Variability is hereditary. Hereditary variability: examples, forms of variability

Heredity and variability are among the determining factors in the evolution of the organic world.

Heredity- this is the property of living organisms to preserve and transmit to their offspring the features of their structure and development. Thanks to heredity, the characteristics of a species, variety, breed, strain are preserved from generation to generation. The connection between generations is carried out during reproduction through haploid or diploid cells (see sections “Botany” and “Zoology”).

Of the cell organelles, the leading role in heredity belongs to chromosomes, which are capable of self-duplication and the formation, with the help of genes, of the entire complex of characteristics characteristic of the species (see the chapter “Cell”). The cells of every organism contain tens of thousands of genes. Their entire set, characteristic of an individual of a species, is called a genotype.

Variability is the opposite of heredity, but is inextricably linked with it. It is expressed in the ability of organisms to change. Due to the variability of individual individuals, the population becomes heterogeneous. Darwin distinguished two main types of variation.

Non-hereditary variability(see about modifications in the chapter “Fundamentals of Genetics and Selection”) arises in the process of individual development of organisms under the influence of specific environmental conditions, causing similar changes in all individuals of the same species, which is why Darwin called this variability definite. However, the extent of such changes may vary among individuals. For example, in grass frogs, low temperatures cause dark coloration, but its intensity varies among individuals. Darwin considered modifications not essential for evolution, since they, as a rule, are not inherited.

Hereditary variability(see about mutations in the chapter “Fundamentals of Genetics and Selection”) is associated with a change in the genotype of an individual, so the resulting changes are inherited. In nature, mutations appear in single individuals under the influence of random external and internal factors. Their character is difficult to predict, which is why Darwin showed this variability. named uncertain. Mutations can be minor or significant and affect various traits and properties. For example, in Drosophila, under the influence of X-rays, wings, bristles, eye and body coloring, fertility, etc. change. Mutations can be beneficial, harmful, or indifferent to the body.

Hereditary variability includes combinative variability. It occurs during free crossings in populations or during artificial hybridization. As a result, individuals are born with new combinations of characters and properties that were absent in the parents (see about dihybrid crossing, new formations during crossing, chromosome crossing in the chapter “Fundamentals of Genetics and Selection”). Relative variability also hereditary; it is expressed in the fact that changes in one organ cause dependent changes in others (see the chapter “Fundamentals of Genetics and Selection” for multiple gene actions). For example, peas with purple flowers always have the same shade of petioles and leaf veins. Wading birds have long limbs and necks that are always accompanied by a long beak and tongue. Darwin considered hereditary variability to be especially important for evolution, since it serves as material for natural and artificial selection in the formation of new populations, species, varieties, breeds and strains.

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

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

Forms of variability	Reasons for appearance	Meaning	Examples
Non-hereditary modification (phenotypic)	Changes in environmental conditions, as a result of which the organism changes within the limits of the reaction norm specified by the genotype	Adaptation - adaptation to given environmental conditions, survival, preservation of offspring	White cabbage does not form a head in hot climates. Breeds of horses and cows brought to the mountains become stunted
Mutational	The influence of external and internal mutagenic factors, resulting in changes in genes and chromosomes	Material for natural and artificial selection, since mutations can be beneficial, harmful and indifferent, dominant and recessive	The appearance of polyploid forms in a plant population or in some animals (insects, fish) leads to their reproductive isolation and the formation of new species and genera - microevolution
Hereditary (genotypic) Kombinatnaya	Arises spontaneously within a population during crossing, when the descendants acquire new combinations of genes	Distribution of new hereditary changes in a population that serve as material for selection	The appearance of pink flowers when crossing white-flowered and red-flowered primroses. When crossing white and gray rabbits, black offspring may appear
Hereditary (genotypic) Correlative (correlative)	Arises as a result of the ability of genes to influence the formation of not one, but two or more traits	Constancy of interrelated characteristics, integrity of the organism as a system	Long-legged animals have long necks. In table varieties of beets, the color of the root crop, petioles and leaf veins changes consistently

Modification variability

Modification 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: in optimal conditions the maximum capabilities inherent in a given genotype are revealed. Thus, the productivity of outbred animals in conditions of improved housing and care increases (milk yield, meat fattening). In this case, all individuals with the same genotype respond to external conditions in the same way (C. Darwin called this type of variability definite variability). However, another trait - the fat content of milk - is slightly susceptible to changes in environmental conditions, and the color of the animal is an even more stable trait. Modification variability usually fluctuates within certain limits. The degree of variation of a trait in an organism, i.e., the limits of modification variability, is called the reaction norm.

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

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

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

Genotypic variability

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

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

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

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

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

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

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

Gene mutations affect the structure of the gene itself and entail changes in the properties of the body (hemophilia, color blindness, albinism, color of flower corollas, etc.). Gene mutations occur in both somatic and germ cells. They can be dominant or recessive. The former appear in both homozygotes and. in heterozygotes, the second - only in homozygotes. In plants, somatic gene mutations that have arisen are retained during vegetative propagation. Mutations in germ cells are inherited when seed propagation 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.

Properties of mutations:

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

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

Combinative variability

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

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

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

Characteristic	Modification variability	Mutational variability
Change object	Phenotype within the normal range of reaction	Genotype
Selective factor	Changing environmental conditions environment	Changes in terms and conditions environment
Inheritance at signs	Not inherited	Inherited
Susceptibility to chromosome changes	Not exposed	Subject to chromosomal mutation
Susceptibility to changes in DNA molecules	Not exposed	Subject to in case gene mutation
Value for an individual	Raises or reduces vitality. productivity, adaptation	Useful changes lead to victory in the struggle for existence, harmful - to death
Meaning for view	Contributes survival	Leads to the formation of new populations, species, etc. as a result of divergence
Role in evolution	Device organisms to environmental conditions	Material for natural selection
Form of variability	Certain (group)	Indefinite (individual), combinative
Subordination to regularity	Statistical pattern variation series	Law of homology series of hereditary variability

Subject. Variability

Questions

1. Classification of variability. Non-hereditary variability and its types.

2. Hereditary variability and its types.

3. Mutagens and metagenesis.

4. Classification of mutations at the chromosomal level.

The presence of common species characteristics allows us to unite all people on earth into a single species, Homo sapiens. Nevertheless, we easily, with one glance, single out the face of a familiar person in a crowd of strangers. The extreme diversity of people - both intragroup (for example, diversity within an ethnic group) and intergroup - is due to their genetic differences.

Any population exhibits external or phenotypic variability in most qualitative and quantitative traits. Human populations are heterogeneous in height, skin pigmentation, facial features, blood types and many other characteristics.

Moreover, calculations of combinations of human genetic material indicate that in the entire history of mankind there has not been, is not, and will not occur in the foreseeable future, genetic repetition, i.e. Every born person is a unique phenomenon in the Universe. The uniqueness of the genetic constitution largely determines the characteristics of the development of the disease in each individual person.

How is the infinite diversity of the human population achieved?

All methods are based on the ability of organisms to acquire new properties in the process of ontogenesis (individual development from the moment of fertilization to death), i.e. change.

Classification of variability. Non-hereditary variability and its types

Variability can be non-hereditary and hereditary.

Non-hereditary variability includes ontogenetic and modification variability. The essence of ontogenetic variability is that the phenotype of an organism changes throughout life, while the genotype does not change, but only a switch of genes occurs.

