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 the 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 toes 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 occurrence 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 be due to an increase or decrease in 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. A certain role can be played by antimutagens - 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 adaptations 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.

Variability (in genetics) is the property of a living organism to respond to the effects of the external or internal environment by acquiring a new biological trait.

Depending on the causes, nature and nature of the changes, variability is distinguished between hereditary (mutational, or genotypic) and non-hereditary (phenotypic, or modification).

At the heart of hereditary variability are changes in the genetic apparatus at any level of its organization - gene, chromosome (see), genome. The resulting change is copied and reproduced from generation to generation.

Hereditary, or genotypic, variability is divided into combinative and mutational.

Combinative variability is associated with obtaining new combinations of genes in the genotype (see. Heredity), which is achieved as a result of two processes: 1) independent divergence of chromosomes during meiosis (see) and their random combination during fertilization (see); 2) gene recombination due to crossing over; the hereditary factors themselves (genes) do not change, but new combinations of them with each other lead to the appearance of organisms with a new phenotype.

Mutational variability is the result of sudden hereditary changes in the genetic material - mutations that are not associated by their nature with the processes of splitting or recombination.

A mutation can occur in any gene, any cell, at any stage of development. At the same time, the ability of individual cell genes to mutate (mutability) is different for different cells. In addition, the same changes in genotypes can manifest themselves differently in different cells. A mutation occurs either under normal conditions of existence (spontaneous mutations), or under the influence of special conditions, such as radiation, physical, chemical and other agents (induced mutations). The agent that causes a mutation is called a mutagen, the modified organism is called a mutant.

The mutation process can proceed with equal probability in any direction - from the original (wild) to the mutant and from the mutant to the wild. In the latter case, one speaks of back or true back mutations. The occurrence of back mutations is judged by the restoration (reversion) of the phenotype. Nevertheless, the restoration of the original phenotype is not an absolute indicator of genotype reversion, because it can also be due to a mutation in a completely different locus (section) of the genetic material. Such mutations are called suppressor.

All changes in the genetic material are divided into gene and chromosomal.

Genetic, or point, mutations are limited to one gene and are caused by the replacement of one base (see) with another, their rearrangement or loss.

Chromosomal mutations affect changes in the number of chromosomes or their structure. The latter can be limited to the limits of one - deletion or duplication (i.e., loss or doubling of part of the chromosomes), inversion (reversal of a chromosome section by 180º), insertions (gene rearrangement) or can capture non-homologous chromosomes - translocations (change in the linkage group of genes due to exchange of regions between non-homologous chromosomes).

Changes in the number of chromosomes usually occur as a result of a violation of the normal processes of meiosis and are expressed in an increase or decrease in the number of complete sets of chromosomes (polyploidy, haploidy) or the number of individual chromosomes of a set (heteroploidy, aneuploidy). Sometimes these changes are referred to by the general term "genomic mutations".

The evolutionary significance of mutations in different cells is not the same and depends on the type of organism. From this point of view, in individuals that reproduce sexually, mutations are generative (mutations of the cells of the reproductive system) and somatic. The mutation of a somatic cell, if it is not detailed for it, will be reproduced in a generation and will lead to the formation of cell systems consisting of normal and mutant cells (so-called mosaics); the number of mutant cells will be proportional to the number of divisions following the mutation.

This mechanism is also applicable to generative mutations. Consequently, the earlier in relation to the terms of development a mutation occurs, the more significant will be the number of altered gametes - or eggs - and the more likely it is that the mutant sex will take part in fertilization. Somatic mutations are not transmitted to gametes and disappear with the death of the organism. Thus, if a somatic mutation (from the point of view of its inheritance) does not matter for sexually reproducing organisms, then the significance of generative mutations in the occurrence of hereditary pathology is enormous. For organisms that reproduce asexually, the division into somatic and generative mutations is not essential.

Non-hereditary variability is not associated with a change in the genotype and is observed as a change in the morphological, physiological and biochemical characteristics of the organism during the development of the organism (ontogenetic variability, phenotypic variability) or as a result of varying environmental conditions (modification variability). However, in both cases, all changes are controlled by the genotype. In the first case - the time and order of occurrence of changes, in the second - the limits of these changes ().

A specific phenotypic or modificational change is not inherited, while the ability of a genotype to respond to a corresponding environmental change is hereditary.

Thus, if the role of non-hereditary changes in the evolution of living nature is limited, then hereditary variability, regardless of its type, served as the main initial mechanism, in combination with artificial and natural selection, which led to the emergence of the whole variety of forms of living nature.

