Examples of modification and hereditary variability. Modification variability. Mechanism, meaning, examples

Abstract on the topic:

Modification variability

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11th grade student a

Sagiev Alexander

Modification (phenotypic) variability- changes in the body associated with changes in phenotype due to influence environment and are, in most cases, adaptive in nature.

The genotype does not change. Generally modern concept“adaptive modifications” corresponds to the concept of “definite variability”, which was introduced into science by Charles Darwin.

Conditional classification modification variability

According to changing signs of the body:

1) morphological changes

2) physiological and biochemical adaptations - homeostasis (increased level of red blood cells in the mountains, etc.)

According to the range of the reaction norm:

1) narrow (more typical for qualitative characteristics)

2) broad (more typical for quantitative traits)

By value:

1) modifications (useful for the body - manifested as an adaptive reaction to environmental conditions)

2) morphoses (non-hereditary changes in phenotype under the influence of extreme environmental factors or modifications that arise as an expression of newly emerged mutations that do not have an adaptive nature)

3) phenocopies (various non-hereditary changes that copy the manifestation of various mutations) - a type of morphosis

By duration:

1) is present only in an individual or group of individuals that have been influenced by the environment (not inherited)

2) long-term modifications - persist for two or three generations

Characteristics of modification variability

1) reversibility - changes disappear when the specific environmental conditions that provoked them change

2) group character

3) changes in the phenotype are not inherited, the reaction norm of the genotype is inherited

4) statistical regularity of variation series

5) affects the phenotype without affecting the genotype itself

Mechanism of modification variability

1) Environment as a reason for modifications

Modifying variability is the result not of changes in the genotype, but of its response to environmental conditions. With modification variability, the hereditary material does not change, but the expression of genes changes.

Under the influence of certain environmental conditions on the body, the course of enzymatic reactions (enzyme activity) changes and the synthesis of specialized enzymes can occur, some of which (MAP kinase, etc.) are responsible for the regulation of gene transcription, depending on environmental changes. Thus, environmental factors are able to regulate gene expression, that is, the intensity of their production of specific proteins, the functions of which respond to specific environmental factors. For example, four genes, located on different chromosomes, are responsible for the production of melanin. Nai large quantity dominant alleles of these genes - 8 - are found in people of the Negroid race. When exposed to a specific environment, for example, intense exposure to ultraviolet rays, epidermal cells are destroyed, which leads to the release of endothelin-1 and eicosanoids. They cause activation of the enzyme tyrosinase and its biosynthesis. Tyrosinase, in turn, catalyzes the oxidation of the amino acid tyrosine. Further education melanin occurs without the participation of enzymes, however, a larger amount of enzyme causes more intense pigmentation.

2) Norm of reaction

The limit for the manifestation of modification variability of an organism with an unchanged genotype is the reaction norm. The reaction rate is determined by the genotype and varies among different individuals of a given species. In fact, the reaction norm is a spectrum of possible gene expression levels, from which the expression level most suitable for given environmental conditions is selected. The reaction norm has a limit for each species - for example, increased feeding will lead to an increase in the weight of the animal, but it will be within the reaction norm characteristic of a given species or breed. The reaction rate is genetically determined and inherited.

For different changes there are different reaction norm limits. For example, the amount of milk yield and the productivity of cereals vary greatly (quantitative changes), the intensity of the color of animals, etc. varies weakly (qualitative changes). In accordance with this, the reaction norm can be broad (quantitative changes - the size of the leaves of many plants, the body size of many insects, depending on the feeding conditions of their larvae) and narrow (qualitative changes - the color of the pupae and adults of some butterflies). However, some quantitative traits are characterized by a narrow reaction rate (milk fat content, the number of toes in guinea pigs), while some qualitative traits are characterized by a wide reaction rate (for example, seasonal color changes in many animal species of northern latitudes).

