Scheme of the evolutionary role of different forms of hereditary variability. Heredity and variability

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 the individual life of an organism, under the influence of environmental factors, two types of changes can occur in it: 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:

mutation occurs suddenly, without any transitions;
the new forms are completely constant, that is, they are stable;
Mutations, unlike non-hereditary changes (fluctuations), do not form continuous series, they are not grouped around an average type (mode). Mutations are qualitative changes;
mutations go in different directions, they can be both beneficial and harmful;
mutation detection depends on the number of individuals analyzed for mutation detection;
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.” As we will see later, modern natural science is 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 patterns 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 those cases when mutations arise 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.

In Darwin's evolutionary theory, the prerequisite for evolution is hereditary variability, and the driving forces of evolution are the struggle for existence and natural selection. When creating the evolutionary theory, Ch. Darwin repeatedly refers to the results of breeding practice. He showed that the diversity of varieties and breeds is based on variability. Variability is the process of the emergence of differences in descendants compared to ancestors, which determine the diversity of individuals within a variety or breed. Darwin believes that the causes of variability are the impact on organisms of environmental factors (direct and indirect), as well as the nature of the organisms themselves (since each of them reacts specifically to the impact of the external environment). Variability serves as the basis for the formation of new features in the structure and functions of organisms, and heredity reinforces these features. Darwin, analyzing the forms of variability, singled out three of them: definite, indefinite and correlative.

A certain, or group, variability is a variability that occurs under the influence of some environmental factor that acts equally on all individuals of a variety or breed and changes in a certain direction. Examples of such variability are an increase in body weight in animal individuals with good feeding, a change in the hairline under the influence of climate, etc. A certain variability is massive, covers the entire generation and is expressed in each individual in a similar way. It is not hereditary, that is, in the descendants of the modified group, under other conditions, the traits acquired by the parents are not inherited.

Indefinite, or individual, variability manifests itself specifically in each individual, i.e. unique, individual in nature. It is associated with differences in individuals of the same variety or breed under similar conditions. This form of variability is indefinite, i.e., a trait under the same conditions can change in different directions. For example, in one variety of plants, specimens appear with different colors of flowers, different intensity of color of petals, etc. The reason for this phenomenon was unknown to Darwin. Indefinite variability is hereditary, that is, it is stably transmitted to offspring. This is its importance for evolution.

With correlative, or correlative, variability, a change in any one organ causes changes in other organs. For example, dogs with poorly developed coats usually have underdeveloped teeth, pigeons with feathered legs have webbing between the fingers, pigeons with a long beak usually have long legs, white cats with blue eyes are usually deaf, etc. Of the factors of correlative variability, Darwin makes important conclusion: a person, selecting any feature of the structure, almost "probably will unintentionally change other parts of the body on the basis of the mysterious laws of correlation."

Having determined the forms of variability, Darwin came to the conclusion that only heritable changes are important for the evolutionary process, since only they can accumulate from generation to generation. According to Darwin, the main factors in the evolution of cultural forms are hereditary variability and human selection (Darwin called such selection artificial). Variability is a necessary prerequisite for artificial selection, but it does not determine the formation of new breeds and varieties.

Forms of natural selection

Selection proceeds continuously over an endless series of successive generations and preserves mainly those forms that are more suitable for given conditions. Natural selection and elimination of some individuals of a species are inextricably linked and are a necessary condition for the evolution of species in nature.

The scheme of action of natural selection in the species system according to Darwin is as follows:

1) Variability is inherent in any group of animals and plants, and organisms differ from each other in many respects;

2) The number of organisms of each species that are born into the world exceeds the number of those that can find food and survive. However, since the abundance of each species is constant under natural conditions, it should be assumed that most of the offspring perish. If all the descendants of any species survived and multiplied, they would very soon supplant all other species on the globe;

3) Since more individuals are born than can survive, there is a struggle for existence, competition for food and habitat. This may be an active struggle not for life, but for death, or less obvious, but no less effective competition, as, for example, for plants during a period of drought or cold;

4) Among the many changes observed in living beings, some facilitate survival in the struggle for existence, while others lead to the fact that their owners die. The concept of "survival of the fittest" is the core of the theory of natural selection;

5) Surviving individuals give rise to the next generation, and thus "successful" changes are transmitted to subsequent generations. As a result, each next generation is more adapted to the environment; as the environment changes, further adaptations occur. If natural selection has been operating for many years, then the last offspring may turn out to be so dissimilar to their ancestors that it would be advisable to single them out as an independent species.

