The role of hereditary variability in the evolution of species and its forms. Hereditary variability: features and significance

There are 2 types of hereditary variability: mutational and combinative.

Combinative variability is based on the formation of recombinations, i.e. such combinations of genes that the parents did not have. Phenotypically, this can manifest itself not only in the fact that parental traits are found in some of the offspring in other combinations, but also in the formation of new traits in the offspring that are absent in the parents. This happens when two or more non-allelic genes that differ in the parents affect the formation of the same trait.

The main sources of combinative variability are:

Independent divergence of homologous chromosomes in the first meiotic division;

Gene recombination based on the phenomenon of chromosome crossing (recombination chromosomes, once in the zygote, cause the appearance of signs that are not typical for parents);

Accidental meeting of gametes during fertilization.

At the core mutational variability mutations lie - persistent changes in the genotype that affect entire chromosomes, their parts or individual genes.

1) Types of mutations according to the consequences of influence on the body are divided into beneficial, harmful and neutral.

2) According to the place of origin, mutations can be generative if they arise in germ cells: they can manifest themselves in the generation that develops from germ cells. Somatic mutations occur in somatic (non-sex) cells. Such mutations can be transmitted to offspring only through asexual or vegetative reproduction.

3) Depending on which part of the genotype they affect, mutations can be:

Genomic, leading to a multiple change in the number of chromosomes, for example, polyploidy;

Chromosomal, associated with a change in the structure of chromosomes, the addition of an extra section due to a crossover, a rotation of a certain section of chromosomes by 180 °, or with a change in the number of individual chromosomes. Thanks to chromosomal rearrangements, the evolution of the karyotype occurs, and individual mutants that have arisen as a result of such rearrangements may be more adapted to the conditions of existence, multiply and give rise to a new species;

Gene mutations are associated with a change in the sequence of nucleotides in a DNA molecule. This is the most common type of mutation.

4) According to the method of occurrence, mutations are divided into spontaneous and induced.

Spontaneous mutations occur in natural conditions under the influence of mutagenic environmental factors without human intervention.

Induced mutations occur when mutagenic factors are directed to the body. Physical mutagens include various types of radiation, low and high temperatures; to chemical - various chemical compounds; to biological - viruses.

So, mutations are the main source of hereditary variability - a factor in the evolution of organisms. Due to mutations, new alleles appear (they are called mutant). However, most mutations are harmful to living beings, because they reduce their fitness, the ability to produce offspring. Nature makes many mistakes, creating, thanks to mutations, many modified genotypes, but at the same time, it always unmistakably and automatically selects those genotypes that give the phenotype most adapted to certain environmental conditions.

Thus, the mutation process is the main source of evolutionary change.

2. Give general characteristics class Dicotyledonous plants. What is the significance of dicotyledonous plants in nature, human life?

Class dicotyledonous plants Plants in which the seed embryo contains

two cotyledons.

Dicot class - 325 families.

Consider large families of dicotyledonous plants.

Family	Features of a flower, inflorescence	flower formula	Fetus	Representatives
Compositae	Flowers - small, tubular and reed-shaped - asymmetric. Inflorescence - basket.	Ch (5) L 5 Tn P 1 - tubular flowers Ch (5) L 5 Tn P 1 - reed flowers	seed, nut	Herbaceous plants (medicinal and oilseeds) - dandelion, chicory, cornflower, chamomile, aster and many others.
cruciferous	Perianth - four-membered. Inflorescence raceme, rarely in the form of a shield.	W 4 L 4 T 4+2 R 1	Pod, pod	Annual and perennial herbaceous plants - turnip, radish, turnip, radish, swede, cabbage and many others.
Rosaceae	Flowers are solitary	P (5) L 5 Tn P 1 P 5+5 L 5 Tn P 1	Drupe, compound drupe, polynutlet, apple	Herbs, shrubs, trees. Rosehip, raspberry, strawberry, plum, apple tree, pear and many others.
Legumes	brush head	W 5 L 1+2+(2) T (9)+1 P 1	Bean	Shrubs. Herbaceous plants - beans, peas, lentils, peanuts, clover, alfalfa, lupins and many others.
Nightshade	Single flowers or inflorescences - brush, curl	W (5) L (5) T (5) R 1	berry, box	Trees. Herbaceous plants - eggplants, tomatoes, peppers, potatoes, nightshade, dope, henbane and many others. others

SIGNIFICANCE IN NATURE: - plants of this class are producers in ecosystems, i.e. they photosynthesize organic substances; - these plants are the beginning of all food chains; - these plants determine the type of biogeocenosis (birch forest, fireweed steppe); They are active participants in the cycle of substances and water.