Modification variability occurs under the influence of environmental factors, but its scope is determined by the genotype, i.e. genetically determined reaction norm.

Hereditary variability is divided into combinative and mutational. Combinative variability is associated with recombination of parental genes.

Hereditary variability and its types.

Combinative variability occurs in the genotypes of descendants due to random recombination of alleles. The genes themselves do not change, but the genotypes of parents and children are different. Combinative variability arises as a result of several processes:

independent divergence of chromosomes during meiosis;

· recombination of genes during crossing over;

· chance meeting of gametes during fertilization.

Combinative variation is the main source of observed genetic diversity. It is known that the human genome contains approximately 30-40 thousand genes. About a third of all genes have more than one allele, that is, they are polymorphic. However, even in the presence of only a small number of loci containing several alleles, only through recombination (due to the mixing of gene complexes) does a colossal amount arise; many unique genotypes.

So, with only 10 genes containing 4 alleles each, the theoretical number of unique diploid genotypes is 10 billion!

Since about one third of the genes in the human genome are polymorphic, it is only through recombination that the inexhaustible genetic diversity of humans is created. In turn, the uniqueness of the genetic constitution largely determines the uniqueness and originality of each person.

Mutational variability is caused by mutations - stable changes in the genetic material and, accordingly, the inherited trait.

Mutational variability occurs due to mutations. Mutations are a violation of genetic material that are persistent and occur suddenly, spasmodically (de Vries).

Heredity - this is the property of living organisms to preserve and transmit characteristics over a series of generations. Thanks to heredity, the characteristics of a species or breed are preserved from generation to generation.

Hereditary variability (mutational or genotypic) is associated with a change in the genotype of an individual, so the resulting changes are inherited. It is the material for natural selection. Darwin called this heredity indeterminate. The basis of hereditary variability is mutations - sudden abrupt and undirected changes in the original form. They lead to the appearance in living organisms of qualitatively new hereditary characteristics and properties that did not previously exist in nature. The source of hereditary variability is the mutation process. There are several types of mutations: genomic, chromosomal and gene.

Genomic mutations (polyploidy and aneuploidy) - These are changes in the number of chromosomes. Polyploidy is a multiple increase in the haploid set of chromosomes (3p, 4p, etc.). Most often, polyploidy is formed when the divergence of chromosomes to the cell poles in meiosis or mitosis is disrupted under the influence of mutagenic factors. It is widespread in plants and extremely rare in animals.

Aneuploidy - increase or decrease in the number of chromosomes in individual pairs. It occurs when chromosomes do not separate in meiosis or chromatids in mitosis. Aneuploids are found in plants and animals and are characterized by low viability.

Chromosomal mutations - These are changes in the structure of chromosomes. The following types of chromosomal mutations are distinguished:

Deficiency - loss of the terminal sections of chromosomes.

Deletions - loss of a section of a chromosome arm.

Duplication - repetition of a set of genes in a certain region of the chromosome.

Inversion - rotation of a chromosome section by 180°.

Translocation - transfer of a section to the other end of the same chromosome or to another, non-homologous chromosome.

Gene mutations - changes in the nucleotide sequence of a DNA molecule (gene). Their result is a change in the sequence of amino acids in the polypeltide chain, and the appearance of a protein with new properties. Most gene mutations do not manifest themselves phenotypically because they are recessive.

Cytoplasmic mutations - associated with changes in cytoplasmic organelles containing DNA (mitochondria and plastids). These mutations are inherited on the maternal line, because When a zygote is formed, it receives all its cytoplasm from the egg. Example: plant variegation is associated with mutations in chlorollasts.

Significance in evolution and ontogenesis Mutations affecting germ cells (generative mutations) appear in the next generation. Mutations of somatic cells manifest themselves in those organs that include altered cells. In animals, somatic mutations are not inherited, since a new organism does not arise from somatic cells. In plants that reproduce vegetatively, somatic mutations can persist. Mutational variability plays the role of the main provider of hereditary changes in evolution. It is this that is the primary material of all evolutionary transformations.

Genotypic variability and its types. Significance in ontogenesis and evolution.

Genotypic, or hereditary variability, represents changes in phenotype caused by changes in genotype.

It is caused by mutations and their combinations during sexual reproduction (for example, inherited polledness in cattle).

Depending on the nature of the variation in the genetic material, a distinction is made between combinative and mutational hereditary variability. Combinative variability is caused by the formation in descendants of new combinations of genes in genotypes that are formed as a result of the recombination of genes and chromosomes during sexual reproduction. The infinite variety of genotypes of living organisms, the uniqueness of each genotype is due to combinative variability. With this type of variability, the combinations of genes and the nature of their interaction in the genotype change, while the genes themselves remain unchanged.

Combinative variability , being the result of recombination of genes of parental individuals in the genotypes of offspring, is based on three main mechanisms.

1. Independent divergence into daughter cells (spermatocytes II, oocyte II and the first reduction body) of homologous chromosomes from each pair (occurs during the first division of meiosis during gametogenesis). For example, even for 2 pairs of chromosomes, 2 variants of chromosome divergence into daughter cells and 4 types of sperm are possible (Fig. 76).

2. Random combination of gametes, and, consequently, homologous (paternal and maternal) chromosomes during fertilization. For the 4 types of sperm noted above, the participation of one of them in the fertilization of the egg will be purely random, and the results of a specific combination of one of the variants of male chromosomes with one (also out of 4 possible, since three variants are carried away by reduction bodies and ceased to exist) will be different ) from variants of female chromosomes homologous to them.

3. Exchange of individual alleles between homologous chromosomes during the process of crossing over of meiosis. After it, combinations of alleles in sperm chromosomes are characterized by new variants that differ from those of somatic cells of the body (Fig. 77).

Crossing over occurs at the beginning of meiosis when homologous chromosomes line up against each other. In this case, sections of homologous chromosomes intersect, break off, and then reattach, but to another chromosome. Ultimately, four chromosomes are formed with different combinations of genes. Chromosomes, called “recombinant”, carry new combinations of genes (Ab and aB) that were absent in the original chromosomes (AB and ab)

Combinative variability explains why children exhibit new combinations of characteristics of relatives on the maternal and paternal lines, and in such specific variants that were not characteristic of either the father, mother, grandfather, grandmother, etc.

Thanks to combinative variability, a variety of genotypes is created in the offspring, which has great value for the evolutionary process due to the fact that: 1) the diversity of material for the evolutionary process increases without reducing the viability of individuals; 2) the ability of organisms to adapt to changing environmental conditions expands and thereby ensures the survival of a group of organisms (population, species) as a whole.

Combinative variability is used in breeding in order to obtain a more economically valuable combination of hereditary characteristics. In particular, the phenomenon of heterosis, increased viability, growth intensity and other indicators is used during hybridization between representatives of different subspecies or varieties. The opposite effect is produced by the phenomenoninbreeding or inbreeding - crossing of organisms that have common ancestors. The common origin of crossed organisms increases the likelihood of them having the same alleles of any genes, and therefore the likelihood of the appearance of homozygous organisms. The greatest degree of inbreeding is achieved during self-pollination in plants and self-fertilization in animals. Homozygosity increases the possibility of the manifestation of recessive allelic genes, mutagenic changes of which lead to the appearance of organisms with hereditary abnormalities.

The results of studying the phenomenon of combinative variability are used in medical genetic counseling, especially in its second and third stages: prognosis of offspring, drawing up a conclusion and explaining the meaning of genetic risk.

