hereditary variability in organisms. Hereditary variability and its types. Chromosomal diseases are divided into two types

Named this type of variability uncertain, since it is initially impossible to determine what changes will appear, in addition, they are always individual.

In each sufficiently long-existing set of individuals, various mutations spontaneously and undirectedly arise, which are later combined more or less randomly with different hereditary properties already present in the set.

Variability due to the occurrence of mutations is called mutational, and due to further recombination of genes as a result of crossing - combinative.

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  Combinative variability - variability that occurs due to recombination of genes during gamete fusion. Main reasons:

  • independent segregation of chromosomes during meiosis;
  • a random meeting of gametes, and as a result, a combination of chromosomes during fertilization;
  • recombination of genes due to crossing over.

  Mutational variability

  Mutational variability - variability caused by the action of mutagens on the body, resulting in mutations (reorganization of the reproductive structures of the cell). Mutagens are physical, chemical and biological.

  mutation theory

  The main provisions of the mutation theory in 1901-1903 were developed by Hugo de Vries, and wrote about it in his work The Mutation Theory. This work rejected the then current understanding of inheritance as the main mechanism of variability in Darwin's theory. Instead, he introduced the term "mutation", denoting the unexpected appearance of new traits in the phenotype, not caused by heredity. The main provisions of the theory:

  1. Mutations occur suddenly, abruptly, as discrete changes in traits.
  2. Unlike non-hereditary changes, mutations are qualitative changes that are passed down from generation to generation.
  3. Mutations manifest themselves in different ways and can be both beneficial and harmful, both dominant and recessive.
  4. The probability of detecting mutations depends on the number of individuals studied.
  5. Similar mutations can occur repeatedly.
  6. Mutations are undirected (spontaneous), that is, any part of the chromosome can mutate, causing changes in both minor and vital signs.

  Almost any change in the structure or number of chromosomes, in which the cell retains the ability to reproduce itself, causes a hereditary change in the characteristics of the organism. According to the nature of the change in the genome, that is, the totality of genes contained in the haploid set of chromosomes, gene, chromosomal and genomic mutations are distinguished.

  Role in evolution

  The whole variety of individual differences is based on hereditary variability, which include:

  • Both sharp qualitative differences, not connected with each other by transitional forms, and purely quantitative differences, forming continuous series, in which close members of the series can differ from each other as little as desired;
  • Both changes in individual traits and properties (independent variability) and interrelated changes in a number of traits (correlative variability);
  • Both changes that have an adaptive value (adaptive variability) and changes that are “indifferent” or even reduce the viability of their carriers (non-adaptive variability).

  All these types of hereditary changes constitute the material of the evolutionary process (see Microevolution). In the individual development of an organism, the manifestation of hereditary traits and properties is always determined not only by the main genes responsible for these traits and properties, but also by their interaction with many other genes that make up the genotype of the individual, as well as by the environmental conditions in which the organism develops.

  Accuracy is undeniably important in the transmission of genetic information over a number of generations, however, excessive conservation of genetic information contained in individual genetic loci can be harmful to the organism and the species as a whole.

  The evolutionarily established relationships between the accuracy of the functioning of genetic systems and the frequency of errors that occur when reproducing the genetic information of individual genetic loci are clearly balanced among themselves, and it has already been established that in a number of cases they are adjustable. Programmed and random inherited changes in the genome, called mutations, can be accompanied by enormous quantitative and qualitative changes in gene expression.

  Didactic purpose: to create conditions for studying and understanding a block of new educational information.

  The purpose of the lesson: to study the phenomenon of hereditary variability, its patterns and evolutionary meaning

  • educational: consider the types of mutational variability, their material basis; reveal the essence of the factors causing mutations; determine the significance of hereditary variability for evolution and selection.
  • Educational: to continue the formation of scientific ideas about the basic properties of living organisms of heredity and variability; the ability to explain these properties on the basis of the acquired knowledge;

  Lesson type: combined.

  Method of conducting: independent work, storytelling, conversation, demonstration, testing

  Program requirements:

  The student must

  • have ideas about the mechanisms of the occurrence of mutations;
  • know definitions of “heredity”, “variability”, “mutation”, “polyploidy”, etc.;
  • be able to explain the phenomena of hereditary variability on the basis of cytological and genetic knowledge.

  New concepts: hereditary variability, mutation, mutagenic factors.

  Methodological support:

  • tables by general biology“Hereditary variability”;
  • didactic material for test task(Annex 1) Modification mutability.test.doc
  • appendix: Mutational variability.ppt worksheet (appendix 2) Worksheet.doc

  During the classes

  1. Organizational moment.

  Greetings; preparing the class for work.

