Genotypic

1. Combinative

2. Mutational

The formation of various types of variability is a consequence of the interaction between the environment and the genotype.

Characteristics of phenotypic variability.

Phenotypic variability - changes in the phenotype that occur under the influence of environmental conditions that do not affect the genotype, although the degree of their severity is determined by the genotype.

Ontogenetic variability - this is a constant change of signs in the process of development of an individual (ontogeny of amphibians, insects, development of morphophysiological and mental signs in humans).

Modification variability - phenotypic changes arising from the influence of environmental factors on the body.

Modification variability is determined by the genotype. Modifications are not inherited and are seasonal and environmental.

Seasonal modifications - genetically determined change of traits as a result of seasonal changes in climatic conditions.

Environmental modifications - adaptive changes in the phenotype in response to changes in environmental conditions. Phenotypically, they manifest themselves in the degree of expression of the trait. Ecological modifications affect quantitative (weight of animals, offspring) and qualitative (human skin color under the influence of UV rays) signs.

Mod properties:

Modifications are not inherited.

Occur gradually, have transitional forms.

Modifications form continuous series and are grouped around the average value.

Arise directionally - under the influence of the same environmental factor, a group of organisms changes in a similar way.

Adaptive ( adaptive ) character have all the most common modifications.

Thus, an increase in the number of erythrocytes and the content of Hb in the blood of animals and humans in the mountains represent an adaptation for a better use of oxygen. Sunburn is an adaptation to the effects of excessive insolation. It has been established that only those modifications that are caused by ordinary changes in natural conditions are adaptive. It has no adaptive value modifications caused by various chemical and physical factors. Thus, by exposing Drosophila pupae to elevated temperatures, individuals with twisted wings can be obtained, with clippings on them, which resembles mutations.

Environmental modifications reversible and with a change of generations, subject to changes in the external environment, they may not appear (fluctuations in milk yield, a change in the number of erythrocytes and leukocytes in diseases or changes in living conditions). If conditions do not change in a number of generations, then the degree of expression of the trait in the offspring is preserved. Such modifications are called long-term. When the conditions of development change, long-term modifications are not inherited. The opinion is erroneous that by upbringing and external influence it is possible to fix a new trait in the offspring (an example of dog training).

Modifications are worn adequate character, i.e. the degree of manifestation of the trait is directly dependent on the type and duration of the factor. Thus, the improvement of livestock conditions causes an increase in the mass of animals.

One of the main properties of modifications is their mass character - the same factor causes the same change in individuals that are similar genotypically. The limit and severity of modifications is controlled by the genotype.

Modifications have varying degrees of durability: long and short term. So, a tan in a person disappears after the end of the action of insolation. Other modifications that have arisen in the early stages of development may persist throughout life (buck-legged after rickets).

Modifications are unambiguous for the most primitive and highly organized organisms. These modifications include phenotypic changes associated with nutrition. Changes not only in the quantity, but also in the quality of food can cause the following modifications: human beriberi, dystrophy, rickets. The most common human modifications include phenotypic signs caused by physical activity: an increase in muscle volume as a result of training, an increase in blood supply, negative changes in a sedentary lifestyle.

Since modifications are not inherited, it is important in medical practice to distinguish them from mutations. Modifications that occur in humans are amenable to correction, while mutational changes cause incurable pathologies.

Variations in gene expression are not unlimited. They are limited by the normal reaction of the body.

reaction rate - this is the limit of the modification variability of the trait. The reaction rate is inherited, not the modifications themselves, i.e. the ability to develop a trait, and the form of its manifestation depends on environmental conditions. The reaction rate is a specific quantitative and qualitative characteristic of the genotype. There are signs with a wide reaction rate and a narrow one. The broad one includes quantitative indicators: the mass of animals, the yield of crops. A narrow reaction rate is manifested in qualitative signs: the percentage of fat content in milk, the content of proteins in the blood of a person. An unambiguous reaction rate is also characteristic of most qualitative features - hair color, eyes.

Under the influence of some harmful factors that a person does not encounter in the process of evolution, modification variability may occur that lies outside the norm of the reaction. Deformities or anomalies occur, which are called morphoses. These are changes in morphological, biochemical, physiological characteristics in mammals. For example, 4 hearts, one eye, two heads; in humans - the absence of limbs in children at birth, intestinal obstruction, swelling of the upper lip. The cause of such changes are teratogens: the drug thalidomide, quinine, the hallucinogen LSD, drugs, alcohol. Morphosis dramatically changes a new trait, in contrast to modifications that cause changes in the severity of a trait. Morphoses can occur during critical periods of ontogeny and are not of an adaptive nature.

Phenotypically, morphoses are similar to mutations and in such cases they are called phenocopies. The mechanism of phenocopies is a violation of the implementation of hereditary information. They arise due to the suppression of the function of certain genes. In their manifestation, they resemble the function of known genes, but are not inherited.

Genotypic variability. The value of combinative variability in ensuring the genetic polymorphism of mankind.