Variation in biology is the occurrence of individual differences between individuals of the same species. Due to variability, the population becomes heterogeneous, and the species has a better chance of adapting to changing environmental conditions.

In a science like biology, heredity and variation go hand in hand. There are two types of variability:

• Non-hereditary (modification, phenotypic).
• Hereditary (mutational, genotypic).

Non-hereditary variability

Modification variability in biology is the ability of a single living organism (phenotype) to adapt to environmental factors within its genotype. Due to this property, individuals adapt to changes in climate and other conditions of existence. underlies the adaptation processes occurring in any organism. So, in outbred animals, with the improvement of conditions of detention, productivity increases: milk yield, egg production, and so on. And the animals brought to the mountainous regions grow undersized and with a well-developed undercoat. Changes in environmental factors and cause variability. Examples of this process can be easily found in everyday life: human skin becomes dark under the influence of ultraviolet rays, muscles develop as a result of physical exertion, plants grown in shaded places and in the light have different leaf shapes, and hares change coat color in winter and summer.

Non-hereditary variability is characterized by the following properties:

• group character of changes;
• not inherited by offspring;
• change in trait within the genotype;
• the ratio of the degree of change with the intensity of the impact of an external factor.

hereditary variability

In biology, hereditary or genotypic variability is the process by which the genome of an organism changes. Thanks to her, the individual acquires features that were previously unusual for her species. According to Darwin, genotypic variation is the main engine of evolution. There are the following types of hereditary variability:

• mutational;
• combinative.

Occurs as a result of the exchange of genes during sexual reproduction. At the same time, the traits of the parents are combined in different ways in a number of generations, increasing the diversity of organisms in the population. Combinative variability obeys the rules of Mendelian inheritance.

An example of such variability is inbreeding and outbreeding (closely related and unrelated crossing). When the traits of an individual producer want to be fixed in the breed of animals, then inbreeding is used. Thus, the offspring becomes more uniform and reinforces the qualities of the founder of the line. Inbreeding leads to the manifestation of recessive genes and can lead to the degeneration of the line. To increase the viability of the offspring, outbreeding is used - unrelated crossing. At the same time, the heterozygosity of the offspring increases and the diversity within the population increases, and, as a result, the resistance of individuals to the adverse effects of environmental factors increases.

Mutations, in turn, are divided into:

• genomic;
• chromosomal;
• genetic;
• cytoplasmic.

Changes affecting sex cells are inherited. Mutations in can be transmitted to offspring if the individual reproduces vegetatively (plants, fungi). Mutations can be beneficial, neutral or harmful.

Genomic mutations

Variation in biology through genomic mutations can be of two types:

• Polyploidy - a mutation often found in plants. It is caused by a multiple increase in the total number of chromosomes in the nucleus, is formed in the process of violation of their divergence to the poles of the cell during division. Polyploid hybrids are widely used in agriculture - in crop production there are more than 500 polyploids (onion, buckwheat, sugar beet, radish, mint, grapes and others).
• Aneuploidy is an increase or decrease in the number of chromosomes in individual pairs. This type of mutation is characterized by low viability of the individual. A widespread mutation in humans - one in the 21st pair - causes Down's syndrome.

Chromosomal mutations

Variability in biology by way appears when the structure of the chromosomes themselves changes: loss of the terminal section, repetition of a set of genes, rotation of a single fragment, transfer of a chromosome segment to another place or to another chromosome. Such mutations often occur under the influence of radiation and chemical pollution of the environment.

Gene mutations

A significant part of these mutations does not appear externally, as it is a recessive trait. Gene mutations are caused by a change in the sequence of nucleotides - individual genes - and lead to the appearance of protein molecules with new properties.

Gene mutations in humans cause the manifestation of some hereditary diseases - sickle cell anemia, hemophilia.

Cytoplasmic mutations

Cytoplasmic mutations are associated with changes in the structures of the cell cytoplasm containing DNA molecules. These are mitochondria and plastids. Such mutations are transmitted through the maternal line, since the zygote receives all the cytoplasm from the maternal egg. An example of a cytoplasmic mutation that has caused variability in biology is plant pinnateness, which is caused by changes in chloroplasts.

All mutations have the following properties:

• They appear suddenly.
• Passed down by inheritance.
• They don't have any direction. Mutations can be subjected to both an insignificant area and a vital sign.
• Occur in individuals, that is, individual.
• In their manifestation, mutations can be recessive or dominant.
• The same mutation can be repeated.

Each mutation is caused by specific causes. In most cases, it cannot be accurately determined. Under experimental conditions, to obtain mutations, a directed factor of the external environment is used - radiation exposure and the like.