Hereditary changes in genetic material are called mutations. According to the nature of their manifestation, they can be dominant or recessive. This circumstance is very important for the existence of the species and its populations.

Mutations, as a rule, turn out to be harmful because they disrupt the finely balanced system of biochemical transformations and rearrange the genetic apparatus. Holders of harmful dominant mutations, which immediately manifest themselves in homo- and heterozygous organisms, often turn out to be non-viable and die at the earliest stages of ontogenesis. As a result of mutations, anomalies in the structure of the body and hereditary human diseases appear and are inherited.

Mutations that sharply reduce viability, partially or completely stop development, are called semi-lethal and lethal, respectively. In humans, such mutations include the recessive hemophilia gene.

Based on the nature of changes in the genetic apparatus, mutations are distinguished: genomic, caused by a change in the number of the complete set of chromosomes.

• Chromosomal mutations are associated with changes in the structure of chromosomes or their number.
• Polyploidy is an increase in the number of chromosomes, multiples of the haploid set. Among plants, there are triploids (3p), tetraploids (4p), etc. More than 500 polyploids are known in plant growing (sugar beets, buckwheat, mint, etc.). All of them are distinguished by a large vegetative mass and are of great value.
• Heteroploidy is a change in the number of chromosomes that is not a multiple of the haploid set. These are mutations associated with an excess or deficiency of one chromosome from a pair of homologous chromosomes. Such mutations occur when meiosis is disrupted, when after conjugation a pair of chromosomes does not diverge and both homologous chromosomes end up in one gamete, and none in the other.
• Heteroploidy is harmful to the body. For example, in humans, the appearance of an extra chromosome in pair 21 causes Down syndrome (dementia).
• Gene mutations affect the structure of the gene itself and entail changes in the properties of the body (hemophilia, color blindness, albinism, etc.).
• Point, or gene mutations, are caused by the replacement of one or more nucleotides within one gene. They entail a change in the structure of proteins, consisting in the appearance of a new sequence of amino acids in the polypeptide chain.

Mutations occur in both somatic and generative cells. Their biological significance for humans is ambiguous. Somatic mutations are not inherited and do not have much significance in the process of evolution. However, in individual development they can influence the formation of traits. If a mutation occurs in the generative cells from which gametes develop, then new characteristics appear in the next or subsequent generations.

The events of our century have shown what potential dangers are fraught with irradiation of living organisms, including humans. From a biological point of view, the most dangerous is ionizing radiation, which includes x-rays and radioactive radiation. In large doses, ionizing radiation destroys and kills cells. Smaller doses lead to other defects: breaks in DNA molecules, which make cell division impossible. Less pronounced damage manifests itself in the form of mutations, which are passed on to descendants during cell division. These types of mutations in somatic cells cause cancer and other diseases.

The nature of mutations does not depend on the external environment, however, factors such as ionizing radiation and certain chemicals increase the frequency of mutations. Exposure of humans to high doses of short-wave radiation causes the development of radiation sickness.

The genetic effect of radiation rarely manifests itself immediately, but the risk of accumulation of harmful genes in the population that threatens future generations should not be underestimated.

When developing new varieties of plants and strains of microorganisms, induced mutations are used (artificially caused by various mutagenic factors: chemicals, X-rays or ultraviolet rays). Then the resulting mutants are selected, preserving the most productive ones.

N.I. Vavilov, studying mutations in related species, established the law of homological series in hereditary variability. .u.

Genetically close species and genera are characterized by similar series of hereditary variability with such regularity that, knowing the number of forms within one species, one can predict the presence of parallel forms in other species of genera.

Guided by the law, it is possible to predict what mutational forms should arise in closely related species of domestic animals, new varieties of cultivated plants, as well as new expected forms (species, genera) in taxonomy.

The application of the laws of heredity and variability to the theory of selection led to a better understanding and significant improvement of a number of important selection methods, the development of new methods, and made it possible to draw up various breeding programs.

Selection (from the Latin “seliktio” - selection) is the science of breeding new and improving existing varieties of cultivated plants, breeds of domestic animals and strains of microorganisms that meet human needs and the level of productive forces of society.

Variety, breed and strain are populations artificially created by man, having certain hereditary characteristics: a complex of morphological and physiological characteristics, productivity and reaction rate.

The creator of the modern genetic basis of selection is N.I. Vavilov. In his opinion, selection is human-directed evolution.

Basic selection methods: hybridization and selection.

Stages of selection work

Stage I of selection work

The original varietal and species diversity of plants and animals are objects of breeding work (without knowledge of the source material, without studying its origin and evolution, it is impossible to improve the existing forms of animals and plants).

At this stage, the work of N. I. Vavilov is used to establish the centers of origin of cultivated plants in the centers of ancient agriculture, create their collection and use them as source material. There are eight such centers.

1. South Asian tropical center. Tropical India, Indochina, Southern China, islands of Southeast Asia. Exceptionally rich in cultivated plants (about ½ of the known species of cultivated plants). The homeland of rice, sugar cane, and many fruit and vegetable crops.
2. East Asian Center. Central Eastern China, Japan, Taiwan Island, Korea. The homeland of soybeans, several types of millet, and many fruit and vegetable crops. This center is also rich in species of cultivated plants - about 20% of the world's diversity.
3. South-West Asian Center. Asia Minor, Central Asia, Iran, Afghanistan, North-West India. The homeland of several forms of wheat, rye, many grains, legumes, grapes, and fruits. 14% of the world's cultural flora originated there.
4. Mediterranean center. Countries located on the shores of the Mediterranean Sea. This center, where the greatest ancient civilizations were located, produced about 11% of cultivated plant species. These include olives, many forage plants (clover, single-flowered lentils), many vegetables (cabbage) and forage crops.
5. Abyssinian center. A small region of the African continent with a very distinctive flora of cultivated plants. Obviously, a very ancient center of original agricultural culture. The homeland of grain sorghum, one type of banana, the oilseed chickpea plant, and a number of special forms of wheat and barley.
6. Central American Center. Southern Mexico. The homeland of corn, long-fiber cotton, cocoa, a number of pumpkins, beans - in total about 90 species of cultivated plants.
7. Andean (South American) center. Includes parts of the Andean mountain range along the west coast of South America. The homeland of many tuberous plants, including potatoes, some medicinal plants (cocaine bush, cinchona tree, etc.)