Genotypic variability - the variability of an organism due to a change in the genetic material of a cell or combinations of genes in the genotype, which can lead to the appearance of new traits or to a new combination of them.

The variability that occurs when crossing, as a result of various combinations of genes, their interaction with each other, is called combinative. In this case, the structure of the gene does not change.

Mechanisms for the occurrence of combinative variability:

crossing over;

independent divergence of chromosomes in meiosis;

random combination of gametes during fertilization.

Combination variability is inherited according to Mendel's rules. The manifestation of traits in combinative variability is influenced by the interaction of genes from one and different allelic pairs, multiple alleles, the pleiotropic effect of genes, gene linkage, penetrance, gene expressivity, etc.

Due to combinative variability, a wide variety of hereditary traits in humans is provided.

The manifestation of combinative variability in humans is influenced by the system of crossing or the system of marriages: inbreeding and outbreeding.

Inbreeding - consanguineous marriage. It can be close to varying degrees, depending on the degree of kinship of those entering into marriage. The marriage of brothers with sisters or parents with children is called the first degree of kinship. Less close - between cousins ​​​​and sisters, nephews with uncles or aunts.

The first important genetic consequence of inbreeding is an increase in the homozygosity of offspring with each generation for all independently inherited genes.

The second is the decomposition of the population into a number of genetically different lines. The variability of the inbred population will increase, while the variability of each isolated line will decrease.

Inbreeding often leads to weakening and even degeneration of offspring. In humans, inbreeding is generally harmful. This increases the risk of disease and premature death of offspring. But examples of long-term close inbreeding, not accompanied by harmful consequences, are known, for example, the genealogy of the pharaohs of Egypt.

Since the variability of any kind of organisms at any given moment is a finite value, it is clear that the number of ancestors in any generation should exceed the number of the species, which is impossible. This implies that among the ancestors there were marriages in varying degrees of kinship, as a result of which the actual number of different ancestors was reduced. This can be shown by the example of a person.

A person has an average of 4 generations per century. So, 30 generations ago, i.e. around 1200 AD. each of us should have had 1,073,741,824 ancestors. In fact, the number at that time did not reach 1 billion. We have to conclude that in the pedigree of each person there were many marriages between relatives, although mostly so distant that they did not suspect their relationship.

In fact, such marriages occurred much more often than follows from the above consideration, since. for most of its history, mankind has existed in the form of isolated peoples and tribal groups.

Therefore, the brotherhood of all people is indeed a real genetic fact.

Outbreeding - unrelated marriage. Unrelated individuals are individuals that do not have common ancestors in 4-6 generations.

Outbreeding increases the heterozygosity of offspring, combines alleles in hybrids that existed separately in parents. Harmful recessive genes found in parents in a homozygous state are suppressed in offspring heterozygous for them. The combination of all genes in the genome of hybrids increases and, accordingly, combinative variability will be widely manifested.

Combinative variability in the family concerns both normal and pathological genes that can be present in the genotype of spouses. When addressing issues of medical and genetic aspects of the family, it is necessary to accurately establish the type of inheritance of the disease - autosomal dominant, autosomal recessive or sex-linked, otherwise the prognosis will be incorrect. If both parents have a recessive abnormal gene in a heterozygous state, the probability of a child having a disease is 25%.

The frequency of Down syndrome in children born to mothers of 35 years of age - 0.33%, 40 years and older - 1.24%.

mutational variability. Theory of H. De Vries. Classification and characteristics of mutations.

Mutational variability - this is a type of variability in which there is an abrupt, intermittent change in a hereditary trait. Mutations - these are sudden persistent changes in the genetic apparatus, including both the transition of genes from one allelic state to another, and various changes in the structure of genes, the number and structure of chromosomes, and cytoplasmic plasmogens.

Term mutation was first proposed by H. de Vries in his work Mutation Theory (1901-1903). The main provisions of this theory:

Mutations occur suddenly, new forms are quite stable.

Mutations are qualitative changes.

Mutations can be beneficial or harmful.

The same mutations can occur repeatedly.

All mutations are divided into groups (Table 9). The primary role belongs generative mutations that occurs in germ cells. Generative mutations that cause a change in the characteristics and properties of the organism can be detected if the gamete carrying the mutant gene is involved in the formation of a zygote. If the mutation is dominant, then a new trait or property appears even in a heterozygous individual that originated from this gamete. If the mutation is recessive, then it can only appear after several generations when it passes into the homozygous state. An example of a generative dominant mutation in humans is the appearance of blistering of the skin of the feet, cataracts of the eye, brachyphalangia (short fingers with insufficiency of the phalanges). An example of a spontaneous recessive generative mutation in humans is hemophilia in individual families.

Table 9 - Classification of mutations

Somatic mutations by their nature, they are no different from generative ones, but their evolutionary value is different and is determined by the type of reproduction of the organism. Somatic mutations play a role in organisms with asexual reproduction. Thus, in vegetatively propagating fruit and berry plants, a somatic mutation can give rise to plants with a new mutant trait. The inheritance of somatic mutations is currently of particular importance in connection with the study of the causes of cancer in humans. It is assumed that for malignant tumors, the transformation of a normal cell into a cancer cell occurs according to the type of somatic mutations.

