The idea that living things are characterized by heredity and variability have been formed in antiquity. It was noticed that during the reproduction of organisms from generation to generation, a complex of signs and properties inherent in a particular species (manifestation of heredity) is transmitted. However, it is equally obvious that there are some differences between individuals of the same species (manifestation of variability).

The knowledge of the presence of these properties was used in the development of new varieties of cultivated plants and breeds of domestic animals. From time immemorial in agriculture used hybridization, that is, the crossing of organisms that differ from each other in some way. However, until the end of the XIX century. such work was carried out by trial and error, since the mechanisms underlying the manifestation of such properties of organisms were not known, and the hypotheses that existed in this regard were purely speculative.

In 1866, the work of Gregor Mendel, a Czech researcher, "Experiments on plant hybrids" was published. It described the patterns of inheritance of traits in the generations of plants of several species, which G. Mendel revealed as a result of numerous and carefully performed experiments. But his research did not attract the attention of his contemporaries, who failed to appreciate the novelty and depth of ideas that surpassed the general level of biological sciences of that time. Only in 1900, after the laws of G. Mendel were discovered anew and independently of each other by three researchers (H. de Vries in Holland, K. Correns in Germany and E. Cermak in Austria), did the development of a new biological science, genetics, begin. patterns of heredity and variability. Gregor Mendel is rightly considered the founder of this young, but very rapidly developing science.

Basic concepts of modern genetics.

Heredity is called the property of organisms to repeat in a series of generations a complex of features (features external structure, physiology, chemical composition, nature of metabolism, individual development, etc.).

Variability- a phenomenon opposite to heredity. It consists in changing combinations of traits or the appearance of completely new traits in individuals of a given species.

Thanks to heredity, the preservation of species is ensured over significant periods (up to hundreds of millions of years) of time. However, the conditions environment change (sometimes significantly) over time, and in such cases the variability leading to the diversity of individuals within the species ensures its survival. Some of the individuals turn out to be more adapted to new conditions, this allows them to survive. In addition, variability allows species to expand the boundaries of their habitat, to develop new territories.

The combination of these two properties is closely related to the evolutionary process. New traits of organisms appear as a result of variability, and due to heredity, they are preserved in subsequent generations. The accumulation of many new traits leads to the emergence of other species

Types of variability

Distinguish between hereditary and non-hereditary variability.

Hereditary (genotypic) variability b is associated with a change in the genetic material itself. Non-hereditary (phenotypic, modification) variability is the ability of organisms to change their phenotype under the influence of various factors. Modification variability is caused by changes in the external environment of the organism or its internal environment.

Reaction rate

These are the boundaries of the phenotypic variability of a trait that occurs under the influence of environmental factors. The rate of reaction is determined by the genes of the organism, therefore, the rate of reaction for the same trait is different for different individuals. The range of reaction rates for various signs also varies. Those organisms in which the reaction rate is wider in terms of a given trait have higher adaptive capabilities in certain environmental conditions, i.e., modification variability in most cases is of an adaptive nature, and most of the changes that have arisen in the body under the influence of certain environmental factors are useful. However, phenotypic changes sometimes lose their adaptive character. If the phenotypic variation is clinically similar to an inherited disease, then such changes are called phenocopy.

Combinative variability

It is associated with a new combination of unchanged parental genes in the genotypes of the offspring. Factors of combinative variability.

1. Independent and random divergence of homologous chromosomes in anaphase I of meiosis.

2. Crossing over.

3. Random combination of gametes during fertilization.

4. Random selection of parental organisms.

Mutations

These are rare, accidentally occurring persistent changes in the genotype that affect the entire genome, entire chromosomes, parts of chromosomes, or individual genes. They arise under the influence of mutagenic factors of physical, chemical or biological origin.

Mutations are:

1) spontaneous and induced;

2) harmful, useful and neutral;

3) somatic and generative;

4) gene, chromosomal and genomic.

Spontaneous mutations are mutations that have arisen in an undirected manner, under the influence of an unknown mutagen.

Induced mutations are mutations artificially caused by the action of a known mutagen.

Chromosomal mutations are changes in the structure of chromosomes during cell division. There are the following types of chromosomal mutations.

1. Duplication - duplication of a chromosome section due to unequal crossing over.

2.Deletion - loss of a portion of the chromosome.

3. Inversion - rotation of the chromosome section by 180 °.

4. Translocation - moving a portion of a chromosome to another chromosome.

Genomic mutations are changes in the number of chromosomes. Types of genomic mutations.

1.Polyploidy - a change in the number of haploid sets of chromosomes in a karyotype. A karyotype is understood as the number, shape and number of chromosomes characteristic of a given species. Distinguish between nullisomy (the absence of two homologous chromosomes), monosomy (the absence of one of the homologous chromosomes) and polysomy (the presence of two or more extra chromosomes).

2. Heteroploidy - a change in the number of individual chromosomes in a karyotype.

Gene mutations are the most common.

Causes of gene mutations:

1) nucleotide loss;

2) insertion of an extra nucleotide (this and the previous reasons lead to a shift in the reading frame);

3) replacement of one nucleotide with another.

The transfer of hereditary traits in a number of generations of individuals is carried out in the process of reproduction. With sex - through the germ cells, with asexual, hereditary traits are transmitted with somatic cells.

Genes are the units of heredity (its material carriers). Functionally, a particular gene is responsible for the development of some trait. This does not contradict the definition that we gave to the gene above. From a chemical point of view, a gene is a section of a DNA molecule. It contains genetic information about the structure of the synthesized protein (i.e. the sequence of amino acids in a protein molecule). The totality of all genes in the body determines the totality of specific proteins synthesized in it, which ultimately leads to the formation of specific traits.

In a prokaryotic cell, genes are part of a single DNA molecule, while in a eukaryotic cell, genes are included in DNA molecules contained in chromosomes. At the same time, in a pair of homologous chromosomes, in the same regions, there are genes responsible for the development of some trait (for example, flower color, seed shape, human eye color). They are called allelic genes. One pair of allelic genes can include either the same (in the composition of nucleotides and the trait they define) or different genes.

The concept of "trait" is associated with some separate quality of an organism (morphological, physiological, biochemical), by which we can distinguish it from another organism. For example: blue or brown eyes, colored or uncolored flowers, tall or short height, blood type I (0) or II (A), etc.

The totality of all genes in an organism is called a genotype, and the totality of all traits is called a phenotype.

The phenotype is formed on the basis of the genotype under certain environmental conditions in the course of the individual development of organisms.

The laws of inheritance were formulated in 1865 by Gregory Mendel in his work "Experiments on plant hybrids". In his experiments, he crossed various varieties of peas (Czech Republic / Austria-Hungary). In 1900, the patterns of inheritance were rediscovered by Correns, Cermak and Gogo de Vries.

Mendel's first and second laws are based on monohybrid crossing, and the third on di and polyhybrid. Monohybrid crossing goes on one pair of alternative traits, dihybrid in two pairs, polyhybrid - more than two. Mendel's success is due to the peculiarities of the applied hybridological method:

The analysis begins with crossing pure lines: homozygous individuals.

Separate alternative mutually exclusive features are analyzed.

Accurate quantitative accounting of offspring with different combinations of traits

The inheritance of the analyzed characters can be traced in a number of generations.

The rule of writing out gametes by the formula 2n , where n is the number of heterozygotes: for monohybrids - 2 varieties of gametes, for dihybrids - 4, for trihybrids - 8.

1st Mendel's Law: "The Law of Uniformity of 1st Generation Hybrids"

When crossing homozygous individuals analyzed for one pair of alternative traits, hybrids of the 1st generation show only dominant traits and uniformity in phenotype and genotype is observed.

In his experiments, Mendel crossed pure lines of pea plants with yellow (AA) and green (aa) seeds. It turned out that all offspring in the first generation are the same in genotype (heterozygous) and phenotype (yellow).

2nd Mendel's law: "The law of splitting"

When crossing heterozygous hybrids of the 1st generation, analyzed according to one pair of alternative traits, in hybrids of the second generation there is a splitting according to the phenotype 3: 1, and according to the genotype 1: 2: 1

In his experiments, Mendel crossed the hybrids (Aa) obtained in the first experiment with each other. It turned out that in the second generation, the suppressed recessive trait reappeared. The data from this experience indicate the splitting out of a recessive trait: it is not lost, but manifests itself again in the next generation.

Cytological foundations of the 2nd Mendel's law

The cytological foundations of Mendel's 2nd law are revealed in the hypothesis of "purity of gametes" ... It can be seen from the crossing schemes that each trait is determined by a combination of two allelic genes. When heterozygous hybrids are formed, allelic genes do not mix, but remain unchanged. As a result meiosis in gametogenesis, only 1 of a pair of homologous chromosomes gets into each gamete. Therefore, only one of a pair of allelic genes, i.e. a gamete is pure relative to another allelic gene.

