Mutation theory is one of the foundations of genetics. It originated shortly after the rediscovery of Mendel's laws by T. Morgan at the beginning of the 20th century. It can be considered that it almost simultaneously originated in the minds of the Dutchman Hugo De Vries (1903) and the domestic botanist S. I. Korzhinsky (1899). However, the priority in the primacy and in the greater coincidence of the initial provisions belongs to the Russian scientist. Recognition of the main evolutionary significance for discrete variability and the denial of the role of natural selection in the theories of Korzhinsky and De Vries was associated with the insolvability at that time of the contradiction in the evolutionary teaching of Charles Darwin between the important role of small deviations and their "absorption" during crossings (see Jenkin's nightmare) .
The main provisions of the Korzhinsky-De Vries mutation theory can be reduced to the following points:
1. Mutations are sudden, like discrete changes in traits
2.New molds are resistant
3. Unlike hereditary changes, mutations do not form continuous series, they are not grouped around any average type. They represent qualitative leaps of change.
4. Mutations manifest themselves in different ways and can be both beneficial and harmful.
5. The probability of detecting mutations depends on the number of individuals studied
6. Similar mutations can occur repeatedly
GENOMIC MUTATIONS:
Mutation is a random phenomenon, i.e. it is impossible to predict: where, when and what change will occur. One can only estimate the probability of mutation in populations by knowing the actual frequencies of certain mutations.
Gene mutations are expressed in a change in the structure of individual sections of DNA. According to their consequences, gene mutations are divided into two groups: mutations without a frameshift and mutations with a frameshift.
Mutations without a frameshift occur as a result of the replacement of nucleotide pairs, while the total length of DNA does not change. As a result, amino acid replacement is possible, but due to the degeneracy of the genetic code, the protein structure can also be preserved.
Frameshift mutations (frameshifts) occur as a result of the insertion or loss of nucleotide pairs, while the total length of the DNA changes. The result is a complete change in the structure of the protein.
However, if after the insertion of a nucleotide pair, a loss of a nucleotide pair occurs (or vice versa), then the amino acid composition of proteins can be restored. Then the two mutations at least partially compensate each other. This phenomenon is called intragenic suppression.
Nonsense mutations. A special group of gene mutations are nonsense mutations with the appearance of stop codons (replacement of a sense codon by a stop codon). Nonsense mutations can occur as a result of substitutions of nucleotide pairs, as well as with losses or insertions. With the appearance of stop codons, the synthesis of the polypeptide generally stops. As a result, null alleles can occur that do not correspond to any protein. Accordingly, the opposite phenomenon is also possible: the replacement of a nonsense codon with a sense codon. Then the length of the polypeptide can increase.
Methods for detecting gene mutations
The difficulty of detecting gene mutations is associated, firstly, with the recessive nature of most mutations (the probability of their phenotypic manifestation is negligible), and secondly, with the lethality of many of them (mutants do not survive).
The whole set of methods for detecting gene mutations can be divided into two groups: methods of genetic analysis and biochemical methods.
1. Genetic analysis methods are based on crossing possible mutation carriers with tester lines (analyzer lines). The simplest method is to cross the carriers of the putative mutation with the corresponding recessive-homozygous line, i.e. conventional test cross.
However, this method does not allow detection of unknown mutations, as well as lethal mutations. Therefore, special tester lines are created to account for lethal mutations.
2. Biochemical methods for detecting mutations are extremely diverse and based on the use of various techniques.
a). Methods based on the detection of certain biochemical products of mutant genes. The easiest way to detect mutations is to change the activity of enzymes or to lose any biochemical trait. For example, in microorganisms on selective nutrient media, auxotrophic forms are detected that are not capable of synthesizing certain substances (compared to normal, prototrophic forms).
b). Methods based on the direct detection of altered nucleic acids and proteins using gel electrophoresis in combination with other methods (blot hybridization, autoradiography).