The vast majority of cultivated plants are associated in their origin with one or more of the geographic centers listed above.

Stage II - crossing (hybridization)

There are two types:

1. Closely related - inbreeding (allowing recessive genes to be transferred to a homozygous state);
2. Unrelated (helping to combine valuable characteristics of different forms in one organism).

Stage III - selection - the final stage of selection.

There are two known forms of selection:

• mass (selection of a group of individuals similar in phenotype, but causing splitting during reproduction)
• individual (selection of single valuable forms and separate cultivation of the Offspring of each individual) leads to the creation of a variety or breed of a pure line.

Inbreeding, polyploidy, artificial mutagenesis, and distant hybridization are widely used in plant breeding.

Well-known geneticist breeders have done a lot in the field of plant breeding: I.V. Michurin and G.D. Karnechenko, II. V. Tsitsin, P. II. Lukyanenko, V. N. Craft, V. S. Pustovoit and l r.

They developed high-yielding varieties of sugar beet, buckwheat, and cotton; highly productive Kuban wheat varieties, Ukrainian varieties “Mironovskaya-808”, “Yubileinaya-50”, “Kharkovskaya-63”, etc.

Selection of animals differs from that of plants: animals produce few offspring, they reach sexual maturity later, they do not reproduce vegetatively, and there is no self-fertilization.

In animal selection, hybridization and selection (mass and individual), inbreeding and other methods are used (M. F. Ivanov, N. S., Baturin, etc.)

Selection of microorganisms is a young, developing branch of breeding. Its task is to obtain highly productive microorganisms by exposing the original forms to X-rays, ultraviolet rays and chemical mutagens.

Alternating treatment with mutagens with selection makes it possible to isolate strains that are tens of times more productive than the original one.

Genetics

Population genetics is the science of the genetic structure of natural populations and the genetic processes occurring in it, such as genetic drift, migration, mutation and selection.

All organisms consist of large populations in which, according to the laws of genetics, a balance of genetic material is maintained. However, this balance is constantly disrupted by mutation processes, migrations, genetic drift and other factors.

All diversity in human populations is the result of mutational changes. Prominent geneticist S. S. Chetverikov (1882-1959) made a significant contribution to proving the connection between genetics and evolution. He showed that the first elementary processes begin in populations. Natural populations, with relative phenotypic homogeneity in genetic structure, are heterogeneous and saturated with many open mutations that form a reserve (genetic load) of hereditary variability.

Genetic structure is understood as the ratio of different genotypes and allelic genes in it. The English mathematician Hardy and the German physician Weinberg established that under ideal conditions - a large population without mutations, migrations and selection - the ratio of genotypes and allelic genes is constant in all generations.

The reserve of hereditary variability in a population is formed due to mutation. Dominant mutations occur rarely, appear immediately and are subject to selection,

Recessive mutations in heterozygous organisms do not manifest themselves phenotypically, but upon crossing they saturate the gene pool of the population and form new genotypes.

The gene pool of populations is also replenished due to gene flow - the migration of individuals from other populations bringing new genes. They, just like mutations, do not appear at first in heterozygous organisms during crossings. One way for gene frequencies to change relatively quickly is through the random distribution of genes, called genetic drift.

Genetic drift, a random, untargeted change in the frequency of occurrence of alleles in a population, caused by periodic population waves. Most often, genetic drift occurs in small populations. As a result of genetic drift in a population, the frequency of occurrence of rare alleles may increase, and some alleles may disappear; Mutant alleles can persist for a long period, which reduces the adaptability of individuals to living conditions.

The reserve of hereditary information is also formed due to combinative variability, in which multidirectional mutations are combined and neutralized in one genotype.

As latent mutations accumulate in a population, they partially transform into a homozygous state and then manifest themselves phenotypically. Under constant conditions, stabilizing selection (selection in favor of the norm of a trait) eliminates them as inappropriate to environmental conditions.

In changing conditions, under the influence of driving selection (selection of certain deviations from the established norm of traits), the reserve of hereditary variability allows the population to adapt to new environmental conditions. The more genotypes there are in a population, the wider its reaction norm, the more likely its survival in changing conditions and the ability to more fully use new habitats.

Each biological species has a unique gene pool, therefore one of the most important tasks of humanity is to protect the gene pool of natural populations of organisms.

Variation is the ability of organisms to acquire differences from other individuals of their species. There are three types - mutations, combinations and modifications.

MUTATIONAL VARIABILITY- these are changes in the DNA of a cell (changes in the structure and number of chromosomes). Occur under the influence of ultraviolet radiation, radiation (X-rays), etc. They are inherited and serve as material for (the mutation process is one of).

COMBINATIVE VARIABILITY occurs when the genes of the father and mother are recombined (mixed). Sources:
1) Crossing over during meiosis (homologous chromosomes come close together and change sections).
2) Independent chromosome segregation during meiosis.
3) Random fusion of gametes during fertilization.

Example: the night beauty flower has a gene for red petals A, and a gene for white petals A. The Aa organism has pink petals; this trait occurs when the red and white genes are combined.

MODIFICATION VARIABILITY occurs under the influence of the environment. It is not inherited, because during modifications only the phenotype (trait) changes, and the genotype does not change.

Examples:
1) You can cut the dandelion root into 2 parts and plant them in different conditions; Plants that look different will grow, although they have the same genotype.
2) If a person is in the sun, he will tan; If he does physical exercise, he will increase his muscles.
3) With good maintenance, chickens increase egg production, cows give more milk.

Modification variability is not unlimited; for example, a white man can never tan like a black man. The boundaries within which modification changes can occur are called "reaction norm", they are inherent in the genotype and are inherited.

1. Below is a list of characteristics of variability. All of them, except two, are used to describe the characteristics of mutational variability. Find two characteristics that “fall out” from the general series and write down the numbers under which they are indicated.

