Variability is a universal property of living systems associated with changes in phenotype and genotype that arise under the influence of the external environment or as a result of changes in hereditary material:
hereditary (genotypic) variability is associated with changes in the genotype. Genotype - the totality of all genes of one organism, interacting with each other and transmitted by inheritance (this is the genetic basis of traits).
non-hereditary (modification) variability is associated with changes in phenotype. Phenotype - the totality of all external signs of an organism that we observe (morphological, physiological, biochemical, histological, anatomical, behavioral, etc.).
Non-hereditary (modification, phenotypic) variability - changes in the characteristics and properties of the organism, the formation of the phenotype of an individual under the influence of its genotype and environmental conditions in which development takes place:
modifications- non-hereditary changes in phenotype that occur under the influence of environmental factors, are adaptive in nature, most often reversible (increase in red blood cells in the blood with a lack of oxygen)
morphoses- non-hereditary changes in phenotype that occur under the influence of extreme environmental factors, are not adaptive in nature, are irreversible (burns, scars)
phenocopies- non-hereditary changes in phenotype that resemble a hereditary disease (enlargement of the thyroid gland in residents of areas where there is iodine deficiency).
The manifestation of the gene depends on other genes of the genotype and regulatory influences from the endocrine system. With the same genotype, under different environmental conditions, traits may be different. Inherited not the trait itself, but the ability to form a certain phenotype under specific environmental conditions (a certain reaction norm ).
Norm of reaction of a trait - limits, degree, range of variability of a characteristic depending on environmental conditions. The breadth of the reaction norm is determined by the genotype and depends on the importance of the trait in the life of the organism. Different signs of one organism have different reaction norm. Qualitative features have narrow reaction norm , allowing for a single implementation option (for example, ensuring a constant structure and size of organs for organisms of a given type; human height, eye color). Quantitative characteristics usually have broad reaction rate (milk yield of cows, egg production of chickens).
The presence of a reaction norm allows organisms to adapt to changing environmental conditions and leave offspring. The wider the reaction norm, the more plastic the trait, the greater the likelihood of survival of the species in changing environmental conditions. Man uses knowledge about reaction norms to obtain higher productivity of plants and animals, creating optimal conditions for their cultivation and maintenance. Thus, modification variability is characterized by a number of features :
affects only the phenotype of the individual (the genotype does not change, therefore this form of variability is not inherited);
determined by the conditions of existence;
has a group character of similar changes occurring in accordance with the action of environmental factors and the norm of reaction;
usually has an adaptive nature to environmental conditions;
changes are gradual;
promotes the survival of individuals, increases vitality, and leads to the formation of modifications.
Modifications form variation series of trait variability within the normal range of reaction from the smallest to the largest value. Reason for variations associated with the influence of various conditions on the development of the trait. To determine the limit of variability of a trait, the frequency of occurrence of each variant is calculated and a variation curve is constructed.
Variation curve - graphic expression of the nature of the variability of a characteristic. The middle members of the variation series are more common, which corresponds to the average value of the trait.
Hereditary (genotypic) variability presented as follows forms :
combinative variability - variability caused by genetic recombination that occurs during meiosis and leads to the appearance of new combinations of genes and traits in descendants. The source of recombination is the sexual process , where possible:
random combination of chromosomes during fertilization;
recombination of genes (crossing over) inherited from parents;
random segregation of chromosomes during meiosis.
mutational variability - variability due to mutations - qualitative or quantitative changes in the genotype.
Mutations - abrupt, persistent hereditary changes in the structure (quality) or quantity of DNA of a given organism, occurring suddenly and affecting various characteristics, properties and functions of the organism.
Thus, mutational variability is characterized by the following features :
Affects the genotype and is inherited;
Has an individual, spasmodic character;
Inadequate to environmental conditions;
It can lead to the formation of new traits, populations, or the death of the organism.
There are various approaches to mutation classification :
A. In relation to the cell type (generative pathway ):
Somatic mutations , arising in somatic cells, are not inherited (with the exception of organisms that reproduce vegetatively). They spread to the part of the body that developed from the changed cell. For species that reproduce sexually, they are not significant, but for vegetatively reproducing plants they are important.
Generative mutations , arising in germ cells, are inherited (passed on through generations).
B. For reasons of occurrence :
Spontaneous (natural) mutations occurring in nature without human intervention.
Induced (artificial) mutations caused by special exposure to artificial sources (chemical, radiation).
B. According to the degree of adaptability:
Beneficial mutations.
Harmful mutations (most often harmful).
Indifferent Mutations .
D. In the direction of flow:
Direct mutations.
Back mutations .
D . According to the nature of manifestation in a heterozygote:
Dominant mutations.
Recessive mutations (usually mutations are recessive and do not manifest themselves phenotypically in heterozygotes).
E. By localization in the cell:
Nuclear mutations associated with changes in the chromosomal material of the cell nucleus.