Causes of Mutations
According to the causes of occurrence, spontaneous and induced mutations are distinguished.
Spontaneous (spontaneous) mutations occur for no apparent reason. These mutations are sometimes considered as errors of the three Ps: the processes of DNA replication, repair, and recombination. This means that the process of occurrence of new mutations is under the genetic control of the organism. For example, mutations are known that increase or decrease the frequency of other mutations; therefore, there are mutator genes and antimutator genes.
At the same time, the frequency of spontaneous mutations also depends on the state of the cell (organism). For example, under conditions of stress, the frequency of mutations can increase.
Induced mutations occur under the action of mutagens.
Mutagens are a variety of factors that increase the frequency of mutations.
For the first time, induced mutations were obtained by domestic geneticists G.A. Nadson and G.S. Filippov in 1925 when yeast was irradiated with radium radiation.
There are several classes of mutagens:
– Physical mutagens: ionizing radiation, thermal radiation, ultraviolet radiation.
– Chemical mutagens: nitrogenous base analogs (eg 5-bromouracil), aldehydes, nitrites, methylating agents, hydroxylamine, heavy metal ions, some drugs and plant protection products.
– Biological mutagens: pure DNA, viruses, antiviral vaccines.
– Automutagens are intermediate metabolic products (intermediates). For example, ethyl alcohol by itself is not a mutagen. However, in the human body, it is oxidized to acetaldehyde, and this substance is already a mutagen.
Gene mutations
1. Variability, its causes and methods of study. Classification of forms of variability. Phenotypic variability and its components. Heritability of traits
2. Mutational variability. The main provisions of the mutation theory. General properties of mutations.
3. Gene mutations. consequences of mutations. Methods for detecting gene mutations.
4. General patterns of the mutation process. Mechanisms of occurrence of gene mutations.
Variability, its causes and methods of study. Classification of forms of variability. Phenotypic variability and its components. Heritability of traits
Self-reproduction with change is one of the basic properties of life. The term "variability" serves to refer to various concepts; like most other terms, it polysemantic(polysemantic). Yuri Alexandrovich Filipchenko distinguished two main approaches to the definition of variability.
1. Variability as a state. In this sense, the term "variability" is used to denote the differences of biological objects from each other at a given time. There are always differences between parts of one organism, between different organisms in a population, between different intrapopulation groups, between populations.
2. Variability as a process. In this sense, the term "variability" refers to the change in a biological object over time. In this case, variability reflects the development of an individual, the difference between offspring and parents.
Any observed variability is phenotypic. In turn, phenotypic or general variability includes three components:
– hereditary(genetic, or genotypic variation) - largely due to the influence of genetic factors. For example, several varieties of the same plant species are grown under similar conditions. Then the differences between the results of the experiment (for example, yield) are due to the genetic characteristics of each variety. The basis of genetic variability is mutational and combinative variability.
– non-hereditary(modification)variability - largely due to the action of non-genetic ( exogenous) factors. For example, one variety of plants is grown in different conditions. Then the differences between the experimental results (for example, yield) are due to the influence of plant growing conditions.
– uncontrollable (residual variability) – due to uncontrollable (at least in this experiment) factors.
For different traits, the influence of the genotype and environmental conditions on the overall phenotypic variability is not the same. For example, coat color, milk fat content in cattle, egg weight in chickens depend mainly on the characteristics of the breed (i.e., on the genotype) - these traits are highly heritable. Other signs: wool quality, overall milk production in cattle, egg production in chickens - depend mainly on growing and keeping conditions - these signs have low heritability.
mutational variability. The main provisions of the mutation theory. General properties of mutations
The term "mutation" (from lat. mutatio- change) has long been used in biology to refer to any abrupt changes. For example, the German paleontologist W. Waagen called the transition from one fossil form to another a mutation. A mutation was also called the appearance of rare traits, in particular, melanistic forms among butterflies.