Analysis and patterns of modification variability

1) Variation series

A ranked display of the manifestation of modification variability is a variation series - a series of modification variability of a property of an organism, which consists of individual properties of modifications, arranged in order of increasing or decreasing quantitative expression of the property (leaf size, change in the intensity of coat color, etc.). A single indicator of the relationship between two factors in a variation series (for example, the length of the coat and the intensity of its pigmentation) is called a variant. For example, wheat growing in one field can differ greatly in the number of ears and spikelets due to different soil indicators and moisture content in the field. By compiling the number of spikelets in one ear and the number of ears of corn, we can obtain a variation series in statistical form:

Variation series of modification variability of wheat

2) Variation curve

A graphical display of the manifestation of modification variability - a variation curve - displays both the range of variation of a property and the frequency of individual variants. The curve shows that the most common are the average variants of manifestation of the trait (Quetelet’s law). The reason for this, apparently, is the effect of environmental factors on the course of ontogenesis. Some factors suppress gene expression, while others, on the contrary, enhance it. Almost always, these factors, while simultaneously acting on ontogeny, neutralize each other, that is, neither a decrease nor an increase in the value of the trait is observed. This is the reason why individuals with extreme expressions of the trait are found in significantly smaller numbers than individuals with average size. For example, average height men - 175 cm - most common in European populations. When constructing a variation curve, you can calculate the value of the standard deviation and, based on this, construct a graph of the standard deviation from the median - the most common value of the attribute.

Graph of standard deviation coming from the variation curve “modification variability of wheat”

Modification variability in the theory of evolution

1) Darwinism

In 1859, Charles Darwin published his work on evolutionary topics entitled On the Origin of Species by Means of natural selection, or the preservation of favorable races in the struggle for life." In it, Darwin showed the gradual development of organisms as a result of natural selection.

Natural selection consists of the following mechanism:

1) first an individual appears with new, completely random properties (formed as a result of mutations)

2) then she is or is not able to leave offspring, depending on these properties

3) finally, if the outcome of the previous stage is positive, then she leaves offspring and her descendants inherit the newly acquired properties

New properties of an individual are formed as a result of hereditary and modification variability. And if hereditary variability is characterized by changes in the genotype and these changes are inherited, then with modification variability the ability of the genotype of organisms to change the phenotype when exposed to the environment is inherited. With constant exposure to the same environmental conditions, mutations can be selected on the genotype, whose effect is similar to the manifestation of modifications, and, thus, modification variability turns into hereditary variability (genetic assimilation of modifications). An example is the constant large percentage of melanin pigment in the skin of black and Mongoloid race compared to Caucasians. Darwin called modification variability definite (group). A certain variability appears in all normal individuals of a species exposed to a certain influence. A certain variability expands the limits of existence and reproduction of an organism.

2) Natural selection and modification variability

Modification variability is closely related to natural selection. Natural selection has four directions, three of which are directly aimed at the survival of organisms with in different forms Not hereditary variability. This is stabilizing, driving and disruptive selection. Stabilizing selection is characterized by the neutralization of mutations and the formation of a reserve of these mutations, which determines the development of the genotype with a constant phenotype. As a result, organisms with an average reaction rate dominate in constant conditions of existence. For example, generative plants maintain a flower shape and size that matches the shape and size of the insect that pollinates the plant. Disruptive selection is characterized by the opening of reserves with neutralized mutations and the subsequent selection of these mutations for the formation of new genotypes and phenotypes that are suitable for the environment. As a result, organisms with extreme reaction rates survive. For example, insects with large wings have greater resistance to gusts of wind, while insects of the same species with weak wings are blown away. Driving selection characterized by the same mechanism as the disruptive one, however, it is aimed at the formation of a new average reaction norm. For example, insects become resistant to chemicals.

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

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

Modification variability

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

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

The phenotype is formed as a result of interactions between the genotype and environmental factors. Phenotypic characteristics are not transmitted from parents to offspring; only the reaction norm is inherited, that is, the nature of the response to changes in environmental conditions. In heterozygous organisms, changing environmental conditions can cause different manifestations of this trait.

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

Genotypic variability

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

Classification of mutations. Mutations can be combined into groups - classified according to the nature of their manifestation, by location, or by the level of their occurrence.