Gene or point mutations - these are cytologically invisible changes in chromosomes. Gene mutations can be either dominant or recessive. The molecular mechanisms of gene mutations are manifested in a change in the order of nucleotide pairs in a nucleic acid molecule at individual sites. The essence of local intragenic changes can be reduced to four types of nucleotide rearrangements:

Replacement base pairs in a DNA molecule:

a) Transition: replacement of purine bases with purine bases or pyrimidine bases with pyrimidine bases;

b) Transversion: substitution of purine bases for pyrimidine bases and vice versa.

deletion (loss) of one pair or group of bases in a DNA molecule;

Insert one pair or group of bases in a DNA molecule;

duplication – repeat of a nucleotide pair;

permutation positions of nucleotides within a gene.

Changes in the molecular structure of a gene lead to new forms of writing off genetic information from it, which is necessary for the flow of biochemical processes in the cell, and leads to the emergence of new properties in the cell and the organism as a whole. Apparently, point mutations are the most important for evolution.

According to the influence on the nature of the encoded polypeptides, point mutations can be represented as three classes:

Missense mutations - occur when a nucleotide is replaced within a codon and cause the substitution of one incorrect amino acid at a certain place in the polypeptide chain. The physiological role of the protein is changing, which creates a field for natural selection. This is the main class of point, intragenic mutations that appear in natural mutagenesis under the influence of radiation and chemical mutagens.

Nonsense mutations - the appearance of terminal codons within the gene due to changes in individual nucleotides within the codon. As a result, the translation process is interrupted at the site of the appearance of the terminal codon. The gene is able to encode only fragments of the polypeptide up to the point where the terminal codon appears.

Frameshift mutations reading occur when insertions and deletions occur within a gene. In this case, after the modified site, the entire semantic content of the gene changes. This is caused by a new combination of nucleotides in triplets, since triplets, after dropping out or insertion, acquire a new composition due to a shift by one nucleotide pair. As a result, the entire polypeptide chain acquires other wrong amino acids after the site of the point mutation.

Chromosomal rearrangements arise as a result of rupture of sections of the chromosome and their recombinations. Distinguish:

Deficiencies and deletions - lack, respectively, of the terminal and middle portion of the chromosome;

Duplications - doubling or multiplication of certain sections of the chromosome;

Inversions - a change in the linear arrangement of genes in the chromosome due to a 180˚ flip of individual sections of the chromosome.

Interchromosomal rearrangements associated with the exchange of regions between non-homologous chromosomes. Such changes are called translocations.

Genomic mutations affect the genome of the cell and cause a change in the number of chromosomes in the genome. This may occur by increasing or decreasing the number of haploid sets or individual chromosomes. Genomic mutations are polyploidy and aneuploidy.

Polyploidy - genomic mutation, consisting in an increase in the number of chromosomes, a multiple of the haploid. Cells with different numbers of haploid sets of chromosomes are called: 3n - triploids, 4n - tetraploids, etc. Polyploidy leads to a change in the characteristics of the organism: an increase in fertility, cell size, and biomass. Used in plant breeding. Polyploidy is also known in animals, for example, in ciliates, silkworms, and amphibians.

Aneuploidy - change in the number of chromosomes that is not a multiple of the haploid set: 2n+1; 2n-1; 2n-2; 2n+2. In humans, such mutations cause pathologies: trisomy syndrome on the X chromosome, trisomy on the 21st chromosome (Down's disease), monosomy on the X chromosome, etc. The phenomenon of aneuploidy shows that a violation of the number of chromosomes leads to a change in the structure and a decrease in the viability of the organism.

Cytoplasmic mutations - this is a change in plasmogens, leading to a change in the signs and properties of the organism. Such mutations are stable and are passed down from generation to generation, such as the loss of cytochrome oxidase in yeast mitochondria.

According to the adaptive value, mutations are divided into: useful, harmful(lethal and semi-lethal) and neutral. This division is conditional. There are almost continuous transitions between beneficial and lethal mutations due to gene expressivity. An example of lethal and sublethal mutations in humans is epiloia (a syndrome characterized by skin proliferation, mental retardation) and epilepsy, as well as the presence of tumors of the heart, kidneys, congenital ichthyosis, amaurotic idiocy (deposition of fatty matter in the central nervous system, accompanied by degeneration of the medulla, blindness) , thalassemia, etc.

Spontaneous Mutations occur naturally without special exposure to unusual agents. The mutation process is characterized mainly by the frequency of occurrence of mutations. A certain frequency of occurrence of mutations is characteristic of each type of organism. Some species have higher mutational variability than others. The established regularities in the frequency of spontaneous mutations are reduced to the following provisions:

different genes in the same genotype mutate at different frequencies (there are mutable and stable genes);

similar genes in different genotypes mutate at different rates.