  2. Testing students' knowledge.

  3. Motivation for learning activities.

  The message of the topic, the purpose of the lesson.

  4. Presentation of new material.

  1. Variability as a property of living organisms.

  2. Visible variability.

  3. Hereditary variability, its significance.

  4. Mutational variability.

  5. Mutagenic factors.

  5. Reporting homework.

  Chapter 15, P.41, pp. 196-200; questions 1-5

  6. Summing up the lesson.

  1. Assess the degree of implementation of the goals.

  2. Evaluation of the work of students in the lesson.

  7. Consolidation of the studied material.

  What is the evolutionary meaning of hereditary variability?

  1. Variability as a property of living organisms.

  What is variability? What does it cause? What are the reasons for the variability?

  In the last lesson, we got acquainted with one of the forms of variability - this is non-hereditary variability (modification). How is it characterized? What is the character? Test execution (Appendix 1) . And today we have to study another form of variability - this is hereditary variability.

  Completing task #1 in the “Worksheet”, column 1 (Annex 2)

  Hereditary variability (genotypic, indefinite) - due to the emergence of new genotypes and, as a rule, leads to a change in the phenotype.

  2. Types of hereditary variability (slide 3)

  • mutational
  • combinative
  • correlative

  3. Mutational variability (slide 4)

  Mutational variation is the result of mutations.

  Mutation (from the Latin “mutazio” - change, change) is a hereditary change in the genotype (this is a change in the hereditary material that leads to the appearance of new signs of the organism that can be transmitted to the next generation. The term “mutation” was introduced into science in 1901 by the Dutch geneticist G. de Vries, who described spontaneous mutations in plants.

  Are mutations a pattern or a paradox?

  Examples of mutations (slide 5)

  Mutations are persistent changes affecting both whole chromosomes, their parts, individual genes. Most often, mutations are small, barely noticeable deviations from the norm.

  Darwin called hereditary variability indeterminate (individual), emphasizing its random and relatively rare character.

  Mutations are a source of genetic diversity, constituting a reserve of hereditary variability.

  Classification of mutations (slide 7)

  • according to the nature of the manifestation
  • at the place of origin
  • by level of occurrence

  1. According to the nature of the manifestation:

  manifestations are dominant and recessive. Mutations often reduce viability or fertility. Mutations that sharply reduce viability, partially or completely stop development, are called semi-lethal and incompatible with life - lethal. (slide 8.9)

  2. According to the place of occurrence:

  A mutation that has arisen in germ cells does not affect the signs given organism, and appears only in the next generation. Such mutations are called generative. If genes are changed in somatic cells, such mutations appear in this organism and are not transmitted to offspring during sexual reproduction. But with asexual reproduction, if an organism develops from a cell or group of cells that has a changed - mutated - gene, mutations can be transmitted to offspring. Such mutations are called somatic (slide 10,11)

  3. According to the level of occurrence:

  Gene (point) mutations are associated with a change in the enomic sequence of nucleotides in a DNA molecule. The mutant gene contributes to the emergence of new alleles, which is of great evolutionary importance. (slide 12.13)

  Chromosomal mutations are associated with changes in the structure of chromosomes. There are the following types of chromosome rearrangements: detachment of various sections of the chromosome, doubling of individual fragments, rotation of a section of the chromosome by 180 °, or attachment of a separate section of the chromosome to another chromosome. Such a change entails a violation of the function of genes in the chromosome and the hereditary properties of the organism, and sometimes its death (slide 14)

  Genomic mutations - mutations, as a result of which there is a change in the number of chromosomes. They arise as a result of a violation of mitosis or meiosis.

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

  polyploidy– an increase in the number of chromosomes, a multiple of the haploid set: triploid (3n), tetraploid (4n). Polyploidy is more often observed in protozoa and in plants. heteroploidy(aneuploidy) - an increase or decrease in the number of chromosomes, not a multiple of the haploid set (slide 15, 16, 17, 18,19).

  5. Mutagenic factors (slide 20.21)

  Spontaneous mutations - occur under normal living conditions, depend on external and internal factors, occur in somatic and generative cells .

  Induced mutations are the artificial production of mutations using mutagens of various nature. For the first time, the ability of ionizing radiation to cause mutations was discovered by G.A. Nadson and G.S. Filippov. In 1927, the American scientist Joseph Muller proved that the frequency of mutations increases with increasing exposure dose. Scientists believe that the fact that mutations are inherited raises some concerns, as this can increase the risk of developing cancer. Mutant gene protects Asians from alcoholism. Why the percentage of alcoholics in Asian countries is much lower than in countries where the main part of the population is the so-called white population.

  The environmental factors that cause mutations are called – mutagens .