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

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

A wide variety of polyploids is observed in floriculture: if one original form in the haploid set had 9 chromosomes, then cultivated plants of this species can have 18, 36, 54 and up to 198 chromosomes. Polyploids develop as a result of exposure of plants to temperature, ionizing radiation, and chemicals (colchicine), which destroy the cell division spindle. In such plants, the gametes are diploid, and when fused with the haploid germ cells of a partner, a triploid set of chromosomes appears in the zygote (2n + n = 3n). Such triploids do not form seeds, they are sterile, but highly productive. Even-numbered polyploids form seeds.

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

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

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

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

Properties of mutations:

1. Mutations occur suddenly, spasmodically.
2. Mutations are hereditary, that is, they are persistently transmitted from generation to generation.
3. Mutations are undirected - any locus can mutate, causing changes in both minor and vital signs.
4. The same mutations can occur repeatedly.
5. According to their manifestation, mutations can be beneficial and harmful, dominant and recessive.

The ability to mutate is one of the properties of a gene. Each individual mutation is caused by some reason, but in most cases these reasons are unknown. Mutations are associated with changes in the external environment. This is convincingly proven by the fact that by influencing external factors manages to sharply increase their number.

Combinative variability

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

In selection important has the law of homological series of hereditary variability, formulated by the Soviet scientist N.I. Vavilov. It reads: inside different types and genera that are genetically close (i.e., having the same origin), similar series of hereditary variability are observed. This type of variability has been identified in many cereals (rice, wheat, oats, millet, etc.), in which the color and consistency of the grain, cold resistance and other qualities vary similarly. Knowing the nature of hereditary changes in some varieties, it is possible to foresee similar changes in related species and, by influencing them with mutagens, induce similar useful changes in them, which greatly facilitates the production of economically valuable forms. Many examples of homological variability are known in humans; for example, albinism (a defect in the synthesis of dye by cells) was found in Europeans, blacks and Indians; among mammals - in rodents, carnivores, primates; short dark-skinned people - pygmies - are found in the tropical forests of equatorial Africa, on the Philippine Islands and in the jungles of the Malacca Peninsula; Some hereditary defects and deformities inherent in humans are also noted in animals. Such animals are used as a model to study similar defects in humans. For example, cataracts of the eye occur in mice, rats, dogs, and horses; hemophilia - in mice and cats, diabetes - in rats; congenital deafness - in guinea pigs, mice, dogs; cleft lip - in a mouse, dog, pig, etc. These hereditary defects are convincing confirmation of the law of homological series of hereditary variability by N. I. Vavilov.

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

Modification (phenotypic) variability- changes in the body associated with changes in phenotype due to environmental influences and, in most cases, of an adaptive nature. The genotype does not change. In general, the modern concept of “adaptive modifications” corresponds to the concept of “definite variability”, which was introduced into science by Charles Darwin.

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✪ Hereditary variability. Combinative and mutational variability

Subtitles

Conditional classification of modification variability

• According to changing signs of the body:
  • morphological changes
  • physiological and biochemical adaptations - homeostasis (increased levels of red blood cells in the mountains, etc.)
• According to the range of the reaction norm
  • narrow (more typical for qualitative traits)
  • broad (more typical for quantitative traits)
• By value:
  • modifications (useful for the body - manifested as an adaptive response to environmental conditions)
  • morphoses (non-hereditary changes in phenotype under the influence of extreme environmental factors or modifications that arise as an expression of newly emerged mutations that do not have an adaptive nature)
  • phenocopies (various non-hereditary changes that copy the manifestation of various mutations)
• By duration:
  • exists only in an individual or group of individuals that have been influenced by the environment (not inherited)
  • long-term modifications - persist for two or three generations

Mechanism of modification variability

Environment as a reason for modifications

Modifying variability is the result not of changes in the genotype, but of its response to environmental conditions. With modification variability, the hereditary material does not change, but the expression of genes changes.

Under the influence of certain environmental conditions on the body, the course of enzymatic reactions (enzyme activity) changes and the synthesis of specialized enzymes can occur, some of which (MAP kinase, etc.) are responsible for the regulation of gene transcription, depending on changes in the environment. Thus, environmental factors are able to regulate gene expression, that is, the intensity of their production of specific proteins, the functions of which respond to specific environmental factors.