2) rotation of a chromosome section by 180 degrees
3) reduction in the number of chromosomes in the karyotype
4) changes in phenotype within the normal reaction range of the trait
5) gene recombination during crossing over

Answer

2. All the characteristics given below, except two, are used to describe mutational variability. Identify two characteristics that “fall out” from the general list and write them down in the numbers under which they are indicated.
1) formed under the influence of x-rays
2) has a directional modification
3) varies within the reaction norm
4) formed as a result of disruption of meiosis
5) occurs suddenly in individuals

Answer

3. All of the characteristics below, except two, are used to describe mutational variability. Find two characteristics that “fall out” from the general series and write down the numbers under which they are indicated.
1) depends on the effect of radiation
2) can occur with the loss of several nucleotides
3) characterized by the appearance of an additional chromosome
4) depends on the breadth of the reaction norm of the trait
5) determined by the combination of gametes during fertilization

Answer

4. All the processes below, except two, are characteristic of mutational variability. Find two processes that “drop out” from the general list and write down the numbers under which they are indicated.
1) change in a sign within the normal range of reaction
2) autosomal inheritance
3) change in the number of chromosomes in a cell
4) loss of a chromosome section
5) polyploidy

Answer

5. All of the characteristics below, except two, are used to describe mutational variability. Find two characteristics that “fall out” from the general list and write down the numbers under which they are indicated.
1) random combination of non-homologous chromosomes in meiosis
2) transfer of a chromosome section to a non-homologous chromosome
3) reduction in the number of chromosomes in the karyotype
4) changes in the nucleotide sequence in the DNA structure
5) gene recombination during crossing over

Answer

6f. All of the characteristics below, except two, are used to describe mutational variability. Identify two characteristics that “fall out” from the general list and write down the numbers under which they are indicated.
1) increase in the number of chromosomes in a cell
2) independent chromosome segregation in meiosis
3) conjugation and crossing over during reduction division
4) loss of a chromosome section
5) change in the sequence of triplets in nucleic acid

Answer

Choose three options. Mutations lead to change
1) primary protein structure
2) stages of fertilization
3) gene pool of the population
4) reaction norms of the trait
5) sequence of mitosis phases
6) sexual composition of the population

Answer

Choose one, the most correct option. An adaptive change in a particular trait within certain genetic limits is called
1) reaction norm
2) relative variability
3) mutation
4) combinative variability

Answer

Choose one, the most correct option. Norm of reaction of a trait
1) inherited
2) depends on the environment
3) formed in ontogenesis
4) depends on the number of chromosomes

Answer

1. Establish a correspondence between the trait and the type of variability as a result of which it arises: 1) combinative, 2) modification
A) the appearance of a green body color in euglena in the light
B) a combination of parents' genes
C) darkening of human skin when exposed to ultraviolet rays
D) accumulation of subcutaneous fat in bears with excess nutrition
D) the birth in a family of children with brown and blue eyes in a ratio of 1:1
E) the appearance of children with hemophilia in healthy parents

Answer

2. Establish a correspondence between examples and forms of variability: 1) combinative, 2) modification. Write numbers 1 and 2 in the order corresponding to the letters.
A) change in fur color in the white hare depending on temperature
B) difference in weight between bulls of the same calving kept under different conditions
C) the appearance of wrinkled seeds in peas when crossing plants with smooth seeds
D) the presence of leaves of different lengths on one plant
D) the birth of a colorblind child to healthy parents

Answer

Establish a correspondence between the characteristic and the type of variability: 1) mutational, 2) combinative
A) occurs when exposed to radiation
B) formed by the fusion of gametes
B) is caused by independent divergence of pairs of chromosomes
D) is caused by the exchange of genes between homologous chromosomes
D) is associated with an increase in the number of chromosomes in the karyotype

Answer

1. Below is a list of characteristics of variability. All of them, except two, are used to describe the characteristics of combinative variability. Find two characteristics that “fall out” from the general series and write down the numbers under which they are indicated in the table.
1) occurrence under the influence of radiation
2) random combination of non-homologous chromosomes in meiosis
3) random combination of gametes during fertilization

5) change in the nucleotide sequence in mRNA

Answer

2. The following characteristics, except two, are used to describe the causes of combinational variability. Identify these two characteristics that “fall out” from the general list, write down the numbers under which they are indicated.
1) random meeting of gametes during fertilization
2) chromosome spiralization
3) DNA replication in interphase
4) gene recombination during crossing over
5) independent chromosome segregation in meiosis

Answer

3. All of the characteristics below, except two, are used to describe combinative variability. Find two characteristics that “fall out” from the general list and write down the numbers under which they are indicated.
1) random combination of non-homologous chromosomes in a gamete
2) change in the sequence of nucleotides in DNA
3) random meeting of gametes during fertilization
4) gene recombination during crossing over
5) adequacy of phenotypic changes to environmental conditions

Answer

4. All of the characteristics below, except two, are used to describe combinative variability. Identify two characteristics that “fall out” from the general list and write down the numbers under which they are indicated.
1) combination of genes during the formation of gametes
2) formation of the genotype during fertilization
3) the appearance in the offspring of combinations of traits that are absent in the parents
4) change in DNA in the mitochondria of the egg
5) loss of amino acid and change in protein structure

Answer

5. All the examples below, except two, characterize combinative variability. Identify two examples that “fall out” from the general list and write down the numbers under which they are indicated in the table.
1) a combination of characteristics of both parents in the offspring
2) the appearance of a child with hemophilia in healthy parents
3) the appearance of green body color in euglena in the light
4) the birth of a blue-eyed child from brown-eyed parents
5) darkening of human skin when exposed to ultraviolet rays

Answer

1. Analyze the table. For each lettered cell, select the appropriate term from the list provided.
1. somatic
2. non-hereditary
3. the birth of offspring with a new phenotype as a result of gene recombination due to crossing over
4. different body weights of bulls of the same litter
5. mutational
6. hereditary

Answer

2. Analyze the table. For each lettered cell, select the appropriate term from the list provided.
1) somatic
2) hereditary
3) the birth of an individual with reduced wings from the parent organisms of Drosophila
4) different shapes of the leaf blade of arrowhead
5) mutational
6) non-hereditary

