viability of individuals.

This classification is very arbitrary, since the same mutation in some conditions can be useful, and in others - harmful.

¨ dominantthat can make the owners of these mutations unviable and cause their death in the early stages of ontogenesis (if the mutations are harmful);

¨ recessive - mutations that are not manifested in heterozygotes; therefore, they remain in the population for a long time and form a reserve of hereditary variability (carriers of such mutations may gain an advantage in the struggle for existence if environmental conditions change).

¨ large - Well-noticeable mutations that strongly change the phenotype (terry in flowers);

¨ small - mutations that practically do not give phenotypic manifestations (slight elongation of the spikes in the ear).

¨ direct - the transition of the gene from the wild type to a new state;

¨ reverse - the transition of a gene from a mutant state to a wild type.

¨ spontaneous - mutations that occur naturally under the influence of environmental factors;

¨ induced - mutations artificially caused by mutagenic factors.

¨ genes;

¨ chromosomal;

¨ genomic.

Genemutations are changes in the structure of a DNA molecule in the region of a particular gene that encodes the structure of a particular protein molecule. These mutations entail a change in the structure of proteins, that is, a new amino acid sequence appears in the polypeptide chain, resulting in a change in the functional activity of the protein molecule. Due to gene mutations, a series of multiple alleles of the same gene occurs. Most often, gene mutations occur as a result of:

Chromosomal mutations

Chromosomal mutations - mutations causing changes in the structure of chromosomes . They arise as a result of chromosome breakup with the formation of “sticky” ends, “Sticky” ends are single-stranded fragments at the ends of a double-stranded DNA molecule. These fragments are able to bind to other fragments of chromosomes that also have sticky ends. Perestroika can be carried out both within the same chromosome - intrachromosomal mutations, and between non-homologous chromosomes - interchromosomal mutations.

¨ deletion - loss of a part of the chromosome (ABCD ® AB);

¨ inversion - 180 ° rotation of the chromosome region (ABCD ® ACBD);

¨ duplication - Doubling of the same chromosome region; (ABCD ® ABCBCD);

¨ translocation - exchange of sites between non-homologous chromosomes (ABCD ® AB34).

Genomic mutations

Genomic mutations are called, as a result of which a change in the number of chromosomes occurs in the cell. Genomic mutations result from a violation of mitosis or meiosis, leading either to an uneven divergence of chromosomes at the poles of the cell, or to a doubling of chromosomes, but without division of the cytoplasm.

Depending on the nature of the change in the number of chromosomes, there are:

¨ Haploidy - a decrease in the number of complete haploid sets of chromosomes.

¨ Polyploidy - an increase in the number of complete haploid sets of chromosomes. Polyploidy is more often observed in protozoa and in plants. Depending on the number of haploid sets of chromosomes contained in the cells, there are: triploids (3n), tetraploids (4n), etc. They can be:

§ autopolyploids - polyploids resulting from the multiplication of genomes of one species;

§ allopolyploids - polyploids resulting from the multiplication of genomes of different species (typical for interspecific hybrids).

¨ Heteroploidy (aneuploidy) - a multiple increase or decrease in the number of chromosomes. Most often, there is a decrease or increase in the number of chromosomes by one (less often two or more). Due to the non-divergence of any pair of homologous chromosomes in meiosis, one of the formed gametes contains one less chromosome, and the other one more. The fusion of such gametes with a normal haploid gamete during fertilization leads to the formation of a zygote with fewer or more chromosomes compared with the diploid set characteristic of this species. Among aneuploids are found:

§ trisomics - organisms with a set of chromosomes 2n + 1;

§ monosomics - organisms with a set of chromosomes 2n -1;

§ nullsomiki - organisms with a set of chromosomes 2n –2.

For example, Down's disease in humans occurs as a result of trisomy on the 21st pair of chromosomes.