Each gene mutates relatively infrequently, but since the number of genes in the genotype is large, then the total mutation frequency of all genes is quite high. Thus, in humans, the frequency of occurrence of mutations in the population is 4·10 -4 for thalassemia, 2.8·10 -5 for albinism, and 3.2·10 -5 for hemophilia.

The frequency of spontaneous mutagenesis can be influenced by specific genes - mutator genes , which can dramatically change the mutability of the organism. Such genes have been discovered in Drosophila, corn, Escherichia coli, yeast, and other organisms. It is assumed that mutator genes change the properties of DNA polymerase, the influence of which leads to mass mutation.

Spontaneous mutagenesis is influenced by the physiological and biochemical state of the cell. Thus, it has been shown that in the process of aging, the frequency of mutations increases significantly. Among the possible causes of spontaneous mutation is the accumulation in the genotype of mutations that block the biosynthesis of certain substances, as a result of which there will be an excessive accumulation of precursors of such substances that may have mutagenic properties. A certain role in the spontaneous mutation of a person can be played by natural radiation, due to which from 1/4 to 1/10 of spontaneous mutations in humans can be attributed.

Based on the study of spontaneous mutations within populations of one species and when comparing populations of different species, N. I. Vavilov formulated law of homologous series hereditary variability: “Species and genera that are genetically close are characterized by similar series of hereditary variability with such regularity that, knowing the number of forms within one species, one can foresee the finding of parallel forms in other species and genera.” The genetically closer the genera are located in the general system, the more complete is the similarity of variability in their series. The main thing in the law of homologous series was a new approach to understanding the principles of mutations in nature. It turned out that hereditary variability is a historically established phenomenon. Mutations are random when taken individually. However, in general, in the light of the law of homologous series, they become a natural phenomenon in the system of species.

Mutations, going as if by chance in different directions, when combined, reveal a common law.

induced mutation process the occurrence of hereditary changes under the influence of a special impact of factors of the external and internal environment.

Mechanisms for the occurrence of mutations. Mutagenesis and carcinogenesis. Genetic danger of environmental pollution by mutagens.

All mutagenesis factors can be divided into three types: physical, chemical and biological.

Among physical factors of greatest importance are ionizing radiation. Ionizing radiation is divided into:

electromagnetic (wave), these include x-rays with a wavelength of 0.005 to 2 nm, gamma rays and cosmic rays;

corpuscular radiation - beta particles (electrons and positrons), protons, neutrons (fast and thermal), alpha particles (nuclei of helium atoms), etc. Passing through living matter, ionizing radiation knocks out electrons from the outer shell of atoms and molecules, which leads to to their chemical transformations.

Different animals are characterized by different sensitivity to ionizing radiation, which ranges from 700 roentgens for humans to hundreds of thousands and millions of roentgens for bacteria and viruses. Ionizing radiation primarily causes changes in the genetic apparatus of the cell. It has been shown that the cell nucleus is 100 thousand times more sensitive to radiation than the cytoplasm. Immature germ cells (spermatogonia) are much more sensitive to radiation than mature ones (spermatozoa). Chromosomal DNA is most sensitive to the effects of radiation. Developing changes are expressed in gene mutations and rearrangements of chromosomes.

It has been shown that the frequency of mutations depends on the total radiation dose and is directly proportional to the radiation dose.

Ionizing radiation affects the genetic apparatus not only directly, but also indirectly. They cause radiolysis of water. The resulting radicals (H + , OH -) have a damaging effect.

Strong physical mutagens include ultraviolet rays (wavelength up to 400 nm), which do not ionize atoms, but only excite their electron shells. As a result, chemical reactions develop in the cells, which can lead to mutation. The frequency of mutations increases with increasing wavelength up to 240-280 nm (corresponds to the absorption spectrum of DNA). UV rays cause gene and chromosomal rearrangements, but in a much smaller amount than ionizing radiation.

A much weaker physical mutagen is elevated temperature. An increase in temperature by 10 increases the mutation rate by 3-5 times. In this case, gene mutations occur mainly in lower organisms. This factor does not affect warm-blooded animals with a constant body temperature and humans.

Chemical mutagens There are many different substances and their list is constantly updated. The most powerful chemical mutagens are:

alkylating compounds: dimethyl sulfate; mustard gas and its derivatives - ethyleneimine, nitrosoalkyl-nitromethyl, nitrosoethylurea, etc. Sometimes these substances are supermutagens and carcinogens.

The second group of chemical mutagens are nitrogenous base analogs (5-bromouracil, 5-bromodeoxyurodine, 8-azoguanine, 2‑aminopurine, caffeine, etc.).

The third group consists acridine dyes (acridine yellow, orange, proflavin).

The fourth group is various according to the structure of the substance: nitrous acid, hydroxylamine, various peroxides, urethane, formaldehyde.

Chemical mutagens can induce both gene and chromosomal mutations. They cause more gene mutations than ionizing radiation and UV rays.

To biological mutagens include certain types of viruses. It has been shown that most human, animal, and plant viruses induce mutations in Drosophila. It is assumed that DNA virus molecules represent a mutagenic element. The ability of viruses to cause mutations was found in bacteria and actinomycetes.