  Distinguish:

  Physical mutagens

  Ionizing and ultraviolet radiation;

  Excessively high or low temperature;

  Chemical mutagens

  Nitrates, nitrites, pesticides, nicotine, methanol, benzpyrene.

  Some food additives, such as aromatic hydrocarbons;

  Oil refining products;

  organic solvents;

  Medications, mercury preparations, immunosuppressants.

  Biological mutagens

  Certain viruses (measles, rubella, influenza)

  Metabolic products (products of lipid oxidation );

  Mutation properties (slide 22)

  • mutations are hereditary, i.e. are passed down from generation to generation.
  • mutations occur suddenly (spontaneously), non-directionally.
  • mutations are not directed - any locus can mutate, causing changes in both minor and vital signs in any direction.
  • the same mutations can occur repeatedly.
  • mutations are individual, i.e. occur in individual individuals.
  • mutations can be beneficial, harmful, neutral; dominant and recessive.

  The meaning of mutations

  They serve as a reserve of hereditary variability (they are stored in the population in a latent recessive form), they are material for evolution.

  The cause of many hereditary diseases and deformities.

  Induced mutations “supply” material for artificial selection and selection.

  Consolidation of the studied material.

  What is the significance of hereditary variability for humans?

  The health of the current future generations of people largely depends on what genetic load is inherited from the previous ones, how many mutations mankind has accumulated. The problem is that the acceleration of the mutation rate leads to an increase in the number of individuals with congenital defects and harmful deviations that are inherited. In this regard, one of critical tasks nature conservation and ensuring human genetic safety – monitoring environment and identification of contaminants with mutagenic and carcinogenic activity.

  Variability is a process that reflects the relationship of an organism with the environment.

  From a genetic point of view, variability is the result of the reaction of the genotype in the process of individual development of the organism to environmental conditions.

  The variability of organisms is one of the main factors of evolution. It serves as a source for artificial and natural selection.

  Biologists distinguish between hereditary and non-hereditary variability. Hereditary variability includes such changes in the characteristics of an organism that are determined by the genotype and persist over a number of generations. To non-hereditary variability, which Darwin called definite, and is now called modification, or phenotypic, variability, refer to changes in the characteristics of the organism; not preserved during sexual reproduction.

  hereditary variability is a change in the genotype non-hereditary variability- change in the phenotype of the organism.

  During individual life An organism under the influence of environmental factors can experience two types of changes: in one case, the functioning, the action of genes in the process of trait formation, changes, in the other, the genotype itself.

  We have become acquainted with hereditary variability resulting from combinations of genes and their interaction. The combination of genes is carried out on the basis of two processes: 1) independent distribution of chromosomes in meiosis and their random combination during fertilization; 2) chromosome crossing and gene recombination. Hereditary variability due to the combination and recombination of genes is commonly called combinative variability. With this type of variability, the genes themselves do not change, their combination and the nature of interaction in the genotype system change. However, this type of hereditary variability should be considered as a secondary phenomenon, and the mutational change in the gene should be considered primary.

  The source for natural selection is hereditary changes - both mutations of genes and their recombination.

  Modification variability plays a limited role in organic evolution. So, if you take vegetative shoots from the same plant, for example, strawberries, and grow them in different conditions of humidity, temperature, light, on different soils, then despite the same genotype, they will turn out to be different. The action of various extreme factors can cause even greater differences among them. However, seeds collected from such plants and sown under the same conditions will give the same type of offspring, if not in the first, then in subsequent generations. Changes in the signs of the organism, caused by the action of environmental factors in ontogenesis, disappear with the death of the organism.

  At the same time, the capacity for such changes, limited by the limits of the normal reaction of the organism's genotype, has an important evolutionary significance. As shown by A.P. Vladimirsky in the 1920s, V.S. Kirpichnikov and I.I. Shmalgauzen in the 1930s, in the case when modification changes in the adaptive value occur with environmental factors constantly acting in a number of generations, which able to cause mutations that determine the same changes, one may get the impression of hereditary fixation of modifications.

  Mutational changes are necessarily associated with the reorganization of the reproducing structures of germ and somatic cells. The fundamental difference between mutations and modifications is that mutations can be accurately reproduced in a long series of cell generations, regardless of the environmental conditions in which ontogenesis takes place. This is explained by the fact that the occurrence of mutations is associated with a change in the unique structures of the cell - the chromosome.