Modern ideas about mutations were formed by the beginning of the 20th century. For example, the Russian botanist Sergei Ivanovich Korzhinsky in 1899 developed an evolutionary theory of heterogenesis based on the concept of the leading evolutionary role of discrete (discontinuous) changes.
However, the most famous was the mutational theory of the Dutch botanist Hugo (Hugo) De Vries (1901), who introduced the modern, genetic concept of mutation to denote rare variants of traits in the offspring of parents who did not have this trait.
De Vries developed a mutation theory based on observations of a widespread weed plant, biennial aspen, or evening primrose ( Oenothera biennis). This plant has several forms: large-flowered and small-flowered, dwarf and giant. De Vries collected seeds from a plant of a certain form, sowed them and received 1 ... 2% of plants of a different form in the offspring. Later it was found that the appearance of rare variants of the trait in evening primrose is not a mutation; this effect is due to the peculiarities of the organization of the chromosome apparatus of this plant. In addition, rare trait variants may be due to rare combinations of alleles (for example, the white color of plumage in budgerigars is determined by a rare combination aabb).
The main provisions of the mutation theory of De Vries remain valid to this day (of course, with some modern clarifications):
| The provisions of the mutation theory of De Vries | Modern refinements | |
| Mutations occur suddenly, without any transitions. | there is a special type of mutations that accumulate over a number of generations (progressive amplification in introns). | |
| Success in identifying mutations depends on the number of individuals analyzed. | without changes | |
| Mutant forms are quite stable. | under the condition of 100% penetrance (the mutant phenotype corresponds to the mutant genotype) and 100% expressivity (the same mutation is equally manifested in different individuals) | |
| Mutations are characterized by discreteness (discontinuity); these are qualitative changes that do not form continuous series, are not grouped around an average type (mode). | there are face mutations, as a result of which there is a slight change in the characteristics of the final product | |
| The same mutations can occur repeatedly. | it concerns gene mutations; chromosomal aberrations are unique and inimitable | |
| Mutations occur in different directions, they can be harmful and beneficial. | mutations themselves are not adaptive; only in the course of evolution, in the course of selection, the “usefulness”, “neutrality” or “harmfulness” of mutations under certain conditions is evaluated; while the "harmfulness" and "usefulness" of mutations depends on the genotypic environment |
The following definition of mutations is currently accepted:
Mutations are qualitative changes in the genetic material, leading to a change in certain signs of the organism.
An organism in which a mutation is found in all cells is called mutant. This occurs if the given organism develops from a mutant cell (gametes, zygotes, spores). In some cases, the mutation is not found in all somatic cells of the body; such an organism is called genetic mosaic. This happens if mutations appear during ontogenesis - individual development. And, finally, mutations can occur only in generative cells (in gametes, spores, and in cells of the germ line - precursor cells of spores and gametes). In the latter case, the organism is not a mutant, but some of its descendants will be mutants.
There are "new" mutations (arising de novo) and "old" mutations. Old mutations are mutations that appeared in the population long before they were studied; Usually old mutations are discussed in population genetics and in evolutionary theory. New mutations are mutations that appear in the offspring of non-mutant organisms (♀ AA × ♂ AA → Ah); Usually, it is precisely such mutations that are discussed in the genetics of mutagenesis.
Mutation is a random phenomenon, i.e. it is impossible to predict: where, when and what change will occur. One can only estimate the probability of mutation in populations by knowing the actual frequencies of certain mutations. For example, the chance of E. coli developing resistance to tetracycline is 10–10 (one in ten billionth), since only one in 10 billion cells is resistant to this antibiotic (but all the offspring of this bacterium will be resistant to tetracycline).
It has been established that the mutability of a gene (i.e., the frequency of occurrence of a certain mutation) depends on the nature of the gene: there are genes that are prone to mutation and relatively stable genes.