N.I. Vavilov, studying hereditary variability in cultivated plants and their ancestors, discovered a number of patterns that allowed us to formulate the law of homologous series of hereditary variability: “Species and genera genetically close are characterized by similar series of hereditary variability with such accuracy that, knowing a number of forms within one species, it is possible to predict the presence of parallel forms in other species and genera. The closer the genera and species are genetically located in the general system, the more complete the similarity in the series of their variability. Entire plant families are generally characterized by a specific cycle of variation passing through all the genera and species that make up the family. ”

This law can be illustrated by the example of the Myatlikovy family, which includes wheat, rye, barley, oats, millet, etc. Thus, the black color of the grain was found in rye, wheat, barley, corn and other plants, the elongated form of the grain was found in all studied species of the family. The law of homologous series in hereditary variation allowed N.I. Vavilov himself to find a number of forms of rye, previously unknown, based on the presence of these signs in wheat. These include: spinous and boneless ears, grains of red, white, black and violet color, mealy and glassy grains, etc.

The law discovered by N.I. Vavilov is valid not only for plants, but also for animals. So, albinism is found not only in different groups of mammals, but also in birds and other animals. Short-toedness is observed in humans, cattle, sheep, dogs, birds, the absence of feathers in birds, scales in fish, wool in mammals, etc.

The law of homologous series of hereditary variability is of great importance for breeding practice. It allows one to predict the presence of forms not found in a given species, but characteristic of closely related species, that is, the law indicates the direction of searches. Moreover, the desired form can be detected in the wild or obtained by artificial mutagenesis. For example, in 1927, the German geneticist E. Baur, based on the law of homologous series, suggested the possible existence of a non-alkaloid form of lupine, which could be used for animal feed. However, such forms were not known. It has been suggested that non-alkaloid mutants are less resistant to pests than bitter lupine plants, and most of them die before flowering.

Based on these assumptions, R. Zengbush began the search for non-alkaloid mutants. He studied 2.5 million lupine plants and identified among them 5 plants with low alkaloids that were the ancestors of fodder lupine.

More recent studies have shown the effect of the law of homologous series at the level of variability of morphological, physiological and biochemical characters of various organisms - from bacteria to humans.

In nature, spontaneous mutagenesis is ongoing. However, spontaneous mutations are rare. For example, in Drosophila, a white-eye mutation forms at a frequency of 1: 100,000 gametes; in humans, many genes mutate at a frequency of 1: 200000 gametes.

In 1925, G.A. Nadson and G.S. Filippov discovered the mutagenic effect of radium rays on hereditary variability in yeast cells. Of particular importance for the development of artificial mutagenesis were the works of G. Meller (1927), who not only confirmed the mutagenic effect of radium rays in experiments on Drosophila, but also showed that irradiation increases the frequency of mutations hundreds of times. In 1928, L. Stadler used x-rays to obtain mutations. The mutagenic effect of chemicals was later proven. These and other experiments showed the existence of a large number of factors called mutageniccapable of causing mutations in various organisms.

All mutagens used to obtain mutations are divided into two groups:

Induced mutagenesis is of great importance. It makes it possible to create valuable source material for breeding, hundreds of highly productive plant varieties and animal breeds, to increase the productivity of a number of biologically active substance producers by 10–20 times, and also reveals ways to create human protective agents against mutagenic factors.

Modification Variability

A large role in the formation of signs of organisms is played by its environment. Each organism develops and lives in a certain environment, experiencing the effect of its factors that can change the morphological and physiological properties of organisms, i.e. their phenotype.

A diverse example of the variability of characters under the influence of environmental factors is the variegation of the arrowhead: leaves immersed in water have a ribbon-like shape, leaves floating on the surface of the water are rounded, and those in the air are arrow-shaped. If the whole plant is completely immersed in water, its leaves are only ribbon-like. Some species of salamanders darken on dark ground and lighten on light. Under the influence of ultraviolet rays in people (if they are not albinos), a tan occurs as a result of the accumulation of melanin in the skin, and in different people the intensity of skin color is different. If a person is deprived of the action of ultraviolet rays, his skin does not change color.