Apparently, all mutagens, both physical and chemical, are in principle universal; can cause mutations in any form of life. For all known mutagens, there is no lower threshold for their mutagenic activity.

Mutations cause congenital deformities and hereditary human diseases. Therefore, the urgent task is to protect people from the action of mutagens. Of great importance in this respect was the prohibition of atmospheric testing of nuclear weapons. It is very important to observe the measures to protect people from radiation in the nuclear industry, when working with isotopes, X-rays. Antimutagens can play a certain role - substances that reduce the effect of mutagens (cysteamine, quinacrine, some sulfonamides, derivatives of propionic and gallic acids).

Repair of genetic material. Mutations associated with impaired repair and their role in human pathology.

Not all damage to the genetic apparatus caused by mutagens is realized in the form of mutations. Many of them are corrected with the help of special repair enzymes.

Repair represents evolutionarily developed devices that increase the noise immunity of genetic information and its stability in a number of generations. The repair mechanism is based on the fact that each DNA molecule contains two complete sets of genetic information recorded in complementary polynucleotide strands. This ensures that uncorrupted information is preserved in one thread, even if the other is damaged, and will correct the defect over an undamaged thread.

There are currently three reparation mechanisms known: photoreactivation, dark repair, post-replication repair.

Photoreactivation consists in the elimination by visible light of thymine dimers, especially often occurring in DNA under the influence of UV rays. The replacement is carried out by a special photoreactivating enzyme, the molecules of which have no affinity for intact DNA, but recognize thymine dimers and bind to them immediately after their formation. This complex remains stable until exposed to visible light. Visible light activates the enzyme molecule, it separates from the thymine dimer and simultaneously separates it into two separate thymines, restoring the original DNA structure.

Dark reparation does not require light. It is capable of repairing a wide variety of DNA damage. Dark repair proceeds in several stages with the participation of several enzymes:

molecules endonucleases constantly examine the DNA molecule, identifying the damage, the enzyme cuts the DNA strand near it;

Endo- or exonuclease makes a second incision in this thread, excising the damaged area;

The exonuclease significantly expands the resulting gap, cutting off tens or hundreds of nucleotides;

Polymerase builds up a gap in accordance with the order of nucleotides in the second (intact) strand of DNA.

Light and dark repairs are observed before replication of damaged molecules has occurred. If the damaged molecules do not replicate, then the daughter molecules may undergo postreplicative repair. Its mechanism is not yet clear. It is assumed that with it, gaps in DNA defects can be built up with fragments taken from intact molecules.

Of utmost importance is genetic differences in the activity of repair enzymes. There are similar differences in humans. The person has a known disease xeroderma pigmentosum . The skin of such people is sensitive to the sun's rays and, with their intense exposure, becomes covered with large pigmented spots, ulcerates and can degenerate into skin cancer. Xeroderma pigmentosa is caused by a mutation that disrupts the repair mechanism for damage caused in the DNA of skin cells by UV rays from sunlight.

The phenomenon of DNA repair is widespread from bacteria to humans and is of great importance for maintaining the stability of genetic information transmitted from generation to generation.

Modification variability - changes in the phenotype of the organism, which in most cases are adaptive in nature and are formed as a result of the interaction of the genotype with the environment. Changes in the body, or modifications, are not inherited. In general, the concept of "modification variability" corresponds to the concept of "variability determined", which was introduced by Darwin.

Conditional classification of modification variability

• By the nature of changes in the body
  • Morphological changes
  • Physiological and biochemical adaptations - homeostasis
• According to the reaction norm spectrum
  • Narrow
  • wide
• By value
  • Adaptive modifications
  • morphoses
  • Phenocopies
• By duration
  • Observed only in individuals exposed to certain environmental factors (single term)
  • Observed in the descendants of these individuals (long-term modifications) for a certain number of generations

The mechanism of modification variability

Modifying variability is not the result of changes in the genotype, but of its response to environmental conditions. That is, the structure of genes does not change - the expression of genes changes.

As a result, under the influence of environmental factors on the body, the intensity of enzymatic reactions changes, which is caused by a change in the intensity of their biosynthesis. Some enzymes, such as MAP kinase, mediate the regulation of gene transcription, which is dependent on environmental factors. Thus, environmental factors are able to regulate the activity of genes and their production of a specific protein, the functions of which are most consistent with the environment.

As an example of adaptive modifications, consider the mechanism of formation of the melanin pigment. Its production corresponds to four genes that are located on different chromosomes. The largest number of alleles of these genes - 8 - is present in people with a dark body color. If the integument is intensively affected by the environmental factor, ultraviolet radiation, then when it penetrates into the lower layers of the epidermis, the cells of the latter are destroyed. There is a release of endothelin-1 and eicosanoids (fatty acid breakdown products), which causes activation and increased biosynthesis of the tyrosinase enzyme. Tyrosinase, in turn, catalyzes the oxidation of the amino acid tyrosine. Further formation of melanin occurs without the participation of tyrosinase, but an increase in the biosynthesis of tyrosinase and its activation causes the formation of a tan, corresponding to environmental factors.