Four genes, located on different chromosomes, are responsible for the production of melanin. Largest quantity dominant alleles of these genes - 8 - are found in people of the Negroid race. When exposed to a specific environment, for example, intense exposure to ultraviolet rays, epidermal cells are destroyed, which leads to the release of endothelin-1 and eicosanoids. They cause activation of the enzyme tyrosinase and its biosynthesis. Tyrosinase, in turn, catalyzes the oxidation of the amino acid tyrosine. Further formation of melanin occurs without the participation of enzymes, however, a larger amount of enzyme causes more intense pigmentation.

Norm of reaction

The limit of manifestation of modification variability of an organism with an unchanged genotype is the norm of reaction. The reaction rate is determined by the genotype and varies among different individuals of a given species. In fact, the reaction norm is a spectrum of possible gene expression levels, from which the expression level most suitable for given environmental conditions is selected. The reaction norm has limits or boundaries for each biological species(lower and upper) - for example, increased feeding will lead to an increase in the weight of the animal, but it will be within the normal reaction range characteristic of a given species or breed. The reaction rate is genetically determined and inherited. For different traits, the reaction norm limits vary greatly. For example, wide limits of the reaction norm are the value of milk yield, cereal productivity and many other quantitative characteristics, narrow limits are the color intensity of most animals and many other qualitative characteristics.

However, some quantitative traits are characterized by a narrow reaction rate (milk fat content, the number of toes in guinea pigs), while some qualitative traits are characterized by a wide reaction rate (for example, seasonal color changes in many animal species of northern latitudes). In addition, the boundary between quantitative and qualitative characteristics is sometimes very arbitrary.

Characteristics of modification variability

• reversibility - changes disappear when the specific environmental conditions that provoked them change
• group character
• changes in the phenotype are not inherited, the norm of the genotype reaction is inherited
• statistical regularity of variation series
• affects the phenotype without affecting the genotype itself.

Analysis and patterns of modification variability

Variation series

A ranked display of the manifestation of modification variability is a variation series - a series of modification variability of a property of an organism, which consists of individual modifications arranged in order of increasing or decreasing quantitative expression of the property (leaf size, change in the intensity of coat color, etc.). A single indicator of the relationship between two factors in a variation series (for example, the length of the coat and the intensity of its pigmentation) is called variant. For example, wheat growing in one field can differ greatly in the number of ears and spikelets due to different soil conditions and moisture content in the field. By compiling the number of spikelets in one ear and the number of ears of corn, we can obtain a variation series in statistical form:

Variation curve

A graphical display of the manifestation of modification variability - a variation curve - displays both the range of variation of a property and the frequency of individual variants. The curve shows that the most common are the average variants of manifestation of the trait (Quetelet’s law). The reason for this, apparently, is the effect of environmental factors on the course of ontogenesis. Some factors suppress gene expression, while others, on the contrary, enhance it. Almost always, these factors, while simultaneously acting on ontogeny, neutralize each other, that is, neither a decrease nor an increase in the value of the trait is observed. This is the reason why individuals with extreme expressions of the trait are found in significantly smaller numbers than individuals with an average value. For example, the average height of a man - 175 cm - is most common in European populations.

When constructing a variation curve, you can calculate the value of the standard deviation and, based on this, construct a graph of the standard deviation from the median - the most common value of the attribute.

Modification variability in the theory of evolution

Darwinism

In 1859, Charles Darwin published his work on the topic of evolution entitled “The Origin of Species by Natural Selection, or the Preservation of Favored Races in the Struggle for Life.” In it, Darwin showed the gradual development of organisms as a result of natural selection. Natural selection consists of the following mechanism:

• first, an individual appears with new, completely random properties (formed as a result of mutations)
• then she is or is not able to leave offspring, depending on these properties
• finally, if the outcome of the previous stage is positive, then she leaves offspring and her descendants inherit the newly acquired properties

New properties of an individual are formed as a result of hereditary and modification variability. And if hereditary variability is characterized by changes in the genotype and these changes are inherited, then with modification variability the ability of the genotype of organisms to change the phenotype when exposed to the environment is inherited. With constant exposure to the same environmental conditions, mutations can be selected on the genotype, whose effect is similar to the manifestation of modifications, and, thus, modification variability turns into hereditary variability (genetic assimilation of modifications). An example is the constant large percentage of melanin pigment in the skin of the Negroid and Mongoloid races compared to the Caucasoid race.

