Hereditary variability, its types. Types of mutations, their causes. The role of mutations in the evolution of the organic world and selection. Variability, its types and biological significance

In our article we will talk about unique property all living organisms, which ensured the emergence of a huge number of species of living beings. This is hereditary variation. What is it, what are its features and mechanism of implementation? You will now find answers to these and many other questions.

What does genetics study

The relatively young science of genetics in the 19th century revealed to mankind many secrets of its origin and development. And the subject of its study are only two properties of living organisms: heredity and variability. Thanks to the first, the continuity of generations is ensured and the exact transfer of genetic information is carried out in a number of generations. But variability provides the emergence of new features.

Variability value

Why would an organism acquire these new traits? The answer is quite simple: for the possibility of adaptation. In the photo below you see representatives of several races of the same biological species - Homo sapiens. Their morphological differences at this stage, of course, have no adaptive significance. But their distant ancestors, new features helped to survive in difficult conditions. Thus, representatives of the Mongoloid race have a narrow section of the eyes, since there were often dust storms in the steppes. And Negroids have dark skin as protection from the scorching sun.

Types of variability

Variability is the property of organisms to acquire new features in the course of their historical and individual development. It is of two types. This is a modification and hereditary variability. They are united by a number of features. For example, changes inevitably occur in the external structure of organisms. But in terms of the duration of the existence of modifications and the degree of action, they are absolutely different.

Modification variability

This type of variability is non-hereditary. It is not fixed in the genotype, is not permanent and occurs under the influence of changes in conditions. environment. A prime example modification variability the well-known experiment with the rabbit may serve. He was shaved off a small patch of gray wool. Ice was applied to bare skin. After some time, wool grew in this place. white color, which was also shaved off. But ice was not applied in this case. As a result, dark-colored hair grew back in this area.

hereditary variability

This type of variability is permanent, since it affects the structure of the genotype down to the level of DNA nucleotides. At the same time, new traits are passed on to new generations. hereditary variability, in turn, is also of two types: combinative and mutational. The first occurs when a new combination of genetic material appears. Her most simple example is the fusion of gametes during sexual reproduction. As a result, the body, receiving half of the genetic information from the male and female body, acquires new features.

The second type is mutational hereditary variability. It consists in the occurrence of sharp non-directional changes in the genotype under the influence of various factors. They can be ionizing and ultraviolet radiation, high temperature, nitrogen-containing chemicals and others.

Depending on the level of the structure of the genetic apparatus in which changes occur, several types of such hereditary modifications are distinguished. With genomic changes in the number of chromosomes in the total set. This leads to anatomical and morphological changes in the body. Thus, the appearance of a third chromosome in the 21st pair causes Down's disease. With chromosomal mutations, a rearrangement of this structure occurs. They are much rarer than genomic ones. Sections of chromosomes can be duplicated or absent, twisted, change their position. But gene mutations, which are also called point mutations, disrupt the sequence of monomers in the structure of nucleic acids.

Regardless of the type of mutations, all of them, as a rule, do not carry beneficial features for the body. Therefore, a person learns to manage them artificially. So, in breeding, polyploidy is often used - a multiple increase in the number of chromosomes in a set. As a result, the plant becomes more powerful and produces large fruits in large quantities. You will not surprise anyone with fig peach and other delicious plant hybrids. But they are the result of artificially carried out hereditary variability.

Hereditary variability in the process of evolution

The development of genetics has helped to make a significant step forward in the development of evolutionary doctrine. The fact that only one pair of chromosomes distinguishes man and ape was a significant proof of Darwin's theory. In plants and animals in historical development, one can trace the inheritance of progressive traits that were transmitted and fixed in the genotype. For example, algae came to land due to the fact that the sign of the presence of mechanical and conductive tissues was fixed in the genotype. Each subsequent generation left for itself only the necessary, useful features, which were adjusted depending on the living conditions and the environment. This is how the dominant species of plants and animals appeared, possessing the most progressive features of the structure.