Cytoplasmic mutations associated with changes in the DNA structure of mitochondria and chloroplasts.
G. By change in phenotype:
Biochemical mutations.
Physiological mutations.
Anatomical and morphological mutations.
Lethal mutations sharply reducing viability.
H. According to the nature of changes in the genotype:
1.Gene (point) mutations associated with the replacement, loss or addition of nucleotides in the DNA molecule. They lead to a change in the DNA code, disruption of the reading frame, which affects the composition of amino acids in the polypeptide chain of the protein and its properties. Often such changes cause the formation of new altered proteins, block the synthesis of an enzyme or other substance, which in turn leads to a change in character and even death of the organism.
2.Chromosomal mutations , associated with changes in the structure of chromosomes. They can be detected under a microscope. The following are distinguished: types of structural changes in chromosomes :
Deletion - loss of a chromosome section
Duplication - doubling of a chromosome region
Inversion - 180° inversion of a separate section of chromosomes. In this case, the number of genes does not change, but the sequence of their location changes
Translocation - exchange of regions between non-homologous chromosomes. As a result, linkage groups change and the homology of chromosomes is disrupted
Transposition - movement of a separate small section within one chromosome
Most structural chromosomal mutations are harmful to the body and lead to a decrease in its viability. The exception is the movement of sections from one chromosome to another, leading to the emergence of previously non-existing linkage groups and the appearance of individuals with new qualities, which is important for evolution and selection.
3.Genomic mutations associated with changes in the number of chromosomes.
Autopolyploidy (autopolyploidy ) - a multiple increase in the haploid set of chromosomes in a cell (a multiple increase in the same genome); occurs when the division spindle is destroyed during mitosis or meiosis, or the process of cytokinesis (formation of the cell septum) that completes the division process is lost, or there is no reduction division during meiosis. All this leads to the formation of gametes with a set of (2p) chromosomes and individuals with 4p, 6p or more chromosomes. Polyploidy is almost never found in animals, but is widespread in plants. Polyploids differ from diploids in their more powerful growth, larger sizes of cells, leaves, flowers, fruits, seeds, etc. Most cultivated plants are polyploids.
Amopolyploidy (amphipolyploidy ) - a multiple increase in the number of chromosomes in hybrids obtained as a result of crossing different species (a multiple multiplication of the hybrid genome). For example, when crossing rye and wheat, a hybrid is obtained with a mixed genome (n + t), consisting of a haploid set of rye chromosomes and a haploid set of wheat chromosomes. The organisms obtained in this way are viable but sterile. To restore fertility, the number of chromosomes of each species is doubled (2n + 2t).
Heteropolyploidy (aneuploidy ) - an increase in the number of chromosomes, not a multiple of the haploid; arise when meiosis is disrupted, when after conjugation the chromosomes do not separate, and both homologous chromosomes end up in one gamete, and none in the other. This mutation leads to the formation of gametes with a set of (2n + 1) chromosomes. Heteroploidy is harmful to the body. For example, in humans, the appearance of an extra chromosome in the 21st pair causes Down syndrome (dementia).
Cytoplasmic mutations associated with changes in cytoplasmic organelles containing DNA. For example, the appearance of variegation in plants is associated with changes in chloroplast DNA; respiratory failure mutations in yeast are associated with changes in mitochondrial DNA. Cytoplasmic mutations are inherited through the maternal line, since during fertilization the zygote receives all the cytoplasm from the mother.
Law of homological series N.I. Vavilova. N.I. Vavilov, studying mutations in related species, established the law of homological series of hereditary variability. Species and genera that are genetically close are characterized by similar series of hereditary variability. The reasons for homologous identical mutations are the common origin of genotypes. This law allows us to predict the presence of a certain characteristic in different genera of the same family if its other genera have this characteristic. Examples of similar mutations in animals are albinism and the absence of hair in mammals, albinism and the absence of feathers in birds, and short-fingered feet in cattle, sheep, dogs, and birds.
Thematic assignments
A1. Modification variability is understood as
1) phenotypic variability
2) genotypic variability
3) reaction norm
4) any changes in the characteristic
A2. Indicate the characteristic with the widest reaction norm
1) the shape of a swallow's wings
2) eagle beak shape
3) time for the hare to molt
4) the amount of wool a sheep has
A3. Please indicate the correct statement
1) environmental factors do not affect the genotype of an individual
2) it is not the phenotype that is inherited, but the ability to manifest it
3) modification changes are always inherited
4) modification changes are harmful
A4. Give an example of a genomic mutation
1) the occurrence of sickle cell anemia
2) the appearance of triploid forms of potatoes
3) creation of a tailless dog breed
4) birth of an albino tiger
A5. Changes in the sequence of DNA nucleotides in a gene are associated
1) gene mutations
2) chromosomal mutations
3) genomic mutations
4) combinative rearrangements
A6. A sharp increase in the percentage of heterozygotes in a cockroach population can result from:
1) increase in the number of gene mutations
2) the formation of diploid gametes in a number of individuals
3) chromosomal rearrangements in some members of the population
4) change in ambient temperature
A7. Accelerated skin aging in rural residents compared to urban residents is an example
1) mutational variability
2) combinational variability
3) gene mutations under the influence of ultraviolet radiation
4) modification variability
A8. The main cause of a chromosomal mutation may be
1) nucleotide replacement in a gene
2) change in ambient temperature
3) disruption of meiosis processes
4) insertion of a nucleotide into a gene
IN 1. What examples illustrate modification variability?