Darwin called modification variability definite (group).

A certain variability is manifested in all normal individuals of a species exposed to a certain influence. A certain variability expands the limits of existence and reproduction of an organism.

Natural selection and modification variability

Modification variability is closely related to natural selection. Natural selection has four directions, three of which are directly aimed at the survival of organisms with different forms of non-hereditary variability. This is stabilizing, driving and disruptive selection.

Stabilizing selection is characterized by the neutralization of mutations and the formation of a reserve of these mutations, which determines the development of the genotype with a constant phenotype. As a result, organisms with an average reaction rate dominate in constant conditions of existence. For example, generative plants maintain a flower shape and size that matches the shape and size of the insect that pollinates the plant.

Disruptive selection is characterized by the opening of reserves with neutralized mutations and the subsequent selection of these mutations for the formation of new genotypes and phenotypes that are suitable for the environment. As a result, organisms with extreme reaction rates survive. For example, insects with strong wings have greater resistance to gusts of wind, while insects of the same species with weak wings are blown away.

Driving selection is characterized by the same mechanism as disruptive selection, but it is aimed at the formation of a new average reaction norm. For example, insects become resistant to chemicals.

Epigenetic theory of evolution

According to the main provisions of the epigenetic theory of evolution, published in 1987, the substrate for evolution is a holistic phenotype - that is, morphoses in the development of an organism are determined by the impact of environmental conditions on its ontogenesis (epigenetic system). At the same time, a stable development trajectory is formed, based on morphoses (creod) - a stable epigenetic system is formed, adaptive to morphoses. This development system is based on the genetic assimilation of organisms (modification gene copying), which consists of conforming to any modification of a specific mutation. That is, this means that a change in the activity of a particular gene can be caused by both a change in the environment and a certain mutation. When a new environment acts on an organism, mutations are selected that adapt the organism to new conditions, therefore the organism, first adapting to the environment through modifications, will then become adapted to it genetically (motor selection) - a new genotype arises, on the basis of which a new one arises phenotype. For example, with congenital underdevelopment musculoskeletal system In animals, a restructuring of the supporting and motor organs occurs in such a way that the underdevelopment turns out to be adaptive. This trait is further fixed by hereditary stabilizing selection. Subsequently, a new mechanism of behavior arises aimed at adapting to adaptation. Thus, the epigenetic theory of evolution considers postembryonic morphosis based on special conditions environment as a driving lever of evolution. Thus, natural selection in the epigenetic theory of evolution consists of the following stages:

Thus, synthetic and epigenetic theories of evolution are quite different. However, there may be cases that are a synthesis of these theories - for example, the appearance of morphoses due to the accumulation of neutral mutations in reserves is part of the mechanism of both synthetic (mutations appear in the phenotype) and epigenetic (morphoses can lead to genecopying modifications if the initial mutations did not determine this ) theories.

Forms of modification variability

In most cases, modification variability contributes to the positive adaptation of organisms to environmental conditions - the response of the genotype to the environment improves and a restructuring of the phenotype occurs (for example, the number of red blood cells increases in a person who has climbed the mountains). However, sometimes, under the influence of unfavorable environmental factors, for example, the influence of teratogenic factors on pregnant women, changes in the phenotype occur that are similar to mutations (non-hereditary changes, similar to hereditary ones) - phenocopies. Also, under the influence of extreme environmental factors, organisms may develop morphoses (for example, a disorder of the musculoskeletal system due to injury). Morphoses are irreversible and non-adaptive in nature, and in their labile nature the manifestations are similar to spontaneous mutations. Morphoses are accepted by the epigenetic theory of evolution as the main factor in evolution.

Long-term modification variability

In most cases, modification variability is non-hereditary in nature and is only a reaction of the genotype of a given individual to environmental conditions with a subsequent change in phenotype. However, long-term modifications are also known, described in some bacteria, protozoa and multicellular eukaryotes. To understand the possible mechanism of long-term modification variability, let us first consider the concept of a genetic trigger.