SIGNIFICANCE IN HUMAN LIFE: - among the plants of the Dicotyledonous class, there are many cultivated plants whose organs are used for human food (the Rosaceae family - cherry, apple, plum, raspberry, family Compositae - sunflower, family Solanaceae - tomato, potato, pepper, family. Cruciferous - various varieties of cabbage, legumes - peas, soybeans, beans) - many plants are used for livestock feed; - in the production of natural threads (linen, cotton); - as cultural and decorative (acacia, roses); - Medicinal (mustard, chamomile, nettle, thermopsis). There are also many spices among this class, they produce tobacco, coffee, tea, cocoa, dyes, ropes, ropes, paper, wooden utensils, furniture, musical instruments; - the wood of some dicotyledons (oak, hornbeam, linden) is invaluable for construction.

hereditary variability- this is a form of variability caused by changes in the genotype, which may be associated with mutational or combinative variability.

Mutational variability

Genes undergo changes from time to time, which are called mutations. These changes are random and appear spontaneously. The causes of mutations can be very diverse. Available whole line factors that increase the likelihood of mutations. It may be the effect of certain chemical substances, radiation, temperature, etc. With the help of these means it is possible to cause mutations, however, the random nature of their occurrence is preserved and it is impossible to predict the appearance of a particular mutation.

The resulting mutations are transmitted to descendants, i.e., determine hereditary variability, with one important caveat related to where the mutation occurred. If a mutation occurs in a germ cell, then it has the ability to be passed on to descendants, that is, to be inherited. If a mutation occurs in a somatic cell, then it is transmitted only to those cells that arise from this somatic cell. Such mutations are called somatic, they are not inherited.

There are several main types of mutations:

1. Gene mutations in which changes occur at the level of individual genes, i.e. sections of the DNA molecule. This may be the loss of nucleotides, the replacement of one base with another, the rearrangement of nucleotides, or the addition of new ones.
2. Chromosomal mutations associated with a violation of the structure of chromosomes. They lead to serious changes that can be detected even with a microscope. Such mutations include loss of chromosome sections (deletions), addition of sections, rotation of a chromosome section by 180°, and the appearance of repeats.
3. Genomic mutations caused by a change in the number of chromosomes. Extra homologous chromosomes may appear, in the chromosome set in place of two homologous chromosomes there are three - trisomy. In the case of monosomy, there is a loss of one chromosome from a pair. With polyploidy, a multiple increase in the genome occurs. Another variant of genomic mutation is haploidy, in which only one chromosome from each pair remains.

The frequency of mutations is affected, as already mentioned, by a variety of factors. When a number of genomic mutations occur great importance has, in particular, the age of the mother.

Heredity and variability. Combination variability

This type of variability is determined by the nature of the sexual process. With combinative variability, new genotypes arise due to new combinations of genes. This type of variability is manifested already at the stage of formation of germ cells. As already mentioned, each sex cell (gamete) contains only one homologous chromosome from each pair. Chromosomes fall into the gamete in an absolutely random way, so the germ cells of one person can differ quite a lot in the set of genes in the chromosomes. An even more important stage for the emergence of combinative variability is fertilization, after which in a newly emerged organism 50% of the genes are inherited from one parent, and 50% from the other.

From the history

The idea that living beings are characterized by heredity and variability developed 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).

Knowledge of the presence of these properties was used in the development of new varieties of cultivated plants and breeds of domestic animals. Isstari in agriculture hybridization was used, that is, the crossing of organisms that differ from each other in some way. However, before late XIX V. 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 identified 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 outstripped the general level of the biological sciences of that time. Only in 1900, after the discovery of G. Mendel's laws anew and independently by three researchers (G. de Vries in Holland, K. Korrens in Germany and E. Cermak in Austria), the development of a new biological science - genetics, which studies patterns of heredity and variability. Gregor Mendel is rightly considered the founder of this young, but very rapidly developing science.

Heredity of organisms

The heredity of organisms is called the common property of all organisms to preserve and transmit structural features and functions from ancestors to offspring.