Answer

3. Analyze the table. For each lettered cell, select the appropriate term from the list provided.
1) modification
2) genetic
3) change in fur color in the white hare depending on the time of year
4) hereditary
5) combinative
6) chromosomal
7) the birth of a wingless Drosophila individual from winged parent organisms
8) non-hereditary

Answer

Analyze the table “Types of variability”. For each cell indicated by a letter, select the corresponding concept or corresponding example from the list provided.
1) only genotype
2) genotype and phenotype
3) mutational
4) non-hereditary
5) phenotypic
6) the appearance of a flower with five petals in lilac
7) the appearance of thick undercoat on a fox in winter
8) birth of a child with Down syndrome

Answer

Choose one, the most correct option. The reason for the combinational variability may be
1) changes in genes during DNA replication
2) chromosomal mutation
3) template DNA synthesis
4) random meeting of gametes during fertilization

Answer

Choose one, the most correct option. A change in the egg production of chickens within certain limits, depending on the conditions of detention and feeding ration, is a manifestation
1) mutational variability
2) adaptation
3) reaction norms of the trait
4) self-regulation

Answer

Choose two correct statements and write down the numbers under which they are indicated in the table.
1) The form of hereditary variability caused by a random combination of gametes is called combinative variability.
2) Phenotypic variability is associated with changes in the genotype.
3) Hereditary variability is associated with changes in the genotype.
4) Modification is a spontaneously occurring natural or artificially caused change in genetic material.

Answer

Establish a correspondence between the characteristics of organisms and the ranges of their reaction norm: 1) narrow reaction norm, 2) wide reaction norm. Write numbers 1 and 2 in the order corresponding to the letters.
A) body weight of cattle
B) the size of the human eyeball
B) the number of vertebrae in the cervical spine of mammals
D) the thickness of mammalian fur
D) the size and shape of a plant flower
E) egg production of chickens

Answer

Establish a correspondence between examples and types of variability: 1) combinative, 2) modification, 3) mutational. Write numbers 1-3 in the order corresponding to the letters.
A) the birth of a right-handed child from left-handed parents
B) change in fur color in an ermine rabbit
C) the formation of green smooth and yellow wrinkled seeds in peas
D) the birth of a blue-eyed child from brown-eyed parents
D) the birth of smooth-haired offspring in guinea pigs with shaggy hair
E) the appearance of a flower with five petals in lilac

Answer

© D.V. Pozdnyakov, 2009-2019

Hereditary changes in genetic material are now called mutations. Mutations- sudden changes in genetic material, leading to changes in certain characteristics of organisms.

The term “mutation” was first introduced into science by the Dutch geneticist G. de Vries. While conducting experiments with evening primrose (an ornamental plant), he accidentally discovered specimens that differed in a number of characteristics from the rest (large growth, smooth, narrow and long leaves, red veins of the leaves and a wide red stripe on the calyx of the flower, etc.). Moreover, during seed propagation, plants persistently retained these characteristics from generation to generation. As a result of generalizing his observations, de Vries created a mutation theory, the main provisions of which have not lost their significance to this day:

© mutations occur suddenly, spasmodically, without any transitions;

© mutations are hereditary, i.e. persistently passed on from generation to generation;

© mutations do not form continuous series, are not grouped around an average type (as with modification variability), they are qualitative changes;

© mutations are non-directional - any locus can mutate, causing changes in both minor and vital signs in any direction;

© the same mutations can occur repeatedly;

© mutations are individual, that is, they occur in individual individuals.

The process of mutation occurrence is called mutagenesis, organisms in which mutations have occurred - mutants, and environmental factors causing mutations are mutagenic.

The ability to mutate is one of the properties of a gene. Each individual mutation is caused by some reason, usually associated with changes in the external environment.

Classification of mutations

There are several classifications of mutations:

© Mutations according to their place of origin:

¨ Generative- originated in germ cells . They do not affect the characteristics of a given organism, but appear only in the next generation.

¨ Somatic - arising in somatic cells . These mutations appear in this organism and are not transmitted to offspring during sexual reproduction (a black spot against the background of brown wool in astrakhan sheep). Somatic mutations can be preserved only through asexual reproduction (primarily vegetative).

© Mutations by adaptive value:

¨ Useful- increasing the viability of individuals.

¨ Harmful:

§ lethal- causing death of individuals;

§ semi-lethal- reducing the viability of an individual (in men, the recessive hemophilia gene is semi-lethal, and homozygous women are not viable).

¨ Neutral - not affecting the viability of individuals.

This classification is very conditional, since the same mutation can be beneficial in some conditions and harmful in others.

© Mutations by nature of manifestation:

¨ dominant, which can make the owners of these mutations unviable and cause their death in the early stages of ontogenesis (if the mutations are harmful);

¨ recessive- mutations that do not appear in heterozygotes, therefore remaining in the population for a long time and forming a reserve of hereditary variability (when environmental conditions change, carriers of such mutations can gain an advantage in the struggle for existence).

© Mutations according to the degree of phenotypic manifestation:

¨ large- clearly visible mutations that greatly change the phenotype (double flowers);

¨ small- mutations that practically do not give phenotypic manifestations (slight lengthening of the awns of the ear).

© Mutations by changing the state of a gene:

¨ straight- transition of a gene from wild type to a new state;

¨ reverse- transition of a gene from a mutant state to a wild type.

© Mutations according to the nature of their appearance:

¨ spontaneous- mutations that arose naturally under the influence of environmental factors;

¨ induced- mutations artificially caused by the action of mutagenic factors.

© Mutations according to the nature of the genotype change:

¨ genes;

¨ chromosomal;

¨ genomic.

Mutations according to the nature of the genotype change

Mutations can cause various changes in the genotype, affecting individual genes, entire chromosomes, or the entire genome.