3rd Mendel's law: "The law of independent combination of features"

When homozygous organisms are crossed, analyzed for two or more pairs of alternative traits, in hybrids of its 3rd generation (obtained by crossing hybrids of the 2nd generation), an independent combination of traits and the corresponding genes of different allelic pairs is observed.

To study the patterns of inheritance plants , differing in one pair of alternative features, Mendel used monohybrid cross ... Then he moved on to experiments on crossing plants, differing in two pairs of alternative characters: dihybrid crossing , where he used homozygous pea plants, differing in color and seed shape. As a result of crossing smooth (B) and yellow (A) with wrinkled (b) and green (a), in the first generation all plants were with yellow smooth seeds.

Thus, the law of uniformity of the first generation manifests itself not only in mono, but also in polyhybrid crossing, if the parental individuals are homozygous.

During fertilization, a diploid zygote is formed due to the fusion of different varieties of gametes. English geneticist Bennet to facilitate the calculation of the options for their combination, he proposed a record in the form lattice - tables with the number of rows and columns according to the number of gamete types formed by crossing individuals.

Analyzing cross

Since individuals with a dominant trait in the phenotype can have a different genotype (Aa and AA), Mendel suggested crossing this organism with recessive homozygote .

A homozygous individual will give uniform generation,

and heterozygous - split by phenotype and genotype 1: 1.

Mohran's chromosomal theory. Chained inheritance

Establishing the patterns of inheritance, Mendel crossed pea plants. Thus, his experiments were carried out at the organismic level. The development of the microscope at the beginning of the 20th century made it possible to identify cells - the material carrier of hereditary inf, transferring research to the cellular level. Based on the results of numerous experiments with fruit flies, in 1911, Thomas Morgan formulated the main provisions of the chromosomal theory of heredity .

The genes on the chromosome are located linearly in certain loci ... Allelic genes occupy the same loci of homologous chromosomes.

Genes located on the same chromosome form clutch group and are inherited primarily together. The number of linkage groups is equal to n set of chromosomes.

Between homologous chromosomes is possible crossing over - exchange of sites, which can disrupt gene linkage. The likelihood that genes will remain linked is directly proportional to the distance between them: the closer the genes are located on the chromosome, the higher the likelihood of their linking. This distance is calculated in morganids: 1 morganid corresponds to 1% of the formation of crossover gametes.

For his experiments, Morgan used fruit flies differing in 2 pairs of traits: gray (B) and black (b); wing length norm (V) and short (v).

1) Dihybrid crossing - first, homozygous individuals AABB and aabb were crossed. Thus, results similar to Mendel were obtained: all individuals with a gray body and normal wings.

2) Analyzing crossing was carried out with the aim of breeding the genotype of hybrids of the 1st generation. A diheterozygous male was crossed with a recessive dihomozygous female. According to his third Mendel's law, the appearance of 4 phenotypes could be expected due to an independent combination of traits: ch (BbVv), chk (bbvv), ck (Bbvv), chn (bbVv) in a ratio of 1: 1: 1: 1. However, only 2 combinations were obtained: ch (BbVv) chk (bbvv).

Thus, in the second generation, only parent phenotypes in a 1: 1 ratio.

This deviation from the free combination of traits is due to the fact that the genes that determine body color and wing length in Drosophila flies are located in the same chromosome and are inherited linked ... It turns out that the diheterozygous male produces only 2 varieties of non-crossover gametes, and not 4, as in the case of dihybrid crossing of organisms with unlinked characters.

3) Analyzing Prescription Crossing - a system of crossbreeding, in which genotypically different parental individuals are used once as a maternal form, another time as a paternal one.

This time, Morgan used a diheterozygous female and a homozygous recessive male. So 4 phenotypes were obtained, but their ratio did not correspond to that observed in Mendel with an independent combination of traits. The number of sc and chs was 83% of the total offspring, and the number of sc and chs was only 17%.

The linkage between genes localized on the same chromosome is disrupted as a result crossing over ... If the break point of the chromosomes lies between linked genes, then the link is broken, and one of them passes into a homologous chromosome. So, in addition to two varieties non-crossover gametes , two more varieties are formed crossover gametes , in which chromosomes have exchanged homologous regions. From them, crossover individuals develop during fusion. According to the position of the chromosomal theory, the distance between genes that determine body color and wing length in Drosophila is 17 morganids - 17% of crossover gametes and 83% of non-crossover ones.

Allelic interaction of genes

1) Incomplete dominance: when crossing homozygous sweet pea plants with red and white flowers, all offspring in the first generation have pink flowers - an intermediate form. In the second generation, phenotypic cleavage corresponds to genotype cleavage in the ratio 1cr: 2roz: 1bel.

2) Overdominance : in the dominant allele in heterozygote the trait is more pronounced than in the homozygote. Moreover, the heterozygous organism Aa has better fitness than both types of homozygotes.

Sickle cell anemia is caused by the mutant allele s. In areas where malaria is common, Ss heterozygotes are more resistant to malaria than SS homozygotes.

3) Codominance : in the phenotype of heterozygotes, both allelic genes appear, as a result of which a new trait is formed. But it is impossible to call one allele dominant, and the other cannot be recessive, since they equally affect the phenotype.

Formation of the 4th blood group in humans. Allele Ia determines the presence of antigen a on erythrocytes, allele Ib - the presence of antigen b. The presence of both alleles in the genotype determines the formation of both antigens on erythrocytes.

4) Multiple alleles: there are more than two allelic genes in the population. Such genes result from mutation of the same chromosome locus. In addition to the dominant and recessive genes, intermediate alleles , which in relation to the dominant behave as recessive, and in relation to the recessive - as dominant. Each diploid individual may have no more than two allelic genes, but their number is not limited in the population. The more allelic genes, the more variants of their combinations. All alleles of one gene are designated by one letter with different indices: A1, A2, A3, etc.

In guinea pigs, the color of the coat is determined by 5 alleys of one locus, which in various combinations give 11 color options. In humans, according to the type of multiple alleles, blood groups are inherited according to the ABO system. Three genes Io, Ia, Ib determine the inheritance of 4 human blood groups (genes Ia Ib are dominant in relation to Io).

Non-allelic gene interactions

1) Complementarity or complementary gene interaction is a phenomenon in which two non-allelic dominant or recessive genes give new feature ... Such interaction of genes is observed when inheriting the forms of the comb in chickens:

A pea (A-cc); B - pink (aaB-); AB nutty; aavv leaf-shaped.

When crossing chickens with pea and pink combs, all 1st generation hybrids will have a nut comb. When crossing dihybrids of the 1st generation with walnut-like ridges, in the 2nd generation, individuals with all types of ridges appear in the ratio 9op: 3roz: 3g: 1sheet. However, in contrast to the splitting with the 3rd Mendel's law, there is no splitting of each allele in the ratio of 3: 1. In other cases of complementarity, perhaps 9: 7 and 9: 6: 1.

2) Epistasis or epistatic gene interaction - suppression action of the genes of one allele by the genes of another. The suppressor gene is a suppressor or inhibitor.

Dominant epistasis - dominant suppressor gene: inheritance of feather color in chickens. C - pigment synthesis, I - suppressor gene. Chickens with genotype C-ii will be stained. The rest of the individuals will be white, since in the presence of a dominant suppressor gene, the suppressed color gene does not appear, or the gene responsible for the synthesis of pigment (ccii) is absent. In the case of crossbreeding dihybrids, the split in the second generation will be 13: 3 or 12: 3: 1.

Recessive epistasis - the suppressor genome is a recessive gene, for example, the inheritance of the color of mice. B - synthesis of gray pigment, b - black; And it promotes the manifestation of color, and - suppresses it. Epistasis will manifest itself only in those cases where there are two aa suppressor genes in the genotype. When crossing dihybrid individuals with recessive epistasis, the splitting in the second generation is 9: 3: 4.

Bombay phenomenon manifests itself in the inheritance of blood groups according to the ABO system. A woman with blood group 1 (IoIo), who married a man with group 2 (IaIo), gave birth to two girls with 4 (IaIb) and 1 (IoIo) groups. This is due to the fact that their mother possessed the Ib allele, but its effect was suppressed by a rare recessive gene, which, in a homozygous state, had its epistatic effect. As a result, the woman showed phenotypically 1 group.

3) Polymerism - the same sign is defined by several alleys. In this case, dominant genes from different allelic pairs affect the degree of manifestation of one trait. It depends on the number of dominant genes in the genotype (the more dominant genes, the more pronounced the trait) and on the effects of environmental conditions.

Polymeric genes are usually denoted by one letter of the Latin alphabet with digital indices A 1 A 2 a 3 and so on. They define polygenic traits ... This is how many quantitative and some qualitative features in animals and humans: height, weight, skin color. Wheat grain color inheritance: each of the dominant genes determines the color red, the recessive genes determine the white color. With an increase in the number of dominant genes, the intensity of the color increases. And only if the organism is homozygous for all pairs of recessive genes, the grains are not colored. So, when crossing dihybrids, splitting in the ratio of 15cr: 1bel.