  On the question of the role of variability in evolution, there was a long discussion in biology in connection with the problem of inheritance of the so-called acquired traits, put forward by J. Lamarck in 1809, partly accepted by Charles Darwin and still supported by a number of biologists. But the vast majority of scientists considered the very formulation of this problem unscientific. At the same time, it must be said that the idea that hereditary changes in the body arise adequately to the action of an environmental factor is completely absurd. Mutations occur in a variety of ways; they cannot be adaptive for the organism itself, since they arise in single cells

  And their action is realized only in offspring. Not the factor that caused the mutation, but only selection evaluates the adaptive knowledge of the mutation. Since the direction and pace of evolution are determined by natural selection, and the latter is controlled by many factors of the internal and external environment, a false idea is created about the initial adequate expediency of hereditary variability.

  Selection on the basis of single mutations "constructs" systems of genotypes that meet the requirements of those permanent conditions in which the species exists.

  The term " mutation"was first proposed by G. de Vries in his classic work" Mutation Theory "(1901-1903). Mutation he called the phenomenon of a spasmodic, discontinuous change in a hereditary trait. The main provisions of the theory of de Vries have not lost their significance so far, and therefore they should be given here:

  1. mutation occurs suddenly, without any transitions;
  2. the new forms are completely constant, that is, they are stable;
  3. Mutations, unlike non-hereditary changes (fluctuations), do not form continuous series, they are not grouped around an average type (mode). Mutations are qualitative changes;
  4. mutations go in different directions, they can be both beneficial and harmful;
  5. mutation detection depends on the number of individuals analyzed for mutation detection;
  6. the same mutations can occur repeatedly.

  However, G. de Vries made a fundamental mistake by opposing the theory of mutations to the theory of natural selection. He incorrectly believed that mutations could immediately give rise to new species adapted to the external environment, without the participation of selection. In fact, mutations are only a source of hereditary changes that serve as material for selection. As we will see later, gene mutation is only evaluated by selection in the genotype system. The error of G. de Vries is connected, in part, with the fact that the mutations he studied in evening primrose (Oenothera Lamarciana) subsequently turned out to be the result of splitting a complex hybrid.

  But one cannot but admire the scientific foresight that H. de Vries made regarding the formulation of the main provisions of the mutation theory and its significance for selection. Back in 1901, he wrote: “...mutation, mutation itself, should become the object of study. And if we ever succeed in elucidating the laws of mutation, then not only will our view of the mutual relationship of living organisms become much deeper, but we also dare to hope that the possibility of mastering mutability should open up as well as the breeder dominates variability, variability. Of course, we will come to this gradually, mastering individual mutations, and this will also bring many benefits to agricultural and horticultural practice. Much that now seems unattainable will be within our power, if only we can learn the laws on which the mutation of species is based. Obviously, here we are waiting for an boundless field of persistent work of high importance both for science and for practice. This is a promising area for dominating mutations.” How can we further verify modern natural science stands on the threshold of understanding the mechanism of gene mutation.

  The theory of mutations could develop only after the discovery of Mendel's laws and the laws established in the experiments of the Morgan school of gene linkage and their recombination as a result of crossing over. Only since the establishment of the hereditary discreteness of chromosomes, the theory of mutations received a basis for scientific research.

  Although at present the question of the nature of the gene has not been completely elucidated, a number of general patterns of gene mutation have nevertheless been firmly established.

  Gene mutations occur in all classes and types of animals, higher and lower plants, multicellular and unicellular organisms, bacteria and viruses. Mutational variability as a process of qualitative spasmodic changes is universal for all organic forms.

  Purely conventionally, the mutation process is divided into spontaneous and induced. In cases where mutations occur under the influence of ordinary natural environmental factors or as a result of physiological and biochemical changes in the organism itself, they are referred to as spontaneous mutations. Mutations that occur under the influence of special influences (ionizing radiation, chemicals, extreme conditions, etc.) are called induced. There are no fundamental differences between spontaneous and induced mutations, but the study of the latter leads biologists to master hereditary variability and unravel the mystery of the gene.

  If you find an error, please highlight a piece of text and click Ctrl+Enter.

  The textbook complies with the Federal State Educational Standard of Secondary (Complete) general education recommended by the Ministry of Education and Science of the Russian Federation and included in the Federal List of Textbooks.

  The textbook is addressed to students in grade 10 and is designed to teach the subject 1 or 2 hours per week.

  Modern design, multi-level questions and tasks, additional information and the possibility of parallel work with an electronic application contribute to the effective assimilation of educational material.

  Book:

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  Remember!

  Give examples of features that change under the influence of the external environment.

  What are mutations?

  Variability- one of the most important properties living organisms, the ability of living organisms to acquire differences from individuals of both other species and their own species.

  There are two types of variability: non-hereditary(phenotypic, or modification) and hereditary(genotypic).