For example, bacterial operons contain, in addition to structural genes, two sections - a promoter and an operator. The operator of some operons is located between the promoter and structural genes (in others it is part of the promoter). If the operator is associated with a protein called a repressor, then together they prevent RNA polymerase from moving along the DNA chain. In bacteria E. coli a similar mechanism can be observed. When there is a lack of lactose and an excess of glucose, a repressor protein (Lacl) is produced, which attaches to the operator, preventing RNA polymerase from synthesizing mRNA for translation of the enzyme that breaks down lactose. However, when lactose enters the cytoplasm of the bacterium, lactose (an inducer substance) attaches to the repressor protein, changing its conformation, which leads to the dissociation of the repressor from the operator. This causes the beginning of the synthesis of an enzyme to break down lactose.

In bacteria, when dividing, the inductor substance (in the case of E. coli- lactose) is transferred to the cytoplasm of the daughter cell and triggers the dissociation of the repressor protein from the operator, which entails the manifestation of enzyme activity (lactase) to break down lactose in rods even in the absence of this disaccharide in the medium.

If there are two operons and if they are interconnected (the structural gene of the first operon encodes a repressor protein for the second operon and vice versa), they form a system called a trigger. When the first operon is active, the second one is disabled. However, under the influence of the environment, the synthesis of the repressor protein by the first operon can be blocked, and then the trigger switches: the second operon becomes active. This trigger condition can be inherited by subsequent generations of bacteria. Molecular triggers can provide long-lasting modifications in eukaryotes as well. This can occur, for example, through the cytoplasmic inheritance of cytoplasmic inclusions in bacteria during their reproduction.

The trigger switching effect can be observed in non-cellular life forms, such as bacteriophages. When bacteria are introduced into a cell due to a lack of nutrients, they remain inactive, integrating into the genetic material. When favorable conditions appear in the cell, phages multiply and break out of the bacterium - the trigger switches due to a change in the environment.

Cytoplasmic inheritance

Comparative characteristics of forms of variability

Together, hereditary and modificational variability provide the basis for natural selection. In this case, qualitative or quantitative changes in the manifestations of the genotype in the characteristics of the phenotype (hereditary variability) determine the result of natural selection - the survival or death of the individual.

Modification variability in human life

The practical use of patterns of modification variability has great importance in crop production and animal husbandry, as it allows one to foresee and plan in advance the maximum use of the capabilities of each plant variety and animal breed (for example, individual indicators of sufficient light for each plant). The creation of known optimal conditions for the implementation of the genotype ensures their high productivity.

This also makes it possible to expediently use the child’s innate abilities and develop them from childhood - this is the task of psychologists and teachers who are still school age trying to determine the inclinations of children and their abilities for one or another professional activity, increasing within the normal reaction level the level of realization of genetically determined abilities of children.

1. What is the role of the genotype and environmental conditions in the formation of the phenotype? Give examples.

Some traits are formed only under the influence of the genotype and their manifestation does not depend on the environmental conditions in which the organism develops. For example, in a person who has genes I A and I B in his genotype, blood group IV is formed, regardless of living conditions. At the same time, height, body weight, the number of red blood cells in the blood and many other characteristics depend not only on the genotype, but also on environmental conditions. Therefore, organisms that have the same genotypes (for example, monozygotic twins) may differ from each other in phenotype.

In 1895, the French botanist G. Bonnier conducted the following experiment: he divided a young dandelion plant into two parts and began to grow them in different conditions- on the plain and high in the mountains. The first plant reached normal height, and the second turned out to be dwarf. This experience shows that the formation of the phenotype (i.e., traits) is influenced not only by the genotype, but also by environmental conditions.

Another example illustrating the influence of the external environment on the manifestation of traits is the change in coat color in Himalayan rabbits. Usually at 20°C their fur all over their body is white, with the exception of black ears, paws, tail and muzzle. At 30°C, rabbits grow completely white. If you shave the hair on the side or back of a Himalayan rabbit and keep it at an air temperature below 2°C, then instead of white wool it will grow black.