1) human tan
2) birthmark on the skin
3) the thickness of the fur of a rabbit of the same breed
4) increase in milk yield in cows
5) six-fingered humans
6) hemophilia
AT 2. Indicate events related to mutations
1) multiple increase in the number of chromosomes
2) change of undercoat of a hare in winter
3) replacement of an amino acid in a protein molecule
4) the appearance of an albino in the family
5) growth of the root system of a cactus
6) formation of cysts in protozoa
1. Establish a correspondence between a plant trait and the type of variability to which it is classified: 1-mutation, 2-modification
A) the appearance in individual inflorescences of flowers with five petals instead of four
B) increased shoot growth in favorable conditions
C) the appearance of single leaves devoid of chlorophyll
D) increased growth and development of shoots under heavy shading
D) the appearance of double flowers among plants of the same variety
Answer
A1 B2 C1 D2 D1
1+. Establish a correspondence between an example of a biological phenomenon and the form of variability that it illustrates: 1-mutation, 2-modification
A) the appearance of a short-legged sheep in a herd of sheep with normal limbs
B) the appearance of an albino mouse among gray mice
C) the formation of different leaf shapes in arrowhead in water and in air
D) manifestation in children of the eye color of one of the parents
D) changing the size of a head of cabbage depending on the intensity of watering
Answer
A1 B1 C2 D1 D2
1++. Establish a correspondence between the characteristic and the type of variability: 1-hereditary, 2-modification
A) Associated with changes in genes and chromosomes
B) Does not affect the genotype
B) Manifests itself in individual individuals
D) Changes appear in all individuals of the species
D) Changes are random
E) Changes are adaptive in nature
Answer
A1 B2 C1 D2 D1 E2
1+++. Which of the following examples refers to modification variability?
A) variation in the size of tubers of one potato plant
B) different eye colors among people in the same family
C) different forms of underwater and above-water arrowhead leaves
D) birth of children with Down syndrome
D) the difference in the length of birch leaves on the north and south sides
E) the appearance in a herd of sheep of individual individuals with short legs
Answer
1++++. Indicate the features of modification variability
A) occurs suddenly
B) manifests itself in individual individuals of the species
C) changes are due to the reaction norm
D) manifests itself similarly in all individuals of the species
D) is adaptive in nature
E) passed on to offspring
Answer
2. Establish a correspondence between the characteristic of variability of organisms and its type: 1-non-hereditary, 2-hereditary
A) occurs as a result of changes in genotype
B) corresponds to environmental conditions and is adaptive
C) manifests itself within the normal range of reaction
D) occurs randomly in single individuals
D) is caused by a combination of genes and mutations
Answer
A2 B1 C1 D2 D2
3. What characterizes a genomic mutation?
A) change in the nucleotide sequence of DNA
B) loss of one chromosome in the diploid set
B) a multiple increase in the number of chromosomes
D) changes in the structure of synthesized proteins
D) doubling a section of a chromosome
E) change in the number of chromosomes in the karyotype
Answer
4. 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
A1 B2 C2 D2 D1
4a. Establish a correspondence between the sign 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
A2 B1 C2 D2 D1 E1
5. Mutations lead to change
A) primary protein structure
B) stages of fertilization
B) the gene pool of the population
D) reaction norms for a trait
D) sequence of mitotic phases
E) sexual composition of the population
Answer
6. Establish a correspondence between the characteristics of the mutation and its type: 1-gene, 2-chromosomal, 3-genome
A) change in the sequence of nucleotides in a DNA molecule
B) change in chromosome structure
B) change in the number of chromosomes in the nucleus
D) polyploidy
D) change in the sequence of gene location
Answer
A1 B2 C3 D3 D2
7. Establish a correspondence between the characteristic of variability and its type: 1-cytoplasmic, 2-combinative
A) occurs during independent chromosome segregation in meiosis
B) occurs as a result of mutations in mitochondrial DNA
B) occurs as a result of chromosome crossing
D) manifests itself as a result of mutations in plastid DNA
D) occurs when gametes meet by chance
Answer
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 one 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 was found in rye, wheat, barley, corn and other plants, and the elongated shape of the caryopsis was 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 one 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.