The probability of an event is a mathematical abstraction, the mathematical expectation of one or another event. The probability of a random event lies in the range from 0 to 1. The mathematical expectation is determined outside of experience (a priori), on the basis of deductive reasoning. For example, when tossing a coin, the probability of getting heads is equal to the probability of getting tails and is equal to 50% or 0.5: R O = R P = 0.5.
However, in biology, the probability of many events cannot be found outside of experience, such as the probability of having a child with Down syndrome. Then the concept of mathematical probability is replaced by the concept statistical probability. The statistical probability is determined empirically (a posteriori). Numerically statistical, or posterior probability event is relative frequency this event. For example, for every 700 newborns, there is one child with Down's disease. Then the statistical probability of having a child with this disease is 1/700 ≈ 0.0014.
The relative frequency fluctuates around some constant number, which is the mathematical expectation of the event. The more observations are made, the more the a posteriori probability approaches the mathematical expectation of this event.
Several mutations can occur in the same cell. However, a single mutation is a rare event. Therefore, to find the probability of the simultaneous occurrence of two, three or more mutations, one cannot use the multiplication rule of probabilities. The probability that no mutation will occur in a cell, one or more mutations will occur is calculated according to Poisson's law (the higher the probability of a single mutation, the more symmetrical the distribution curve becomes).
§ 2. Mutation theory
Discovery of intermittent, sudden, hereditary non-directional changes - mutations(from lat. mutation- change) *, the distribution of which is purely random, served as an impetus for the even more rapid development of classical genetics at the beginning of the 20th century and for elucidating the role of hereditary changes in evolution.
* (Suddenly occurring hereditary changes have long been called mutations (in the 17th and 18th centuries). This term was resurrected by G. De Vries.)
In 1898 a Russian botanist S. I. Korzhinsky, and two years later, the Dutch botanist De Vries (one of those who rediscovered Mendel's law - see Chapter IV, § 3) independently made another extremely important genetic generalization, called mutation theory.
The essence of this theory lies in the fact that mutations arise suddenly and undirected, but once having arisen, the mutation becomes stable. The same mutation can occur repeatedly.
One day, passing by a potato field (near the Dutch village of Gilversum), overgrown with an aspen weed brought from America, a night candle or evening primrose ( Oenothera Lamarckiana) from the fireweed family (which includes the well-known fireweed, or Ivan tea), De Vries noticed specimens among ordinary plants that differed sharply from them. The scientist collected the seeds of these exceptional plants and planted them in his experimental garden. For 17 years, De Vries observed evening primrose (thousands of plants). First, he discovered three mutants: one of them was dwarf, the other giant - its leaves, flowers, fruits, seeds turned out to be large, long stems (Fig. 29), the third had red veins on leaves and fruits. For 10 years, De Vries received many new forms from normal plants, differing in a number of features. The scientist closely followed mutants(the so-called carriers of mutations) and their descendants for several years. On the basis of observations, supplementing the teachings of Darwin, he came to the conclusion about the paramount importance of sharp hereditary deviations - mutations for the emergence of new species. Mutations appear in a variety of directions in representatives of any of the species. Since not all mutations allow the mutant to survive (in a certain environment), the further existence of the corresponding form is decided by the Darwinian struggle for existence through natural selection.
Soon, many descriptions of various mutations in animals and plants appeared in the scientific literature.
Not knowing the mechanism of the occurrence of mutations, De Vries believed that all such changes occur spontaneously, spontaneously. This provision is valid only for a part of mutations.
The inevitability of spontaneous mutations follows from the inevitability of the movement of atoms, in which sooner or later, but statistically inevitably, transitions of electrons from one orbit to another occur. As a result, individual atoms and whole molecules change even under the most constant environmental conditions. This inevitable change in any physical and chemical structure is reflected in the appearance of spontaneous mutations (DNA molecules, the custodians of hereditary information, are such a structure).