4) Pleiotropy- one gene affects several traits. The phenomenon was described by Mendel, who discovered that a hereditary factor in pea plants can determine several traits: the red color of the flowers, the gray color of the seeds, and a pink spot at the base of the leaves. Often extends to evolutionarily important traits: fertility, life expectancy, the ability to survive in extreme environmental conditions.

In some cases, the pleetropic gene is dominant with respect to one trait, and with respect to another, recessive. If the pleetropic gene is only dominant or only recessive in relation to all the traits it defines, then the nature of inheritance is similar to the laws of Mendel.

A kind of splitting is observed when one of the signs is recessive or lethal (homozygote leads to death). For example, black wool of Karakul sheep and scar development are determined by one gene, while gray wool and an underdeveloped scar are determined by its allelic gene. Gray dominates black, norm over anomaly. Homozygous individuals for the rumen underdevelopment gene and gray die, therefore, when heterozygous individuals are crossed, a fourth of the offspring (gray homozygotes) are not viable. Splitting in a ratio of 2: 1.

Penetrance and expressiveness

The genotype of an individual determines only the potential for the development of a trait: the implementation of a gene into a trait depends on the influence of other genes and environmental conditions, therefore, the same hereditary information manifests itself differently in different conditions. Consequently, it is not a ready-made trait that is inherited, but the type of reaction to the action of the environment.

Penetrance - the penetration of a gene into a sign. Expressed as a percentage of the number of individuals carrying the trait, to the total carriers of a gene that is potentially capable of being realized in this trait. Full penetrance (100%) - all carriers of the gene have a phenotypic manifestation of the trait. Incomplete - the effect of the gene is not manifested in all carriers.

If a gene is beaten into a trait, it is penetrant, but it can manifest itself in different ways. Expressiveness - the severity of the sign. The gene that causes a decrease in the number of eye facets in Drosophila has different expressiveness. Homozygotes have different number facets, up to their complete absence.

Penetrance and expressiveness depend on the influence of other genes and the external environment.

Variability

Variability - the ability to acquire new characters under the influence of external and internal environmental factors (morphological, physiological, biochemical). Variability is associated with the diversity of individuals of one species, which serves as material for evolutionary processes. The unity of heredity and variability is a condition for incessant biological evolution. There are several types:

1) Hereditary, genotypic, indeterminate, individual

It is hereditary in nature, and is due to the recombination of genes in the genotype and mutations, is inherited. There are combinative and mutational

2) Non-hereditary, modification, phenotypic, group, definite

Modification variability is an evolutionarily fixed adaptive reactions of an organism in response to changes in environmental conditions, a consequence of the interaction of the environment and the genotype. It is not inherited, since it does not lead to a change in the genotype. Unlike mutations, many modifications are reversible: tanning, cow milk production, etc.

Short feedback form

1. Features of the method of hybridological analysis. Mendel's laws.
2. Types of gene interaction.
3. Linked inheritance of traits.
4. Cytoplasmic inheritance.

Method hybridological analysis , which consists in crossing and subsequent accounting for splits (ratios of phenotypic and genotypic varieties of offspring), was developed by the Czech naturalist G. Mendel (1865). The peculiarities of this method include: 1) taking into account, when crossing, not the entire diverse complex of traits in parents and offspring, but analysis of the inheritance of individual alternative traits identified by the researcher; 2) quantitative accounting in a series of successive generations of hybrid plants, differing in individual characteristics; 3) individual analysis of the offspring from each plant.

Working with self-pollinating plants of garden peas, G. Mendel chose varieties (pure lines) for the experiment, differing from each other by alternative manifestations of traits. Mendel processed the obtained data mathematically, as a result of which a clear pattern of inheritance of individual traits of parental forms by their descendants in a number of subsequent generations was revealed. Mendel formulated this pattern in the form of the rules of heredity, which were later named Mendel's laws.

The crossing of two organisms is called hybridization. Monohybrid (monogenic) is called the crossing of two organisms, in which the inheritance of one pair of alternative manifestations of a trait is traced (the development of this trait is due to a pair of alleles of the same gene). First generation hybrids are uniform in the studied trait. Only one of the pair appears in F1 alternative options a trait of seed color called dominant. These results illustrate Mendel's first law of uniformity for first-generation hybrids, as well as the rule of dominance.

Mendel's first law can be formulated as follows: when crossing homozygous individuals differing in one or more pairs of alternative traits, all hybrids of the first generation will be uniform in these traits. The hybrids will show the dominant traits of the parents.

In the second generation, splitting was found according to the studied trait

The ratio of offspring with dominant and recessive manifestations of the trait turned out to be close to ¾ to ¼. Thus, Mendel's second law can be formulated as follows: during monohybrid crossing of heterozygous individuals (F1 hybrids) in the second generation, splitting is observed according to the variants of the analyzed trait in a ratio of 3: 1 in phenotype and 1: 2: 1 in genotype. To explain the distribution of traits in hybrids of successive generations, G. Mendel suggested that each hereditary trait depends on the presence in somatic cells of two hereditary factors derived from the father and mother. To date, it has been established that Mendel's hereditary factors correspond to genes - chromosome loci.

Homozygous plants with yellow seeds (AA) form gametes of the same variety with the A allele; plants with green seeds (aa) form gametes with a. Thus, using modern terminology, the hypothesis “ purity of gametes"Can be formulated as follows:" In the process of formation of germ cells, only one gene from an allelic pair gets into each gamete, because in the process of meiosis one chromosome from a pair of homologous chromosomes gets into the gamete.

Crossing, in which inheritance is traced in two pairs of alternative traits, is called dihybrid, for several pairs of signs - polyhybrid. In Mendel's experiments, when crossing a pea cultivar that had yellow (A) and smooth (B) seeds, with a pea cultivar with green (a) and wrinkled (b) seeds, F1 hybrids had yellow and smooth seeds, i.e. dominant traits appeared (hybrids are uniform).

Hybrid seeds of the second generation (F2) were divided into four phenotypic groups in the ratio: 315 - with smooth yellow seeds, 101 - with wrinkled yellow seeds, 108 - with smooth green seeds, 32 - with green wrinkled seeds. If the number of offspring in each group is divided by the number of offspring in the smallest group, then in F2 the ratio of phenotypic classes will be approximately 9: 3: 3: 1. So according to Mendel's third law, genes of different allelic pairs and their corresponding traits are passed on to offspring whatever apart, combining in all kinds of combinations.

With the complete dominance of one allele over another, heterozygous individuals are phenotypically indistinguishable from homozygous for the dominant allele and can only be distinguished using hybridological analysis, i.e. by offspring, which is obtained from a certain type of crossing, called analyzing... Analyzing is a type of crossing in which a test individual with a dominant trait is crossed with an individual homozygous for the recessive aplel.

If the dominant individual is homozygous, the offspring from such a cross will be uniform and no splitting will occur. In the event that an individual with a dominant trait is heterozygous, splitting will occur in a ratio of 1: 1 in phenotype and genotype.

Interaction of genes

In some cases, the action of different genes is relatively independent, but, as a rule, the manifestation of traits is the result of the interaction of products of different genes. These interactions can be associated with both allelic and with non-allelic genes.

Interaction between allelic genes are carried out in the form of three forms: complete dominance, incomplete dominance and independent manifestation (codominance).

Previously, Mendel's experiments were considered, which revealed the complete dominance of one allele and the recessiveness of the other. Incomplete dominance is observed when one gene from a pair of alleles does not provide the formation of its protein product sufficient for the normal manifestation of a trait. With this form of gene interaction, all heterozygotes and homozygotes differ significantly in phenotype from each other. At co-dominance in heterozygous organisms, each of the allelic genes causes the formation of a trait controlled by it in the phenotype. An example of this form of interaction of alleles is the inheritance of human blood groups according to the ABO system, determined by gene I. There are three alleles of this gene, Io, Ia, Ib, which determine the antigens of blood groups. The inheritance of blood groups also illustrates the phenomenon plural allelism: in the gene pools of human populations, gene I exists in the form of three different alleles, which are combined in individual individuals only in pairs.

Interaction of non-allelic genes. In some cases, one trait of an organism can be influenced by two (or more) pairs of non-allelic genes. This leads to significant numerical deviations of phenotypic (but not genotypic) classes from those established by Mendel during dihybrid crossing. The interaction of non-allelic genes is subdivided into the main forms: complementarity, epistasis, and polymeria.

At complementary In interaction, the sign is manifested only in the case of the simultaneous presence of two dominant non-allelic genes in the genotype of the organism. An example of a complementary interaction is the crossing of two different varieties of sweet pea with white flower petals.

The next type of interaction of non-allelic genes is epistasis, in which the gene of one allelic pair suppresses the action of the gene of the other pair. The gene that suppresses the action of another is called epistatic genome(or suppressor). The suppressed gene is called hypostatic. Epistasis can be dominant and recessive. An example of dominant epistasis is the inheritance of the color of the plumage of chickens. Gene C in the dominant form determines the normal production of pigment, but the dominant allele of another gene I is its suppressor. As a result, chickens with a dominant allele of the color gene in their genotype turn out to be white in the presence of a suppressor. The epistatic effect of the recessive gene illustrates the inheritance of coat color in house mice. Agouti color (reddish-gray coat color) is determined by the dominant gene A. Its recessive allele and in the homozygous state causes black coloration. The dominant gene of the other pair C determines the development of the pigment; homozygotes for the recessive allele c are albinos with white hair and red eyes (no pigment in the coat and iris of the eyes).