Along with marriage systems, there are two types of formation of married couples:

1) positive assortative (selective) formation of marital pairs, or more frequent marriage of individuals similar in certain phenotypic characteristics (marriages between deaf-mute people, or similar in height, mental development, etc.);

2) negative assortative formation of mating pairs, or more rare marriage of individuals with similar certain characteristics (for example, red-haired individuals avoid marrying each other).

Both inbreeding and positive assortative formation of mating pairs increase (the latter, although to a lesser extent) the level of homozygosity of offspring, including at loci of harmful recessive alleles. Outbreeding, on the contrary, increases the degree of heterozygosity and in many cases increases the level of viability. The possible consequences of inbreeding and positive assortative formation of marital pairs are used in medical and genetic counseling of potential marriage partners.

Mutations - these are heritable changes in genetic material that lead to changes in the characteristics of the organism. The foundations of the doctrine of mutations were laid by G. de Vries already in 1901, who described mutations in Elotera, but their molecular mechanisms were studied much later. According to G. de Vries, a mutation is an abrupt, intermittent change in a hereditary trait.

The essence of the mutation theory of G. de Vries comes down to the following provisions:

1) mutation occurs discretely, without transitions;

2) new forms are constant;

3) mutations are multidirectional (beneficial and harmful);

4) the detection of mutations depends on the sample size of the studied organisms;

5) the same mutations can occur repeatedly.

Mutational changes are extremely diverse. They can affect almost all morphological, physiological and biochemical characteristics of the body, and can cause sharp or, conversely, barely noticeable phenotypic deviations from the norm.

Mutational variability is based on structural changes in genes and chromosomes. Depending on the nature of the changes in the genetic material, there are:

1) gene (point) mutations, which are an insertion, deletion, replacement or change of a pair of nucleotides;

2) insertions - insertions (“cuts”) of DNA molecules or their fragments into a gene, most often leading to its inactivation or to a strong polar effect in operons;

3) chromosomal rearrangements, or aberrations - transformations of the structure of chromosomes based on their break;

4) genomic (genotypic) mutations, which consist in changing the number of chromosomes in a cell.

Phenotypic variability and its types. Adaptive nature of modifications. Norm of reaction of a trait. Expressiveness and penetrance of the trait.

Modification (phenotypic) variability is caused only by the influence of external conditions and is not associated with changes in the genotype. Specific variants of the phenotype state with modification variability are called modifications. Of greatest interest areadaptive modifications - non-inherited changes beneficial to the body that contribute to its survival in changed conditions. Unlike mutations (rare, isolated and random events), adaptive modifications are directed and at the same time often reversible, predictable and often characteristic of large groups organisms. The basis for the existence of modifications is that the phenotype is the result of the interaction of the genotype and external conditions. Therefore, changes in external conditions can cause changes in phenotype that are not accompanied by changes in the genotype. The mechanism for the occurrence of modifications is that environmental conditions affect enzymatic reactions (metabolic processes) occurring in a developing organism and, to a certain extent, change their course, and, consequently, the result - the state of the trait formed on their basis.

Modifications have the following properties:

1) the degree of severity of the modification is proportional to the strength and duration of action on the body of the factor causing the modification (this pattern fundamentally distinguishes modifications from mutations, especially gene mutations);

2) in the vast majority of cases, modification is a useful adaptive reaction of the body in response to the action of one or another external factor

3) only those modifications are adaptive , which are caused by ordinary changes in natural conditions that the ancestors of individuals of a given species repeatedly “faced” throughout its past evolutionary history;

4) modifications caused by experimental influences, especially chemical and physical factors that the body does not encounter in nature, as a rule, do not have adaptive significance, and often represent malformations and deformities. Modifications induced in this way are often called morphoses.

5) unlike mutations, which are characterized by high constancy, modifications have varying degrees of persistence. Many of them are reversible, i.e. the changes that arise gradually disappear if the effect of the factor that caused them ceases. Thus, a person’s tan goes away when the skin stops being exposed to insolation, muscle volume decreases after stopping training, etc.

6) modifications, unlike mutations, are not inherited, i.e. are non-hereditary. This is consistent with the “central dogma of molecular biology” by F. Crick, according to which the transfer of information is possible only from genetic material to gene products-proteins, but not in the opposite direction.

External conditions have a huge impact on all the signs and properties of a developing organism.

Norm of reaction. With modification variability, a trait can change within certain limits (range), characteristic of each state of the genotype. The range within which the same genotype is capable of causing the development of different phenotypes is called the reaction norm. In other words, the normreactions are the amplitude of possible variability in the ontogenesis of an organism with a specific unchanged genotype. The reaction rate is best observed in organisms with the same genotypes, for example, in vegetatively propagated plants and identical twins. In this case, it is possible to identify the reaction norm of the genotype in its most “purest” form. The reaction norm, controlled by the genotype, is the result of the evolutionary process.

The main factors that can provide variation in symptoms within the normal reaction range are:

1) polygenic determination of the trait and reaction of the organism;

2) pleiotropy of the gene action;

3) dependence of the manifestation of mutation on environmental conditions;

4) heterozygosity of the organism;

5) interaction of genes at the level of gene products (subunits of protein molecules);

6) alternative paths of development in the body system and the implementation of biosynthesis in the cell (blocking one path is compensated by another).

Penetrance characterized by the frequency or probability of manifestation of an allele of a certain gene and is determined by the percentage of individuals in a population in which it is phenotypically manifested. A distinction is made between complete (manifestation of the trait in all individuals) and incomplete (in some) penetrance. Quantitatively, penetrance is expressed by the percentage of individuals in which a given allele is manifested. For example, the penetrance of congenital hip dislocation in humans is 25%, which indicates that only 1/4 of the genotypes carrying a particular gene exhibit its phenotypic effect.

Based on incomplete penetrance lies the interaction of genetic and environmental causes. Knowledge of the penetrance of certain alleles is necessary in medical genetic counseling to determine the possible genotype of “healthy” people who have hereditary diseases in their family. Cases of incomplete penetrance include manifestations of genes that control sex-limited and sex-dependent traits.

Expressivity - the degree of phenotypic manifestation of a gene, as a measure of the strength of its action, determined by the degree of development of the trait. Expressiveness in both sexes can be the same or different, constant or variable, if the severity of the trait with the same genotype varies from individual to individual. In the absence of variability in a trait controlled by a given allele, one speaks of constant expressivity (an unambiguous reaction norm). For example, the ABO blood group alleles in humans have virtually constant expressivity. Another type of expressiveness is changeable or variable. It is based on various reasons: the influence of external environmental conditions (modifications), genotypic environment (during the interaction of genes).

The degree of expressiveness is assessed quantitatively using statistical indicators. In cases of extreme variants of changes in expressivity (complete absence of a trait), an additional characteristic is used - penetrance. Huntington's chorea can serve as an example of incomplete penetrance and variable expressivity of the manifestation of a dominant gene. The age at which Huntington's chorea first appears varies. It is known that in some carriers it will never manifest itself (incomplete penetrance), in addition, this gene has variable expressivity, since carriers become ill at different ages.

Modification variability ensures the relatively rapid formation of adaptations of the organism to changing environmental conditions during ontogenesis, thereby promoting the survival of the organism. Consequently, modifications are the most important factor in the normal course and completion of ontogenesis of a living organism.