Another example is the seasonal change in fur color in animals (molting). Shedding and subsequent coloring are due to the action of temperature indicators on the pituitary gland, which stimulates the production of thyroid-stimulating hormone. This causes an effect on the thyroid gland, under the action of hormones of which molting occurs.

reaction rate

The reaction rate is the spectrum of gene expression with an unchanged genotype, from which the most appropriate level of activity of the genetic apparatus is selected, and forms a specific phenotype. For example, there is an allele of the X a gene, which causes the production of more ears of wheat, and an allele of the Y b gene, which produces a small number of ears of wheat. The expression of alleles of these genes is interrelated. The entire expression spectrum is located between the maximum expression of the a allele and the maximum expression of the b allele, and the intensity of the expression of these alleles depends on environmental conditions. Under favorable conditions (with a sufficient amount of moisture, nutrients), the allele "dominates" and under unfavorable conditions, the manifestation of the b allele predominates.

The reaction rate has a limit of manifestation for each species - for example, increased feeding of animals will cause an increase in its mass, however, it will be within the range of detection of this trait for a given species. The reaction rate is genetically determined and inherited. For various changes, there are different facets of the manifestation of the reaction norm. For example, the amount of milk yield, the productivity of cereals (quantitative changes) vary greatly, the color intensity of animals varies slightly, etc. (qualitative changes). In accordance with this, the reaction rate can be narrow (qualitative changes - the color of the pupae and adults of some butterflies) and wide (quantitative changes - the size of the leaves of plants, the size of the body of insects depending on the nutrition of their pupae. However, some quantitative changes are characterized by a narrow reaction rate (fat content of milk, number of toes in porpoises), and for some qualitative changes wide (seasonal color changes in animals of northern latitudes).In general, the reaction rate and the intensity of gene expression based on it predetermine the dissimilarity of intraspecific units.

Characteristics of modification variability

• turnover - changes disappear when the specific environmental conditions that led to the modification appear disappear;
• Group character;
• Changes in the phenotype are not inherited - the norm of the genotype reaction is inherited;
• Statistical regularity of variation series;
• Modifications differentiate the phenotype without changing the genotype.

Analysis and patterns of modification variability

Displays of the manifestation of modification variability are ranked - a variation series - a series of modification variability of an organism's property, consisting of individual interconnected properties of the organism's phenotype, arranged in ascending or descending order of the quantitative expression of the property (leaf size, changes in fur color intensity, etc.). A single indicator of the ratio of two factors in a variation series (for example, the length of the fur and the intensity of its pigmentation) is called a variant. For example, wheat growing in one field can vary greatly in the number of spikelets and ears due to different soil parameters. Comparing the number of spikelets in one spikelet and the number of ears, you can get the following variation series:

Variation curve

A graphical representation of the manifestation of modification variability - a variation curve - reflects both the range of power variation and the frequency of occurrence of individual variants.

After plotting the curve, it can be seen that the most common are the average variants of the manifestation of the property (Quetelet's law). The reason for this is the effect of environmental factors on the course of ontogeny. Some factors suppress gene expression, while others increase it. Almost always, these factors, acting equally on ontogeny, neutralize each other, i.e. extreme manifestations of the trait are minimized in terms of frequency of occurrence. This is the reason for the greater occurrence of individuals with an average manifestation of the trait. For example, the average height of a man - 175 cm - is most common.

When constructing a variation curve, one can calculate the value of the standard deviation and, on the basis of this, construct a graph of the standard deviation from the median - the manifestation of the trait that occurs most often.

Graph of the standard deviation, built on the basis of the variation curve "modification variability of wheat"

Forms of modification variability

Phenocopies

Phenocopies - changes in the phenotype under the influence of adverse environmental factors, similar to mutations. The genotype does not change. Their causes are teratogens - certain physical, chemical (drugs, etc.) and biological agents (viruses) with the occurrence of morphological anomalies and malformations. Phenocopies are often similar to hereditary diseases. Sometimes phenocopies originate from embryonic development. But more often examples of phenocopies are changes in ontogeny - the spectrum of phenocopies depends on the stage of development of the organism.

morphoses

Morphoses are changes in the phenotype under the influence of extreme environmental factors. For the first time, morphoses manifest themselves precisely in the phenotype and can lead to adaptive mutations, which is taken by the epigenetic theory of evolution as the basis for the movement of natural selection based on modification variability. Morphoses are non-adaptive and irreversible in nature, that is, like mutations, they are labile. Examples of morphoses are scars, certain injuries, burns, etc.

Long-term modification variability

Most modifications are not inherited and are only a reaction of the genotype to environmental conditions. Of course, the offspring of an individual that has been exposed to certain factors that have formed a wider reaction rate can also have the same wide changes, but they will only appear when exposed to certain factors, which, by acting on genes that cause more intense enzymatic reactions. However, in some protozoa, bacteria, and even eukaryotes, there is a so-called long-term modification variability due to cytoplasmic heredity. To elucidate the mechanism of long-term modification variability, let us first consider the regulation of the trigger by environmental factors.