So, hereditary variability is the ability of organisms to acquire new traits that are fixed in the genotype. Such changes are of a long-term nature, do not disappear when environmental conditions change, and are inherited.

1. What is heredity?

Answer. Heredity is the property of organisms to repeat in a number of generations similar types of metabolism and individual development in general. It is provided by self-reproduction of the material units of heredity - genes localized in specific structures of the cell nucleus (chromosomes) and cytoplasm. Together with variability, heredity ensures the constancy and diversity of life forms and underlies the evolution of living nature.

2. What is variability?

Answer. Variability - a variety of signs and properties in individuals and groups of individuals of any degree of kinship. Variation is inherent in all living organisms. Distinguish variability: hereditary. and non-hereditary. individual and group. Hereditary variability is caused by the occurrence of mutations, non-hereditary - by the influence of environmental factors. The phenomena of heredity and variability underlie evolution.

Questions after § 46

1. What types of variability do you know?

Answer. There are two types of variability: modification (phenotypic) and hereditary (genotypic).

Changes in the traits of an organism that do not affect its genes and cannot be passed on to the next generations are called modification, and this type of variability is called modification.

The following main characteristics of modification variability can be listed:

– modification changes are not passed on to descendants;

- modification changes occur in many individuals of the species and depend on the impact of the environment;

- modification changes are possible only within the limits of the reaction norm, i.e., ultimately they are determined by the genotype

Hereditary variability is due to changes in the genetic material and is the basis of the diversity of living organisms, as well as main reason evolutionary process, as it supplies material for natural selection.

The occurrence of changes in the hereditary material, i.e., in DNA molecules, is called mutational variability. Moreover, changes can occur both in individual molecules (chromosomes), and in the number of these molecules. Mutations occur under the influence of various factors of the external and internal environment.

2. What are the main signs of modification variability?

Answer. Most often, quantitative signs are subject to modifications - height, weight, fertility, etc. A classic example modification variability can be the variability of the shape of the leaves of the arrowhead plant, which takes root under water. One arrowhead individual has three types of leaves, depending on where the leaf develops: under water, on the surface or in the air. These differences in the shape of the leaves are determined by the degree of their illumination, and the set of genes in the cells of each leaf is the same.

For various signs and properties of the organism, a greater or lesser dependence on environmental conditions is characteristic. For example, in humans, the color of the iris and the blood type are determined only by the corresponding genes, and living conditions cannot influence these signs. But height, weight, physical endurance are highly dependent on external conditions e.g. nutrition, exercise, etc.

3. What is the reaction rate?

Answer. The limits of modification variability of any trait are called the reaction norm. The reaction rate is genetically determined and inherited.

The variability of a trait is sometimes very large, but it cannot go beyond the limits of the reaction norm. In some traits, the reaction rate is very wide (for example, shearing wool from sheep, milk yield of cows), while other features are characterized by a narrow reaction rate (hair color in rabbits).

A very important conclusion follows from the above. It is not the trait itself that is inherited, but the ability to express this trait in certain conditions, in other words, the norm of the body's reaction to external conditions is inherited

4. What forms of hereditary variability do you know?

Answer. Hereditary variability manifests itself in two forms - combinative and mutational.

Mutational variability is a change in the DNA of a cell (a change in the structure and number of chromosomes). Arise under the influence of ultraviolet, radiation (X-rays), etc. They are inherited, serve as material for natural selection (the mutation process is one of driving forces evolution).

Combination variability occurs when the genes of the father and mother are recombined (mixing). Sources:

1) Crossing over during meiosis (homologous chromosomes closely approach and change areas).

2) Independent divergence of chromosomes during meiosis.

3) Random fusion of gametes during fertilization.

5. What are the reasons for combinative variability?

Answer. The basis of combinative variability is the sexual process, which results in a huge set of diverse genotypes.