The inheritance of a trait, the transmission and development of which, as a rule, is caused by two alleles of the same gene, is called monogenic... In addition, genes from different allelic pairs are known (they are called polymeric or polygenes), approximately equally influencing the sign.

The phenomenon of simultaneous action on a trait of several non-allelic genes of the same type is called polymerization. Although polymeric genes are not allelic, since they determine the development of one trait, they are usually denoted by one letter A (a), numbers indicating the number of allelic pairs. The action of polygenes is most often cumulative.

Chained inheritance

The analysis of the simultaneous inheritance of several traits in Drosophila, carried out by T. Morgan, showed that the results of the analyzing crossing of F1 hybrids sometimes differ from those expected in the case of their independent inheritance. In the offspring of such a crossing, instead of a free combination of traits from different pairs, a tendency towards inheritance of predominantly parental combinations of traits was observed. This inheritance of traits was called linked. Linked inheritance is explained by the location of the corresponding genes on the same chromosome. As part of the latter, they are transmitted from generation to generation of cells and organisms, preserving the combination of parental alleles.

The dependence of linked inheritance of traits on the localization of genes in one chromosome gives reason to consider chromosomes as separate clutch groups. Analysis of the inheritance of the trait of eye color in Drosophila in the laboratory of T. Morgan revealed some features that made it necessary to distinguish traits as a separate type of inheritance sex-linked inheritance.

The dependence of the experimental results on which of the parents was the carrier of the dominant variant of the trait made it possible to suggest that the gene that determines the color of the eyes in Drosophila is located on the X chromosome and has no homologue on the Y chromosome. All the features of sex-linked inheritance are explained by the unequal dose of the corresponding genes in representatives of different - homo- and heterogametic sex. The X chromosome is present in the karyotype of each individual, therefore, the traits determined by the genes of this chromosome are formed in both females and males. Individuals of the homogametic sex receive these genes from both parents and pass them on to all descendants through their gametes. Representatives of the heterogametic sex receive a single X chromosome from a homogametic parent and pass it on to their homogametic offspring. In mammals (including humans), the male sex receives X-linked genes from the mother and passes them on to their daughters. At the same time, the male sex never inherits the paternal X-linked trait and does not pass it on to his sons.

Actively functioning genes of the Y-chromosome, which do not have alleles in the X-chromosome, are present in the genotype only of the heterogametic sex, and in the hemizygous state. Therefore, they manifest themselves phenotypically and are transmitted from generation to generation only in representatives of the heterogametic sex. So, a person has a sign of hypertrichosis auricle("Hairy ears") occurs exclusively in males and is inherited from father to son.

REGULARITIES OF INHERITANCE OF CHARACTERS

Genetics(Greek genetikos - which refers to origin) is a biological science, the subject of which is heredity and variability. Heredity and variability are the basic properties of all living organisms. The term “genetics” was first proposed in 1906 by the English scientist W. Betson.

Heredity - the property of organisms to transmit their characteristics and features of development to offspring; property to provide material and functional continuity between generations. Heredity is realized during reproduction. Each kind of organisms preserves and recreates its own kind in a number of generations. In the process of reproduction, not only the like is recreated, but the new also arises. Children always look like their parents, but they are never exact copies of them. They differ both from their parents and from each other.

The main task of genetics - to learn the laws of heredity and variability in order to develop ways of managing them in the interests of all mankind. To accomplish this task, genetics uses a number of methods, the main one being genetic analysis. It is based on a hybridological method - the study of the patterns of inheritance of traits by hybridization (crossing). The method was developed by G. Mendel (1865). Genetics also uses methods of other sciences: microscopic, ultramicroscopic, statistical, physicochemical, population, cybernetic. The study of heredity is carried out at different objects and at different levels (molecular, chromosomal, cellular, organismic, population). The variety of objects and research methods led to the emergence of the following sections of genetics: genetics of microorganisms, plant genetics, animal genetics, human genetics, cytogenetics, molecular genetics, biochemical genetics, radiation genetics, population genetics. There is also such a section as behavior genetics. A feature of modern genetics is penetration into all areas of the molecular level of research, deepening ties with other sciences.

The value of genetics. Genetic patterns underlie all biological phenomena. Genetics is the leading science of modern natural history. It forms the theoretical basis for selection. New breeds of animals, plant varieties, strains of microorganisms have been created with the help of genetic methods. The methods of genetics are used to solve food, environmental, space and other global problems of mankind. Genetic knowledge is an integral part of all scientific programs for nature protection and public health.

Genetics is closely related to medicine, because about 5% of children are born with various genetic defects. All genetic sciences are important for medicine. This is due to the universality of the laws of genetics, which were first established on experimental objects, and then turned out to be acceptable for humans as well. Experimental genetics data are used for diagnostics, treatment and prevention of hereditary diseases. Using the methods of genetic engineering and biotechnology, insulin, interferon, and antibiotics, which are necessary for practical medicine, are obtained in vitro (outside the body) in industrial quantities.

Human genetics - a section of general genetics, which studies the heredity and variability of a person. The main task of human genetics is to learn the patterns of heredity and tempering of a person in order to preserve the health of present and future generations. Methods for studying human heredity - genealogical, twin, cytogenetic, biochemical, population statistical, dermatoglyphics, molecular genetic.

Human heredity as an independent subject of research was first identified in 1865 by the English scientist F. Galton (1822-1911), who is considered one of the founders of genetics. He was born in the same year as G. Mendel (1822-1884). F. Galton is a cousin of Charles Darwin (1809-1882) - the author of the first scientific evolutionary theory. Both of them are the grandchildren of the English physician and naturalist E. Darwin (1731-1802), known for his progressive views of nature. F. Galton proposed a number of methods for human genetic analysis (genealogical, twin, statistical, dermatoglyphics), studied the issues of quantitative assessment of human traits (character, intelligence, talent, ability to work) and their inheritance, created a special direction in genetics - eugenics (Greek: kind, genesis - race, origin) and determined its main goal - to improve man and the human race as a whole. He saw the ways of such “improvement” in selective reproduction of some people (for example, gifted, talented) and restriction of other marriages. Theoretically, eugenics was based on real facts of the hereditary dependence of normal and pathological signs, but in practice it was carried out in a number of countries (Nazi Germany) as an inhuman recognition of certain categories of the population as inferior, which were legally subject to forced sterilization (“racial hygiene”). Eugenics programs have long delayed the development of human genetics.

The main scientific directions of the development of modern human genetics:

Cytogenetics studies human chromosomes, their structural and functional organization, mapping, and develops methods of chromosomal analysis. Achievements of cytogenetics are used to diagnose human chromosomal diseases. Population genetics studies the genetic structure of human populations, the frequency of alleles of individual genes (normal and pathological) in human populations, predicts and evaluates the genetic consequences of environmental pollution, the influence of anthropogenic environmental factors on biological processes that occur in human populations (mutation process). These studies allow predicting the frequency of certain hereditary diseases in generations and planning preventive measures. Biochemical genetics studies using biochemical methods the ways of realizing genetic information from gene to trait. Using biochemical methods, developed express methods for the diagnosis of a number of hereditary diseases, including methods of prenatal (prenatal) diagnosis. The development of a system for protecting the gene pool of people from ionizing radiation is one of the main tasks of radiation genetics. Immunological genetics (immunogenetics) studies the genetic dependence of the body's immunological characteristics, immune reactions. Pharmacological genetics (pharmacogenetics) studies the genetic dependence of the reactions of individuals to medicinal products and the effect of the latter on the hereditary apparatus.

Features of human genetics

Unlike classical objects of genetics, a person is a specific and complex object of genetic analysis. The specificity of man lies in the fact that it combines the laws of organic evolution and the laws of social life. The hybridological method, which is based on a system of experimental crosses, is unacceptable for humans. Experimental marriages are impossible for humans. Genetic experiments on humans are prohibited. There are other features that make it difficult to study heredity and human variability.

The main ones are:

1. Slow generation change (after about 25-30 years). The lifespan of a person, as an object of observation, may exceed the lifespan of a researcher.

2. A small number of children in each family.

3. Complex karyotype, which includes 46 chromosomes (24 linkage groups - 22 pairs of autosomes, X-, Y-chromosomes). For comparison, Drosophila has 8 chromosomes (4 linkage groups).

4.Human is characterized by significant genotypic polymorphism, which, together with different ecological and social conditions, causes a high degree of phenotypic polymorphism.

Medical genetics as a science

Medical genetics - a section of human genetics that studies the role of heredity in human pathology. The subject of the study of medical genetics is hereditary human diseases and diseases with a hereditary tendency.