One of the central problems of genetics is elucidating the correlation between the genotype and environmental conditions in the formation of the phenotype of an organism. When developing under different conditions, identical twins differ in phenotype. That is, in this case, non-hereditary variability manifests itself. Its study makes it possible to find out how hereditary information is realized in certain living conditions.
Modification variability
– These are changes in the characteristics of an organism (its phenotype) caused by changes in environmental conditions and not associated with changes in the genotype. Hence, modification changes (modifications)
- these are reactions to changes in the intensity of certain environmental conditions, the same for all genotypically homogeneous organisms.
The degree of severity of modifications is directly proportional to the intensity and duration of action of a certain factor on the body.
For a long time, there have been discussions about whether changes in the states of characteristics acquired by an organism during individual development are inherited or not. The fact that modifications are not inherited was proved by the German scientist A. Weissmann. For many generations, he cut off the tails of mice, but tailed offspring were born to tailless parents.
As numerous studies have shown, modifications can disappear throughout the life of one individual if the effect of the factor that caused them ceases. For example, a summer tan disappears in the fall. Some modifications may persist throughout life, but are not passed on to descendants. For example, rickets persists throughout life, but is not passed on to descendants.
Modification changes play an extremely important role in the life of organisms, ensuring adaptability to changing environmental conditions. For example, molting in mammals plays a protective role; tanning protects against the harmful effects of sunlight.
But not all codification changes are adaptive in nature. When the body finds itself in unusual conditions. For example, when the lower part of a potato stem is shaded, tubers form on it.
Modification variability obeys statistical laws. For example, any sign can change only within certain limits. These limits, determined by the genotype of the organism, are called reaction norm . Thus, a given allelic gene does not determine the specific state of the trait it encodes, but only the limits within which it can vary depending on the intensity of the action of certain environmental factors. Among the traits there are those whose state is almost completely determined by the genotype (eye location, blood type, etc.) The degree of manifestation of the state of other traits (height, weight of the body) is significantly influenced by environmental conditions.
Research has shown that the reaction norm for certain traits has different limits. The narrowest reaction norm is for traits that determine the viability of organisms (for example, the location of internal organs), and for traits that do not have such significance, it can be wider (weight, height...)
To study the variability of a particular trait, they are variation series– sequence of options - quantitative indicators of the manifestation of states of a certain characteristic, arranged in ascending or descending order. The length of the variation series indicates the scope of modification variability. It is determined by the genotype of organisms (reaction norm), but also depends on environmental conditions: the more stable the living conditions of organisms, the shorter the variation series will be, and vice versa.
If we trace the distribution of individual variants within the variation series, we can note that the largest number of them are located in its middle part, that is, they have the average value of a certain characteristic. This distribution is explained by the fact that the minimum and maximum values of trait development are formed when the majority of environmental factors act in one direction: the most or least favorable. But the body, as a rule, feels their different influence: some factors contribute to the development of the trait, while others, on the contrary, inhibit it, therefore the degree of its development in most individuals of the species is average. So, most people are of average height and only some of them are giants or dwarfs.
The distribution of variants within a variation series is depicted as a variation curve. A variation curve is a graphical representation of the variability of a particular trait, illustrating both the range of variability and the frequency of occurrence of individual variants. Using a variation curve, you can establish the average indicators and reaction norm of a particular trait.
In addition to non-hereditary modification variability, there is also hereditary variability associated with changes in the genotype. Hereditary variability can be combinative and mutational.
Combinative variability associated with the emergence of different combinations of allelic genes (recombinations). The source of combinative variability is the conjugation of homologous chromosomes in prophase and their independent divergence in anaphase of the first division of meiosis, as well as the random combination of allelic genes during the fusion of gametes. Consequently, combinative variability, which provides a variety of combinations of allelic genes, also ensures the appearance of individuals with different combinations of character states. Combinative variability is also observed in organisms that reproduce asexually or vegetatively.
Mutations - these are suddenly occurring persistent changes in the genotype, leading to changes in certain hereditary characteristics of the body. The foundations of the doctrine of mutations were laid by the Dutch scientist Hugo de Vries, who proposed this term.
The ability to mutate is a universal property of all organisms. Mutations can occur in any cells of the body and cause any changes in the genetic apparatus and, accordingly, the phenotype. Mutations that occur in the germ cells of the body are inherited during sexual reproduction, and in non-reproductive cells they are inherited only during asexual or vegetative reproduction.
Depending on the nature of the effect on the life activity of organisms, lethal, sublethal and neutral mutations are distinguished. Lethal mutations , manifested in the phenotype, cause the death of organisms before birth or the end of their development period. Sublethal mutations reduce the viability of organisms, leading to the death of some of them (from 10 to 50%), and neutral under these conditions do not affect the viability of organisms. The likelihood that a new mutation will be beneficial is small. But in some cases, especially when environmental conditions change, neutral mutations may be beneficial for the organism.