Spontaneous mutations are constantly found in nature with a certain frequency, which is relatively close in the most diverse species of living organisms. The frequency of occurrence of spontaneous mutations varies according to individual traits from one mutation per 10 thousand gametes to one mutation per 10 million gametes. However, due to the large number of genes in each individual in all organisms, 10-25% of all gametes carry certain mutations. Approximately every tenth individual is a carrier of a new spontaneous mutation.
It should be noted that most of the newly emerging mutations are usually in a recessive state, increasing only the latent, potential, variability characteristic of organisms of any kind. When the conditions of the external environment change, for example, when the action of natural selection changes, this latent hereditary variability can manifest itself, since individuals carrying recessive mutations in the heterozygous state will not be destroyed in the process of the struggle for existence under new conditions, but will remain and give offspring. Spontaneous, spontaneous mutations appear without any outside intervention. However, there are many so-called induced mutations. Factors that cause (induce) mutations can be a variety of environmental influences temperature, ultraviolet radiation, radiation (both natural and artificial), the action of various chemical compounds - mutagens. Mutagens are agents of the external environment that cause certain changes in the genotype - a mutation, and the process of formation of mutations itself - mutagenesis.
Radioactive mutagenesis began to be studied in the 20s of our century. In 1925 Soviet scientists G. S. Filippov and G. A. Nadson For the first time in the history of genetics, X-rays were used to generate mutations in yeast. A year later, an American researcher G. Meller(later twice Nobel Prize winner), who worked for a long time in Moscow, at an institute led by N. K. Koltsov, applied the same mutagen on Drosophila.
Numerous mutations were found in Drosophila, two of them vestigial and curled are shown in Fig. thirty.
At present, work in this area has grown into one of the sciences - radiation biology, a science that has great practical application. For example, some mutations of fungi - producers of antibiotics - give hundreds and even thousands of times greater yield of medicinal substances. In agriculture, due to mutations, high-yielding plants have been obtained. Radiation genetics is important in the study and exploration of outer space.
Chemical mutagenesis was first purposefully studied by the employee of N.K. Koltsova V.V. Sakharov in 1931 on Drosophila when its eggs were exposed to iodine, and later M. E. Lobashov.
Chemical mutagens include a wide variety of substances (alkylating compounds, hydrogen peroxide, aldehydes and ketones, nitrous acid and its analogues, various antimetabolites, salts of heavy metals, dyes with basic properties, aromatic substances), insecticides (from Latin insecta - insects , cida - killer), herbicides (from the Latin herba - grass), drugs, alcohol, nicotine, some medicinal substances and many others.
In recent years, work has begun in our country on the use chemical mutagens to create new breeds of animals. Interesting results have been achieved in changing the color of wool in rabbits and in increasing the length of wool in sheep. It is essential that these achievements were obtained at such dosages of mutagens that do not cause the death of experimental animals. The strongest chemical mutagens (nitrosoalkylureas, 1,4-bisdiazoacetylbutane) are widely used.
One of the main tasks of selection agricultural plants is the creation of varieties resistant to fungal and viral diseases. Chemical mutagens are an effective tool for obtaining plant forms resistant to various diseases. In cereals (spring and winter wheat, barley, oats), forms resistant to powdery mildew, with increased resistance to various types of rust, were obtained. It is important that in some mutants an increase in the amount of protein does not correlate with a deterioration in its quality, and it is possible to obtain forms with an increased content of protein and essential amino acids in it (lysine, methionine, threonine).
Among the mutants induced by chemical mutagens, forms with a complex of positive traits are of great interest. There are frequent cases of obtaining such forms in wheat, peas, tomatoes, potatoes and other crops. Mutations are the stuff of both natural, and for artificial selection(selections).
In 1920, at that time, still young, but one of the largest geneticists of the 20th century, Nikolai Ivanovich Vavilov established that there is parallelism of variability among the most diverse systematic units of living beings. This provision is called the rule homologous(from lat. homologis- agreement, common origin) of the series, which to a certain extent allows you to predict what mutations can occur in related (and sometimes distant) forms. This rule lies in the fact that between different systematic groups (species, genera, classes, and even types) there are repeating series of forms that are similar in their morphological and physiological properties. This similarity is due to the presence of common genes and their similar mutation.