Despite the fact that modifications are not inherited by offspring, modification variability in general is important for the evolution of the organic world. Modifications can serve in the course of natural selection as a “cover” for mutations, the phenotypic manifestation of which duplicates non-hereditary changes. Favoring the survival of organisms, modification variability contributes to the preservation and participation in reproduction of specific individuals with various genotypes. Along with this, modifications contribute to the development of new habitats by the species (population), which leads to an expansion of the range of this group of organisms. All of these modification effects favor the evolutionary success of a species or population.

Man as a specific object of genetic research. Methods for studying human genetics. Medical and genetic aspect of marriage. Medical genetic counseling. The importance of genetics for medicine.

Man as a specific object of genetic research. The study of human genetics is associated with great difficulties: a complex karyotype - many chromosomes and linkage groups, late puberty and a rare change of generations, a small number of offspring, the impossibility of experimentation, the impossibility of creating identical living conditions. Despite all this, human genetics is better studied today than the genetics of many other organisms (for example, mammals) thanks to the needs of medicine and a variety of modern research methods.

Study methods :

Genealogical method consists of studying pedigrees based on Mendelian laws of inheritance and helps to establish the nature of inheritance of a trait (dominant or recessive). This is how the inheritance of a person’s individual characteristics is established: facial features, height, blood type, mental and mental makeup, as well as some diseases. This method revealed the harmful consequences of consanguineous marriages, which are especially manifested in cases of homozygosity for the same unfavorable recessive allele. In consanguineous marriages, the likelihood of having children with hereditary diseases and early childhood mortality is tens and even hundreds of times higher than average.

Twin method is to study the differences between identical twins. This method is provided by nature itself. It helps to identify the influence of environmental conditions on the phenotype for the same genotypes. Identical twins raised in the same conditions have a striking similarity not only in morphological characteristics, but also in mental and intellectual characteristics. Using the twin method, the role of heredity in a number of diseases was revealed.

Population statistical method. Population genetics studies genetic differences between individual groups of people (populations) and examines patterns of geographic distribution of genes.

Cytogenetic method . based on the study of variability and heredity at the level of cells and subcellular structures. A connection has been established between a number of serious diseases and chromosomal abnormalities. Chromosome disorders occur in 7 out of every thousand newborns, and they also lead to the death of the embryo (miscarriage) in the first third of pregnancy in half of all cases. If a child with chromosomal abnormalities is born alive, he usually suffers from severe illnesses and is retarded in mental and physical development.

Biochemical methods . The content allows you to identify many hereditary human diseases associated with metabolic disorders. Anomalies of carbohydrate, amino acid, lipid and other types of metabolism are known. For example, diabetes mellitus is caused by a violation of the normal functioning of the pancreas - it does not secrete into the blood required quantity the hormone insulin, which increases blood sugar. This disorder is not caused by one gross error in genetic information, but by a whole set of small errors that together lead to or predispose to the disease.

Methods of somatic cell genetics - studies the heredity and variability of somatic cells, i.e. body cells, not sex cells. Somatic cells have the entire set of genetic information; they can be used to study the genetic characteristics of an entire organism. Human somatic cells are obtained for genetic research from biopsy material (intravital excision of tissues or organs), when a small piece of tissue is taken for research.

Immunogenetic methods . The immunogenetic method includes serological methods, immunoelectrophoresis, etc., which are used to study blood groups, proteins and enzymes in tissue blood serum. With its help, you can establish immunological incompatibility, identify immunodeficiency, mosaicism of twins, etc.

Molecular genetic methods . Universality of methods. Characteristics of the main methodological approaches (DNA isolation, restriction, electrophoresis, blotting, hybridization). Polymerase chain reaction, sequencing. Possibilities and scope of application of molecular genetic methods in the diagnosis of hereditary pathology.

Methods for studying gene linkage . Fundamentals and conditions for applying the method in human genetics and medical genetics.

Biological modeling of hereditary diseases studies human diseases in animals that can suffer from these diseases. It is based on Vavilov’s law of homologous series of hereditary variability, for example, sex-linked hemophilia can be studied in dogs, epilepsy in rabbits, diabetes mellitus, muscular dystrophy in rats, cleft lip and palate in mice.

Medical genetic counseling - specialized medical care is the most common form of prevention of hereditary diseases. Genetic counseling - consists of informing a person about the risk of developing a hereditary disease, transmitting it to descendants, as well as diagnostic and therapeutic actions.

Stage 1 counseling - clarification of the diagnosis of the disease.

Stage 2 consulting - determination of the risk of having a sick child.

Stage 3 counseling - the geneticist must draw a conclusion about the risk of the disease in the children being examined and give them appropriate recommendations.

4 (final) stage counseling - the correct answer and probable complications or outcome of the expected pregnancy in a language they can understand.

The task medical genetics is identification, study, prevention and treatment of hereditary diseases, as well as the development of ways to prevent the harmful effects of environmental factors on human heredity.There are practically no diseases that have absolutely nothing to do with heredity. Conventionally, hereditary diseases can be divided into three large groups: metabolic diseases, molecular diseases, which are usually caused by gene mutations, and chromosomal diseases.

Gene mutations may be expressed in an increase or decrease in the activity of certain enzymes, up to their absence. Phenotypically, such mutations manifest themselves as hereditary metabolic diseases, which are determined by the absence or excess of the product of the corresponding biochemical reaction. Gene mutations are classified according to their phenotypic manifestation, i.e., as diseases associated with disorders of amino acid, carbohydrate, lipid, mineral metabolism, and nucleic acid metabolism.

Chromosomal diseases. This type of hereditary disease is associated with changes in the number or structure of chromosomes. The frequency of chromosomal abnormalities in newborns ranges from 0.6 to 1%, and at the stage of 8-12 weeks about 3% of embryos have them. Among spontaneous miscarriages, the frequency of chromosomal abnormalities is approximately 30%, and in the early stages (up to two months) - 50% and higher. All types of chromosomal and genomic mutations have been described in humans, including aneuploidy, which can be of two types -moyosomy And polysomy. Monosomies are particularly severe

Shereshevsky syndrome - Turner (44+X), manifested in women who are characterized by pathological changes in physique (short stature, short neck), disturbances in the development of the reproductive system (absence of most female secondary sexual characteristics), and mental limitations. The frequency of occurrence of this anomaly is 1: 4000-5000.

Trisomic women (44+XXX), As a rule, they are distinguished by disorders of sexual, physical and mental development, although in some patients these signs may not appear. There are known cases of fertility in such women. The frequency of the syndrome is 1:1000.

Klinefelter syndrome (44+XXY) characterized by impaired development and activity of the gonads, a eunuchoid body type (shoulders narrower than the pelvis, female-type hair growth and fat deposition on the body, elongated arms and legs compared to the body). Hence the higher growth. These signs, combined with some mental retardation, appear in a relatively normal boy from the moment of puberty. Klinefelter syndrome is observed in polysomy not only on the X chromosome (XXX XXXY, XXXXY), but also on the Y chromosome (XYY. XXYY. XXYYY). The frequency of the syndrome is 1:1000.

Down syndrome ( trisomy on chromosome 21) . According to various authors, the frequency of births of children with Down syndrome is 1:500-700 newborns, and over the past decades the frequency of trisomy-21 has increased.