Gene mutations

Genetic mutations are changes in the structure of a DNA molecule in a region of a specific gene that encodes the structure of a specific protein molecule. These mutations entail a change in the structure of proteins, that is, a new amino acid sequence appears in the polypeptide chain, resulting in a change in the functional activity of the protein molecule. Thanks to gene mutations, a series of multiple alleles of the same gene occurs. Most often, gene mutations occur as a result of:

© replacement of one or more nucleotides with others;

© nucleotide insertions;

© loss of nucleotides;

© nucleotide duplication;

© changes in the order of alternation of nucleotides.

Chromosomal mutations

Chromosomal mutations- mutations that cause changes in chromosome structure . They arise as a result of the breakage of chromosomes with the formation of “sticky” ends. “Sticky” ends are single-stranded fragments at the ends of a double-stranded DNA molecule. These fragments are able to connect with other fragments of chromosomes that also have “sticky” ends. Rearrangements can be carried out both within the same chromosome - intrachromosomal mutations and between non-homologous chromosomes - interchromosomal mutations.

© Intrachromosomal mutations:

¨ deletion- loss of part of a chromosome (АВСD ® AB);

¨ inversion- rotation of a chromosome section by 180˚ (ABCD ® ACBD);

¨ duplication- doubling of the same chromosome section; (ABCD ® ABCBCD);

© Interchromosomal mutations:

¨ translocation- exchange of sections between non-homologous chromosomes (ABCD ® AB34).

Genomic mutations

Genomic mutations are called mutations that result in a change in the number of chromosomes in a cell. Genomic mutations arise as a result of disturbances in mitosis or meiosis, leading either to uneven divergence of chromosomes to the poles of the cell, or to doubling of chromosomes, but without division of the cytoplasm.

Depending on the nature of the change in the number of chromosomes, there are:

¨ Haploidy- reduction in the number of complete haploid sets of chromosomes.

¨ Polyploidy- increase in the number of complete haploid sets of chromosomes. Polyploidy is more often observed in protozoa and plants. Depending on the number of haploid sets of chromosomes contained in cells, they are distinguished: triploids (3n), tetraploids (4n), etc. They can be:

§ autopolyploids- polyploids resulting from the multiplication of genomes of one species;

§ allopolyploids- polyploids resulting from the multiplication of genomes of different species (typical of interspecific hybrids).

¨ Heteroploidy (aneuploidy) - a multiple increase or decrease in the number of chromosomes. Most often, there is a decrease or increase in the number of chromosomes by one (less often two or more). Due to the nondisjunction of any pair of homologous chromosomes in meiosis, one of the resulting gametes contains one less chromosome, and the other one more. The fusion of such gametes with a normal haploid gamete during fertilization leads to the formation of a zygote with a smaller or larger number of chromosomes compared to the diploid set characteristic of a given species. Among aneuploids there are:

§ trisomics- organisms with a set of chromosomes 2n+1;

§ monosomics- organisms with a set of chromosomes 2n -1;

§ nullosomics- organisms with a set of chromosomes 2n–2.

For example, Down syndrome in humans occurs as a result of trisomy on the 21st pair of chromosomes.

N.I. Vavilov, studying hereditary variability in cultivated plants and their ancestors, discovered a number of patterns that made it possible to formulate the law of homological series of hereditary variability: “Species and genera that are genetically close are characterized by similar series of hereditary variability with such correctness that, knowing a number of forms within one species, one can foresee the finding of parallel forms in other species and genera. The closer the genera and species are genetically located in the general system, the more complete the similarity in the series of their variability. Whole families of plants are generally characterized by a certain cycle of variation passing through all the genera and species that make up the family.”

This law can be illustrated by the example of the Poa family, which includes wheat, rye, barley, oats, millet, etc. Thus, the black color of the caryopsis is found in rye, wheat, barley, corn and other plants, and the elongated shape of the caryopsis is found in all studied species of the family. The law of homological series in hereditary variability allowed N.I. Vavilov himself to find a number of forms of rye, previously unknown, based on the presence of these characteristics in wheat. These include: awned and awnless ears, grains of red, white, black and purple color, mealy and glassy grains, etc.

The law discovered by N.I. Vavilov is valid not only for plants, but also for animals. Thus, albinism occurs not only in different groups of mammals, but also in birds and other animals. Short-fingeredness is observed in humans, cattle, sheep, dogs, birds, the absence of feathers in birds, scales in fish, wool in mammals, etc.

The law of homologous series of hereditary variability is of great importance for breeding practice. It allows us to predict the presence of forms not found in a given species, but characteristic of closely related species, that is, the law indicates the direction of searches. Moreover, the desired form can be found in the wild or obtained through artificial mutagenesis. For example, in 1927, the German geneticist E. Baur, based on the law of homological series, suggested the possible existence of an alkaloid-free form of lupine, which could be used as animal feed. However, such forms were not known. It has been suggested that alkaloid-free mutants are less resistant to pests than bitter lupine plants, and most of them die before flowering.

Based on these assumptions, R. Zengbusch began the search for alkaloid-free mutants. He examined 2.5 million lupine plants and identified among them 5 plants with a low content of alkaloids, which were the ancestors of fodder lupine.

Later studies showed the effect of the law of homological series at the level of variability of morphological, physiological and biochemical characteristics of a wide variety of organisms - from bacteria to humans.

Variability.

Modification variability.

Combinative variability.

Marriage system.

Mutational variability.

One of the signs of life is variability. Any living organism is different from other members of its species. Variability– the property of living organisms to exist in different forms. Group And individual variability - classification according to evolutionary significance. Variability realized by a group of organisms is called group, while in one organism or a group of its cells it is individual.

According to the nature of changes in signs and mechanism:

Phenotypic

Random

Modification

Genotypic

Somatic

Generative (mutational, combinative)

a) genetic

b) chromosomal

c) genomic

Modification variability reflects a change in phenotype under the influence of environmental factors (strengthening and development of muscle and bone mass in athletes, increased erythropoiesis in high mountains and the far north). A special case of phenotypic variability is phenocopies. Phenocopies– phenotypic modifications caused by environmental conditions that imitate genetic traits. Under the influence of external conditions, signs of a completely different genotype are copied on a genetically normal organism. The manifestation of color blindness can occur under the influence of nutrition, poor mental constitution, and increased irritability. A person develops the disease vitiligo (1% of people) - a disorder of skin pigmentation. 30% of patients have a genetic defect, the rest have occupational vitiligo (exposure to special chemicals and toxic substances on the body). In Germany 15 years ago, children were born with fecomelia - shortened, flipper-like arms. It revealed. That the birth of such children occurred if the mother took Telidomide (a sedative indicated for pregnant women). As a result, the normal non-mutant genotype received a mutation.