Trigger regulation by modifications

As an example of long-term modification variability, consider the bacterial operon. An operon is a method of organizing genetic material in which genes that code for proteins that work together or in sequence are combined under one promoter. The bacterial operon contains, in addition to gene structures, two sections - a promoter and an operator. The operator is located between the promoter (the site from which transcription begins) and the structural genes. If the operator is associated with certain repressor proteins, then together they prevent the RNA polymerase from moving along the DNA chain, it starts with the promoter. 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), then they form a system called a trigger. When the first component of the trigger is active, the other component is passive. But, under the influence of certain environmental factors, the trigger may switch to the second operon due to interruption of the coding of the repressor protein for it.

The effect of switching triggers can be observed in some non-cellular life forms, such as bacteriophages, and in prokaryotes, such as Escherichia coli. Let's consider both cases.

colibacillus - a set of species of bacteria that interact with certain organisms with a common benefit (mutualism). They have a high enzymatic activity against sugars (lactose, glucose), moreover, they cannot simultaneously break down glucose and lactose. The regulation of the ability to cleave lactose is performed by the lactose operon, which consists of a promoter, operator, and terminator, as well as a gene encoding a repressor protein for the promoter. In the absence of lactose in the environment, the repressor protein binds to the operator and transcription stops. If lactose enters a bacterial cell, it combines with the repressor protein, changes its conformation, and dissociates the repressor protein from the operator.

Bacteriophages are viruses that infect bacteria. When entering a bacterial cell, under adverse environmental conditions, bacteriophages remain inactive, penetrating into the genetic material and being transferred to daughter cells during the binary separation of the mother cell. When favorable conditions appear in the bacterial cell, the trigger switches to the bacteriophage as a result of the ingestion of nutrients-inducers, and the bacteriophages multiply and break out of the bacterium.

This phenomenon is often observed in viruses and prokaryotes, but it almost never occurs in multicellular organisms.

Cytoplasmic inheritance

Cytoplasmic heredity is heredity, which consists in the entry into the cytoplasm of an inductor substance that triggers gene expression (activates the operon) or in the autoreproduction of parts of the cytoplasm.

For example, when a bacterium buds, a bacteriophage is inherited, which is located in the cytoplasm and plays the role of a plasmid. Under favorable conditions, DNA replication is already taking place and the genetic apparatus of the cell is replaced by the genetic apparatus of the virus. A similar example of variability in Escherichia coli is the work of the E. coli lactose operon - in the absence of glucose and the presence of lactose, these bacteria produce an enzyme for the breakdown of lactose due to the switching of the lactose operon. This operon switch can be inherited during budding by passing lactose to the daughter bacterium during its formation, and the daughter bacteria also produce an enzyme (lactase) to break down lactose even in the absence of this disaccharide in the environment.

Also, cytoplasmic inheritance associated with long-term modification variability found in eukaryotic representatives such as the Colorado potato beetle and Habrobracon wasps. Under the action of intense thermal indicators in the pupae of the Colorado potato beetle, the color of the beetles changed. Under the obligatory condition that the female beetle also experienced the effects of intense thermal indicators, in the descendants of such beetles the present manifestation of the trait persisted for several generations, and then the previous norm of the trait returned. This continued modification variability is also an example of cytoplasmic inheritance. The reason for inheritance is the autoreproduction of those parts of the cytoplasm that have undergone changes. Let us consider the mechanism of autoreproduction as the cause of cytoplasmic heredity in detail. In the cytoplasm, organelles that have their own DNA and RNA and other plasmogens can self-reproduce. Organelles that are able to self-reproduce are mitochondria and plastids that are capable of self-duplication and protein biosynthesis through replication and the stages of transcription, processing and translation. Thus, the continuity of the autoreproduction of these organelles is ensured. Plasmogenes are also capable of self-reproduction. If, under the influence of the environment, the plasmogen has undergone changes that determined the activity of this gene, for example, during the dissociation of a repressor protein or associations encoding a protein, then it begins to produce a protein that forms a certain trait. Since plasmogens are able to be transported across the membrane of female eggs and thus inherited, their specific state is also inherited. At the same time, the modifications that the gene caused by activating its own expression are also preserved. If the factor that caused the activation of gene expression and protein biosynthesis by it is preserved during ontogenesis to the offspring of the individual, then the trait will be transmitted to the next offspring. Thus, a long-term modification persists as long as there is a factor that causes this modification. With the disappearance of the factor, the modification slowly fades away over several generations. This is where long-term modifications differ from regular modifications.