Let's take a human as an example. Each human cell contains 23 maternal and 23 paternal chromosomes. During the formation of gametes, only 23 chromosomes will fall into each of them, and how many of them will be from the father and how many from the mother is a matter of chance. This is the first source of combinative variability.

The second reason is crossing over. Not only does each of our cells carry the chromosomes of grandparents, a certain part of these chromosomes received, as a result of crossing over, part of their genes from homologous chromosomes that previously belonged to another line of ancestors. Such chromosomes are called recombinant. Participating in the formation of the organism of a new generation, they lead to unexpected combinations of features that neither the paternal nor the maternal organism had.

Finally, the third reason for combinative variability is the random nature of the meetings of certain gametes in the process of fertilization.

All three processes underlying combinative variability operate independently of each other, creating a huge variety of all possible genotypes.

Hereditary variability 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. This may be the effect of certain chemicals, radiation, temperature, etc. Using these tools, mutations can be caused, 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, 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, the age of the mother, in particular, is of great importance.

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.

LECTURE

TOPIC:Heredity and variability

LECTURE PLAN:

Heredity

Variability

1. hereditary variability

   Non-hereditary variability

1. Heredity

The development of the organic world largely depends on factors such as heredity and variability.heredity called the common property of all organisms to store and transmit their characteristics to offspring. Thanks to heredity, the specific qualities of each biological species are preserved from generation to generation.

The relationship between parents and offspring in organisms is carried out mainly through reproduction. Although offspring are similar to parents and ancestors, they are not their exact copy. The mechanism of heredity has long interested humanity. In 1866 G. Mendel expressed the opinion that the characteristics of organisms are determined by heritable units, which he called "elements". Later they were calledhereditary factors and finallygenes . Genes are located on chromosomes and they are passed from one generation to the next.

Despite the fact that much is now known about chromosomes and the structure of DNA, to give precise definition gene is still difficult. As a result of studying the nature of a gene, it can be defined as a unit of recombination, mutation and function.Gene - this is a factor of heredity, a functionally indivisible unit of genetic material in the form of a section of a nucleic acid molecule (DNA or RNA).It encodes a specific protein structure, t-RNA or r-RNA molecule, or interacts with biologically active substances (for example, enzymes). The gene is an integral functional unit, and any violation of its structure changes the information encoded in it or leads to its loss.

As a result of heredity, the body receives from the parents a set of genes, which is commonly calledgenotype . The eukaryotic genome is more complex than that of prokaryotes because it has large quantity nuclear DNA, structural and regulatory genes. In addition to the hereditary material located in the nucleus, there is alsocytoplasmic inheritance , ornon-nuclear . It lies in the ability of certain structures of the cytoplasm to store and transmit to descendants part of the hereditary information of the parents. Although the leading role in the inheritance of most of the characteristics of an organism belongs to nuclear genes, the role of cytoplasmic heredity is also significant. It is associated with two types of geneticphenomena:

Inheritance of traits that are encodedextranuclear genes located in certain organelles (mitochondria, plastids);

The manifestation in offspring of traits predetermined by nuclear genes, the formation of which is influenced byegg cytoplasm .

The existence of genes in organelles (mitochondria, plastids) capable of self-duplication became known at the beginning of the 20th century. during the study of green and colorless plastids in some flowering plants with mosaic leaf colors. Extranuclear genes, interacting with nuclear genes, influence the formation of a trait. For example, cytoplasmic inheritance associated with plastid genes affects such a trait as variegation in plants (begonia, snapdragon, etc.). And this trait is transmitted through the maternal line.

The cause of variegation is the loss of the ability of some plastids to form the pigment chlorophyll. After cell division with colorless plastids, white spots appear in the leaves, which alternate with green areas. The transmission of such a trait through the maternal line is explained by the fact that during the formation of gametes, plastids get to the eggs, and not to the sperm. When new plastids are formed, green plastids give rise to green, and colorless plastids give rise to colorless ones. During cell division, plastids are distributed randomly, resulting in cells with colorless, green, or both types of plastids at the same time..