Depending on the nature of changes in the genetic apparatus, mutations are distinguished into genomic, chromosomal and gene mutations.
Genomic mutations associated with a multiple increase or decrease in chromosome sets. An increase in their number, leading to polyploidy, most often observed in plants, sometimes in animals (since such organisms die or are unable to reproduce).
Polyploidy can occur in different ways: doubling the number of chromosomes, not accompanied by subsequent cell division, the formation of gametes with an unreduced number of chromosomes as a result of disruption of the meiosis process. The cause of polyploidy can also be the fusion of non-reproductive cells or their nuclei.
Polyploidy leads to an increase in the size of organisms, intensification of their life processes and increased productivity. This is explained by the fact that the intensity of protein biosynthesis depends on the number of homologous chromosomes in the nucleus: the more, the more protein molecules of each type are formed per unit time. However, polyploidy may be accompanied by a decrease in fertility due to disruption of the meiotic process: polyploid organisms may produce gametes with a different number of sets of chromosomes. As a rule, such gametes are not able to fuse.
Polyploidy plays an important role in the evolution of plants as one of the mechanisms for the formation of new species. It is used in plant breeding to develop new highly productive varieties, for example, soft wheat, sugar beets, garden dugouts, etc.
Mutations associated with a decrease in the number of sets of chromosomes lead to exactly the opposite consequences: haploid forms are smaller in size compared to diploid forms, and their productivity and fertility are reduced. In breeding this type of mutations. They are used to obtain forms that are homozygous for all genes: first, haploid forms are obtained, and then the number of chromosomes is doubled.
Chromosomal mutations associated with changes in the number of individual homologous chromosomes or in their structure. A change in the number of homologous chromosomes compared to the norm has a significant impact on the phenotype of mutant organisms. Moreover, the absence of one or both homologous chromosomes has a more negative effect on the vital processes and development of the organism than the appearance of an additional chromosome. For example, a human embryo with chromosome set 44A+X develops into a female body with significant deviations in the structure and vital functions (pterygoid fold of skin on the neck, impaired formation of bones, circulatory and genitourinary systems), while an embryo with chromosome set 44A+XXX develops into a female body , only slightly different from normal. The appearance of a third chromosome in pair 21 causes Down syndrome.
Various options for rearranging the structure of chromosomes are also possible: loss of a section, change in the sequence of genes in a chromosome, etc. When a section is lost, the chromosome becomes shorter and loses some genes. As a result, recessive alleles may appear in the phenotype of heterozygous organisms. In other cases, an additional fragment belonging to a homologous chromosome is inserted into the chromosome. This type of mutation rarely manifests itself in the phenotype.
During chromosomal rearrangements associated with a change in the sequence of gene location, the section of the chromosome formed as a result of two breaks is rotated 180° and, with the help of enzymes, is inserted back into it. This type of mutation often does not affect the phenotype because the number of genes on the chromosome remains unchanged.
There is also an exchange of sections between chromosomes of different pairs, as well as the insertion of an unusual fragment into a certain section of a chromosome.
A common cause of mutations associated with changes in the structure and number of chromosomes may be a disruption of the process of meiosis, in particular, the conjugation of homologous chromosomes.
Gene mutations– these are persistent changes in individual genes caused by a violation of the nucleotide sequence in nucleic acid molecules (loss or addition of individual nucleotides, replacement of one nucleotide by another, etc.). This is the most common type of mutation, which can affect any characteristics of the body and is transmitted from generation to generation for a long time. Different alleles have different degrees of ability to change structure. There are persistent alleles, mutations of which are observed relatively rarely, and unstable alleles, mutations of which occur much more often.
Gene mutations can be dominant, subdominant (partially manifested) and recessive. Most gene mutations are recessive; they appear only in the homozygous state and therefore are quite difficult to identify.
Under natural conditions, mutations of individual alleles are observed quite rarely, but since organisms have a large number of genes, the total number of mutations is also large. For example, in Drosophila, approximately 5% of the tentorium carries various mutations.
The causes of the mutations remained unclear for a long time. And only in 1927, T. Morgan’s employee, G. Meller, established that mutations could be caused artificially. Using X-rays on Drosophila, he observed various mutations in them. Factors that can cause mutations are called mutagenic .
By origin they are chemical, physical and biological. Among physical mutagens Ionizing radiation, in particular X-rays, is of greatest importance. Passing through living matter, X-rays knock out electrons from the outer shell of atoms or molecules, as a result of which they become positively charged, and the knocked-out electrons continue this process, causing chemical transformations of various compounds of living organisms. Physical mutagens also include ultraviolet rays (affect chemical reactions, causing gene and, less commonly, chromosomal mutations), elevated temperature (the number of gene mutations increases, and when increased to the upper limit, chromosomal ones) and other factors.