It may also happen that some members of a given group of individuals will acquire some changes and be adapted to the environment in one way, while other members of it, having a different set of changes, will be adapted in a different way; in this way, two or more species may arise from one ancestral species, provided that such groups are isolated.

driving selection

Natural selection always leads to an increase in the average fitness of populations. Changes in external conditions can lead to changes in the fitness of individual genotypes. In response to these changes, natural selection, using a huge store of genetic diversity for many different traits, leads to significant shifts in the genetic structure of the population. If the external environment is constantly changing in a certain direction, then natural selection changes the genetic structure of the population in such a way that its fitness in these changing conditions remains maximum. In this case, the frequencies of individual alleles in the population change. The average values of adaptive traits in populations also change. In a number of generations, their gradual shift in a certain direction can be traced. This form of selection is called driving selection.

A classic example of motive selection is the evolution of color in the birch moth. The color of the wings of this butterfly imitates the color of the bark of trees covered with lichens, on which it spends daylight hours. Obviously, such a protective coloration was formed over many generations of previous evolution. However, with the beginning of the industrial revolution in England, this device began to lose its importance. Atmospheric pollution has led to the mass death of lichens and the darkening of tree trunks. Light butterflies on a dark background became easily visible to birds. Since the middle of the 19th century, mutant dark (melanistic) forms of butterflies began to appear in populations of the birch moth. Their frequency increased rapidly. By the end of the 19th century, some urban populations of the moth were almost entirely composed of dark forms, while light forms still predominated in rural populations. This phenomenon has been called industrial melanism. Scientists have found that in polluted areas, birds are more likely to eat light forms, and in clean areas - dark ones. The imposition of restrictions on atmospheric pollution in the 1950s caused natural selection to change direction again, and the frequency of dark forms in urban populations began to decline. They are almost as rare today as they were before the Industrial Revolution.

Driving selection brings the genetic composition of populations in line with changes in the external environment so that the average fitness of populations is maximum. On the island of Trinidad, guppy fish live in different water bodies. Many of those that live in the lower reaches of the rivers and in the ponds perish in the teeth of predatory fish. In the upper reaches, life for guppies is much calmer - there are few predators. These differences in environmental conditions led to the fact that the "top" and "grassroots" guppies evolved in different directions. The "grassroots", which are under constant threat of extermination, begin to breed at an earlier age and produce many very small fry. The chance of survival of each of them is very small, but there are a lot of them and some of them have time to multiply. "Horse" reach puberty later, their fertility is lower, but the offspring are larger. When the researchers transferred the "grassroots" guppies to uninhabited reservoirs in the upper reaches of the rivers, they observed a gradual change in the type of development of the fish. 11 years after the move, they became much larger, entered breeding later and produced fewer but larger offspring.

The rate of change in the frequencies of alleles in a population and the average values of traits under the action of selection depends not only on the intensity of selection, but also on the genetic structure of the traits that are being selected. Selection against recessive mutations is much less effective than against dominant ones. In the heterozygote, the recessive allele does not appear in the phenotype and therefore eludes selection. Using the Hardy-Weinberg equation, one can estimate the rate of change in the frequency of a recessive allele in a population depending on the intensity of selection and the initial ratio of frequencies. The lower the allele frequency, the slower its elimination occurs. In order to reduce the frequency of recessive lethality from 0.1 to 0.05, only 10 generations are needed; 100 generations - to reduce it from 0.01 to 0.005 and 1000 generations - from 0.001 to 0.0005.

The driving form of natural selection plays a decisive role in the adaptation of living organisms to external conditions that change over time. It also ensures the wide distribution of life, its penetration into all possible ecological niches. It is a mistake to think, however, that under stable conditions of existence, natural selection ceases. Under such conditions, it continues to act in the form of stabilizing selection.

Stabilizing selection

Stabilizing selection preserves the state of the population, which ensures its maximum fitness under constant conditions of existence. In each generation, individuals that deviate from the average optimal value in terms of adaptive characteristics are removed.

Many examples of the action of stabilizing selection in nature have been described. For example, at first glance it seems that individuals with maximum fecundity should make the greatest contribution to the gene pool of the next generation. However, observations of natural populations of birds and mammals show that this is not the case. The more chicks or cubs in the nest, the more difficult it is to feed them, the smaller and weaker each of them. As a result, individuals with average fecundity turn out to be the most adapted.

Selection in favor of averages has been found for a variety of traits. In mammals, very low and very high birth weight newborns are more likely to die at birth or in the first weeks of life than middle weight newborns. Accounting for the size of the wings of birds that died after the storm showed that most of them had too small or too large wings. And in this case, the average individuals turned out to be the most adapted.