Thus, changes in a number of signs of organisms are caused by the action of environmental factors. Moreover, these changes are not inherited. So, if you get offspring from newts grown on dark soil, and put them on a light, then they will all have a light color, and not dark, like their parents. Or, collecting seeds from the arrowhead, which grew up under complete immersion in water, and planting them in a shallow pond, we get plants whose leaves will have a shape depending on environmental conditions (ribbon-like, roundish, arrow-shaped). That is, this type of variation does not affect the genotype and therefore is not transmitted to descendants.

The variability of organisms that occurs under the influence of environmental factors and does not affect the genotype is called modifying.

© Modification Variability is of a group nature, that is, all individuals of the same species, placed in the same conditions, acquire similar characteristics. For example, if you place a vessel with green euglens in the dark, then all of them will lose their green color, but if you put it back on the light, everyone will turn green again.

© Modification variability is certain, that is, always corresponds to the factors that cause it. So, ultraviolet rays change the color of human skin (as pigment synthesis is enhanced), but do not change the proportions of the body, and increased physical activity affects the degree of muscle development, and not on skin color.

However, one should not forget that the development of any trait is determined primarily by the genotype. At the same time, genes determine the possibility of the development of a trait, and its appearance and severity in many meters is determined by environmental conditions. So, the green color of plants depends not only on genes that control the synthesis of chlorophyll, but also on the presence of light. In the absence of light, chlorophyll is not synthesized.

Despite the fact that under the influence of environmental conditions the signs can change, this variability is not unlimited. Even in the case of the normal development of the sign, the degree of its severity is different. So, on the wheat field, you can find plants with large ears (20 cm or more) and very small (3-4 cm). This is because the genotype defines certain boundaries within which a change in a trait can occur. The degree of variation of the trait, or the limits of modification variability, is called reaction rate.The reaction rate is expressed in the totality of phenotypes of organisms that are formed on the basis of a particular genotype under the influence of various environmental factors. As a rule, quantitative traits (plant height, yield, leaf size, milk yield of cows, egg laying of chickens) have a broader reaction rate, that is, they can vary widely than qualitative traits (coat color, milk fat, flower structure, blood type) .

Knowing the reaction rate is important for agricultural practice.

Thus, modification variability is characterized by the following basic properties:

Statistical patterns of modification variability

The modification variability of many traits of plants, animals, and humans obeys general laws. These patterns are identified on the basis of the analysis of the manifestation of the trait in a group of individuals ( n) The severity of the studied trait among the members of the sample is different.

Fig. 338. The variational curve.

Based on the variation series, variation curve -graphic display of the frequency of occurrence of each variant (Fig. 338).

For example, if you take 100 ears of wheat ( n) and count the number of spikelets in the ear, then this number will be from 14 to 20 - this is the numerical value of the option ( v).

Variation series:

v = 14 15 16 17 18 19 20

Frequency of occurrence of each option

p= 2 7 22 32 24 8 5

The average value of the trait is more common, and variations significantly different from it are much less common. It is called normal distribution. The curve on the chart is usually symmetrical. Variations, both larger than medium and smaller, are equally common.

where M is the average value of the feature, in the numerator the sum of the works is an option for their frequency of occurrence, in the denominator is the number of options. For this characteristic, the average value is 17.13.

Knowledge of the laws of modification variability is of great practical importance, since it allows one to anticipate and plan ahead the severity of many attributes of organisms depending on environmental conditions.

After Gregor Mendel discovered the uniform laws of heredity.

At the beginning of the 20th century, genetic scientists began to conduct many experiments on crossing at a variety of objects. As a result, it was found that the laws established by Mendel are not always manifested.

Mendel crossed digeterozygotes - organisms that differed in two ways. The traits that Mendel considered were localized on different homologous chromosomes.

Recall that the third law of Mendel is formulated as follows: each pair of allelic genes (and alternative characters controlled by them) is inherited independently of each other.

What does it mean, independently of each other?