The phenomenon of cytoplasmic inheritance associated with mitochondrial genes can be observed in yeast. In these microorganisms, genes were found in the mitochondria that predetermine the absence or presence of respiratory enzymes, as well as resistance to the action of certain antibiotics. The influence of the nuclear genes of the mother's organism through the cytoplasm of the egg on the formation of traits can also be traced on the example of a pond snail. This freshwater mollusk has forms with a different direction of twisting of the shell - left or right. The right-handed allele dominates over the left-handed allele, but this trait is predetermined by the maternal genes. For example, individuals homozygous for a recessive gene (left-handed) may have a right-handed shell if the mother had the dominant allele.

2. Variability

variabilitythey call the whole set of discrepancies on a particular trait between organisms that belong to the same population or species. There are two main forms of change:hereditaryAndnon-hereditary.

2.1. hereditary variability

Hereditary variability is called variability, which is transmitted from parents to offspring, i.e. inherited. Such variability is associated with a change in the genetic material caused by mutations. Therefore, hereditary variability is also calledgenotypic , genetic ormutational .

Mutation is a change in chromosomes that occurs under the influence of environmental factors. The concept of mutations was introduced into science by the Dutch botanist Hugo where Fries. They also formulatedmutation theory , a number of provisions of which belong to the famous Russian botanist S.I. Korzhinsky.

The main provisions of modern mutation theory :

Mutations occur suddenly, abruptly, and appear as discrete features;

Mutations are not lost and are passed down from generation to generation;

Mutations manifest themselves in different ways and can be dominant or recessive, beneficial or detrimental, differ in the strength of their effect on the body, cause minor changes in the functioning of the body, or affect vital signs and be lethal;

The probability of detecting mutations depends on the number of individuals studied;

Mutations can occur repeatedly;

Mutations can be caused by the influence of strong physical or chemical agents on the body, but the appearance of one or another mutation is not associated with the type of agent;

Mutations are always spontaneous, independent of one another, and do not have a group orientation. Any part of the chromosome can mutate.

Mutational variability, in contrast to modificational variability, is an important source of evolutionary transformations. Thanks to genetic variability, organisms with new properties and traits are formed, maintained high level and phenotypic variation.

Depending on the nature of the effect on the viability of organisms, there arelethal , sublethal Andneutral mutations. Lethal, as a rule, entail the death of organisms even before the moment of birth or before the onset of sexual maturity. Sublethal - reduce viability, leading to the death of some part (from 10 to 50%). Neutral mutations under normal conditions of existence for organisms do not affect their viability. And in some cases, such mutations can even become useful, especially when the conditions of the organism's existence change.

According to the nature of hereditary changes in the genetic material, three types of mutations are distinguished: gene, chromosomal, genomic.

Genetic ( point ) mutations – are qualitative changes in individual genes. These mutations occur at the level of the primary strand of DNA, and lead to a violation of the amino acid sequence in proteins. Such changes can have negative consequences for the body. After all, the amino acid sequence in each protein is strictly specific, and the replacement of even one of them can lead to a violation of the spatial structure of the protein and, accordingly, functions.

The most common case of a point mutation is the substitution of a nucleotide pair of GA for GC or vice versa. If these changes occur within structural genes, then instead of the AGA triplet, an AHC triplet may appear in the polypeptide chain, respectively, instead of a negatively charged amino acid.arginine will be an uncharged amino acidserine. Such a mutation can lead to a change in the charge of the protein, a violation of its conformation, and if it is an enzyme, then to a decrease in the rate of the chemical reaction that it catalyzes. As a result, failures in the metabolism of the whole organism may begin.