Medical genetics studies the etiology and pathogenesis of hereditary diseases, develops methods of diagnosis, treatment and prevention, investigates the relative role of hereditary and non-hereditary factors in the development of diseases with a hereditary tendency. The main task of medical genetics is to study hereditary human diseases in order to prevent their development in a number of generations, to protect human heredity from harmful environmental factors.

The object of medical genetics is a person with hereditary pathology, as well as his family, healthy and sick relatives. Physicians and nurses of any specialty face hereditary diseases. For catalogs published in recent years, in 1966 1487 were known, 1982 - about 4000, 2000 - 6678 hereditary diseases.

Medical genetics is associated with all clinical sciences. The branch of medical genetics is clinical genetics. The ultimate goal in them is the same - to provide assistance to the patient, to prevent the appearance of hereditary diseases in generations. However, each clinical science studies specific hereditary diseases according to its profile.

Medical genetics examines genetic patterns common to all hereditary diseases or a large group of them. At the same time, it relies on human genetics: it develops in the same directions and uses the same research methods as human genetics.

Modern medical genetics and medicine in the fight for the health of people in each generation are guided primarily by the prevention of hereditary diseases through prenatal (antenatal) diagnostics, medical genetic counseling, identification of heterozygous carriers of pathological genes, advice to married couples with an increased risk of having a sick child, development of legislative acts against environmental pollution by mutagens.

Medical genetics studies the structure of human genes, artificially synthesizes them, and, together with clinical sciences, develops methods for treating hereditary diseases using genes (gene therapy and genosurgery).

The theory considering the genotype as an integral system is based on two postulates:

1. One gene influences the formation of several traits.

2. Each trait of an organism develops as a result of the interaction of many genes.

The interaction of genes is understood as not the direct effect of one gene on another (one section of the DNA molecule on another section). In reality, gene interactions are biochemical in nature. It is based on the interaction of genetic products (RNA, then proteins) synthesized under the control of genes.

Proteins can enter into various reactions with each other: some proteins suppress the action of other proteins or, conversely, complement each other's action, can cause various gene mutations, as a result of which the gene encodes a protein in an altered form. All these interactions between proteins, synthesized under the control of genes, lead to the formation of organisms with a certain set of traits.

Known two types of gene interaction: allelic and non-allelic.

There are two main groups of gene interaction: interaction between allelic genes and interaction between non-allelic genes. However, it should be understood that this is not a physical interaction of the genes themselves, but the interaction of primary and secondary products that will determine this or that trait. In the cytoplasm, there is an interaction between proteins - enzymes, the synthesis of which is determined by genes, or between substances that are formed under the influence of these enzymes.

Genes that occupy identical (homologous) loci on homologous chromosomes are called allelic. Each organism has two allelic genes.

Allelic gene interactions

The genotype includes a large number of genes that function and interact as an integral system. G. Mendel in his experiments discovered only one form of interaction between allelic genes - complete dominance of one allele and complete recession of the other. The genotype of an organism cannot be viewed as a simple sum of independent genes, each of which functions independently of the others. Phenotypic manifestations of a particular trait are the result of the interaction of many genes.

Such forms of interaction between allelic genes are known: complete dominance, incomplete dominance, codominance and overdominance.

The main form of interaction is complete domination, which was first described by G. Mendel. Its essence lies in the fact that in a heterozygous organism, the manifestation of one of the alleles dominates over the manifestation of the other. With complete dominance of genotype cleavage 1: 2: 1 does not coincide with phenotype cleavage - 3: 1. In medical practice, with two thousand monogenic hereditary diseases, almost half of the dominant manifestation of pathological genes takes place over normal ones. In heterozygotes, the pathological allele manifests itself in most cases as signs of the disease (dominant phenotype).

Incomplete dominance - a form of interaction in which in a heterozygous organism (Aa) the dominant gene (A) does not completely suppress the recessive gene (a), as a result of which an intermediate between the parental traits appears. Here splitting by genotype and phenotype coincides and amounts to 1: 2: 1

At codominance in heterozygous organisms, each of the allelic genes causes the formation of a product dependent on it, that is, the products of both alleles appear. A classic example of such a manifestation is the blood group system, in particular the ABO system, when human erythrocytes carry antigens on the surface that are controlled by both alleles. This form of manifestation is called codominance.

The boundaries between codominance, incomplete dominance and intermediate inheritance are phenotypically rather vague. So, in some sources, codominance is considered as the absence of dominant-recessive relations, that is, it is an intermediate inheritance. At the same time, some cases of incomplete dominance (for example, in some species pink flowers appear in F 1 hybrids from crossing red-flowered and white-flowered plants) can also be considered as intermediate inheritance. The reason for the confusion is that in all three cases, the first generation hybrids have an intermediate variant of the trait.

Co-dominance and incomplete dominance, despite the phenotypic similarity, have different mechanisms of appearance. Co-dominance occurs when two alleles are fully manifested; incomplete dominance occurs when the dominant allele does not completely suppress the recessive one, that is, in heterozygotes, the dominant allele is manifested weaker than in homozygotes for this allele. With incomplete dominance, these genotypes are distinguished by expressiveness, that is, by the degree of severity of the trait.

Overdominance - when the dominant gene in the heterozygous state manifests itself more strongly than in the homozygous one. So, in Drosophila with the AA genotype, the normal life span; Aa - the elongated trivacy of life; aa - death.

Phenotypically, as a rule, in the case of overdominance, heterozygotes do not have special external characteristics. The advantage is related to biochemical characteristics.

One of the characteristic examples of overdominance is the increased frequency of the allele of the sickle cell anemia gene in human populations living with a high probability of contracting malaria. The mutant allele protects the body from malaria. Homozygotes for the normal allele can contract malaria and die, homozygotes for the mutant allele are more likely to die from anemia. Heterozygotes for this gene do not suffer from sickle cell anemia and are resistant to malaria.

The advantage of heterozygotes has also been shown for many genes and in many organisms. For Drosophila melanogaster shows the effects of overdominance by the alcohol dehydrogenase gene in laboratory populations.

In some cases, the allele of the gene associated with overdominance is recessively lethal and is maintained in the population due to the advantage of heterozygotes. Such cases include, for example, the system of lethal alleles of the gene lethal giant larvae... Heterozygotes with normal and mutant variants of this gene, in some cases, are characterized by increased viability.

Like any phenomenon that leads to a change in the fitness of individuals in populations, overdominance is associated with a genetic burden. More adapted heterozygous organisms, when crossed both with each other and with representatives of other genetic classes, should give less adapted offspring. The genetic burden associated with maintaining genetic diversity in a population under over-dominance is called segregation.

An extreme case of overdominance is the complete nonviability of homozygotes. Such situations are typical for laboratory populations of Drosophila melanogaster carrying balanced flying. Obviously, in this case, when heterozygotes are crossed with each other, half of the offspring will belong to nonviable genotypic classes. Consider a hypothetical case when the number of genes for which overdominance takes place is large and overdominance is so strong that homozygotes for any of the genes are not viable. Then the fecundity of individuals in the population should be very high in order to compensate for the decline in the population due to the cleavage of individuals of non-viable genotypic classes. For each of these overdominant genes, cleavage leads to the nonviability of half of the offspring. For 10 genes, only 1/1024 of the offspring will be viable.

A consequence of the model is that in natural populations, overdominance cannot simultaneously give great advantages to heterozygotes and spread to a large number of genes. Otherwise, the payment for the increased fitness of some individuals will be the need to maintain fertility at an unattainable level.

The molecular basis for dominance was unknown to Mendel. It is now clear that the locus corresponding to a particular gene consists of long sequences, including hundreds and thousands of DNA nucleotides. The central dogma of molecular biology is that DNA → RNA → protein, that is, DNA is transcribed into mRNA, and mRNA is translated into protein. In this process, different alleles may or may not be transcribed, but, being transcribed, translated into different forms of the same protein isoform. Proteins often function as enzymes that catalyze chemical reactions in the cell, which directly or indirectly determine the phenotype. In any diploid organism, the alleles corresponding to one locus are either the same (in homozygotes) or different (in heterozygotes). Even if alleles are different at the level of DNA sequences, their proteins can be identical. In the absence of differences between protein products, it is impossible to say which of the alleles is dominant (in this case, codominance takes place). Even if the two protein products are slightly different from each other, they probably give the same phenotype and can carry out the same enzymatic reactions (if they are enzymes). In this case, it is also impossible to say which of the alleles is dominant.