When the 1st generation organisms are crossed during meiosis, 4 types of gametes are formed.

Where genes are combined in various combinations. Such combinations were obtained because the genes were on different chromosomes.

But in 1906, William Betson and Reginald Pennet, crossing plants of sweet peas and analyzing the inheritance of several traits of pollen shape and color of flowers, found that these traits do not give an independent distribution in the offspring in a 3: 1 ratio, hybrids always repeated the traits of parental forms.

It became clear that not all characteristics are characterized by an independent distribution in the offspring and free combination.

The fact is that the pollen form gene and the flower coloring gene lie on the same chromosome.

There are much more signs in the body than chromosomes in which these signs are localized. Therefore, each chromosome carries not one gene, but a whole group of genes responsible for the development of different characters.

The study of the inheritance of traits whose genes are localized on the same chromosome was carried out by Thomas Morgan.

He proposed the law of linked inheritance (Morgan law): genes that are on the same chromosome with meiosis fall into one gamete, that is, they are inherited linked.

What does it mean linked?That is, between genes that are on the same chromosome, adhesion forces arise, that is, interaction forces. And the closer these genes are, the stronger the interaction.

If Mendel conducted his experiments on peas, then for Morgan the fruit fly Drosof became the main object and la, which had a diploid set of 8 chromosomes.

Small size, short life cycle and ease of cultivation allows the use of a number of Drosophila species as exemplary objects of genetic research.

The male and the female are clearly distinguishable externally - in the male, the abdomen is smaller and darker.

Drosophila fruit is the most important species of Drosophila for scientific research. Its main characteristics as a model object is a small number of chromosomes. Drosophila every two weeks at a temperature of 25 ° C is quite easy to breed in test tubes and gives numerous offspring.

Consider one of Thomas Morgan's first experiments on the study of linked inheritance.

Crossing a fly with a gray body and normal wings and a fly with a dark colored body and rudimentary wings, in the first generation Morgan received hybrids that had a gray body and normal wings.

The A-large gene is responsible for the gray body, the recessive a-small gene for black body color, the dominant B-large gene for the development of long wings, and the recessive be-small gene for not the development of wings, that is, the wings remain in their infancy.

This means that the gene that determines the gray color of the abdomen dominates over the dark color, and the gene that causes the development of normal wings dominates the underdeveloped gene.

Which have a gray body and long wings and a black body with rudimentary wings.

That is, here the splitting goes precisely into 2 classes, and not into four, as in the case of Mendel's hybrid hybrid.

Why two?The fact is that body coloring genes and wing lengths are linked in the chromosome.

Symbols AB AB ab ab are not located next to each other as we wrote them before, but as if under each other with two dashes. By dashes we conventionally denote chromosomes.

In the first generation, the body is heterozygous for both genes, but with the formation of gametes, this heterozygosity does not give all possible combinations. That is, the parental genes remain interconnected and gametes are obtained of 2 types.

With the combination of this kind of gametes in the offspring, only 3 genotypic classes of offspring arise.

Morgan, researching the inheritance of linked genes, found that there was a violation of this rule for Mendel's hybrid crossbreeding.

He performed an analysis cross.

He took a digeterozygous individual, which was obtained by crossing in the first generation and crossed it with a black fly with rudimentary wings, that is, both recessive traits. He got an unusual result.

Morgan reasoned.If the body color genes and the shape of the wings are localized on the same chromosome, then with this crossing two groups of individuals should be obtained, repeating the signs of the parental forms, since the mother body should form gametes of only two types - AB and ab, and the father - one type - ab . Therefore, in the offspring, two groups of individuals with the genotype AB ab and BB aa should be formed.

However, in the offspring appear individuals (albeit in a small amount) with recombined characters, that is, having the genotype Aa bb and aa Bb.

In the offspring, individuals with signs of parental forms clearly dominated (41.5% were gray long-winged and 41.5% black with rudimentary wings), and only a small part of the flies had a different combination of characters than their parents (8.5% were gray with rudimentary wings and 8.5% are black long-winged).