Substitutions can also be neutral, for example, substitutions of amino acids with the same properties. To extremely negative consequences result in stop codon mutations or mutations in the deletion or incorporation of one of the nucleotides. As a result, part or all of the sequence of triplets is changed, which contributes to a serious violation of the amino acid structure of the protein and this is almost always incompatible with the normal functioning of the body.

Chromosomal mutations - mutations associated with visible transformations of chromosomes. These can be moving one part of the chromosome to another, turning a part of the chromosome by 180°, embedding extra parts of the chromosome, or, conversely, the loss of any parts. In most cases, chromosomal rearrangements do not pass without consequences for the body. Most often they lead to death even in the very early stages of embryo development. If chromosomal changes do not concern genes that are responsible for important functions of the body, then they usually lead to violations of meiosis, which means infertility of the individual. However, there are also completely neutral chromosomal mutations.(chromosomal polymorphisms).

Genomic mutations associated with a change in the number of chromosomes. They are caused by gross violations of meiosis. One type of chromosomal mutation isaneuploidy- an increase in homologous chromosomes by one or more, or, conversely, a lack of most often one chromosome. Usually in animals, such disorders are incompatible with the normal functioning of the organism and lead either to death in the early stages or to numerous developmental disorders. A human hereditary disease, the so-called Down syndrome, is caused by the appearance of a third extra chromosome in the 21st pair. And the appearance of the third chromosome in the 15th pair causes another human hereditary anomaly - polydactyly - the appearance of the sixth finger on the limbs.

Genomic mutations associated with a multiple increase in the number of chromosomes are calledpolyploidy (from Greek.polyploethia- lots of, a large number of). If the number of chromosome sets increases by one, then it is a triploid, if by two, it is a tetraploid, etc. The largest increase in the number of chromosome sets found in organisms are organisms with a tenfold chromosome set.

Polyploidy contributes to an increase in the size of the organism, accelerates vital processes, and can cause disturbances in the process of reproduction. This is especially true for polyploid forms with an unpaired set of chromosomes, which can only reproduce by parthenogenesis or vegetatively.

Polyploidy is very common in nature. For the most part, it is represented by paroploid (tetra- or octoploid) forms, in which meiosis normally occurs. There are a lot of polyploid species among plants and much less of them among animals. Quite often they are found among invertebrates (crustaceans, mollusks, worms). There are polyploids among vertebrates. In fish, for example, there are even whole families (sturgeon) and orders (salmon-like), the species of which are exclusively polyploid. Polypoids occur less frequently in amphibians and reptiles, and in birds and mammals, such individuals die in the early stages of development.

Somatic mutations Mutations that occur only in individual somatic cells. In sexually reproducing organisms (most animals), such mutations are not inherited. Plants are another matter vegetative reproduction allows you to save the change that has occurred and make it hereditary.

Most mutations that occur in an organism tend to be recessive, and wild type(this is the name of the usual phenotype characteristic of individuals that live in natural conditions) - dominant. For example,albinism(from lat.albus- white) - a recessive trait that manifests itself in the homozygous (aa) state in the form of a lack of skin pigment, hair, in the iris of the eyes. As it turned out, in albino individuals, the tyrosinase enzyme, which catalyzes the formation of melanin pigment, does not function. Heterozygous individuals (Aa) have a wild color.

Dominant mutations also appear in the heterozygous state, but they occur much less frequently than recessive ones. The consequence of such mutations are, for example, most cases of the appearance of melanistic animals, in which, unlike non-mutated individuals, a lot of melanin is synthesized. Usually such organisms have a darker color..

Another an important factor genetic variation isrecombination (from lat.re– a prefix that indicates a repeated action andcombinare,- connection) - the redistribution of genetic material in the offspring. The main causes of gene recombination are:

The combination of gametes from different parents in the case of random breeding in animals and cross-pollination in plants;

Independent distribution of chromosomes after the first division of meiosis;

Crossing over - exchange of regions of homologous chromosomes during conjugation in metaphaseI meiosis.