Chemical mutagens were discovered later than the physical ones. A significant contribution to their study was made by the Ukrainian school of geneticists, headed by Academician S. M. Gershenzon. Many chemical mutagens are known, and more and more are discovered every year. For example, the alkaloid colchicine destroys the spindle, which leads to a doubling of the number of chromosomes in the cell. Mustard gas increases the mutation rate by 90 times. Chemical mutagens can cause mutations of all types.
TO biological mutag We include viruses. It has been established that in cells infected with viruses, mutations are observed much more often than in healthy ones. Viruses cause both gene and chromosomal mutations, introducing a certain amount of their own genetic information into the genotype of the host cell. These processes are thought to have played an important role in the evolution of prokaryotes because viruses can transfer genetic information between cells of different species.
Spontaneous (involuntary) mutations occur without a noticeable influence of mutagenic factors, for example, as errors in the reproduction of the genetic code. Their reasons have not yet been fully elucidated. They can be: natural background radiation, cosmic rays reaching the surface of the Earth, etc.
Living organisms are able to protect their genes from mutations in a certain way. For example, most amino acids are encoded not by one, but by several triplets; many genes are repeated in the genotype. Protection against mutations is also provided by the removal of altered sections from the DNA molecule: with the help of enzymes, two breaks are formed, the mutated section is removed, and in its place a section with a nucleotide sequence inherent to this part of the molecule is inserted.
The ability to mutate is inherent in all living organisms. They arise suddenly, and the changes caused by mutations are stable and can be inherited. Mutations can be harmful, neutral, or, very rarely, beneficial to the body. Mutagens are universal, meaning they can cause mutations in any type of organism. Unlike modifications, mutations do not have a specific direction: the same mutagenic factor, acting with the same intensity on genetically identical organisms, can cause different types of mutations in them. At the same time, various mutagens can cause identical hereditary changes in genetically distant organisms. The severity of mutational changes in the phenotype does not depend on the intensity and duration of action of the mutagenic factor. Thus, a weak mutagenic factor, acting for a short time, can sometimes cause more significant changes in the phenotype than a stronger one. However, with increasing intensity of the action of the mutagenic factor, the frequency of mutations increases to a certain level.
For all mutagenic factors there are no lower limit their actions, that is, the limit below which they are not able to cause mutations. This property of mutagenic factors has important theoretical and practical significance, since it indicates that the genotype of organisms must be protected from all mutagenic factors, no matter how low the intensity of their action.
Different types of living organisms and even different individuals of the same species are unequally sensitive to the action of mutagenic factors.
The significance of mutations in nature is that they are the main source of hereditary variability - a factor in the evolution of organisms. Thanks to mutations, new alleles appear - mutant. Most mutations are harmful to living beings because they reduce their adaptability to living conditions. However, neutral mutations may be beneficial under certain environmental changes.
Mutations are widely used in breeding, as they allow increasing the diversity of the starting material and increasing the efficiency of breeding work.
The outstanding Russian geneticist N.I. Vavilov formulated law of homological series: genetically close species and genera are characterized by similar series of hereditary variability with such regularity that, knowing a number of forms within one species or genus, one can predict the presence of forms with a similar combination of characters within close species or genera. Moreover, the closer the family ties between organisms, the more similar the series of their hereditary variability is. This pattern, discovered by Vavilov in plants, turned out to be universal for all organisms. The genetic basis of this law is that the degree of historical relatedness of organisms is directly proportional to the number of their common genes. Therefore, the mutations of these genes may be similar. In the phenotype, this is manifested by the same pattern of variability in many characters in closely related species, genera, and other taxa.
The law of homological series explains the direction of historical development of related groups of organisms. Based on it and having studied the hereditary variability of closely related species, breeding plans to work on creating new plant varieties and animal breeds with a certain set of hereditary traits. In the taxonomy of organisms, this law makes it possible to predict the existence of systematic groups unknown to science if forms with similar combinations of characteristics are identified in closely related groups.
Variability- the ability of living organisms to acquire new characteristics and properties. Thanks to variability, organisms can adapt to changing environmental conditions.
There are two main forms of variability: hereditary and non-hereditary.
Hereditary, or genotypic, variability- changes in the characteristics of the organism due to changes in the genotype. It, in turn, is divided into combinative and mutational. Combinative variability arises due to the recombination of hereditary material (genes and chromosomes) during gametogenesis and sexual reproduction. Mutational variability arises as a result of changes in the structure of hereditary material.
Non-hereditary, or phenotypic, or modification, variability- changes in the characteristics of the organism that are not due to changes in the genotype.
Mutations
Mutations- these are persistent, sudden changes in the structure of the hereditary material at various levels of its organization, leading to changes in certain characteristics of the organism.
The term “mutation” was introduced into science by De Vries. Created by him mutation theory, the main provisions of which have not lost their significance to this day.