Such results could be obtained only if the genes responsible for body color and wing shape are on the same chromosome.

In the prophase of the first meiotic division, homologous chromosomes (that is, identical chromosomes of the same pair) conjugate (approach each other), and an exchange of sites can occur between them at this moment - crossing over - crossing over.

Crossingover or crossing is the process of exchanging regions of homologous chromosomes during conjugation in the prophase of the first meiosis. As a result, red-colored gametes are formed.

Organisms that result from the fusion of crossover gametes are called recombinant .

So, as a result of crossing-over in some cells, chromosome regions are exchanged between genes A and B, gametes AB and AB appear, and, as a result, four groups of phenotypes are formed in the offspring, as with free combination of genes.

However, crossing over does not occur after each conjugation (chromosome convergence). And to determine in which parts of the chromosomes it will happen is quite difficult.

During the experiment, Thomas Morgan was able to prove that the crossover frequency between genes is directly proportional to the distance between them in the chromosome. That is, we can say that the farther the genes are from each other on the chromosome, the more often crossing over occurs between them.

If we consider 2 genes A and B, we can see 2 cases.

In the first case, genes A and B are on opposite sides of the cross. Then, after crossing over, we will see new combinations of alleles of these two genes. In this case, Ab and AB.

In the 2nd case, genes A and B are located on one side of the intersection. Then, after crossing over the new combinations of alleles of these two genes, we will not see.

Thus, there are concepts of full and incomplete grip.

Incomplete linkage is a type of linked inheritance in which the genes of the analyzed traits are located at a certain distance from each other, which makes crossing over between them possible.

Full linkage is a form of linked inheritance in which the genes of the analyzed traits are so close to each other that crossing over between them becomes impossible.

This discovery allowed the laboratory of Thomas Morgan to develop a method. Which allows you to build chromosome maps .

Chromosomal cards is a graphical representation of a chromosome in which certain loci (genes) are marked according to the distance between them.

Chromosomal maps are compiled using genetic analysis, which allows you to accurately determine the location in the chromosome of any gene.

Chromosomal theory of heredity

Morgan's study of the inheritance of parental traits with Drosophila hybrids showed that the number of linked inheritance groups was equal to the number of pairs of homologous chromosomes.

For example, a person has 46 chromosomes, hence 23 cohesion groups. Drosophila has 8 chromosomes, that is, 4 cohesion groups.

On this basis, it was concluded that the strict localization of specific genes in certain pairs of chromosomes.

The appearance of crossover (recombinant) Drosophila individuals could be explained only by the linear arrangement of genes in the chromosomes and their exchange during crossing over in the prophase of the first meiosis.

Thomas Morgan substantiated the chromosome theory of heredity. According to this theory, transmission of hereditary information is associated with chromosomes in which genes are linearly, in a certain sequence, localized . Thus, it is the chromosomes that represent the material basis of heredity.

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

The chromosomal theory of heredity was formulated in 1911 by the American scientist Thomas Morgan. Its essence is as follows:

· The main material carrier of heredity are chromosomes with genes localized in them.

· Genes in the chromosomes are linearly located, each gene has a specific place (locus) in the chromosome;

· Genes located on the same chromosome form a linkage group and are inherited together;

· The number of linkage groups is equal to the haploid set of chromosomes in homogametic individuals and n + 1 in heterogametic individuals.

· Between homologous chromosomes, an exchange of sites (crossingover) can occur; As a result of crossing over, gametes arise whose chromosomes contain new gene combinations.

· Gene linkage may be impaired as a result of crossing over;

· Crossover frequency between homologous chromosomes depends on the distance between genes localized on the same chromosome. The greater this distance, the higher the crossover frequency.

The significance of this theory lies in the fact that it gave an explanation of the laws of Mendel, revealed the cytological foundations of the inheritance of traits and the genetic foundations of the theory of natural selection.

Chromosomes

Law

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