As a result of sexual reproduction, recombination leads to the formation of offspring with a wide variety of genotypic combinations. As a result, it is impossible to meet two genetically identical individuals in one population. Recombination plays an important role in the evolution of organisms. Its properties are used in the process of breeding new varieties of plants and animal breeds..

2.2. Non-hereditary variability

The development of the phenotype of an organism occurs when its hereditary basis - the genotype - interacts with environmental conditions. The signs of the organism vary to varying degrees under the influence of various environmental factors. Some of them are very plastic and changeable, others are less changeable, others practically do not change under the influence of environmental conditions. For example, milk cattle largely depends on the conditions of detention (feeding, care). While the fat content of milk is more breed dependent and difficult to change, it is possible to achieve some results by changing the diet. Even more constant sign is the suit. Under all possible conditions, it almost does not change.

Modification (from lat.modulus- measure, type andfacies- shape, appearancevariability – these are changes in the characteristics of an organism (its phenotype) caused by changes in the conditions of the environment of existence and not associated with changes in the genotype.Due to the fact that modification variability is not associated with a change in the genotype, it is not inherited.

Actually modificationchanges (modifications)- these are the reactions of organisms to a change in the intensity of the action of certain environmental factors. They are the same for all genotypes of closely related organisms. For example, all arrowhead plants immersed in water form long and thin leaves, while those that grow on dry land have arrow-shaped leaves. Arrowhead plants partially submerged in water produce leaves of both types.

In the diurnal moth, the variegated wing color depends on the temperature at which the pupae developed. From those pupae that have overwintered, butterflies come out with a brick-red color, and from those that developed in summer at elevated temperatures, with a black background of the wings. The degree of severity of modifications directly depends on the intensity and duration of the action of a certain factor on the body. So, in a small crustacean-brine shrimp, the degree of hairiness of the back of the abdomen depends on the salinity of the water: it is the greater, the lower the concentration of salts.

Numerous studies have shown that modifications can disappear during the life of one individual if the action of the factor that caused them ceases. For example, a tan acquired by a person in summer gradually disappears during the autumn-winter period. If the arrowhead plant is transplanted from water to dry land, then the new leaves will not have an elongated, but an arrow-shaped shape. The resulting modifications can persist throughout the life of an individual, especially those that arose at the early stages of individual development. But they are not passed on to offspring. For example, the curvature of the bones of the lower extremities as a result of rickets persists throughout life. But in parents who had rickets in childhood, children are born normal if, during their development, they receive required amount vitamin D. Another example of modifications that persist throughout life is the differentiation of honey bee larvae into queens and workers. The larvae, which develop in special large cells of the honeycombs and feed only on "royal jelly", which is produced by special glands of worker bees, develop into queens. And those who are fed with bee bread (a mixture of honey and pollen) subsequently become working individuals - underdeveloped females, incapable of reproduction. Therefore, the differentiation of female honey bee larvae depends on the food they receive during their development. If at the early stages of development the larvae are interchanged, from which the queen and the worker bee should develop in the future, then the nature of their nutrition and subsequent differentiation will change accordingly. However, at later stages of development, this becomes impossible.

Modification variability plays an exceptional role in the life of organisms, ensuring their adaptability to changes in environmental conditions. Thus, the change in the shape of the leaves of the arrowhead from arrow-shaped to ribbon-shaped (linear) when this plant is immersed in water protects it from damage by the current. Changing the coat of mammals during the autumn molt to a thicker one provides protection from the action low temperatures, and human tan - from the harmful effects of solar radiation. All this gives reason to believe that such modifications arose in the process historical development species as certain adaptive responses to changing environmental conditions of existence that organisms constantly encounter. However, not all modifications are adaptive in nature. For example, if you shade lower part potato stalk, then above-ground tubers will begin to form on it. Modifications devoid of adaptive significance arise when organisms find themselves in conditions that are unusual for themselves, which their ancestors did not have to face.