Phenocopies appear in most cases under the influence of the external environment in the early stages of embryogenesis, which leads to congenital diseases and developmental defects. The presence of phenocopies makes it difficult to diagnose diseases.

Somatic variability not inherited.

Combinative variability- the result of independent chromosome divergence during the process of meiosis, fertilization, crossing over with gene recombination. With combinative variability, recombination of genes occurs, a new individual set of chromosomes arises, and therefore a new genotype and phenotype. For combinative variability in the human system, the marriage system is of great importance. The simplest is random selection of pairs (panmixia). Strictly panmix populations do not exist, because There are restrictions: social, religious, individual, economic and others. Therefore, in human populations there are deviations from panmixia in two directions:

People who are related to each other marry more often than with random selection - inbreeding - inbreeding (consanguineous marriages).

People marry more often through random selection of couples than through consanguineous marriages - autobreeding.

Inbred marriages are of great medical importance. Because the likelihood that both spouses have the same recessive genes is much higher if the spouses are related to each other, especially closely. The relationship is natural. From a medical point of view, selective marriages based on phenotypic characteristics are considered to be close in genetic effect. If the choice of a marriage partner influences the genotype of the offspring - assortative marriages. People who are phenotypically similar are more likely to marry than with a random selection of pairs - positive assortative marriages, if less often - negative ones. Examples include marriages between deaf and mute people, tall people, and people of the same skin color. Negative assortative marriages between red-haired people.

Consanguineous marriages were common in the early stages of human development.

There are 3 inbreeding groups:

between first-degree relatives

consanguineous marriages of isolated populations

encouraged consanguineous marriages for social, religious and other reasons.

Incestuous (forbidden) marriages between relatives of the first kinship: mother-son, father-daughter, brother-sister. Took place in Egypt, the Ptolemaic dynasty. In a number of eastern countries, the family of Ivan the Terrible (starting with Ivan Kalita - several similar marriages).

Legal restrictions: marriages between cousins, nephews and aunts, nieces and uncles are allowed. Although there are restrictions in some countries. USA and UK - uncle-niece, half-uncle-niece - are prohibited. In the USA, cousins ​​are prohibited, in the UK they are allowed.

Consanguineous marriages in isolated areas (isolates), incl. and religious isolates are inevitable, because otherwise the population dies out.

In large non-isolated populations, consanguineous marriages account for 1% in the city and 3% in villages, up to second cousins. Consanguineous marriages are encouraged among Jews in eastern countries. There's up to 12%.

In Samarkand region

Uncle-niece 46

Nephew-Aunt 14

Cousins ​​42

Incest 2

Inbreeding coefficient - average identical by origin.

USA, Catholics – 0.00009

Israel and Jordan – 0.432

India – 0.32

Japan – 0.0046

In India, half of the marriages are between relatives - infant mortality for any income is 50%.

Genetic effect of consanguineous marriages: rare autosomal recessive diseases become common.

The frequency of occurrence of recessive genes, compared to marriages between people who are not related, increases sharply in marriages between relatives.

disease	Frequency of occurrence (typical)	Frequency of occurrence (consanguineous marriages)
Phenylketanuria
Xeroderma pigmentosum
Orusha disease
Microcephaly
Amaurotic idiocy
Anatalasemia

Mutational variability- the only type of variability that can result in the appearance of new genes that may not have been encountered before. The genotype changes and, as a result, the phenotype changes. In accordance with the three levels of organization of genetic material, 3 types of mutations are distinguished: gene, chromosomal and genomic.

Mutation- a sudden hereditary change in any phenotypic trait caused by a sharp structural or functional change.

Gene mutations are associated with changes in the internal structure of genes, which transforms one allele into another. Several types of gene mutations can be distinguished at the molecular level:

Substitution of nucleotide pairs

Deletion

Nucleotide insertion

Rearrangement (inversion) of a gene region.

Substitution of nucleotide pairs. Replacing a purine base with another purine base, or one pyrimidine base with another pyrimidine base is a transition. Replacing a purine base with a pyrimidine base and vice versa is a transversion. When nucleotides are replaced in structural genes, the meaning of the gene changes - missense mutations occur. In this case, one amino acid in the polypeptide is replaced by another. The phenotypic manifestation of the mutation depends on the position of the amino acid in the polypeptide. When the CTC sequence is replaced by CAC, sickle cell anemia occurs. A new polypeptide is formed and hemoglobin has completely different properties. Some missense mutations result in an enzyme that is highly active under some conditions and moderately active under other conditions. Because the genetic code is degenerate, then when replacing triplets encoding the same amino acid, mutations do not appear. Another type of mutation is nonsense mutation. With these mutations, when one nucleotide is replaced by another, meaningless triplets are formed. The synthesis of the polypeptide stops and the protein has completely different properties.

UAG. UAA. UGA. meaningless triplets.

Deletion or insertion of one or more nucleotides results in the loss or insertion of one or more amino acids in the polypeptide. there may be no effect. If a deletion or insertion of 1 nucleotide (or another number of nucleotides not a multiple of 3) occurs, a shift in the reading frame is observed, and the structure of the polypeptide is disrupted.

Most changes in the molecular structure of genes lead to new forms of reading genetic information, which is implemented in metabolic pathways and biochemical reactions, new properties of cells and the whole organism appear. A large number of mutations occur in the body. They affect intelligence, behavior, metabolic traits, etc. mutations that change visible morphological characteristics - visible (albinism mutation). A normal dominant gene turns into a recessive one, melanin production stops, and is phenotypically manifested by white hair and eyes. There is a group of biochemical mutations that are detected using complex biochemical methods. For example, humans synthesize a number of enzymes that convert lactose into galactose. In the absence of the enzyme lactase, fermentation occurs in the large intestine, gas formation, etc. There can be child and adult forms. When galactose accumulates, galactosemia occurs, which can lead to mental retardation.