So, among the varieties of wheat and rye, there are similar forms, winter and spring, with awns, short awns or no awns of the ear; both have lowered, smooth-spiked, red-, white- and black-spiked races, races with a brittle and unbreakable spike, and other features. A similar parallelism between organisms belonging to different species, genera, families, and even different classes, is observed in animals. An example would be gigantism, dwarfism, or lack of pigmentation- albinism in mammals, birds, as well as in other animals and plants.
Having found a series of forms A, B, C, D, D, E in one biological species and establishing forms A 1, B 1, D 1, E 1 in another species related to it, it can be assumed that there are still undiscovered forms C 1 and G 1 .
In humans, the mutation rate in natural conditions is 1:1,000,000, but if we take into account the huge number of genes, then at least 10% of gametes, both male and female, carry any newly emerging mutation.
The term "mutation" (from lat. mutatio- change) has long been used in biology to refer to any abrupt changes. For example, the German paleontologist W. Waagen called the transition from one fossil form to another a mutation. The appearance of rare traits, in particular, melanistic forms among butterflies, was also called a mutation.
Modern ideas about mutations were formed by the beginning of the 20th century. For example, the Russian botanist Sergei Ivanovich Korzhinsky in 1899 developed evolutionary theory of heterogenesis, based on ideas about the leading evolutionary role of discrete (discontinuous) changes.
However, the most famous mutation theory of the Dutch botanist Hugo (Hugo) De Vries(1901), who introduced the modern, genetic concept of mutation to refer to rare variants of a trait in the offspring of parents who did not have that trait.
De Vries developed a mutation theory based on observations of a widespread weed plant, biennial aspen, or evening primrose ( Oenothera biennis). This plant has several forms: large-flowered and small-flowered, dwarf and giant. De Vries collected seeds from a plant of a certain form, sowed them and received 1 ... 2% of plants of a different form in the offspring. Later it was found that the appearance of rare variants of the trait in evening primrose is not a mutation; this effect is due to the peculiarities of the organization of the chromosome apparatus of this plant. In addition, rare trait variants may be due to rare combinations of alleles (for example, the white color of plumage in budgerigars is determined by a rare combination aabb).
The main provisions of the mutation theory of De Vries remain true to this day:
- Mutations occur suddenly, without any transitions.
- Success in identifying mutations depends on the number of individuals analyzed.
- Mutant forms are quite stable.
- Mutations are characterized by discreteness (discontinuity); these are qualitative changes that do not form continuous series, are not grouped around an average type (mode).
- The same mutations can occur repeatedly.
- Mutations occur in different directions, they can be harmful and beneficial
The following definition of mutations is currently accepted:
Mutations are qualitative changes in the genetic material, leading to a change in certain signs of the organism.
Mutation is random phenomenon, i.e. it is impossible to predict: where, when and what change will occur. One can only estimate the probability of mutation in populations by knowing the actual frequencies of certain mutations.
Gene mutations expressed in a change in the structure of individual sections of DNA. According to their consequences, gene mutations are divided into two groups:
- mutations without frameshift,
- frameshift mutations.
Mutations without frameshift readings occur as a result of the replacement of nucleotide pairs, while the total length of DNA does not change. As a result, amino acid replacement is possible, but due to the degeneracy of the genetic code, the protein structure can also be preserved.
Frameshift Mutations readouts (frameshifts) occur as a result of the insertion or loss of nucleotide pairs, while the total length of DNA changes. The result is a complete change in the structure of the protein.
However, if after the insertion of a nucleotide pair, a loss of a nucleotide pair occurs (or vice versa), then the amino acid composition of proteins can be restored. Then the two mutations at least partially compensate each other. This phenomenon is called intragenic suppression.