2. What is modification variability? Give examples.

Modifying variability is a change in phenotype under the influence of environmental factors that occurs without changing the genotype within the normal reaction limits.

For example, dandelion leaf length and shape vary significantly even within the same plant. It was noticed that the lower the temperature at which the leaves formed, the smaller they were and the larger the cutouts. leaf blade. On the contrary, at higher temperatures larger leaves with small cutouts of the leaf blade are formed.

In an adult, depending on nutrition and lifestyle, body weight changes; in cows, milk yield can change; in chickens, egg production can change. In a person who finds himself high in the mountains, the content of red blood cells in the blood increases over time to provide the body cells with oxygen.

3. What is the reaction norm? Prove it on specific examples the validity of the statement that it is not the trait itself that is inherited, but its reaction norm.

The reaction norm is the limits of modification variability of a trait. Some characteristics, for example, leaf length, plant height, animal body weight, milk yield of large cattle, egg production of chickens, have a wide reaction rate. Others, for example, the size of flowers and their shape, the color of seeds, flowers and fruits, the color of animals, the fat content of milk - have a narrower reaction norm.

The reaction rate is determined by the genotype and is inherited. For example, the more time a person spends in direct sunlight, the more melanin is synthesized in open areas skin and, accordingly, its color is darker. As you know, the intensity of tanning is not inherited, but is determined by the specific living conditions of a particular person. In addition, even for someone who is constantly under direct sunlight person Caucasian, the skin cannot synthesize the amount of melanin that is typical, for example, for representatives of the Negroid race. This example indicates that the range of variability of a trait (reaction norm) is predetermined by the genotype and it is not the trait itself that is inherited, but the ability of the organism to form a certain phenotype under the influence of environmental conditions.

4. Describe the main properties of modifications. Why non-hereditary variability also called group? Certain?

Modifications have the following basic properties:

● Reversibility – with a change in external conditions, individuals change the degree of expression of certain characteristics.

● In most cases they are adequate, i.e. the degree of severity of a symptom is directly dependent on the intensity and duration of action of a particular factor.

● They have an adaptive (adaptive) nature. This means that in response to changing environmental conditions, an individual exhibits phenotypic changes that contribute to its survival.

● Mass distribution - the same factor causes approximately the same changes in individuals that are genotypically similar.

● Modifications are not inherited, because modification variability is not accompanied by a change in genotype.

Non-hereditary (modification) variability is called group variability, since certain changes in environmental conditions cause similar changes in all individuals of a particular species (mass property). Modification variability is also called definite, because modifications are adequate, predictable and accompanied by a change in the phenotype of individuals in a certain direction.

5. What statistical methods are they used to analyze the variability of quantitative traits?

To characterize the degree of variability of quantitative characteristics, statistical methods such as constructing a variation series and a variation curve are most often used.

For example, the number of spikelets in complex ears of wheat of the same variety varies over a fairly wide range. If you arrange the ears in increasing order of the number of spikelets, you will get a variation series of the variability of this trait, consisting of individual variants. The frequency of occurrence of a particular variant in the variation series is not the same: the most common are ears with an average number of spikelets and less often those with more and less.

The distribution of variants in this series can be depicted graphically. To do this, the values ​​of option (v) are plotted on the abscissa axis in the order of their increase, and on the ordinate axis – the frequency of occurrence of each option (p). A graphical expression of the variability of a trait, reflecting both the range of variations and the frequency of occurrence of individual variants, is called a variation curve.

6. How important is it in practice to know the norm of reaction of traits in plants, animals and humans?

Knowledge of the patterns of modification variability and reaction norms is of great practical importance, as it allows one to anticipate and plan many indicators in advance. In particular, creating optimal conditions for the implementation of the genotype makes it possible to achieve high animal productivity and plant yields. Knowledge of reaction norms various signs a person is necessary in medicine (it is important to know how certain physiological indicators correspond to the norm), pedagogy (upbringing and training taking into account the abilities and capabilities of the child), light industry (sizes of clothes, shoes) and many other areas of human activity.