Modification variability is subject to certainstatistical regularities . In particular, any sign can change only within certain limits. Such limits of modification variability (from min to max) of traits are predetermined by the genotype of the organism and are calledreaction rate . Consequently, a particular allelic gene does not predetermine the development of a certain state of a trait encoded by it, but only the limits in which it can change depending on the intensity of the action of certain environmental factors. Among the characters there are those whose different states are almost completely determined by the genotype (for example, the location of the eyes, the number of fingers on the limbs, the blood type, the nature of the leaf venation, etc.). But the degree of manifestation of states of other signs (height and weight of organisms, the size of a puff plate, etc.) is significantly influenced by environmental conditions. For example, the development of the color of the coat of some animals (for example, ermine rabbits, Siamese cats) depends on temperature. If such animals shave off a body area covered with white hair and apply ice to it, then black hair will grow in this area at low temperatures.

Reaction rate for different signs has its own limits. Signs that determine the viability of organisms (for example, the relative position of internal organs) have a very narrow reaction rate, and for signs that do not carry an important vitality, it can be much wider (body weight, height, hair color).

Usually a single manifestation of a trait is calledoption . To study the variability of a particular trait, i.e. option, make upvariational (from lat.variatio-change)row – a sequence of quantitative indicators of the manifestations of the states of a certain feature (variant), arranged in ascending or descending order.

The length of the variation series indicates the range of modification variability (reaction rate). It is predetermined by the genotype of the organism, but depends on environmental conditions: the more stable they are, the shorter the variation series will be, and vice versa. If we trace the distribution of individual variants within the variation series, we can see that the largest number of them is located in its middle part, i.e. they have an average value of this attribute.

This distribution is explained by the fact that the minimum and maximum values ​​of the trait development are formed when most of the environmental factors act in one direction: the most or least favorable. But the body, as a rule, feels their influence differently: some factors contribute to the development of the trait, while others, on the contrary, inhibit it. That is why the degree of development of a certain trait in most individuals of the same species is averaged. So, most people are of average height, and only a small part of them are giants or dwarfs. The distribution of variants within a variation series can be graphically depicted as a variation curve.Variation curve - this graphic image dependencies possible options trait from the frequency of occurrence.With the help of a variation curve, you can set the average indicators and the reaction rate of a certain trait.

GENERALIZATION

The manifestation of the phenotype of each organism depends on heredity and variability. Due to heredity, an individual receives a genetic set from parental forms, thus preserving specific features of each species, and variability violates this pattern - due to variability in the world, it is impossible to meet two genetically identical individuals.

There are two types of variability: non-hereditary (phenotypic, modification) and hereditary (genotypic, genetic). Factors of genetic variability are mutations and recombinations of genetic material. And therefore, hereditary variability is also called mutational. Modification variability is caused by the body's reactions to environmental factors. And since the conditions for the formation of each organism are largely different, then individuals, even if they are representatives of the same species, have their own unique phenotype.

Heredity and variability play an important role in the evolution of organisms. Their properties are also used in the process of breeding new varieties of plants and animal breeds.

QUESTIONS TO CONTROL

1. What is a gene from a biochemical and genetic point of view?

2. Why are heredity and variability called alternative phenomena? Define heredity and variability.

3. What is cytoplasmic inheritance and what causes it?

4. What are mutations? What types of mutations do you know?

5. What is aneuploidy and polyploidy?

6. Why do mutations associated with a multiple decrease in the chromosome set negatively affect the viability of organisms compared to those caused by a multiple increase in the genome?

7. Are most mutations recessive or dominant?

8. What is the difference between modification and mutational variability?

9. What is called the reaction rate of modification variability?

10. What is included in the statistical processing of modification variability data?

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 did not make it possible to strictly distinguish different forms 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. special case 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.