If a sick child is born, drug, dietary and hormonal treatment is sometimes possible. A clear example confirming the capabilities of medicine in the fight against hereditary diseases is polio. This disease is characterized by a hereditary predisposition, but the direct cause of the disease is viral infection. Carrying out mass immunization against the causative agent of the disease made it possible to save all children hereditarily predisposed to it from the severe consequences of the disease. Dietary and hormonal treatment has been successfully used in the treatment of phenylketonuria, diabetes mellitus and other diseases

Ontogenesis as the process of realizing hereditary information under certain environmental conditions. Main stages of ontogenesis. Types of ontogenetic development. Periodization of ontogeny.

Ontogenesis, or individual development of the organism , is carried out on the basis of a hereditary program obtained through the reproductive cells of the parents that have entered into fertilization (in asexual reproduction, this program is contained in the unspecialized cells of the only parent giving offspring). During the implementation of hereditary information in the process of ontogenesis, specific and individual morphological, physiological and biochemical properties are formed in the organism, in other words - phenotype. In the process of development, the body naturally changes its characteristics, nevertheless remaining an integral system. Therefore, a phenotype must be understood as a set of properties throughout the entire course of individual development, each stage of which has its own characteristics.

The leading role in the formation of the phenotype belongs to hereditary information contained in the genotype of an organism. In this case, simple traits develop as a result of a certain type of interaction of the corresponding allelic genes. At the same time, the entire genotype system has a significant influence on their formation. The formation of complex traits occurs as a result of various interactions of non-allelic genes directly in the genotype or products controlled by them. The starting program for the individual development of the zygote also contains so-called spatial information that determines the anteroposterior and dorsoventral (dorsoventral) coordinates for the development of structures.

Along with this, the result of the implementation of the hereditary program contained in the genotype of an individual largely depends on the conditions in which this process is carried out. Environmental factors external to the genotype can promote or hinder the phenotypic manifestation of genetic information, strengthen or weaken the degree of such manifestation. Already at the transcription stage, control of the expression of individual genes is carried out through the interaction of genetic and non-genetic factors. Consequently, even in the formation of the elementary characteristics of an organism - polypeptides - the genotype as a system of interacting genes and the environment in which it is realized take part.

In the genetics of individual development Wednesday is a complex concept. On the one hand, this is the immediate environment in which individual genes and the genotype as a whole carry out their functions. It is formed by the entire set of factors of the internal environment of the body: cellular contents (excluding DNA), the nature of direct intercellular interactions, biologically active substances (hormones). The set of intraorganismal factors influencing the implementation of the hereditary program is designated as 1st order environment. The factors of this environment have a particularly great influence on the function of the genotype during the period of active formative processes, primarily in embryogenesis. On the other hand, the concept of environment is distinguished, or 2nd order environments, as a combination of factors external to the body.

Periodization of ontogeny Individual development is a holistic, continuous process in which individual events are interconnected in space and time. There are several schemes for the periodization of ontogenesis, each of which is most suitable for solving specific scientific or practical problems.

WITH general biological points of view: pre-reproductive, reproductiveand pacutely reproductive.

IN pre-reproductive period the individual is not capable of reproduction. Its main content is the development of a sexually mature phenotype.

Embryonic or embryonic, the period of ontogenesis begins from the moment of fertilization and continues until the embryo emerges from the egg membranes.

Larval The period is typically observed in the development of those vertebrates whose embryos emerge from the egg shells and begin to lead an independent lifestyle without reaching the definitive (mature) organizational features.

metamorphosis consists of the transformation of the larva into the juvenile form.

Juvenile the period begins with the completion of metamorphosis and ends with puberty and the beginning of reproduction.

IN reproductive period the individual performs the function of sexual reproduction.

Post-reproductive period associated with the aging of the body and is characterized by a weakening or complete cessation of participation in reproduction.

• Human ontogeny

Antenatal ontogeny:

Germinal or embryonic period. The first week after conception.

Embryonic period. Second to fifth week of pregnancy.

Fetal period: 32 weeks.

Postnatal ontogenesis:

Neonatal or newborn period. 1-10 days.

Infancy. 10 days – 1 year.

Early childhood. 1-3 years.

First childhood. 4-7 years.

Second childhood. 8-12 years old for boys, 8-11 years old girls.

Adolescence. 13-16 years old for boys, 12-15 years old girls.

Adolescence. 17-21 years for boys, 16-20 years for girls.

Mature age:

Iperiod: 22-35 years old men, 21-35 years old women.

IIperiod: 36-60 years old men, 36-55 years old women.

Old age. Men 61-74 years old, women 56-74 years old.

Senile age. 75-90 years old.

Longevity period. Over 90 years old.

The germinal period is the moment from the beginning of conception to the formation of the embryo. The embryonic period is divided into 2 phases: the phase of histotrophic nutrition and the phase of vitelline circulation. In the fetal period, a transition from yolk to hemo-amniotrophic nutrition occurs. During the neonatal period, the baby is fed colostrum milk. During the period of mature breastfeeding, and then complementary feeding is added to mother's milk and the sensorimotor pattern of standing is implemented. During early childhood, the development of walking and speaking skills occurs. During early childhood, vocabulary increases and the first phase of thinking formation occurs. In second childhood, the analytical and synthetic activity of the brain becomes more complex and the 2nd phase of thinking is formed. In adolescence, the maturation of the visceral systems is basically completed and the 3rd phase of the organization of thinking occurs. The period of adolescence or adolescence is a turning point when personality formation and puberty are completed. The period of maturity or stability is the most productive in social terms and the organization of physiological functions. During old age, involutional changes begin, which are a consequence of physiological changes in homeostasis.In subsequent periods they become more active

Correlation between onto- and phylogeny. K. Baer's law of germinal similarity. Biogenetic law of E. Haeckel and F. Muller

1st Law of Germinal Similarity “The early stages of development of organisms belonging to different classes are more similar to each other than the later stages.”

2nd Law of Development Specialization “As ontogenesis progresses, each organism develops more and more specific characteristics”

F. Muller: "Evolutionary changes in structureadultsanimals occur thanks tochanges in the course of ontogenesis of descendantscompared to those of their ancestors".

E. Haeckel Created a triple parallelism method:

comparative morphology

comparative embryology data

paleontological data

sources for constructing a phylogenetic series

Biogenetic law“Ontogenesis is a quick and brief repetition of phylogeny”

Recapitulation –This is a repetition in the ontogenesis of descendants of the stages of evolution of their ancestors.

• Relationship between onto- and phylogeny . According to modern concepts, most phylogenetic innovations are associated with ontogenetic heterochronies, that is, with shifts in the relative rates of various ontogenetic processes. One of the most evolutionarily significant heterochronies is a shift in the period of puberty in evolutionary descendants to stages corresponding to the larvae of their ancestors. This shift is called neoteny, or paedomorphosis. In this case, the life cycle of evolutionary descendants is usually shortened (for example, due to the loss of the metamorphosis phase inherent in the ancestors). Neoteny is considered one of the ways to achieve rapid evolutionary progress.

  Further development of problems of ontogenesis is of paramount importance both for fundamental natural science and for a number of medical, biotechnological and environmental problems.

Characteristics and significance of the main stages of embryonic development: prezygotic period, fertilization, zygote, cleavage. Their regulatory mechanisms at the gene and cellular levels.