The relationship between parents and offspring in organisms is carried out mainly through reproduction. Offspring are always like parents and ancestors, but they are not an exact copy.

Everyone knows that an oak tree grows from an acorn, and cuckoo chicks hatch from eggs. From the seeds of cultivated plants of a certain variety, plants of the same variety grow. In domestic animals, descendants of the same breed retain their properties.

Why do offspring look like their parents? In Darwin's time, the causes of heredity were little understood. It is now known that the material basis of heredity is the genes located on the chromosomes. A gene is a section of a molecule organic matter DNA, under the influence of which signs are formed. In the cells of organisms different types contains units and tens of chromosomes and hundreds of thousands of genes.

Chromosomes with genes located in them are found both in germ cells and in the cells of the body. During sexual reproduction, the fusion of male and female gametes occurs. In the cells of the embryo, male and female chromosomes are combined, therefore, its formation occurs under the influence of the genes of both the maternal and paternal organisms. The development of some traits is more influenced by the genes of the maternal organism, others - by the paternal organism, and maternal and paternal genes have an equal influence on the third traits. Therefore, the offspring, in some respects, turns out to be similar to the mother's organism, in others - to the father's, in the third - it combines the signs of the father and mother, that is, it has an intermediate character.

The variability of organisms

The variability of organisms is called the general property of organisms to acquire new features - differences between individuals within a species.

All signs of organisms are changeable: features of external and internal structure, physiology, behavior, etc. In the offspring of one pair of animals or among plants grown from the seeds of one fruit, it is impossible to meet completely identical individuals. In a herd of sheep of the same breed, each animal differs in subtle features: body size, length of legs, head, color, length and density of the curl of wool, voice, habits. The number of marginal reed flowers in the inflorescences of the golden rod (composite family) ranges from 5 to 8. The number of petals of the oak anemone (buttercup family) is 6, and sometimes 7 and 8. Plants of the same species or variety differ somewhat from each other in terms of flowering, ripening fruits, degree of drought resistance, etc. Due to the variability of individuals, the population is heterogeneous.

Darwin distinguished two main forms of variability - non-hereditary and hereditary.

Non-hereditary or modification variability

It has long been noted that all individuals of a given breed, variety or species, under the influence of a certain cause, change in one direction. Varieties of cultivated plants, in the absence of the conditions in which they were bred by man, lose their qualities. For example, white cabbage, when cultivated in hot countries, does not form a head. It is known that with good fertilizer, watering, and lighting, plants bush abundantly and bear fruit. Breeds of horses brought to the mountains or to islands where the food is not nutritious become stunted over time. The productivity of outbred animals in conditions of improved maintenance and care increases. All these changes are non-hereditary, and if plants or animals are transferred to their original conditions of existence, then the signs again return to their original ones.

The causes of non-hereditary, or modification, variability of organisms at the time of Darwin were poorly understood. To date, it has been found out that the formation of an organism occurs both under the influence of genes and under the influence of environmental conditions. These conditions are the cause of non-hereditary, modification, variability. They can speed up or slow down growth and development, change the color of flowers in plants, but the genes do not change. Thanks to non-hereditary variability individuals of populations are adapted to changing environmental conditions.

hereditary variability

In addition to modification, there is another form of variability - the hereditary variability of organisms, which affects chromosomes or genes, that is, the material foundations of heredity. Hereditary changes were well known to Darwin, he assigned them a large role in evolution.

The causes of hereditary variation in Darwin's time were also little explored. It is now known that hereditary changes are due to changes in genes or the formation of new combinations of them in the offspring. So, one type of hereditary variability - mutations - is due to a change in genes; another species - combinative variability - is caused by a new combination of genes in the offspring; the third - correlative variability - is associated with the fact that the same gene influences the formation of not one, but two or more traits. Thus, the basis of all types of hereditary variability is a change in a gene or a set of genes.

Mutations can be minor and affect a variety of morphological and physiological features of the organism, for example, in animals - size, color, fertility, milkiness, etc. Sometimes mutations manifest themselves in more significant changes. Such changes were used to create fat-tailed, merino and astrakhan breeds of sheep, terry varieties of many ornamental plants, trees with weeping and pyramidal crowns. Known hereditary changes in strawberries with simple ovate leaves, celandine with dissected leaves.