Mutations that disrupt life - lethal, semi-lethal and sublethal.

Lethal - death of a zygote or a developing organism at a certain stage of embryogenesis - miscarriages.

Semi-lethal and sublethal weaken the viability of the organism or individual cells (for example, brachydactyly - homozygotes die).

Chromosomal mutations (chromosomal aberrations)– structural rearrangements affecting one or more chromosomes. With all the variety of structural rearrangements, they are all associated with the loss or addition of a chromosome section. Partial monosomy and trisomy (see lecture 8). Chromosome mutations account for 7% of chromosomal diseases. Clinically, they are accompanied by multiple malformations and anomalies.

Genomic mutations. Polyploidy is an increase in the number of chromosomes, a multiple of the diploid set (liver cells are normal). Aneuploidy (heteroploidy) is a decrease or increase in the number of chromosomes that is not a multiple of the diploid one. Haploidy is the presence of a haploid set of chromosomes in some cells (as a rule, cell death occurs).

Mutations may be beneficial, harmful, or have no apparent effect—i.e. be neutral. Ordinary genes in a population are adaptive, the owners adapt better, and newly emerging mutations most often have already been encountered before and were lost because they did not contribute to better adaptation to certain living conditions. The mutant gene may accumulate and may be beneficial. Yet most mutations are harmful.

Modification variability

Modifying variability does not cause changes in the genotype; it is associated with the reaction of a given, one and the same genotype to changes in the external environment: under optimal conditions, the maximum capabilities inherent in a given genotype are revealed. Thus, the productivity of outbred animals in conditions of improved housing and care increases (milk yield, meat fattening). In this case, all individuals with the same genotype respond to external conditions in the same way (C. Darwin called this type of variability definite variability). However, another trait - the fat content of milk - is slightly susceptible to changes in environmental conditions, and the color of the animal is an even more stable trait. Modification variability usually fluctuates within certain limits. The degree of variation of a trait in an organism, that is, the limits of modification variability, is called the reaction norm. A wide reaction rate is characteristic of such characteristics as milk yield, leaf size, and color in some butterflies; a narrow reaction norm - the fat content of milk, egg production in chickens, the color intensity of the corollas of flowers, and more. The phenotype is formed as a result of interactions between the genotype and environmental factors. Phenotypic characteristics are not transmitted from parents to offspring; only the reaction norm is inherited, that is, the nature of the response to changes in environmental conditions. In heterozygous organisms, changing environmental conditions can cause different manifestations of this trait.
Modification properties:

1) non-heritability;

2) the group nature of the changes;

3) correlation of changes to the influence of a certain environmental factor;

4) the dependence of the limits of variability on the genotype.

Genotypic (hereditary)variability

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

Mutational variability

Mutation(lat. mutatio- change) - a persistent (that is, one that can be inherited by the descendants of a given cell or organism) transformation of the genotype, occurring under the influence of the external or internal environment. The term was proposed by Hugo de Vries. The process of mutation occurrence is called mutagenesis.

Mutations appear constantly during processes occurring in a living cell. The main processes leading to the occurrence of mutations are DNA replication, DNA repair disorders, transcription and genetic recombination.

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

Mutations, according to the nature of their manifestation, are - dominant And recessive. Mutations often reduce viability or fertility. Mutations that sharply reduce viability, partially or completely stop development, are called semi-lethal. And incompatible with life - lethal. Mutations are also divided into spontaneous And induced. Spontaneous mutations occur spontaneously throughout the life of an organism in normal environmental conditions with a frequency of about 10 -9 - 10 -12 per nucleotide per cellular generation of the organism.

Induced mutations are heritable changes in the genome that arise as a result of certain mutagenic effects in artificial (experimental) conditions or under adverse environmental influences.

Mutations are divided according to the place of their occurrence. A mutation that occurs in germ cells does not affect the characteristics of a given organism, but appears only in the next generation. Such mutations are called generative. If genes change in somatic cells, such mutations appear in this organism and are not transmitted to offspring during sexual reproduction. But with asexual reproduction, if an organism develops from a cell or group of cells that has a changed - mutated - gene, mutations can be transmitted to offspring. Such mutations are called somatic.
Mutations are classified according to the level of their occurrence. There are chromosomal, gene and genomic ( karyotype change( change in the number of chromosomes)) mutations.

Genomic:

Polyploidization(the formation of organisms or cells whose genome is represented by more than two (3n, 4n, 6n, etc.) sets of chromosomes) and aneuploidy (heteroploidy) - a change in the number of chromosomes that is not a multiple of the haploid set (see Inge-Vechtomov, 1989). Depending on the origin of chromosome sets among polyploids there are allopolyploids, which have sets of chromosomes obtained by hybridization from different species, and autopolyploids, in which the number of chromosome sets of their own genome increases by a multiple of n.

Polyploidy or an increase in the number of chromosomes that is a multiple of the haploid set. In accordance with this, plants are distinguished into triploids (3p), tetraploids (4p), etc. More than 500 polyploids are known in plant growing (sugar beets, grapes, buckwheat, mint, radishes, onions, etc.). All of them are distinguished by a large vegetative mass and have great economic value.

A wide variety of polyploids is observed in floriculture: if one original form in the haploid set had 9 chromosomes, then cultivated plants of this species can have 18, 36, 54 and up to 198 chromosomes. Polyploids are obtained as a result of exposure of plants to temperature, ionizing radiation, and chemicals (colchicine), which destroy the cell division spindle. In such plants, the gametes are diploid, and when fused with the haploid germ cells of a partner, a triploid set of chromosomes appears in the zygote (2n + n = 3n). Such triploids do not form seeds; they are sterile, but highly productive. Even-numbered polyploids form seeds. Heteroploidy is a change in the number of chromosomes that is not a multiple of the haploid set. In this case, the set of chromosomes in a cell can be increased by one, two, three chromosomes (2n + 1; 2n + 2; 2n + 3) or decreased by one chromosome (2l-1). For example, a person with Down syndrome has one extra chromosome on the 21st pair, and the karyotype of such a person is 47 chromosomes. People with Shereshevsky-Turner syndrome (2n-1) are missing one X chromosome and 45 chromosomes remain in the karyotype. These and other similar deviations in numerical relationships in a person’s karyotype are accompanied by health disorders, mental and physical disorders, decreased vitality, etc.