• Fertilization - This is the process of fusion of germ cells. The diploid cell formed as a result of fertilization iszygote -represents initial stage development of a new organism. The fertilization process consists of three successive phases:

  a) bringing together gametes (gamons(gamete hormones), on the one hand, activate the movement of sperm, and on the other, their gluing.) At the moment of contact of the sperm with the shell of the egg,acrosome reaction,during which, under the action of proteolytic enzymes of the acrosome, the egg membranes dissolve. Next, the plasma membranes of the egg and sperm merge and, through the resulting cytoplasmic bridge, the cytoplasm of both gametes are combined. Then the nucleus and centriole of the sperm pass into the cytoplasm of the egg, and the sperm membrane is embedded in the membrane of the egg. The tail part of the sperm in most animals also enters the egg, but then separates and dissolves without playing any role in further development;

  b) activation of the egg Due to the fact that a section of the sperm membrane is permeable to sodium ions, the latter begin to enter the egg, changing the membrane potential of the cell. Then, in the form of a wave propagating from the point of contact of the gametes, an increase in the content of calcium ions occurs, after which the cortical granules also dissolve in a wave. The specific enzymes released in this process promote the detachment of the vitelline membrane; it hardens, itfertilization membrane.All the described processes represent the so-calledcortical reaction.;

  c) fusion of gametes, or syngamy When the egg meets the sperm, it is usually at one of the stages of meiosis, blocked by a specific factor. In most vertebrates, this block occurs at the metaphase II stage; in many invertebrates, as well as in three species of mammals (horses, dogs and foxes), the block occurs at the stage of diakinesis. In most cases, the meiosis block is removed after the activation of the egg due to fertilization. While meiosis completes in the egg, the nucleus of the sperm that penetrates it is modified. It takes the form of an interphase and then a prophase nucleus. During this time, DNA doubles andmale pronucleusreceives an amount of hereditary material correspondingn2 With,those. contains a haploid set of reduplicated chromosomes. The nucleus of the egg, having completed meiosis, turns intofemale pronucleus,also purchasingn2 With.Both pronuclei undergo complex movements, then come closer and merge (synkarion) , forming a common metaphase plate. This, in fact, is the moment of the final fusion of gametes -syngamy.The first mitotic division of the zygote leads to the formation of two embryonic cells (blastomeres) with a set of chromosomes 2n2 cin everyone.

  Zygote - diploid(containing a complete double setchromosomes) cell resulting fromfertilization(mergerseggsAndspermatozoon). The zygote istotipotent(that is, capable of giving birth to any other)cell.

  Man's firstmitoticdivision of the zygote occurs approximately 30 hours after fertilization, which is due to complex processes of preparation for the first act of fragmentation. The cells formed as a result of fragmentation of the zygote are called

  blastomeres. The first divisions of the zygote are called “fragmentations” because the cell is fragmented: the daughter cells become smaller after each division, and there is no stage of cell growth between divisions.

  Crushing - this is a series of successive mitotic divisions of the zygote and then blastomeres, ending with the formation of a multicellular embryo -blastulas. Between successive divisions, cell growth does not occur, but DNA is necessarily synthesized. All DNA precursors and necessary enzymes are accumulated during oogenesis. First, the blastomeres are adjacent to each other, forming a cluster of cells calledMorula . Then a cavity forms between the cells -blastocoel, filled with liquid. Cells are pushed to the periphery, forming the wall of the blastula -blastoderm. The total size of the embryo at the end of cleavage at the blastula stage does not exceed the size of the zygote. The main result of the cleavage period is the transformation of the zygote intomulticellular single-layer embryo .

  Morphology of crushing. As a rule, blastomeres are located in in strict order relative to each other and the polar axis of the egg. The order, or method, of crushing depends on the quantity, density and nature of the distribution of the yolk in the egg. According to the Sachs-Hertwig rules, the cell nucleus tends to be located in the center of the yolk-free cytoplasm, and the cell division spindle tends to be located in the direction of the greatest extent of this zone.

  In oligo- and mesolecithal eggs crushingcomplete,orholoblastic.This type of cleavage occurs in lampreys, some fish, all amphibians, as well as in marsupials and placental mammals. With complete crushing, the plane of the first division corresponds to the plane of bilateral symmetry. The plane of the second division runs perpendicular to the plane of the first. Both grooves of the first two divisions are meridian, i.e. begin at the animal pole and spread to the vegetative pole. The egg cell turns out to be divided into four more or less equal in size blastomeres. The plane of the third division runs perpendicular to the first two in the latitudinal direction. After this, uneven cleavage appears in mesolecithal eggs at the stage of eight blastomeres. At the animal pole there are four smaller blastomeres -micromeasures,on the vegetative - four larger ones -macromeres.Then the division again occurs in the meridian planes, and then again in the latitude planes.

  In polylecithals in the eggs of bony fish, reptiles, birds, as well as monotreme mammals, crushingpartial,ormerob-lastic,those. covers only yolk-free cytoplasm. It is located in the form of a thin disk at the animal pole, which is why this type of crushing is calleddiscoidal.When characterizing the type of crushing, they also take into account relative position and the rate of blastomere division. If blastomeres are arranged in rows above each other along radii, cleavage is calledradial.It is typical of chordates and echinoderms. In nature, there are other variants of the spatial arrangement of blastomeres during crushing, which determines such types as spiral in mollusks, bilateral in roundworms, anarchic in jellyfish.

  A relationship was observed between the distribution of yolk and the degree of synchrony in the division of animal and vegetative blastomeres. In oligolecithal eggs of echinoderms, cleavage is almost synchronous; in mesolecithal egg cells, synchrony is broken after the third division, since vegetative blastomeres divide more slowly due to the large amount of yolk. In forms with partial crushing, divisions are asynchronous from the very beginning andblastomeres occupying a central position divide faster.

  By the end of crushing, a blastula is formed. The type of blastula depends on the type of cleavage, and therefore on the type of egg.

  Features of molecular genetic and biochemical processes during crushing. As noted above, mitotic cycles during the cleavage period are greatly shortened, especially at the very beginning.

  For example, the entire division cycle in eggs sea ​​urchin lasts 30-40 minutes with the duration of the S-phase only 15 minutes. GI- andG2-periods are practically absent, since the necessary reserve of all substances has been created in the cytoplasm of the egg cell, and the larger the cell, the larger it is. Before each division, DNA and histones are synthesized.

  The rate at which the replication fork moves along DNA during cleavage is normal. At the same time, more initiation points are observed in the DNA of blastomeres than in somatic cells. DNA synthesis occurs in all replicons simultaneously, synchronously. Therefore, the time of DNA replication in the nucleus coincides with the doubling time of one, and shortened, replicon. It has been shown that when the nucleus is removed from the zygote, fragmentation occurs and the embryo reaches in its development almost to the blastula stage. Further development stops.

  At the beginning of cleavage, other types of nuclear activity, such as transcription, are practically absent. IN different types eggs, gene transcription and RNA synthesis begin on different stages. In cases where there are many different substances in the cytoplasm, as, for example, in amphibians, transcription is not immediately activated. Their RNA synthesis begins at the early blastula stage. On the contrary, in mammals, RNA synthesis already begins at the stage of two blastomeres.

  During the fragmentation period, RNA and proteins are formed, similar to those synthesized during oogenesis. These are mainly histones, cell membrane proteins and enzymes necessary for cell division. The named proteins are used immediately along with proteins previously stored in the cytoplasm of the eggs. Along with this, during the crushing period, the synthesis of proteins that were not there before is possible. This is supported by data on the presence of regional differences in the synthesis of RNA and proteins between blastomeres. Sometimes these RNAs and proteins begin to act at later stages.