7*. If a primrose, which under normal conditions has red flowers, is transferred to a greenhouse with a temperature of 30–35ºC and high humidity, the new flowers on this plant will already be white. If this plant is returned to relatively low temperature conditions (15-20ºC), it begins to bloom red flowers again. How can this be explained?

This is a typical example of modification variability. Most likely, an increase in temperature causes a decrease in the activity of enzymes that ensure the synthesis of red pigment in the petals, up to their complete inactivation (at 30–35ºС).

8*. Why in poultry farms is the daylight hours artificially extended for laying hens to 20 hours, and for broiler cockerels reduced to 6 hours per day?

Length of daylight hours – important factor, influencing the sexual behavior of birds. Increasing the duration of daylight hours activates the production of sex hormones - thus, laying hens are stimulated to increase egg production. Short daylight hours cause a decrease in sexual activity, so broiler cockerels move less, do not fight with each other, and direct all the body’s resources to increasing body weight.

*Tasks marked with an asterisk require students to put forward various hypotheses. Therefore, when marking, the teacher should focus not only on the answer given here, but take into account each hypothesis, assessing the biological thinking of students, the logic of their reasoning, the originality of ideas, etc. After this, it is advisable to familiarize students with the answer given.

Modifying variability is non-hereditary and is therefore also called phenotypic. It produces external differences within a species. Modification changes, although not fixed in genes, are determined by them and have boundaries determined by the genotype.

Modifications

In biology, modifications are phenotypic differences between organisms that have the same genotype.

Rice. 1. Modifications in plants

Such differences are caused by environmental factors, which may include:

• soil fertility;
• climatic conditions;
• animal feeding;
• lighting for plants and more.

Modifications arise as adaptive responses and in most cases are useful adaptations.

A variety of changes are called modifications.
Examples of modification variability:

• change of color in the mountain hare (seasonal modifications);
• formation of horns in adult ungulates (age-related modifications);
• weight gain with increased feeding;
• increase in muscle volume during training;
• darkening of human skin in the sun and much more.

For each type, you can create a variation series that shows all possible modifications.

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Rice. 2. Variation series

Morphoses

If organisms are exposed to high-intensity harmful factors, they may develop dramatically altered characteristics that are not adaptive in nature. Such changes are called morphoses.

Externally, morphoses are similar to mutations and in such cases are called phenocopies, because they “copy” the manifestation of hereditary changes.

Rice. 3. Phenocopies

Morphoses are deformities. Unlike mutations, they are not fixed in the genotype.

Effect of genotype

Although modifications occur under the influence of environmental conditions, they are determined by a specific genotype.

For example, people living in high mountain areas have almost a third more red blood cells than residents of the plains. But the ability for increased production of red blood cells is determined by a person’s genotype, i.e. modifications have a hereditary basis.

An organism does not inherit a trait, but the ability to form a certain phenotype. Therefore, differences between individuals are determined by both environmental and genetic factors.

The amplitude within which a sign can vary is called the reaction norm. Morphosis is outside the norm of reaction.

Properties

This type of variability has a group nature and is sometimes also called group variability, since the modification occurs in all individuals of the species placed in the same conditions.

If a vessel with euglena is placed in a dark place, then they all lose their green color. If you return the euglena to the light, the color will also return to all. This also indicates the reversibility of the modifications.

But in humans, increased nutrition will not cause an increase in body weight in everyone, but in those who are genetically predisposed to this.

In one experiment, a researcher removed their tails over 22 generations of mice and crossed them with each other. Of the 1,592 animals examined, not a single one gave birth to offspring in which the artificial modification took hold.

Meaning

The formation of modification changes has adaptive significance in the life of an organism.

For example, the skin, darkening when tanning, limits the penetration of ultraviolet rays into the body, which allows you to stay in the sun longer without negative consequences.

IN agriculture, knowing the reaction rate for each breed, you can achieve optimal productivity indicators.

What have we learned?

While studying the variability of organisms in grade 11, we gave a description of its modification variety. Modification variability is the property of forming variants of characteristics within the limits of the reaction norm. Its features: reversibility and group nature. The reasons for the modifications are environmental factors. The modification is not inherited.

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