- Mutations arise suddenly, spasmodically, without any transitions.
- Mutations are hereditary, i.e. are 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, and environmental factors causing mutations are mutagens.
Based on the type of cells in which the mutations occurred, they are distinguished: generative and somatic mutations.
Generative mutations arise in germ cells, do not affect the characteristics of a given organism, and appear only in the next generation.
Somatic mutations arise in somatic cells, manifest themselves in a given organism and are not transmitted to offspring during sexual reproduction. Somatic mutations can be preserved only through asexual reproduction (primarily vegetative).
According to their adaptive value, they are divided into: beneficial, harmful (lethal, semi-lethal) and neutral mutations. Useful- increase vitality, lethal- cause death semi-lethal- reduce vitality, neutral- do not affect the viability of individuals. It should be noted that the same mutation can be beneficial in some conditions and harmful in others.
According to the nature of their manifestation, mutations can be dominant And recessive. If a dominant mutation is harmful, then it can cause the death of its owner in the early stages of ontogenesis. Recessive mutations do not appear in heterozygotes, therefore they remain in the population for a long time in a “hidden” state and form a reserve of hereditary variability. When environmental conditions change, carriers of such mutations may gain an advantage in the struggle for existence.
Depending on whether the mutagen that caused this mutation has been identified or not, they distinguish induced And spontaneous mutations. Typically, spontaneous mutations occur naturally, while induced mutations are caused artificially.
Depending on the level of hereditary material at which the mutation occurred, gene, chromosomal and genomic mutations are distinguished.
Gene mutations
Gene mutations- changes in gene structure. Since a gene is a section of a DNA molecule, a gene mutation represents changes in the nucleotide composition of this section. Gene mutations can occur as a result of: 1) replacement of one or more nucleotides with others; 2) nucleotide insertions; 3) loss of nucleotides; 4) doubling of nucleotides; 5) changes in the order of alternation of nucleotides. These mutations lead to changes in the amino acid composition of the polypeptide chain and, consequently, to changes in the functional activity of the protein molecule. Gene mutations result in multiple alleles of the same gene.
Diseases caused by gene mutations are called genetic diseases (phenylketonuria, sickle cell anemia, hemophilia, etc.). The inheritance of gene diseases obeys Mendel's laws.
Chromosomal mutations
These are changes in the structure of chromosomes. Rearrangements can occur both within one chromosome - intrachromosomal mutations (deletion, inversion, duplication, insertion), and between chromosomes - interchromosomal mutations (translocation).
Deletion— loss of a chromosome section (2); inversion— rotation of a chromosome section by 180° (4, 5); duplication- doubling of the same chromosome section (3); insertion— rearrangement of the section (6).
Chromosomal mutations: 1 - parachromosomes; 2 - deletion; 3 - duplication; 4, 5 — inversion; 6 - insertion.
Translocation- transfer of a section of one chromosome or an entire chromosome to another chromosome.
Diseases caused by chromosomal mutations are classified as chromosomal diseases. Such diseases include “cry of the cat” syndrome (46, 5p -), translocation variant of Down syndrome (46, 21 t21 21), etc.
Genomic mutation called a change in the number of chromosomes. Genomic mutations occur as a result of disruption of the normal course of mitosis or meiosis.
Haploidy- reduction in the number of complete haploid sets of chromosomes.
Polyploidy- increase in the number of complete haploid sets of chromosomes: triploids (3 n), tetraploids (4 n) etc.
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).
The most likely cause of heteroploidy is the nondisjunction of any pair of homologous chromosomes during meiosis in one of the parents. In this case, one of the resulting gametes contains one less chromosome, and the other contains 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: nullosomia (2n - 2), monosomy (2n - 1), trisomy (2n + 1), tetrasomy (2n+ 2) etc.
The genetic diagrams below show that the birth of a child with Klinefelter syndrome or Turner-Shereshevsky syndrome can be explained by the nondisjunction of sex chromosomes during anaphase 1 of meiosis in the mother or father.
1) Nondisjunction of sex chromosomes during meiosis in the mother
| R | ♀46,XX | × | ♂46,XY | ||
| Types of gametes | 24, XX 24, 0 | 23, X 23, Y | |||
| F | 47, XXX trisomy on the X chromosome |
47, XXY syndrome Klinefelter |
45, X0 Turner syndrome- Shereshevsky |
45,Y0 death zygotes |
|
2) Nondisjunction of sex chromosomes during meiosis in the father
| R | ♀46,XX | × | ♂46,XY |
| Types of gametes | 23, X | 24, XY 22, 0 | |
| F | 47, XXY syndrome Klinefelter |
45, X0 Turner syndrome- Shereshevsky |
Diseases caused by genomic mutations also fall into the chromosomal category. Their inheritance does not obey Mendel's laws. In addition to the above-mentioned Klinefelter or Turner-Shereshevsky syndromes, such diseases include Down syndrome (47, +21), Edwards syndrome (+18), Patau syndrome (47, +15).