  An important role in fragmentation is played by the division of the cytoplasm -cytotomy.It has a special morphogenetic significance, as it determines the type of fragmentation. During cytotomy, a constriction is first formed using a contractile ring of microfilaments. The assembly of this ring occurs under the direct influence of the poles of the mitotic spindle. After cytotomy, the blastomeres of the oligolecithal eggs remain connected to each other only by thin bridges. It is at this time that they are easiest to separate. This occurs because cytotomy leads to a decrease in the contact area between cells due to limited area membrane surfaces Immediately after cytotomy, the synthesis of new areas of the cell surface begins, the contact zone increases and the blastomeres begin to come into close contact. Cleavage furrows run along the boundaries between individual sections of ovoplasm, reflecting the phenomenon of ovoplasmic segregation.Therefore, the cytoplasm of different blastomeres differs in chemical composition.

Characteristics and significance of the main stages of embryonic development: gastrulation, histo- and organogenesis. Formation of 2 and 3 layer embryos. Methods of formation of mesoderm. Derivatives of germ layers. Regulatory mechanisms of these processes at the gene and cellular levels.

• Histogenesis - (from the Greek histos - tissue it ...genesis), a set of processes that has developed in phylogenesis, ensuring the ontogenesis of multicellular organisms education, the existence and restoration of tissues with their inherent organ-specificity. features. In the body, tissues develop from certain embryonic primordia (derivatives of germ layers), formed as a result of proliferation, movement (morphogenetic movements) and adhesion of embryonic cells in the early stages of its development in the process of organogenesis. Beings, factor G. is the differentiation of determined cells, leading to the appearance of various morphol. and physiol. types of cells that are regularly distributed in the body. Sometimes G. is accompanied by the formation of intercellular substance. An important role in determining the direction of G. belongs to intercellular contact interactions and hormonal influences. A set of cells that perform certain functions. G., is divided into a number of groups: ancestral (stem) cells, capable of differentiation and replenishment of the loss of their own kind by division; progenitor cells (so-called semi-stem) - differentiate, but retain the ability to divide; mature differentiated cells. Reparative hygiene in the postnatal period underlies the restoration of damaged or partially lost tissue. G.'s qualities and changes can lead to the appearance and growth of a tumor.

  Organogenesis (from Greekorganon- organ,genesis- development, education) - the process of development, or formation, of organs in the embryo of humans and animals. Organogenesis follows earlier periods of embryonic development (see Embryo) - egg fragmentation, gastrulation and occurs after the main rudiments (anlage) of organs and tissues have separated. Organogenesis proceeds in parallel with histogenesis (see), or tissue development. Unlike tissues, each of which has its source in one of the embryonic rudiments, organs, as a rule, arise with the participation of several (from two to four) different rudiments (see Germ layers), giving rise to different tissue components of the organ. For example, as part of the intestinal wall, the epithelium lining the organ cavity and glands develop from the internal germ layer - endoderm (see), connective tissue with blood vessels and smooth muscle tissue - from mesenchyme (see), mesothelium covering the serous membrane of the intestine - from the visceral layer of the splanchnotome, i.e., the middle germ layer - mesoderm, and the nerves and ganglia of the organ - from the neural rudiment. The skin is formed with the participation of the outer germ layer - ectoderm (see), from which the epidermis and its derivatives (hair, sebaceous and sweat glands, nails, etc.) develop, and dermatomes, from which mesenchyme arises, differentiating into the connective tissue basis of the skin (dermis ). Nerves and nerve endings in the skin, as elsewhere, are derivatives of the neural rudiment. Some organs are formed from one primordium, for example, bone, blood vessels, lymph nodes - from mesenchyme; however, here too, derivatives of the rudiment of the nervous system—nerve fibers—grow into the anlage, and nerve endings are formed.

  If histogenesis consists mainly in the reproduction and specialization of cells, as well as in the formation of intercellular substances and other non-cellular structures by them, then the main processes underlying organogenesis are the formation of folds, invaginations, protrusions, thickenings by the germ layers, uneven growth, fusion or division (separation), as well as mutual germination of various bookmarks. In humans, organogenesis begins at the end of the 3rd week and is generally completed by the 4th month of intrauterine development. However, the development of a number of provisional (temporary) organs of the embryo - chorion, amnion, yolk sac - begins already from the end of the 1st week, and some definitive (final) organs form later than others (for example, lymph nodes - from the last months of intrauterine development to onset of puberty).

  Gastrulation – single-layer embryo - blastula - turns intomultilayer -two- or three-layer, calledgastrula(from Greekgaster -stomach in the diminutive sense).

  In primitive chordates, for example, the lancelet, a homogeneous single-layer blastoderm during gastrulation is transformed into an outer germ layer - ectoderm - and an inner germ layer -endoderm.The endoderm forms the primary gut with a cavity insidegastrocele.The hole leading into the gastrocoel is calledblastoporeor primary mouth.Two germ layersare the defining morphological signs of gastrulation. Their existence at a certain stage of development in all multicellular animals, from coelenterates to higher vertebrates, allows us to think about the homology of the germ layers and the unity of origin of all these animals. In vertebrates, in addition to the two mentioned during gastrulation, a third germ layer is formed -mesoderm,occupying a place between ecto- and endoderm. The development of the middle germ layer, which is chordomesoderm, is an evolutionary complication of the gastrulation phase in vertebrates and is associated with the acceleration of their development in the early stages of embryogenesis. In more primitive chordates, such as the lancelet, chordomesoderm is usually formed at the beginning of the next phase after gastrulation -organogenesis.A shift in the time of development of some organs relative to others in descendants compared with ancestral groups is a manifestationheterochrony.Changes in the time of formation of the most important organs in the process of evolution are not uncommon.

  The gastrulation process is characterizedimportant cellular transformations,such as directed movements of groups and individual cells, selective proliferation and sorting of cells, the beginning of cytodifferentiation and inductive interactions.

  Methods of gastrulation are different. There are four types of spatially directed cell movements that lead to the transformation of the embryo from a single-layer to a multi-layer.

  Intussusception - invagination of one of the sections of the blastoderm inward as a whole layer. In the lancelet, the cells of the vegetative pole invaginate; in amphibians, invagination occurs at the border between the animal and vegetative poles in the region of the gray falx. The process of invagination is possible only in eggs with a small or medium amount of yolk.

  Epiboly - overgrowth of small cells of the animal pole with larger cells of the vegetative pole that lag behind in the rate of division and are less mobile. This process is clearly expressed in amphibians.

  Denomination - separation of blastoderm cells into two layers lying one above the other. Delamination can be observed in the discoblastula of embryos with a partial type of cleavage, such as reptiles, birds, and oviparous mammals. Delamination occurs in the embryoblast of placental mammals, leading to the formation of the hypoblast and epiblast.

  Immigration - movement of groups or individual cells that are not united into a single layer. Immigration occurs in all embryos, but is most characteristic of the second phase of gastrulation in higher vertebrates. In each specific case of embryogenesis, as a rule, several methods of gastrulation are combined.