What is the reason for the constant appearance of poorly adapted forms in constant conditions of existence? Why is natural selection unable to once and for all clear a population of unwanted evasive forms? The reason is not only and not so much in the constant emergence of more and more new mutations. The reason is that heterozygous genotypes are often the fittest. When crossing, they constantly give splitting and homozygous descendants with reduced fitness appear in their offspring. This phenomenon is called balanced polymorphism.

sexual selection

In males of many species, pronounced secondary sexual characteristics are found that at first glance seem maladaptive: the tail of a peacock, the bright feathers of birds of paradise and parrots, the scarlet combs of roosters, the enchanting colors of tropical fish, the songs of birds and frogs, etc. Many of these features make life difficult for their carriers, making them easily visible to predators. It would seem that these signs do not give any advantages to their carriers in the struggle for existence, and yet they are very widespread in nature. What role did natural selection play in their origin and spread?

It is known that the survival of organisms is an important, but not the only component of natural selection. Another important component is attractiveness to members of the opposite sex. C. Darwin called this phenomenon sexual selection. He first mentioned this form of selection in The Origin of Species and later analyzed it in detail in The Descent of Man and Sexual Selection. He believed that "this form of selection is determined not by the struggle for existence in the relationship of organic beings among themselves or with external conditions, but by the rivalry between individuals of the same sex, usually males, for the possession of individuals of the other sex."

Sexual selection is natural selection for success in reproduction. Traits that reduce the viability of their carriers can emerge and spread if the advantages they provide in breeding success are significantly greater than their disadvantages for survival. A male that lives a short time but is liked by females and therefore produces many offspring has a much higher cumulative fitness than one that lives long but leaves few offspring. In many animal species, the vast majority of males do not participate in reproduction at all. In each generation, fierce competition for females arises between males. This competition can be direct, and manifest itself in the form of a struggle for territories or tournament fights. It can also occur in an indirect form and be determined by the choice of females. In cases where females choose males, male competition is shown in displaying their flamboyant appearance or complex courtship behavior. Females choose those males that they like the most. As a rule, these are the brightest males. But why do females like bright males?

The fitness of the female depends on how objectively she is able to assess the potential fitness of the future father of her children. She must choose a male whose sons will be highly adaptable and attractive to females.

Two main hypotheses about the mechanisms of sexual selection have been proposed.

According to the “attractive sons” hypothesis, the logic of female selection is somewhat different. If bright males, for whatever reason, are attractive to females, then it is worth choosing a bright father for your future sons, because his sons will inherit the bright color genes and will be attractive to females in the next generation. Thus, a positive feedback occurs, which leads to the fact that from generation to generation the brightness of the plumage of males is more and more enhanced. The process goes on increasing until it reaches the limit of viability. Imagine a situation where females choose males with a longer tail. Long-tailed males produce more offspring than males with short and medium tails. From generation to generation, the length of the tail increases, because females choose males not with a certain tail size, but with a larger than average size. Eventually, the tail reaches a length where its detriment to the viability of the male is balanced by its attractiveness in the eyes of the females.

In explaining these hypotheses, we tried to understand the logic of the action of female birds. It may seem that we expect too much from them, that such complex fitness calculations are hardly accessible to them. In fact, in choosing males, females are no more and no less logical than in all other behaviors. When an animal feels thirsty, it does not reason that it should drink water in order to restore the water-salt balance in the body - it goes to the watering place because it feels thirsty. When a worker bee stings a predator attacking a hive, she does not calculate how much by this self-sacrifice she increases the cumulative fitness of her sisters - she follows instinct. In the same way, females, choosing bright males, follow their instincts - they like bright tails. All those who instinctively prompted a different behavior, all of them left no offspring. Thus, we discussed not the logic of females, but the logic of the struggle for existence and natural selection - a blind and automatic process that, acting constantly from generation to generation, has formed all that amazing variety of shapes, colors and instincts that we observe in the world of wildlife. .

variability called the common property of all living organisms to acquire differences between individuals of the same species.

Ch. Darwin singled out the following main types of variability: definite (group, non-hereditary, modification), indefinite (individual, hereditary, mutational) and combined. Hereditary variability includes such changes in the characteristics of living beings that are associated with changes in (i.e., mutations) and are transmitted from generation to generation. The transfer of material from parents to offspring must be very accurate, otherwise the species cannot be preserved. However, sometimes there are quantitative or qualitative changes in the DNA, and the daughter cells get distorted compared to the parental genes. Such errors in the hereditary material are passed on to the next generation and are called mutations. An organism that has received new properties as a result of mutations is called a mutant. Sometimes these changes are clearly visible phenotypically, for example, the absence of pigments in the skin and hair - albinism. But most often, mutations are recessive and appear in the phenotype only when they are present in the homozygous state. The existence of hereditary changes was known. All of it follows from the doctrine of hereditary changes. Hereditary variability is a necessary prerequisite for natural and. However, at the time of Darwin there were still no experimental data on heredity and the laws of inheritance were not known. This made it impossible to strictly distinguish between different forms of variability.

mutation theory was developed in the early twentieth century by the Dutch cytologist Hugo de Vries. have a number of properties:

Mutations occur suddenly, and any part of the genotype can mutate.
Mutations are more often recessive and less often dominant.
Mutations can be harmful, neutral or beneficial to the organism.
Mutations are passed down from generation to generation.
Mutations can take place under the influence of both external and internal influences.