Chromosomal:

At chromosomal mutations major rearrangements of the structure of individual chromosomes occur. In this case there are loss (deletion) or doubling of a part (duplication) genetic material of one or more chromosomes, change in the orientation of chromosome segments in individual chromosomes (inversion), and transfer of part of the genetic material from one chromosome to another (translocation), extreme case - unification of whole chromosomes, so-called. Robertsonian translocation, which is a transitional variant from a chromosomal mutation to a genomic one. (That is, with chromosomal mutations, it is possible to separate different sections of a chromosome (deletion), doubling of individual fragments (duplication), rotate a section of a chromosome by 180° (inversion), or attach a separate section of a chromosome to another chromosome (translocation))

Chromosomal mutations are associated with changes in the structure of chromosomes. There are the following types of chromosome rearrangements. Such a change entails disruption of the function of genes in the chromosome and the hereditary properties of the organism, and sometimes its death.

Genetic:

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

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

At the gene level, changes in the primary DNA structure of genes under the influence of mutations are less significant than with chromosomal mutations, but gene mutations are more common. As a result of gene mutations, substitutions, deletions and insertions of one or more nucleotides, translocations, duplications and inversions of various parts of the gene occur. In the case when only one nucleotide changes under the influence of a mutation, they speak of point mutations.

Point mutation, or single base substitution, - a type of mutation in DNA or RNA, which is characterized by the replacement of one nitrogenous base with another. The term also applies to pairwise nucleotide substitutions. The term point mutation also includes insertions and deletions of one or more nucleotides. There are several types of point mutations.

• Base substitution point mutations. Since DNA contains only two types of nitrogenous bases - purines and pyrimidines, all point mutations with base substitutions are divided into two classes: transitions and transversions. Transition is a base substitution mutation when one purine base is replaced by another purine base (adenine to guanine or vice versa), or a pyrimidine base by another pyrimidine base (thymine to cytosine or vice versa. Transversion is a base substitution mutation where one purine base is replaced by a pyrimidine base or vice versa). Transitions occur more often than transversions.

• Frameshift point mutations. They are divided into deletions and insertions. Deletions is a reading frameshift mutation when one or more nucleotides are lost in a DNA molecule. Insertion is a frameshift mutation when one or more nucleotides are inserted into a DNA molecule.

Complex mutations also occur. These are changes in DNA when one section of it is replaced by a section of a different length and a different nucleotide composition.

Point mutations can appear opposite damage to the DNA molecule that can stop DNA synthesis. Such mutations are called target mutations (from the word “target”). Cyclobutane pyrimidine dimers cause both targeted base substitution mutations and targeted frameshift mutations.

Sometimes point mutations occur in so-called undamaged regions of DNA, often in a small vicinity of photodimers. Such mutations are called untargeted base substitution mutations or untargeted frameshift mutations.

Point mutations do not always form immediately after exposure to a mutagen. Sometimes they appear after dozens of replication cycles. This phenomenon is called delayed mutations. With genomic instability, the main cause of the formation of malignant tumors, the number of delayed mutations sharply increases.

There are four possible genetic consequences of point mutations:

1) preservation of the meaning of the codon due to the degeneracy of the genetic code (synonymous nucleotide replacement),

2) a change in the meaning of the codon, leading to the replacement of an amino acid in the corresponding place of the polypeptide chain (missense mutation),

3) formation of a meaningless codon with premature termination (nonsense mutation). There are three meaningless codons in the genetic code: amber - UAG, ocher - UAA and opal - UGA (mutations that lead to the formation of meaningless triplets are named accordingly - for example, the amber mutation),

4) reverse replacement (stop codon to sense codon).

By influence on gene expression, mutations are divided into two categories: mutations such as base pair substitutions And reading frame shift type. The latter are deletions or insertions of nucleotides, the number of which is not a multiple of three, which is associated with the triplet nature of the genetic code.

The primary mutation is sometimes called direct mutation, and a mutation that restores the original structure of the gene is reverse mutation, or reversion. A return to the original phenotype in a mutant organism due to restoration of the function of the mutant gene often occurs not due to true reversion, but due to a mutation in another part of the same gene or even another non-allelic gene. In this case, the recurrent mutation is called a suppressor mutation. The genetic mechanisms due to which the mutant phenotype is suppressed are very diverse.

Kidney mutations (sports) - persistent somatic mutations occurring in the cells of plant growth points. Lead to clonal variability. They are preserved during vegetative propagation. Many varieties of cultivated plants are bud mutations.

Properties of mutations:

1. Mutations occur suddenly, spasmodically.

2. Mutations are hereditary, that is, they are persistently transmitted from generation to generation.

3. Undirected mutations - any locus can mutate, causing changes in both minor and vital signs.

4. The same mutations can occur repeatedly.

5. According to their manifestation, mutations can be beneficial and harmful, dominant and recessive.

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

Mutagenesis models

Currently, there are several approaches to explain the nature and mechanisms of mutation formation. Currently, the polymerase model of mutagenesis is generally accepted. It is based on the idea that the only cause of mutations is random errors in DNA polymerases. In the tautomeric model of mutagenesis proposed by Watson and Crick, the idea was first put forward that mutagenesis is based on the ability of DNA bases to be in different tautomeric forms. The process of mutation formation is considered as a purely physical and chemical phenomenon. Polymerase-tautomeric The ultraviolet mutagenesis model is based on the idea that the formation of cis-syn cyclobutane pyrimidine dimers can change the tautomeric state of their constituent bases. Error-prone and SOS synthesis of DNA containing cis-syn cyclobutane pyrimidine dimers is studied. There are other models.