Polyploidy characteristic of plants. The production of polyploids is widely used in plant breeding.
The law of homological series of hereditary variability N.I. Vavilova
“Species and genera that are genetically close are characterized by similar series of hereditary variability with such regularity that, knowing the series of forms within one species, one can predict the presence 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. himself. Vavilov 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.
| Hereditary variation of traits * | Rye | Wheat | Barley | Oats | Millet | Sorghum | Corn | Rice | Wheatgrass | ||
| Corn | Coloring | Black | + | + | + | — | — | + | + | + | + |
| Purple | + | + | + | — | — | + | + | + | — | ||
| Form | Round | + | + | + | + | + | + | + | + | + | |
| Extended | + | + | + | + | + | + | + | + | + | ||
| Biol. signs | Lifestyle | Winter crops | + | + | + | + | + | ||||
| Spring | + | + | + | + | + | + | + | + | |||
* Note. The “+” sign means the presence of hereditary forms that have the specified trait.
Open N.I. Vavilov’s law 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 toedness is observed in humans, cattle, sheep, dogs, birds, the absence of feathers in birds, scales in fish, wool in mammals, etc.
The law of homological series of hereditary variability is of great importance for selection, since it allows us to predict the presence of forms not found in a given species, but characteristic of closely related species. Moreover, the desired form can be found in the wild or obtained through artificial mutagenesis.
Artificial mutations
Spontaneous mutagenesis constantly occurs in nature, but spontaneous mutations are a fairly rare occurrence, for example, in Drosophila, the white eye mutation is formed with a frequency of 1:100,000 gametes.
Factors whose impact on the body leads to the appearance of mutations are called mutagens. Mutagens are usually divided into three groups. Physical and chemical mutagens are used to artificially produce mutations.
Induced mutagenesis is of great importance because it makes it possible to create valuable starting material for breeding, and also reveals ways to create means of protecting humans from the action of mutagenic factors.
Modification variability
Modification variability- these are changes in the characteristics of organisms that are not caused by changes in the genotype and arise under the influence of environmental factors. The habitat plays a big role in the formation of the characteristics of organisms. Each organism develops and lives in a certain environment, experiencing the action of its factors that can change the morphological and physiological properties of organisms, i.e. their phenotype.
An example of the variability of characteristics under the influence of environmental factors is the different shape of the leaves of the arrowhead: leaves immersed in water have a ribbon-like shape, leaves floating on the surface of the water are round, and those in the air are arrow-shaped. Under the influence of ultraviolet rays, people (if they are not albinos) develop a tan as a result of the accumulation of melanin in the skin, and the intensity of the skin color varies from person to person.
Modification variability is characterized by the following main properties: 1) non-heritability; 2) the group nature of the changes (individuals of the same species placed in the same conditions acquire similar characteristics); 3) correspondence of changes to the influence of environmental factors; 4) dependence of the limits of variability on the genotype.
Despite the fact that signs may change under the influence of environmental conditions, this variability is not unlimited. This is explained by the fact that the genotype determines specific boundaries within which changes in a trait can occur. The degree of variation of a trait, or the limits of modification variability, is called reaction norm. The reaction norm is expressed in the totality of phenotypes of organisms formed on the basis of a certain genotype under the influence of various environmental factors. As a rule, quantitative traits (plant height, yield, leaf size, milk yield of cows, egg production of chickens) have a wider reaction rate, that is, they can vary widely than qualitative traits (coat color, milk fat content, flower structure, blood type) . Knowledge of reaction norms is of great importance for agricultural practice.
Modification variability of many characteristics of plants, animals and humans obeys general laws. These patterns are identified based on the analysis of the manifestation of the trait in a group of individuals ( n). The degree of expression of the characteristic being studied varies among members of the sample population. Each specific value of the characteristic being studied is called option and denoted by the letter v. The frequency of occurrence of individual variants is indicated by the letter p. When studying the variability of a trait in a sample population, a variation series is compiled in which individuals are arranged in ascending order of the indicator of the trait being studied.
For example, if you take 100 ears of wheat ( n= 100), count the number of spikelets in an ear ( v) and the number of ears with a given number of spikelets, then the variation series will look as follows.
| Option ( v) | 14 | 15 | 16 | 17 | 18 | 19 | 20 |
| Frequency of occurrence ( p) | 2 | 7 | 22 | 32 | 24 | 8 | 5 |
Based on the variation series, it is constructed variation curve— graphical display of the frequency of occurrence of each option.
The average value of a characteristic is more common, and variations significantly different from it are less common. It is called "normal distribution". The curve on the graph is usually symmetrical.
The average value of the characteristic is calculated using the formula:
Where M— average value of the characteristic; ∑( v