Mutations are divided into several types:

Point (gene) mutations are changes in individual genes. This can happen when one or more nucleotide pairs in a DNA molecule are replaced, dropped or inserted.
Chromosomal mutations are changes in parts of a chromosome or whole chromosomes. Such mutations can occur as a result of deletion - loss of part of the chromosome, duplication - doubling of any part of the chromosome, inversion - rotation of the chromosome section by 1800, translocation - tearing off part of the chromosome and moving it to a new position, for example, attachment to another chromosome.
mutations consist in changing the number of chromosomes in the haploid set. This can occur due to the loss of a chromosome from the genotype, or, conversely, an increase in the number of copies of any chromosome in the haploid set from one to two or more. A special case of genomic mutations - polyploidy - an increase in the number of chromosomes by a factor. The concept of mutations was introduced into science by the Dutch botanist de Vries. In an aspen (primrose) plant, he observed the appearance of sharp, spasmodic deviations from the typical form, and these deviations turned out to be hereditary. Further studies on various objects - plants, animals, microorganisms showed that the phenomenon of mutational variability is characteristic of all organisms.
Chromosomes are the material basis of the genotype. Mutations are changes that occur in chromosomes under the influence of external factors or. Mutational variability is newly occurring changes in the genotype, while combinations are new combinations of parental genes in the zygote. Mutations affect various aspects of the structure and functions of the body. For example, in Drosophila, mutational changes in the shape of the wings (up to their complete disappearance), body color, development of bristles on the body, shape of the eyes, their color (red, yellow, white, cherry), as well as many physiological signs (lifespan, fertility) are known. ).

They take place in different directions and in themselves are not adaptive, beneficial changes for the body.

Many emerging mutations are unfavorable for the organism and can even cause its death. Most of these mutations are recessive.

Most mutants have reduced viability and are weeded out by natural selection. Evolution or new breeds and varieties require those rare individuals that have favorable or neutral mutations. the significance of mutations lies in the fact that they create hereditary changes that are the material for natural selection in nature. Mutations are also necessary for individuals with new properties valuable to humans. Artificial mutagenic factors are widely used to obtain new breeds of animals, plant varieties and strains of microorganisms.

Combination variability also refers to hereditary forms of variability. It is due to the rearrangement of genes during the fusion of gametes and the formation of a zygote, i.e. during the sexual process.

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. From time immemorial, hybridization has been used in agriculture, that is, the crossing of organisms that differ from each other in some way. However, until the end of the XIX century. such work was carried out by trial and error, since the mechanisms underlying the manifestation of such properties of organisms were not known, and the hypotheses that existed in this regard were purely speculative.

In 1866, the work of Gregor Mendel, a Czech researcher, "Experiments on Plant Hybrids" was published. It described the patterns of inheritance of traits in the generations of plants of several species, which G. Mendel 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.

Basic concepts of modern genetics.

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

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

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

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

Types of variability

Distinguish between hereditary and non-hereditary variability.

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

reaction rate

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

Combination variability

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

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

2. Crossing over.

3. Random combination of gametes during fertilization.

4. Random selection of parental organisms.

Mutations

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

Mutations are:

1) spontaneous and induced;

2) harmful, useful and neutral;

3) somatic and generative;

4) gene, chromosomal and genomic.

Spontaneous mutations are mutations that have arisen undirectedly, under the influence of an unknown mutagen.

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

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

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

2. Deletion - loss of a chromosome segment.

3. Inversion - rotation of a chromosome segment by 180 °.

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

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

1. Polyploidy - a change in the number of haploid sets of chromosomes in a karyotype. Under the karyotype understand the number, shape and number of chromosomes characteristic of a given species. Nullisomy (the absence of two homologous chromosomes), monosomy (the absence of one of the homologous chromosomes) and polysomy (the presence of two or more extra chromosomes) are distinguished.

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

Gene mutations are the most common.

Causes of gene mutations:

1) nucleotide dropout;

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

3) replacement of one nucleotide by another.

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

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

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

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

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

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

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