Dominance usually occurs when one of the alleles is non-functional for molecular level, that is, it is not transcribed or gives a non-functional protein product. This could be the result of a mutation that changes the DNA sequence of the allele. A homozygote for non-functional alleles usually exhibits a characteristic phenotype due to the absence of a certain protein; for example, in humans and other animals, unpigmented albino skin appears due to homozygosity for an allele that interferes with the synthesis of the skin pigment melanin. It is important to understand that recessiveness in an allele is not determined by the absence of any function: in heterozygotes, this is the result of interaction with an alternative allele. There are three main types of such interactions:

1. Typically, a single functional allele produces enough protein to produce a phenotype identical to that of a homozygote for the functional allele. This is called haplosufficiency. For example, if we take the amount of enzyme produced by a functional heterozygote as 100%, then each of the functional alleles will be responsible for the production of 50% of the total amount of the enzyme. The only functional allele of the heterozygote provides 50% of the enzyme, and this is enough to maintain a normal phenotype. If a heterozygote and a homozygote for a functional allele have the same phenotype, then the functional allele dominates over the non-functional one. This happens with the albinism gene: the heterozygote produces an amount of the enzyme, which is enough to form a precursor of melanin, and the individual has normal pigmentation.

(2) Less often, the presence of a single functional allele does not provide a normal phenotype, but its defectiveness is not as pronounced as in a homozygote for non-functional alleles. This occurs when the functional allele is not haplosufficient. Usually these cases include the concepts of haploinsufficiency and incomplete dominance. An intermediate variant of this interaction occurs when the heterozygote has a phenotype intermediate between the two homozygotes. Depending on which of the homozygotes the variant of the heterozygote trait is closer to, they speak of incomplete dominance of one allele over the other. An example of such an interaction is the case with human hemoglobin described above.

3. Rarely, a single functional allele of a heterozygote gives an inferior gene product, and its phenotype is similar to that of a homozygote for non-functional alleles. Such cases of haploinsufficiency are extremely unusual. In these cases, the non-functional allele dominates over the functional one. This situation can occur when a non-functional allele produces a defective protein that suppresses the function of the protein produced by the normal allele. The defective protein “dominates” over the standard one, and the heterozygote phenotype is more similar to that of the homozygote for defective alleles. It should be noted that the dominant is often incorrectly called defective alleles, the phenotype caused by which in the homozygous state has not been studied, but in combination with the normal allele, they give a characteristic phenotype. This phenomenon occurs in some genetic diseases caused by trinucleotide repeats, such as Huntington's disease.

Multiple allelism

Each organism has only two allelic genes. At the same time, the number of alleles in nature can often be more than two, if some locus can be in different states. In such cases, they say multiple alleles or multiple allelomorphism.

Multiple alleles are designated by one letter with different indices, for example: A 1, A 2, A 3 ... Allelic genes are localized in the same regions of homologous chromosomes. Since there are always two homologous chromosomes in a karyotype, even with multiple alleles, each organism can simultaneously have only two identical or different alleles. Only one of them enters the reproductive cell (together with the difference in homologous chromosomes). For multiple alleles, the characteristic influence of all alleles on the same trait. The difference between them lies only in the degree of development of the trait.

The second feature is that somatic cells or cells of diploid organisms contain a maximum of two alleles out of several, since they are located at the same locus of the chromosome.

Another feature is inherent in multiple alleles. By the nature of dominance, allelomorphic traits are placed in a sequential row: more often a normal, unchanged trait dominates others, the second gene of the series is recessive relative to the first, but dominates the next, etc. One example of the manifestation of multiple alleles in humans is the ABO blood groups.

The term "blood group" characterizes the erythrocyte antigen systems controlled by certain loci containing a different number of allelic genes, such as A, B and 0 ("zero") in the AB0 system. The term "blood type" reflects its antigenic phenotype (complete antigenic "portrait", or antigenic profile) - the totality of all group antigenic characteristics of blood, serological expression of the entire complex of inherited blood group genes.

The two most important classifications of the human blood group are the AB0 system and the Rh system.

AB0 system

Proposed by the scientist Karl Landsteiner in 1900. There are several main groups of allelic genes of this system: A¹, A², B and 0. The gene locus for these alleles is located on the long arm of chromosome 9. The main products of the first three genes - genes A¹, A² and B, but not gene 0 - are specific enzymes glycosyltransferases belonging to the class of transferases. These glycosyltransferases carry specific sugars - N-acetyl-D-galactosamine in the case of the A¹ and A² types of glycosyltransferases, and D-galactose in the case of the B-type glycosyltransferase. In this case, all three types of glycosyltransferases attach the transferred carbohydrate radical to the alpha-linking unit of short oligosaccharide chains.

Human blood plasma may contain agglutinins α and β, in erythrocytes - agglutinogens A and B, moreover, of proteins A and α, one and only one is contained, the same is for proteins B and β.

Thus, there are four valid combinations; which of them is characteristic for a given person determines his blood group:

· α and β: first (0)

· A and β: second (A)

· α and B: third (B)

· A and B: 4th (AB)

The A (II) phenotype can be in a person who has inherited from his parents either two genes A (AA), or genes A and 0 (A0). Accordingly, phenotype B (III) - when inheriting either two genes B (BB), or B and 0 (B0). Phenotype 0 (I) manifests itself when two genes 0 are inherited. Thus, if both parents have blood group II (genotypes A0 and A0), one of their children may have the first group (genotype 00). If one of the parents has blood group A (II) with possible genotypes AA and A0, and the other has B (III) with possible genotype BB or B0, children may have blood groups 0 (I), A (II), B (III ) or AB (IV).

A parent with blood group I (0) cannot have a child with blood group IV (AB), regardless of the blood group of the other parent. A parent with blood group IV (AB) cannot have a child with blood group I (0), regardless of the blood group of the second parent. Exceptions are possible in extremely rare cases, when the A and B genes are suppressed by the h-genome (possibly suppression by other genes), the so-called Bombay phenomenon.

Multiple allelicism is of great biological and practical importance, since it enhances combinative variability, especially genotypic one.

Rh system (rhesus system)

The Rh factor of the blood is an antigen (protein) that is found on the surface of red blood cells (erythrocytes). It was discovered in 1940 by Karl Landsteiner and A. Weiner. About 85% of Europeans (99% of Indians and Asians) have Rh and are therefore Rh positive. The remaining 15% (7% among Africans), who do not have it, are Rh-negative. Blood Rhesus plays an important role in the formation of the so-called hemolytic jaundice of newborns, caused by the Rh-conflict of the immunized mother and fetal erythrocytes.

It is known that blood rhesus is a complex system that includes more than 40 antigens, denoted by numbers, letters and symbols. The most common type of Rh antigens are D (85%), C (70%), E (30%), e (80%) - they also have the most pronounced antigenicity. The Rh system does not normally have agglutinins of the same name, but they can appear if a person with Rh-negative blood is transfused with Rh-positive blood.

The Rh factor is inherited in a recessive-dominant mode of inheritance. Rh positive is dominant, negative is recessive. The Rh + phenotype is manifested in both homozygous and heterozygous genotypes (++ or + -), the Rh- phenotype is manifested only in the homozygous genotype (only -).

A couple of Rh- and Rh- can have children only Rh-. A pair of Rh + and Rh-, as well as a pair of Rh + and Rh +, can have children of both Rh + and Rh-, or only Rh +, depending on the genotype of the Rh + parents.

Donors and recipients of blood must have "compatible" blood types. In Russia, for health reasons and in the absence of blood components of the same group according to the AB0 system (with the exception of children), transfusion of Rh-negative blood of group 0 (I) to a recipient with any other blood group in an amount of up to 500 ml is allowed. Rh-negative erythrocyte mass or suspension from donors of group A (II) or B (III), according to vital indications, can be transfused to a recipient with AB (IV) group, regardless of his Rh-affiliation. In the absence of single-group plasma, the recipient can be transfused with AB (IV) plasma.

In the middle of the 20th century, it was assumed that the blood of group 0 (I) Rh- is compatible with any other groups. People with group 0 (I) Rh- were considered “universal donors”, and their blood could be transfused to anyone in need. Currently, such blood transfusions are considered acceptable in desperate situations, but not more than 500 ml.

The incompatibility of blood of group 0 (I) Rh- with other groups was observed relatively rarely, and this circumstance was not paid due attention to for a long time. The table below illustrates people with which blood groups were able to give / receive blood (compatible combinations are marked with Yes). For example, the owner of group A (II) Rh− can receive blood from groups 0 (I) Rh− or A (II) Rh− and donate blood to people who have blood groups AB (IV) Rh +, AB (IV) Rh−, A ( II) Rh + or A (II) Rh−.

Interaction of non-alelic genes

There are many cases where a trait or properties are determined by two or more non-relevance genes that interact with each other. Although here the interaction is conditional, because it is not genes that interact, but the products controlled by them. In this case, there is a deviation from the Mendelivian laws of splitting.

There are four main types of gene interactions: complementarity, epistasis, polymerization and modifying action (pleiotropy).

Complementarity is a type of interaction between non-allelic genes when one dominant gene complements the action of another non-allelic dominant gene, and together they define a new trait that the parents do not have. Moreover, the corresponding trait develops only in the presence of both non-allelic genes. For example, the gray coat color in mice is controlled by two genes (A and B). Gene A determines the synthesis of pigment, however, both homozygotes (AA) and heterozygotes (Aa) are albinos. Another gene, B, provides pigment accumulations mainly at the base and at the ends of the hair. Crossing diheterozygotes (Aabb x Aabb) leads to the splitting of hybrids in a ratio of 9: 3: 4. Numerical ratios for complementary interactions can be as high as 9: 7; 9: 6: 1 (modification of Mendelian splitting).