Modification variability and theories of evolution

Natural selection and its influence on modification variability

Natural selection is the survival of the fittest individuals and the appearance of offspring with fixed successful changes. Four types of natural selection:

Stabilizing selection. This form of selection leads to: a) the neutralization of mutations by selection, neutralizes their oppositely directed action, b) the improvement of the genotype and the process of individual development with a constant phenotype, and c) the formation of a reserve of neutralized mutations. As a result of this selection, organisms with an average rate of reaction dominate under low conditions of existence.

driving selection. This form of selection leads to: a) the disclosure of mobilization reserves, consisting of neutralized mutations, b) the selection of neutralized mutations and their compounds, and c) the formation of a new phenotype and genotype. As a result of this selection, organisms with a new average reaction rate dominate, which is more in line with the changing environmental conditions in which they live.

Disruptive selection. This form of selection brings about the same processes as in motive selection, but it is not aimed at the formation of a new average reaction rate, but at the survival of organisms with extreme reaction rates.

sexual selection. This form of selection results in facilitating the encounter between the sexes, limiting the participation in the reproduction of the species of individuals with less developed sexual characteristics.

In general, most scientists consider the substrate of natural selection, coupled with other constant factors (genetic drift, struggle for existence), hereditary variability. These views were realized in conservative Darwinism and neo-Darwinism (the synthetic theory of evolution). Recently, however, some scientists began to adhere to a different view, according to which the substrate before natural selection is morphosis - a separate type of modification variability. This view has evolved into the epigenetic theory of evolution.

Darwinism and Neo-Darwinism

From the point of view of Darwinism, one of the main factors of natural selection, which determines the fitness of organisms, is hereditary variability. This leads to the dominance of individuals with successful mutations, as a consequence of this - to natural selection, and, if the changes are strongly pronounced, to speciation. Modification variability depends on the genotype. The synthetic theory of evolution, created in the 20th century, adheres to the same view regarding modification variability. M. Vorontsov. As can be seen from the above text, these two theories consider the genotype to be the basis for natural selection, which changes under the influence of mutations, which are one of the forms of hereditary variability. Changes in the genotype cause a change in the norm of the reaction, since it is the genotype that determines it. The reaction rate determines the change in the phenotype, and thus the mutations are manifested in the phenotype, which leads to its greater compliance with environmental conditions if the mutations are expedient. The stages of natural selection according to Darwinism and neo-Darwinism consist of the following stages:

1) First, an individual appears with new properties (which are due to mutations);

2) Then she is able or unable to leave descendants;

3) If an individual leaves offspring, then changes in its genotype are fixed in generations, and this, finally, leads to natural selection.

Epigenetic theory of evolution

The epigenetic theory of evolution considers the phenotype as a substratum of natural selection, and selection not only fixes beneficial changes, but also takes part in their creation. The main influence on heredity is not the genome, but the epigenetic system - a set of factors acting on ontogeny. With morphosis, which is one of the types of modification variability, a stable developmental trajectory (creod) is formed in an individual - an epigenetic system that adapts to morphosis. This system of development is based on the genetic assimilation of organisms, which consists in the modification of a certain mutation - a modification gene copy, due to an epigenetic change in the structure of chromatin. This means that a change in gene activity can be the result of both mutations and environmental factors. Those. on the basis of a certain modification under the intense influence of the environment, mutations are selected that adapt the body to new changes. This is how a new genotype is formed, which forms a new phenotype. Natural selection, according to et, consists of the following stages:

1) Extreme environmental factors lead to morphosis;

2) morphosis leads to destabilization of ontogeny;

3) Destabilization of ontogeny leads to the appearance of an abnormal phenotype, which most closely matches the morphosis;

4) With a successful match of the new phenotype, the modifications are copied, which leads to stabilization - a new reaction norm is formed;

Comparative characteristics of hereditary and non-hereditary variability

Modification variability in human life

Man, in general, has long used the knowledge of modification variability, for example, in the economy. Knowing certain individual characteristics of each plant (for example, the need for light, water, temperature conditions), it is possible to plan the maximum level of use (within the reaction norm) of this plant - to achieve the highest fruitfulness. Therefore, people place different types of plants for their formation in different conditions - in different seasons, etc. The situation is similar with animals - knowledge of the need, for example, cows causes increased production of milk and, as a result, an increase in milk yield.

Since the functional asymmetry of the cerebral hemispheres is formed with the achievement of a certain age, and in illiterate uneducated people it is less, it can be assumed that the asymmetry is a consequence of modification variability. Therefore, at the stages of training, it is very advisable to identify the child's abilities in order to most fully realize its phenotype.

Examples of modification variability

• In insects and animals
• An increase in red blood cells when climbing mountains in animals (homeostasis)
  • Increased skin pigmentation with intense exposure to ultraviolet radiation
  • The development of the motor apparatus as a result of training
  • Scars (morphosis)
  • Change in coloration of Colorado potato beetles with prolonged exposure to high or low temperatures on their pupae
  • Changing the color of the fur in some animals with changing weather conditions
  • The ability of butterflies from the genus Vanessa (Vanessa) to change their color with changes in temperature
• In plants
  • The different structure of the underwater and emersed leaves in water ranunculus plants
  • Development of undersized forms from seeds of lowland plants grown in the mountains

• In bacteria
  • work of the genes of the lactose operon of Escherichia coli