Mutations can occur due to a variety of influences. The source of combinative variability in populations is crossing. Individual individuals of the same population differ somewhat from each other in genotype. As a result of free crossing, new combinations of genes are obtained.

Hereditary changes that have appeared in a population due to random causes gradually spread among individuals due to free crossing, and the population becomes saturated with them. These hereditary changes in themselves cannot lead to the emergence of a new population, let alone a new species, but they are necessary material for selection, a precondition for evolutionary change.

Even Darwin noted the correlative nature of hereditary variability. For example, the long limbs of animals are almost always accompanied by an elongated neck; hairless dogs have underdeveloped teeth; pigeons with feathered legs have webbing between the toes. In table varieties of beets, the color of the root crop, petioles and the underside of the leaves changes in a coordinated manner. In snapdragon with light corollas of flowers, the stem and leaves are green; with dark corollas - the stem and leaves are dark. Therefore, when selecting for one desired trait, one should take into account the possibility of the appearance in the offspring of other, sometimes undesirable, traits relatively associated with it.

Heredity and variability are different properties of organisms that determine the similarity and dissimilarity of offspring with parents and with more distant ancestors. Heredity expresses the stability of organic forms in a number of generations, and variability - their ability to transform.

Darwin repeatedly stressed the need for a deep development of the laws of variability and heredity. Later they became the subject of study of genetics.

Heredity- This the most important feature living organisms, which consists in the ability to transfer the properties and functions of parents to their descendants. This transmission is carried out with the help of genes.

A gene is a unit of storage, transmission and realization of hereditary information. A gene is a specific section of a DNA molecule, in the structure of which the structure of a certain polypeptide (protein) is encoded. Probably, many DNA regions do not encode proteins, but perform regulatory functions. In any case, in the structure of the human genome, only about 2% of DNA are sequences on the basis of which messenger RNA is synthesized (transcription process), which then determines the amino acid sequence during protein synthesis (translation process). It is currently believed that there are about 30,000 genes in the human genome.

Genes are located on chromosomes, which are located in the nuclei of cells and are giant DNA molecules.

Chromosomal theory of heredity was formulated in 1902 by Setton and Boveri. According to this theory, chromosomes are carriers of genetic information that determines the hereditary properties of an organism. In humans, each cell has 46 chromosomes, divided into 23 pairs. Chromosomes that form a pair are called homologous.

Sex cells (gametes) are formed using a special type of division - meiosis. As a result of meiosis, only one homologous chromosome from each pair remains in each germ cell, i.e. 23 chromosomes. Such a single set of chromosomes is called haploid. At fertilization, when the male and female sex cells merge and a zygote is formed, the double set, which is called diploid, is restored. In the zygote of the organism that develops from it, one chromosome from each nara is received from the paternal organism, the other from the maternal one.

A genotype is a set of genes received by an organism from its parents.

Another phenomenon that genetics studies is variability. Variability is understood as the ability of organisms to acquire new features - differences within a species. There are two types of change:
- hereditary;
- modification (non-hereditary).

hereditary variability- this is a form of variability caused by changes in the genotype, which can be associated with mutational or combinative variability.

mutational variability.
Genes undergo changes from time to time, which are called mutations. These changes are random and appear spontaneously. The causes of mutations can be very diverse. There are a number of factors that increase the likelihood of a mutation occurring. This may be exposure to certain chemicals, radiation, temperature, etc. Mutations can be caused by these means, but the random nature of their occurrence remains, and it is impossible to predict the appearance of a particular mutation.

The resulting mutations are transmitted to descendants, that is, they determine hereditary variability, which is associated with where the mutation occurred. If a mutation occurs in a germ cell, then it has the opportunity to be transmitted to descendants, i.e. be inherited. If the mutation occurred in a somatic cell, then it is transmitted only to those of them that arise from this somatic cell. Such mutations are called somatic, they are not inherited.

There are several main types of mutations.
- Gene mutations, in which changes occur at the level of individual genes, i.e. sections of the DNA molecule. This can be a waste of nucleotides, the replacement of one base with another, a rearrangement of nucleotides, or the addition of new ones.
- Chromosomal mutations associated with a violation of the structure of chromosomes lead to serious changes that can be detected using a microscope. Such mutations include loss of chromosome sections (deletions), addition of sections, rotation of a chromosome section by 180°, and the appearance of repeats.
- Genomic mutations are caused by a change in the number of chromosomes. Extra homologous chromosomes may appear: in the chromosome set, in place of two homologous chromosomes, there are three trisomy. In the case of monosomy, there is a loss of one chromosome from a pair. With polyploidy, a multiple increase in the genome occurs. Another variant of genomic mutation is haploidy, in which only one chromosome from each pair remains.