An example of a complementary gene interaction in humans is the synthesis of a protective protein, interferon. Its formation in the body is associated with the complementary interaction of two non-allelic genes located on different chromosomes.

Epistasis is an interaction of non-allelic genes in which one gene suppresses the action of another non-allelic gene. Oppression can be caused by both dominant and recessive genes (A> B, a> B, B> A, B> A), and depending on this, dominant and recessive epistasis is distinguished. The suppressive gene is called an inhibitor or suppressor. Inhibitor genes generally do not determine the development of a certain trait, but only suppress the action of another gene.

The gene, the effect of which is suppressed, is called hypostatic. In case of epistatic interaction of genes, the phenotype splitting in F2 is 13: 3; 12: 3: 1 or 9: 3: 4, etc. The color of the pumpkin fruit, the color of the horses are determined by this type of interaction.

If the suppressor gene is recessive, then cryptomeria occurs (Greek chrishtad - secret, hidden). In humans, such an example can be the "Bombay Phenomenon". In this case, a rare recessive allele "x" in the homozygous state (mm) suppresses the activity of the jB gene (which determines the B (III) blood group of the ABO system). Therefore, a woman with the jв_хх genotype has phenotypically I blood group - 0 (I).

Most of the quantitative traits of organisms are determined by several non-allelic genes (polygenes). The interaction of such genes in the process of trait formation is called polymeric... In this case, two or more dominant alleles equally affect the development of the same trait. Therefore, polymer genes are usually denoted by one letter of the Latin alphabet with a digital index, for example: A 1 A 1 and a 1 a 1. For the first time, unambiguous factors were identified by the Swedish geneticist Nilsson-Ehle (1908) when studying color inheritance in wheat. It was found that this trait depends on two polymeric genes, therefore, when crossing dominant and recessive digomozygotes - colored (A 1 A 1, A 2 A 2) with colorless (a 1 a 1, a 2 a 2) - in F 1, all plants produce colored seeds, although they are lighter than the parental specimens, which have red seeds. In F 2, when crossing individuals of the first generation, splitting by phenotype is manifested in a ratio of 15: 1, therefore only recessive digomozygotes are colorless (a 1 a 1 a 2 a 2). In pigmented specimens, the color intensity is very different depending on the number of dominant alleles they received: the maximum in the dominant digomozygotes (A 1 A 1, A 2 A 2) and the minimum in carriers of one of the dominant alleles.

An important feature of polymerization is the summation of the action of non-allelic genes on the development of quantitative traits. If with monogenic inheritance of a trait, three variants of "doses" of a gene in the genotype are possible: AA, Aa, aa, then with polygenic inheritance, their number increases to four or more. The summation of the "doses" of polymeric genes makes it impossible for continuous series of quantitative changes.

The biological significance of polymerization also lies in the fact that the traits encoded by these genes are more stable than those encoded by one gene. An organism without polymeric genes would be very unstable: any mutation or recombination would lead to a sharp change, which in most cases has an unfavorable character.

Animals and plants have many polygenic traits, among them valuable for the economy: growth intensity, early maturity, egg production. the amount of milk, the content of sugar substances and vitamins, etc.

Human skin pigmentation is determined by five or six polymeric genes. In the indigenous inhabitants of Africa (Negroid race) dominant alleles prevail, in representatives of the Caucasian race - recessive. Therefore, mulattoes have intermediate pigmentation, but in case of mulatto marriages, they may have both more and less intensely pigmented children.

Many morphological, physiological and pathological features of a person are determined by polymeric genes: height, body weight, blood pressure, etc. The development of such signs in a person obeys general laws polygenic inheritance and depends on environmental conditions. In these cases, there is, for example, a tendency to hypertension, obesity, etc. These signs under favorable environmental conditions may not appear or appear slightly. These polygenic traits differ from monogenic ones. By changing environmental conditions, it is possible to ensure the prevention of a number of polygenic diseases.

Pleiotropy

The influence of one gene on the development of two or more traits is called multiple, or pleiotropic, action, and the phenomenon itself is called pleiotropy(from the Greek pleistos - plural, greatest). The biochemical nature of the pleiotropic action of the gene is well understood. One protein-enzyme, formed under the control of one gene, determines the development of not only this trait, but also affects the secondary reactions of the biosynthesis of various other traits and properties, causing their changes.

Pleiotropy is widespread: most genes in all organisms have multiple effects. This phenomenon was first discovered by G. Mendel. He found that the plant with corn flowers always had red spots in the axils of the leaves at the same time, and the seed coat was gray or brown in color. These three traits were determined by the action of one gene. Recently, it was found that many induced mutations in peas are characterized by a high degree of pleiotropy, manifested in changes in up to ten or more traits. NI Vavilov and OV Yakushkina, studying the inheritance of some traits in Persian wheat, found that the dominant gene for the black color of the spike simultaneously causes the drooping of the spikelet scales.

In the human genotype, genes are known that have pleiotropic effects. For example, a gene is known that causes the characteristic picture of Marfan syndrome. Such people are distinguished by prolonged growth of limbs, especially legs and fingers (spider fingers). In addition, this gene causes a defect in the lens of the eye.

Another example of the pleiotropy of a gene in humans is the sickle cell mutation. In this case, a mutation of the normal allele leads to a change in the molecular structure of the hemoglobin protein. As a result, mutated erythrocytes lose their ability to transport oxygen and, instead of normal, round, acquire a sickle shape. People homozygous for this trait develop acute anemia, as a rule, people die at birth. People who are heterozygous for this allele often exhibit sickle cells without impairing oxygen transport and are also highly resistant to malaria mosquitoes. As a result, a paradoxical situation arises in which the gene is lethal in a person in a homozygous state; nevertheless, it becomes widespread. The reason is that heterozygous people are less likely to develop tropical malaria. In this case, the increase goes to heterozygotes, the number of which in populations is greater than that of people homozygous for this mutation. This phenomenon has been found in the Mediterranean and some other areas.

Pleiotropic action of genes - this is the dependence of several traits on one gene, that is, the multiple action of one gene. The Drosophila gene white the eye simultaneously affects the color of the body, length, wings, the structure of the reproductive apparatus, reduces fertility, and reduces life expectancy. In humans, a hereditary disease is known - arachnodactyly ("spider fingers" - very thin and long fingers), or Marfan's disease. The gene responsible for this disease causes a disruption in the development of connective tissue and simultaneously affects the development of several signs: a violation of the structure of the lens of the eye, anomalies in the cardiovascular system.

The pleiotropic effect of a gene can be primary or secondary. In primary pleiotropy, the gene exhibits its multiple effects. For example, in Hartnup's disease, a gene mutation leads to impaired absorption of the amino acid tryptophan in the intestine and its reabsorption in the renal tubules. At the same time, membranes of intestinal epithelial cells and renal tubules are simultaneously affected with disorders of the digestive and excretory systems.

In secondary pleiotropy, there is one primary phenotypic manifestation of the gene, followed by a stepwise process of secondary changes leading to multiple effects. So, with sickle cell anemia, homozygotes have several pathological signs: anemia, an enlarged spleen, damage to the skin, heart, kidneys and brain. Therefore, homozygotes with the sickle cell anemia gene die, as a rule, in childhood. All these phenotypic manifestations of a gene constitute a hierarchy of secondary manifestations. The root cause, the direct phenotypic manifestation of the defective gene is abnormal hemoglobin and sickle-shaped erythrocytes. As a result, successively other pathological processes occur: adhesion and destruction of erythrocytes, anemia, defects in the kidneys, heart, brain - these pathological signs are secondary.

With pleiotropy, a gene, acting on some one basic trait, can also change, modify the manifestation of other genes, in connection with which the concept of modifier genes has been introduced. The latter enhance or weaken the development of traits encoded by the "main" gene.

Indicators of the dependence of the functioning of hereditary inclinations on the characteristics of the genotype is penetrance and expressiveness.

Considering the action of genes, their alleles, it is necessary to take into account the modifying effect of the environment in which the organism develops. If primrose plants are crossed at a temperature of 15-20 ° C, then in F1, according to the Mendelivian scheme, all generations will have pink flowers... But when such a crossing is carried out at a temperature of 35 ° C, then all hybrids will have white flowers. If crosses are carried out at a temperature of about 30 ° C, then a difference ratio (from 3: 1 to 100%) of plants with white flowers occurs.

This fluctuation of classes during splitting, depending on environmental conditions, is called penetrance - the strength of phenotypic manifestation. So, penetrance is the frequency of the manifestation of a gene, the phenomenon of the appearance or absence of a trait in organisms that are identical in genotype.

Penetrancefluctuates significantly among both dominant and recessive genes. Along with genes, the phenotype of which appears only under a combination of certain conditions and rather rare external conditions (high penetrance), a person has genes whose phenotypic manifestation occurs under any combination of external conditions (low penetrance). Penetrance is measured by the percentage of organisms with a phenotypic trait out of the total number of carriers of the corresponding alleles examined.