  Non-hereditary (modification) variability. This type of variability is the process of the emergence of new traits under the influence of environmental factors that do not affect the genotype. Consequently, the modifications of signs that arise in this case - modifications - are not inherited (Fig. 93). Two identical (monozygous) twins, having exactly the same genotypes, but by the will of fate grew up in different conditions, can be very different from each other. A classic example, proving the influence of the external environment on the development of signs, is the arrowhead. This plant develops three types of leaves, depending on the growing conditions - in the air, in the water column or on its surface.

  
  

  Rice. 93. Oak leaves grown in bright light (A) and in a shaded place (B)

  
  

  

  Rice. 94. Changing the color of the coat of the Himalayan rabbit under the influence of various temperatures

  Under the influence of ambient temperature, the coat color of the Himalayan rabbit changes. The embryo, developing in the womb, is in conditions of elevated temperature, which destroys the enzyme necessary for pigment synthesis, so rabbits are born completely white. Shortly after birth, certain protruding parts of the body (nose, tips of the ears and tail) begin to darken, because there the temperature is lower than in other places, and the enzyme is not destroyed. If you pluck out an area of ​​white wool and cool the skin, black wool will grow in this place (Fig. 94).

  Under similar environmental conditions in genetically close organisms, modification variability has group character, for example, in the summer, under the influence of UV rays, a protective pigment, melanin, is deposited in the skin of most people, people sunbathe.

  In the same species of organisms, under the influence of environmental conditions, the variability of various traits can be completely different. For example, in cattle, milk yield, weight, and fertility are very dependent on the conditions of feeding and maintenance, and, for example, the fat content of milk under the influence of external conditions changes very little. Manifestations of modification variability for each trait are limited by their reaction rate. reaction rate- these are the limits in which a change in a trait is possible in a given genotype. In contrast to the modification variability itself, the reaction rate is inherited, and its limits are different for different traits and for individual individuals. The narrowest reaction rate is typical for traits that provide the vital qualities of the body.

  Due to the fact that most modifications have an adaptive value, they contribute to adaptation - the adaptation of the body within the limits of the norm of reaction to existence in changing conditions.

  Hereditary (genotypic) variability. This type of variability is associated with changes in the genotype, and the traits acquired as a result of this are inherited by the next generations. There are two forms of genotypic variability: combinative and mutational.

  Combination variability consists in the appearance of new traits as a result of the formation of other combinations of parental genes in the genotypes of offspring. This type of variability is based on independent divergence of homologous chromosomes in the first meiotic division, a random meeting of gametes in the same parental pair during fertilization, and a random selection of parental pairs. It also leads to the recombination of genetic material and increases the variability of the exchange of sections of homologous chromosomes, which occurs in the first prophase of meiosis. Thus, in the process of combinative variability, the structure of genes and chromosomes does not change, but new combinations of alleles lead to the formation of new genotypes and, as a result, to the appearance of offspring with new phenotypes.

  Mutational variability It is expressed in the appearance of new qualities of the organism as a result of the formation of mutations. The term "mutation" was first introduced in 1901 by the Dutch botanist Hugo de Vries. According to modern concepts mutations- these are sudden natural or artificially induced inherited changes in the genetic material, leading to a change in certain phenotypic characteristics and properties of the organism. Mutations are undirected, that is, random, in nature and are the most important source of hereditary changes, without which the evolution of organisms is impossible. At the end of the XVIII century. in America, a sheep with shortened limbs was born, which gave rise to a new Ancon breed (Fig. 95). in Sweden at the beginning of the 20th century. a mink with platinum fur was born on a fur farm. The huge variety of traits in dogs and cats is the result of mutational variation. Mutations arise abruptly, as new qualitative changes: awnless wheat was formed from spinous wheat, short wings and striped eyes appeared in Drosophila, white, brown, black color appeared in rabbits from the natural color of agouti as a result of mutations.

  According to the place of origin, somatic and generative mutations are distinguished. Somatic mutations arise in the cells of the body and are not transmitted through sexual reproduction to the next generations. Examples of such mutations are age spots and skin warts. generative mutations appear in germ cells and are inherited.

  
  

  

  Rice. 95. Ancona sheep

  According to the level of change in the genetic material, gene, chromosomal and genomic mutations are distinguished. Gene mutations cause changes in individual genes, disrupting the order of nucleotides in the DNA chain, which leads to the synthesis of an altered protein.

  Chromosomal mutations affect a significant portion of the chromosome, disrupting the functioning of many genes at once. A separate fragment of the chromosome can double or be lost, which causes serious disturbances in the functioning of the body, up to the death of the embryo in the early stages of development.

  Genomic mutations lead to a change in the number of chromosomes as a result of violations of the divergence of chromosomes in the divisions of meiosis. The absence of a chromosome or the presence of an extra one leads to adverse consequences. The best-known example of a genomic mutation is Down syndrome, a developmental disorder that occurs when an extra chromosome 21 is added. Such people have total number chromosomes is 47.