An organism in which a mutation is found in all cells is called mutant. This occurs if the given organism develops from a mutant cell (gametes, zygotes, spores). In some cases, the mutation is not found in all somatic cells of the body; such an organism is called genetic mosaic. This happens if mutations appear during ontogenesis - individual development. Finally, mutations can only occur in generative cells(in gametes, spores and in germline cells - precursor cells of spores and gametes). In the latter case, the body is not a mutant, but some of his descendants will be mutants.
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Mutation theory, or, more correctly, the theory of mutations, is one of the foundations of genetics. It originated shortly after the rediscovery of the laws of G. Mendel in the works of G. De Vries (1901 -1903). Even earlier, the Russian botanist S. I. Korzhinsky (1899) came to the idea of an abrupt change in hereditary properties in his work “Heterogenesis and Evolution”. So it is fair to speak about the mutational theory of Korzhinsky - De Vries. The mutational theory is described in much more detail in the works of H. De Vries, who devoted most of his life to studying the problem of mutational variability in plants.
At first, the mutational theory focused entirely on the phenotypic manifestation of hereditary changes, practically without dealing with the mechanism of their occurrence. In accordance with the definition of G. De Vries, a mutation is a phenomenon of an abrupt, intermittent change in a hereditary trait. As will be shown later, the very definition of the concept of "mutation" causes difficulties. Until now, despite numerous attempts, there is no concise definition of mutation better than that given by G. De Vries, although it is not free from shortcomings.
Mutations (from Latin mutatio - change, change) are sudden natural (spontaneous) or artificially induced (induced) persistent changes in the hereditary structures of living matter responsible for the storage and transmission of genetic information. The ability to give M. - to mutate - is a universal property of all forms of life from viruses and microorganisms to higher plants, animals and humans; it underlies hereditary variability (see variability) in living nature. M., arising in germ cells or spores (generative M.), are inherited; M., arising in cells that do not participate in sexual reproduction (somatic mutations), lead to genetic mosaicism: part of the body consists of mutant cells, the other - of non-mutant ones. In these cases, M. can be inherited only through vegetative reproduction with the participation of mutant somatic parts of the body (buds, cuttings, tubers, etc.).
The sudden appearance of hereditary changes was noted by many scientists of the 18th and 19th centuries; it was well known to C. Darwin, but in-depth study of M. began only with the emergence on the threshold of the 20th century. experimental genetics. The term "M." introduced into genetics in 1901 by H. De Vries.
Mutation types . According to the nature of the change in the genetic apparatus, M. is divided into genomic, chromosomal and gene, or point. Genomic M. consist in changing the number of chromosomes in the cells of the body. These include: Polyploidy - an increase in the number of sets of chromosomes, when instead of the usual 2 sets of chromosomes for diploid organisms, there can be 3, 4, etc.; Haploidy - instead of 2 sets of chromosomes, there is only one; Aneuploidy - one or more pairs of homologous chromosomes are absent (nullisomy) or are represented not by a pair, but by only one chromosome (monosomy) or, on the contrary, by 3 or more homologous partners (trisomy, tetrasomy, etc.). Chromosomal M., or chromosomal rearrangements (See. Chromosomal rearrangements), include: inversions - a section of the chromosome is turned 180 °, so that the genes contained in it are located in the reverse order compared to normal; translocations - exchange of sections of two or more non-homologous chromosomes; deletions - loss of a significant portion of the chromosome; shortages (small deletions) - loss of a small portion of the chromosome; duplications - doubling of a section of a chromosome; fragmentation - breakage of a chromosome into 2 parts or more. Genetic M. are persistent changes in the chemical structure of individual genes and, as a rule, are not reflected in the morphology of chromosomes observed under a microscope. M. genes are also known, localized not only in chromosomes, but also in some self-reproducing organelles of the cytoplasm (for example, in mitochondria, plastids; see Cytoplasmic inheritance).