If a gene completely, independently of the environment, determines the phenotypic manifestation, then it has a penetrance of 100 percent. However, some dominant genes appear less regularly.

So, polydactyly has a clear vertical inheritance, but there are generations missing. The dominant anomaly - premature puberty - is inherent only in men, but sometimes diseases can be transmitted from a person who did not suffer from this pathology. Penetrance indicates in what percentage of gene carriers is the corresponding phenotype. So, penetrance depends on genes, on the environment, on both. Thus, this is not a constant gene property, but the function of genes under specific environmental conditions.

Expressiveness (lat. expresio - expression) is a change in the quantitative manifestation of a trait in different individuals-carriers of the corresponding alleles.

In dominant hereditary diseases, expressivity may fluctuate. In the same family, hereditary diseases from lungs, hardly noticeable to severe ones, can manifest themselves: various forms of hypertension, schizophrenia, diabetes mellitus, etc. Recessive hereditary diseases within the family are manifested in the same type and have minor fluctuations in expressivity.

The main patterns of inheritance were discovered by G. Mendel on peas. He carried out intraspecific crosses of forms that differ in a single number of characters and have alternative (contrasting) manifestations. Among the traits he used were the color of seeds, flowers and beans, the shape of seeds and beans, the arrangement of flowers, and the height of plants. Initially, a hybridological analysis of pea forms that differed in one trait was carried out. Crosses in which parental forms are involved that differ in the manifestations of one trait are called monohybrid.

When two original forms belonging to pure lines are crossed, in the first daughter generation, as a rule, the appearance of offspring of the same phenotype is observed. This pattern is known as the law of uniformity for first-generation hybrids. F 1 hybrids can have a manifestation of the trait of one of the parents, and an expression intermediate between the original forms. Moreover, if the differences in parental forms are determined by one gene (monogenic), the record of crossing is as follows: P AA x aa → F 1 Aa. This means that gene A is responsible for the manifestation of this trait, which exists in two different states - A and a. These alternative gene states are called alleles.

Analyzing the results of monohybrid crosses, G. Mendel established a rule (sometimes called the law) of gamete purity. It implies that any gamete of any organism carries one allele of each gene, the alleles in them are not mixed. This means that in individuals of the AA genotype, gametes of the same species are formed - A, in individuals of the aa genotype - also of the same type - a. Such individuals, which form gametes of only one variety (at least for the gene that is in the focus of attention), are homozygous (or homozygous). Thus, it is not difficult to make sure that pure lines consist of homozygous individuals. Px hybrids of genotype Aa form gametes of two varieties - A and a, each of which is "pure" with respect to allele A or a. Such individuals (or genotypes) that form gametes of several species are called heterozygous (or heterozygotes). The law of uniformity of the first generation hybrids is based on the mechanism of chromosome divergence in meiosis. Each of the alleles lies in its own chromosome (or chromatid), and during the divergence of chromosomes (in the first division of meiosis), and then chromatids (in the second division of meiosis), one of the corresponding alleles departs with them into the haploid cells. Thus, the law of uniformity of the first generation hybrids is a consequence of the fundamental rule of gamete purity, which determines other laws of inheritance.

Alleles of a single gene interact with each other in different ways. If the heterozygote Aa exhibits a phenotypic expression of the trait, which is the same as in individuals of the AA genotype, then allele A completely dominates over a, then AA individuals have a dominant manifestation of the trait, and homozygotes for a are recessive. This is another rule of Mendelism - the rule of domination. If the heterozygote has a manifestation of a trait intermediate between the two parental forms (for example, when crossing night beauty plants with red and white flowers, hybrids with a pink corolla are formed), then we are talking about incomplete dominance.

Sometimes heterozygotes show signs of both parents - this is the absence of dominance, or co-dominance.

Splitting law in monohybrid crossing

A monohybrid crossing is a crossing in which the original forms differ in one trait. When crossing hybrids of the first generation, obtained from crossing homozygous forms, a split into 3/4 individuals with a dominant manifestation of a trait and 1/4 with a recessive manifestation of a trait is found.

In the second generation, obtained by crossing P1 hybrids with each other, two phenotypic classes appear in a strictly defined ratio. This is splitting, which is understood as the presence of several phenotypes in the offspring in specific numerical ratios.

First generation hybrids can interbreed not only with their own kind. If a heterozygous P1 individual is crossed with an organism homozygous for the recessive allele of the gene in question, then a splitting is obtained: Aa x aa → 1/2 Aa: 1/2 aa.

This crossing is called the analyzing one. In the analyzing crossing, it is not difficult to establish the types of gametes formed by a heterozygous individual and their numerical ratio, it is easy to determine which organisms are heterozygous and which are homozygous for the trait of interest to us.

The law of splitting in monohybrid crossing is also read in the reverse order: if, when crossing two individuals, one of the splits considered above is obtained (in P2 - 3: 1, 1: 2: 1, 2: 1, and in analyzing crossing - 1: 1), then the original parental forms differ in the alleles of one gene, that is, there is a difference between them in one gene (monogenic difference in the original forms).

Independent inheritance law in dihybrid crossing

A dihybrid crossing is a cross in which the original forms differ in two ways. For each of the traits, the parental forms differ in one gene (according to trait A - according to gene A, according to trait B - according to gene B). When crossing F 1 hybrids obtained from a dihybrid crossing, a splitting according to the phenotype is observed: 9/16 A-B-: 3/16 A-bb: 3/16 aaB-: 3/16 aabb.

In this case, the characters are inherited independently of each other, and for each of them, a 3/4: 1/4 split is observed.

This cleavage is easily obtained as a combined one, combining two monohybrid ones (in the second generation of each of which a 3: 1 cleavage is observed), with one gene responsible for each trait:

(3/4 A- + 1/4 aa) x (3/4 B- + 1/4 bb) = 9/16 A-B- + 3/16 A-bb + 3/16 aaB- + 1/16 aabb.

In analyzing crosses, a split of 1: 1: 1: 1 is similarly obtained.

The implementation of this law is determined by the independent nature of the divergence of chromosomes of non-homologous pairs in meiosis, as well as by the fact that genes A and B are located in different (non-homologous) chromosomes. Independent divergence of chromosomes in meiosis leads to the emergence of new combinations of genes and traits that were not present in parental organisms - recombinants appear in the offspring (individuals carrying recombined combinations of traits).

Splitting is also obtained in polyhybrid crosses (crosses in which the parental forms differ in several or many characteristics).

All the laws of inheritance by G. Mendel illustrate the point of view he postulated about the discrete nature of inheritance: it is not the trait itself that is inherited, but the material factors that determine it. These factors are genes.

Interaction of genes

Some traits are determined not by one gene, but by the simultaneous action of several. In such cases, there is undoubtedly a change and complication of the splitting formulas and methods of analysis. Genes that affect the development of one trait are called interacting. There are several types of such interaction of genes: complementary, epistatic, polymeric.

The dominant alleles of both genes lead to the formation of a new manifestation of the trait, mutually complementing each other (complementing). If the genotype contains only recessive alleles of both genes, then the trait does not appear. Biochemical analysis complements this scheme. The color of the eyes in Drosophila is caused by two pigments (bright red and brown), each of which is formed in a separate biosynthetic chain. The recessive allele "b" in homozygotes interrupts the synthesis of bright red pigment - in such individuals the eyes are brown, allele "a" disrupts the synthesis of brown pigment - in homozygotes aa the eyes are bright red, in individuals "A-B-" both pigments, causing a dark red color of the eyes, and homozygotes for both genes "aabb" have no dyes in the eyes at all - the eyes are colorless (white).

Interaction of genes (or interaction of non-allelic genes) leads to cleavages of the digenous type. In addition to the case discussed above, splits can be observed in the second generation: 9: 7, 9: 6: 1, 9: 3: 4, 12: 3: 1, 13: 3, 15: 1.

Conditions for the implementation of the laws of inheritance

The patterns of inheritance of traits discussed above are fulfilled only if certain conditions are met. It is necessary that all types of gametes are formed with equal probability, have the same viability and participate in fertilization with the same efficiency, forming all types of zygotes with the same frequency, while zygotes should be characterized by equal viability. The severity of the trait should also be unchanged. Failure to meet at least one of these conditions leads to distortion of the splittings.

For example, if in a monohybrid crossing, in which splitting is observed in F 2 1/4 AA: 2/4 Aa: 1/4 aa, selective death of zygotes of the AA genotype is observed, then the phenotypic splitting will look like 2/3 Aa: 1/3 aa.

It should be noted that even if the above conditions are met, the actual splitting does not always exactly match the theoretically calculated one. The fact is that the laws of inheritance discovered by Mendel are manifested in a fairly large amount of statistical material. For their accurate implementation, it is necessary to analyze a sample of a certain size. Thus, the patterns of inheritance are biological in nature, but have a statistical character of manifestation.