  In protozoa and in plants, an increase in the number of chromosomes, a multiple of the haploid set, is often observed. Such a change chromosome set is called polyploidy(Fig. 96). The emergence of polyploids is associated, in particular, with the nondisjunction of homologous chromosomes during meiosis, as a result of which not haploid, but diploid gametes can form in diploid organisms.

  mutagenic factors. The ability to mutate is one of the properties of genes, so mutations can occur in all organisms. Some mutations are incompatible with life, and the embryo that received them dies in the womb, while others cause persistent changes in traits that are significant to varying degrees for the life of the individual. Under normal conditions, the mutation rate of an individual gene is extremely low (10–5), but there are environmental factors that significantly increase this value, causing irreversible damage to the structure of genes and chromosomes. Factors whose impact on living organisms leads to an increase in the frequency of mutations are called mutagenic factors or mutagens.

  
  

  

  Rice. 96. Polyploidy. Chrysanthemum flowers: A - diploid form (2 n); B - polyploid form

  All mutagenic factors can be divided into three groups.

  Physical mutagens are all types of ionizing radiation (?-rays, x-rays), ultraviolet radiation, high and low temperatures.

  Chemical mutagens- these are analogs of nucleic acids, peroxides, salts of heavy metals (lead, mercury), nitrous acid and some other substances. Many of these compounds cause disturbances in DNA replication. Substances used in agriculture for pest and weed control (pesticides and herbicides), industrial waste, certain food colors and preservatives, some medications, components of tobacco smoke.

  Special laboratories and institutes have been set up in Russia and other countries of the world to test all newly synthesized chemical compounds for mutagenicity.

  To the group biological mutagens include foreign DNA and viruses that, embedding in the host's DNA, disrupt the work of genes.

  Review questions and assignments

  1. What kinds of variability do you know?

  2. What is a reaction rate?

  3. Explain why phenotypic variability is not inherited.

  4. What are mutations? Describe the main properties of mutations.

  5. Give a classification of mutations according to the level of changes in the hereditary material.

  6. Name the main groups of mutagenic factors. Give examples of mutagens that belong to each group. Assess if there are mutagenic factors in your environment. What group of mutagens do they belong to?

  Think! Execute!

  1. In your opinion, can environmental factors affect the development of an organism carrying a lethal mutation?

  2. Can combinative variability manifest itself in the absence of the sexual process?

  3. Discuss in class what are the ways to reduce human exposure to mutagenic factors in today's world.

  4. Can you give examples of modifications that are not adaptive in nature?

  5. Explain to someone unfamiliar with biology how mutations differ from modifications.

  6. Perform the study: "The study of modification variability in students (for example, body temperature and pulse rate, periodically measured for 3 days)".

  Work with computer

  Refer to the electronic application. Study the material and complete the assignments.

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  Variation is the occurrence of individual differences. Based on the variability of organisms, a genetic diversity of forms appears, which, as a result of the action of natural selection, are transformed into new subspecies and species. There are modification variability, or phenotypic, and mutational, or genotypic.

  TABLE Comparative characteristics forms of variability (T.L. Bogdanova. Biology. Tasks and exercises. A guide for applicants to universities. M., 1991)

  Variability forms	Reasons for the appearance	Meaning	Examples
  Non-hereditary modification (phenotypic)	A change in environmental conditions, as a result of which the organism changes within the norm of the reaction specified by the genotype	Adaptation - adaptation to given environmental conditions, survival, preservation of offspring	White cabbage in a hot climate does not form a head. Breeds of horses and cows brought to the mountains become stunted
  Mutational	The influence of external and internal mutagenic factors, resulting in a change in genes and chromosomes	Material for natural and artificial selection, since mutations can be beneficial, harmful and indifferent, dominant and recessive	The appearance of polyploid forms in a plant population or in some animals (insects, fish) leads to their reproductive isolation and the formation of new species, genera - microevolution
  Hereditary (genotypic) Combined	Occurs spontaneously within a population when crossing, when offspring have new combinations of genes	Distribution in a population of new hereditary changes that serve as material for selection	The appearance of pink flowers when crossing white-flowered and red-flowered primroses. When crossing white and gray rabbits, black offspring may appear
  Hereditary (genotypic) Correlative (correlative)	Arises as a result of the properties of genes to influence the formation of not one, but two or more traits	The constancy of interrelated features, the integrity of the body as a system	Long-legged animals have a long neck. In table varieties of beets, the color of the root crop, petioles and leaf veins consistently changes.