The frequency of mutations is affected, as already mentioned, by a variety of factors. When a number of genomic mutations occur, the age of the mother, in particular, is of great importance.

Combination variability.
This type of variability is determined by the nature of the sexual process. With combinative variability, new genotypes arise due to new combinations of genes. This type of variability is manifested already at the stage of formation of germ cells. As already mentioned, each sex cell (gamete) contains only one homologous chromosome from each pair. Chromosomes enter the gamete randomly, so the germ cells of one person can differ quite a lot in the set of genes in the chromosomes. An even more important stage for the emergence of combinative variability is fertilization, after which 50% of the genes of the newly emerged organism are inherited from one parent, and 50% from the other.

Modification variability is not associated with changes in the genotype, but is caused by the influence of the environment on the developing organism.

The presence of modification variability is very important for understanding the essence of inheritance. Traits are not inherited. You can take organisms with exactly the same genotype, for example, grow cuttings from the same plant, but place them in different conditions (light, humidity, mineral nutrition) and get quite different plants with different traits (growth, yield, leaf shape). and so on.). To describe the actually formed signs of an organism, the concept of "phenotype" is used.

The phenotype is the whole complex of actually occurring signs of an organism, which is formed as a result of the interaction of the genotype and environmental influences during the development of the organism. Thus, the essence of inheritance lies not in the inheritance of a trait, but in the ability of the genotype, as a result of interaction with developmental conditions, to give a certain phenotype.

Since modification variability is not associated with changes in the genotype, modifications are not inherited. Usually this position is for some reason difficult to accept. It seems that if, say, parents train for several generations in lifting weights and have developed muscles, then these properties must be passed on to children. Meanwhile, this is a typical modification, and training is the influence of the environment that influenced the development of the trait. No changes in the genotype occur during modification, and the traits acquired as a result of modification are not inherited. Darwin called this kind of variation - non-hereditary.

To characterize the limits of modification variability, the concept of the reaction norm is used. Some traits in a person cannot be changed due to environmental influences, such as blood type, gender, eye color. Others, on the contrary, are very sensitive to the effects of the environment. For example, as a result of prolonged exposure to the sun, the skin color becomes darker, and the hair lightens. The weight of a person is strongly influenced by the characteristics of nutrition, illness, the presence of bad habits, stress, lifestyle.

Environmental influences can lead not only to quantitative, but also to qualitative changes in the phenotype. In some species of primrose, at low air temperatures (15-20 C), red flowers appear, but if the plants are placed in a humid environment with a temperature of 30 ° C, then white flowers form.

moreover, although the reaction rate characterizes a non-hereditary form of variability (modification variability), it is also determined by the genotype. This provision is very important: the reaction rate depends on the genotype. The same influence of the environment on the genotype can lead to a strong change in one of its traits and not affect the other in any way.

The textbook complies with the Federal State Educational Standard for Secondary (Complete) General Education, is recommended by the Ministry of Education and Science of the Russian Federation and is 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.

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Remember!

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

What are mutations?

Variability- one of the most important properties of living things, 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 grown 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 traits 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 temperature environment the color of the coat 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 summer period In most people, under the influence of UV rays, a protective pigment, melanin, is deposited in the skin, people sunbathe.

In the same species of organisms, under the influence of environmental conditions, variability various signs 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 signs that provide vital important qualities organism.

Due to the fact that most modifications have an adaptive value, they contribute to adaptation - the adaptation of the organism 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. 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 the independent divergence of homologous chromosomes in the first meiotic division, chance meeting gametes in the same parental pair during fertilization and 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 ideas 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 occur abruptly, like new ones qualitative changes: awnless wheat was formed from awned 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. Most famous example Down syndrome is a genomic mutation, a developmental disorder that occurs when an extra 21st chromosome appears. 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. This change in the 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 are analogues. 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 to control pests and weeds (pesticides and herbicides), waste products from industrial enterprises, certain food dyes and preservatives, some drugs, tobacco smoke components have a mutagenic effect.

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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