  Modification variability

  Modification variability does not cause changes in the genotype, it is associated with the reaction of a given, one and the same genotype to a change in the external environment: under optimal conditions, the maximum of the possibilities inherent in a given genotype is revealed. Thus, the productivity of outbred animals under conditions of improved maintenance and care increases (milk yield, meat fattening). In this case, all individuals with the same genotype respond to external conditions in the same way (Ch. Darwin called this type of variability a certain variability). However, another sign - the fat content of milk - is slightly affected by changes in environmental conditions, and the color of the animal is an even more stable sign. Modification variability usually fluctuates within certain limits. The degree of variation of a trait in an organism, i.e., the limits of modification variability, is called the reaction norm.

  A wide reaction rate is characteristic of such traits as milk yield, leaf size, color in some butterflies; a narrow reaction rate - fat content of milk, egg production in chickens, color intensity of corollas in flowers, etc.

  The phenotype is formed as a result of interactions between the genotype and environmental factors. Phenotypic traits are not transmitted from parents to offspring, only the norm of reaction is inherited, that is, the nature of the response to changes in environmental conditions. In heterozygous organisms, when environmental conditions change, various manifestations of this trait can be caused.

  Properties of modifications: 1) non-heritability; 2) the group nature of the changes; 3) correlation of changes to the action of a certain environmental factor; 4) the conditionality of the limits of variability by the genotype.

  Genotypic variability

  Genotypic variability is subdivided into mutational and combinative. Mutations are called spasmodic and stable changes in units of heredity - genes, entailing changes in hereditary traits. The term "mutation" was first introduced by de Vries. Mutations necessarily cause changes in the genotype that are inherited by offspring and are not associated with crossing and recombination of genes.

  Mutation classification. Mutations can be combined, into groups - classified according to the nature of manifestation, in place or, according to the level of their occurrence.

  Mutations by the nature of manifestation are dominant and recessive. Mutations often reduce viability or fertility. Mutations that sharply reduce viability, partially or completely stop development, are called semi-lethal, and those incompatible with life are called lethal. Mutations are classified according to where they occur. A mutation that has arisen in germ cells does not affect the characteristics of a given organism, but manifests itself only in the next generation. Such mutations are called generative. If genes are changed in somatic cells, such mutations appear in this organism and are not transmitted to offspring during sexual reproduction. But with asexual reproduction, if an organism develops from a cell or group of cells that has a changed - mutated - gene, mutations can be transmitted to offspring. Such mutations are called somatic.

  Mutations are classified according to their level of occurrence. There are chromosomal and gene mutations. Mutations also include a change in the karyotype (a change in the number of chromosomes). Polyploidy is an increase in the number of chromosomes, a multiple of the haploid set. In accordance with this, triploids (3p), tetraploids (4p), etc. are distinguished in plants. More than 500 polyploids are known in plant growing (sugar beet, grapes, buckwheat, mint, radish, onion, etc.). All of them are distinguished by a large vegetative mass and have great economic value.

  A large variety of polyploids is observed in floriculture: if one initial form in the haploid set had 9 chromosomes, then cultivated plants of this species can have 18, 36, 54 and up to 198 chromosomes. Polyploids develop as a result of exposure of plants to temperature, ionizing radiation, chemicals (colchicine), which destroy the spindle of cell division. In such plants, the gametes are diploid, and when they merge with the haploid germ cells of the partner, a triploid set of chromosomes appears in the zygote (2n + n = Zn). Such triploids do not form seeds, they are sterile, but high-yielding. Even polyploids form seeds.

  Heteroploidy is a change in the number of chromosomes that is not a multiple of the haploid set. In this case, the set of chromosomes in a cell can be increased by one, two, three chromosomes (2n + 1; 2n + 2; 2n + 3) or reduced by one chromosome (2n-1). For example, a person with Down syndrome has one extra chromosome in the 21st pair and the karyotype of such a person is 47 chromosomes. People with Shereshevsky-Turner syndrome (2p-1) lack one X chromosome and 45 chromosomes remain in the karyotype. These and other similar deviations of numerical relations in the human karyotype are accompanied by a health disorder, a mental and physique disorder, a decrease in vitality, etc.

  Chromosomal mutations are associated with changes in the structure of chromosomes. There are the following types of chromosome rearrangements: detachment of various sections of the chromosome, doubling of individual fragments, rotation of a section of the chromosome by 180 °, or attachment of a separate section of the chromosome to another chromosome. Such a change entails a violation of the function of genes in the chromosome and the hereditary properties of the organism, and sometimes its death.

  Gene mutations affect the structure of the gene itself and entail a change in the properties of the organism (hemophilia, color blindness, albinism, color of flower corollas, etc.). Gene mutations occur in both somatic and germ cells. They can be dominant and recessive. The first appear both in homozygotes and. in heterozygotes, the second - only in homozygotes. In plants, the resulting somatic gene mutations are preserved during vegetative propagation. Mutations in germ cells are inherited during seed reproduction of plants and during sexual reproduction of animals. Some mutations have a positive effect on the body, others are indifferent, and others are harmful, causing either the death of the organism or a weakening of its viability (for example, sickle cell anemia, hemophilia in humans).

  When breeding new plant varieties and strains of microorganisms, induced mutations are used, artificially caused by certain mutagenic factors (X-rays or ultraviolet rays, chemical substances). Then, the obtained mutants are selected, keeping the most productive ones. In our country, many economically promising varieties of plants have been obtained by these methods: non-lodging wheat with a large ear, resistant to diseases; high-yielding tomatoes; cotton with large bolls, etc.

  Mutation properties:

  1. Mutations occur suddenly, abruptly.
  2. Mutations are hereditary, that is, they are persistently transmitted from generation to generation.
  3. Mutations are not directed - any locus can mutate, causing changes in both minor and vital signs.
  4. The same mutations can occur repeatedly.
  5. According to their manifestation, mutations can be beneficial and harmful, dominant and recessive.

  The ability to mutate is one of the properties of a gene. Each individual mutation is caused by some cause, but in most cases these causes are unknown. Mutations are associated with changes in the external environment. This is convincingly proved by the fact that through the influence of external factors it is possible to sharply increase their number.

  Combination variability

  Combinative hereditary variability arises as a result of the exchange of homologous regions of homologous chromosomes during meiosis, and also as a result of independent divergence of chromosomes during meiosis and their random combination during crossing. Variability can be caused not only by mutations, but also by combinations of individual genes and chromosomes, a new combination of which, during reproduction, leads to a change in certain signs and properties of the organism. This type of variability is called combinative hereditary variability. New combinations of genes arise: 1) during crossing over, during the prophase of the first meiotic division; 2) during independent segregation of homologous chromosomes in the anaphase of the first meiotic division; 3) during the independent divergence of daughter chromosomes in the anaphase of the second meiotic division and 4) during the fusion of different germ cells. The combination of recombined genes in the zygote can lead to the combination of traits of different breeds and varieties.

  In breeding, the law of homologous series of hereditary variability, formulated by the Soviet scientist N. I. Vavilov, is of great importance. It says: within different species and genera that are genetically close (that is, having a common origin), similar series of hereditary variability are observed. Such a character of variability was found in many cereals (rice, wheat, oats, millet, etc.), in which the color and consistency of grain, cold resistance, and other qualities vary in a similar way. Knowing the nature of hereditary changes in some varieties, one can foresee similar changes in related species and, by acting on them with mutagens, cause similar beneficial changes in them, which greatly facilitates the production of economically valuable forms. Many examples of homological variability are also known in humans; for example, albinism (a defect in the synthesis of a dye by cells) was found in Europeans, blacks and Indians; among mammals - in rodents, carnivores, primates; short dark-skinned people - pygmies - are found in the tropical forests of equatorial Africa, the Philippine Islands and the jungles of the Malay Peninsula; some hereditary defects and deformities inherent in man are also noted in animals. Such animals are used as a model for studying similar defects in humans. For example, a cataract of the eye occurs in mice, rats, dogs, horses; hemophilia - in a mouse and a cat, diabetes - in a rat; congenital deafness - in guinea pigs, mice, dogs; cleft lip - in mice, dogs, pigs, etc. These hereditary defects are convincing confirmation of the law of homologous series of hereditary variability by N. I. Vavilov.

  Table. Comparative characteristics of the forms of variability (T.L. Bogdanova. Biology. Tasks and exercises. A guide for applicants to universities. M., 1991)

  Characteristic	Modification variability	Mutational variability
  Object of change	Phenotype within normal limits	Genotype
  Selecting factor	Changing environmental conditions environments	Changing environmental conditions
  Inheritance signs	Not inherited	Inherited
  Susceptibility to chromosome changes	Not exposed	undergo chromosomal mutation
  Susceptibility to changes in DNA molecules	Not exposed	Exposed in case gene mutation
  Significance for an individual	Raises or lowers viability. productivity, adaptation	Helpful Changes lead to victory in the struggle for existence, harmful - to death
  View value	Promotes survival	Leads to the formation of new populations, species, etc. as a result of divergence
  Role in evolution	fixture organisms to environmental conditions	Material for natural selection
  Shape of variability	Certain (group)	Indefinite (individual), combinative
  Subordination of regularity	Statistical regularity variation series	Homological law series of hereditary variability