  Morphology of gastrulation. In the area of ​​the blastula, from the cellular material of which, during gastrulation and early organogenesis (neurulation), completely defined germ layers and organs are usually formed. Intussusception begins at the vegetative pole. Due to faster division, the cells of the animal pole grow and push the cells of the vegetative pole into the blastula. This is facilitated by a change in the state of the cytoplasm in the cells forming the lips of the blastopore and adjacent to them. Due to invagination, the blastocoel decreases and the gastrocoel increases. Simultaneously with the disappearance of the blastocoel, the ectoderm and endoderm come into close contact. In the lancelet, as in all deuterostomes (these include the echinoderm type, the chordate type and some other small types of animals), the blastopore region turns into the tail part of the body, in contrast to protostomes, in which the blastopore corresponds to the head part. The oral opening in deuterostomes is formed at the end of the embryo opposite the blastopore. Gastrulation in amphibians has much in common with gastrulation of the lancelet, but since their eggs have much more yolk and it is located mainly at the vegetative pole, large amphiblastula blastomeres are not able to invaginate.Intussusception goes a little differently. At the border between the animal and vegetative poles in the gray falx region, the cells first strongly stretch inward, taking the form≪ flask-shaped≫ , and then pull the cells of the superficial layer of the blastula along with them. A crescentic groove and a dorsal lip of the blastopore appear. At the same time, smaller cells of the animal pole, dividing faster, begin to move towards the vegetative pole. In the area of ​​the dorsal lip they turn over and invaginate, and larger cells grow on the sides and on the side opposite the falciform groove. Then the processepiboly leads to the formation of the lateral and ventral lips of the blastopore. The blastopore closes into a ring, inside which large light cells of the vegetative pole are visible for some time in the form of the so-called yolk plug. Later they are completely immersed inside, and the blastopore narrows. Using the method of marking with intravital (vital) dyes in amphibians, the movements of blastula cells during gastrulation were studied in detail. It was established that specific areas of the blastoderm, calledpresumptive(from the Latin praesumptio - assumption), during normal development they find themselves first as part of certain rudiments of organs, and then as part of the organs themselves. It is known that in tailless amphibians, the material of the presumptive notochord and mesoderm at the blastula stage lies not on its surface, but in the inner layers of the amphiblastula wall, however, approximately at the same levels as shown in the figure. Analysis of the early stages of amphibian development allows us to conclude thatovoplasmic segregation,which clearly manifests itself in the egg and zygote is of great importance in determining the fate of the cells that inherited one or another section of the cytoplasm. Gastrulation in embryos with a mepoblastic type of cleavage and development has its own characteristics. Ubirdsit begins after cleavage and formation of the blastula during the passage of the embryo through the oviduct. By the time the egg is laid, the embryo already consists of several layers: the top layer is calledepiblastoma,lower -primary hypoblast.Between them there is a narrow gap - the blastocoel. Then it formssecondary hypoblast,the method of formation of which is not entirely clear. There is evidence that primary germ cells originate in the primary hypoblast of birds, and the secondary one forms the extraembryonic endoderm. The formation of primary and secondary hypoblasts is considered a phenomenon preceding gastrulation. The main events of gastrulation and the final formation of the three germ layers begin after oviposition with the onset of incubation. An accumulation of cells occurs in the posterior part of the epiblast as a result of uneven cell division in speed and their movement from the lateral sections of the epiblast to the center, towards each other. The so-calledprimitive streak,which extends towards the head end. In the center of the primitive streak is formedprimary groove,and along the edges there are primary rollers. At the cephalic end of the primary streak a thickening appears -Hensen's node,and in it is the primary fossa. When epiblast cells enter the primary groove, their shape changes. They resemble in shape≪ flask-shaped≫ gastrula cells of amphibians. These cells then become stellate in shape and submerge beneath the epiblast to form mesoderm. The endoderm is formed on the basis of the primary and secondary hypoblast with the addition of a new generation of endoderm cells migrating from the upper layers of the blastoderm. The presence of several generations of endodermal cells indicates that the gastrulation period is extended over time. Some of the cells migrating from the epiblast through Hensen's node form the future notochord. Simultaneously with the initiation and elongation of the notochord, Hensen's node and the primary streak gradually disappear in the direction from the head to the caudal end. This corresponds to the narrowing and closure of the blastopore. As the primitive streak contracts, it leaves behind formed areas of the axial organs of the embryo in the direction from the head to the tail sections. It seems reasonable to consider the movements of cells in the chick embryo as homologous epiboly, and the primitive streak and Hensen's node as homologous to the blastopore in the dorsal lip of the gastrula of amphibians. It is interesting to note that the cells of mammalian embryos, despite the fact that in these animals the eggs have a small amount of yolk and complete fragmentation, during the gastrulation phase they retain the movements characteristic of the embryos of reptiles and birds. This supports the idea that mammals descended from an ancestral group in which eggs were rich in yolk.

  Features of the gastrulation stage. Gastrulation is characterized by a variety of cellular processes. Mitotic continuescell proliferation,and it has different intensity in different parts of the embryo. At the same time, the most characteristic feature gastrulation consists ofmovement of cell masses.This leads to a change in the structure of the embryo and its transformation from a blastula to a gastrula. Happeningsortingcells according to their belonging to different germ layers, within which they≪ find out≫ each other. The gastrulation phase beginscytodifferentiation,which means a transition to the active use of biological information from one’s own genome. One of the regulators of genetic activity is the different chemical composition of the cytoplasm of embryonic cells, established as a result of ovoplasmic segregation. Thus, the ectodermal cells of amphibians have dark color due to the pigment that entered them from the animal pole of the egg, and the endoderm cells are light, since they originate from the vegetative pole of the egg. During gastrulation, the role ofembryonic induction.It has been shown that the appearance of the primitive streak in birds is the result of an inductive interaction between the hypoblast and the epiblast. The hypoblast is characterized by polarity. A change in the position of the hypoblast relative to the epiblast causes a change in the orientation of the primitive streak. All of these processes are described in detail in the chapter. It should be noted that such manifestationsintegrityembryo-likedetermination, embryonic regulationAndintegrationinherent in it during gastrulation to the same extent as during cleavage.

  Mesoderm formation -In all animals, with the exception of coelenterates, in connection with gastrulation (in parallel with it or at the next stage caused by gastrulation), and third germ layer - mesoderm. This is a set of cellular elements lying between the ectoderm and endoderm, i.e. in the blastocoele. Like this. Thus, the embryo becomes not two-layered, but three-layered. In higher vertebrates, a three-layered structure of embryos appears already during the process of gastrulation, while in lower chordates and all other types, as a result of gastrulation proper, a two-layer embryo is formed.

  It is possible to establish two fundamentally different pathways for the emergence of mesoderm: teloblastic, peculiar Protostomia, And enterocoelous, characteristic ofDeute-rosiomia. in protostomes, during gastrulation, at the border between the ectoderm and endoderm, on the sides of the blastopore, there are already two large cells that separate small cells from themselves (due to divisions). Thus, the middle layer is formed - mesoderm. Teloblasts, giving rise to new generations of mesoderm cells, remain at the posterior end of the embryo. For this reason, this method of mesoderm formation is called teloblastic (from Greek telos - end).

  With the enterocoel method, a set of cells of the developing mesoderm appears in the form of pocket-like protrusions of the primary intestine (protrusion of its walls into the blastocoel). These protrusions, into which parts of the primary intestinal cavity enter, are separated from the intestine and separated from it in the form of pouches. The cavity of the sacs turns into in general, i.e., into the secondary body cavity, the coelomic sacs can be divided into segments of the middle germ layer does not reflect the whole variety of variations and deviations that are strictly natural for individual groups of animals. Similar to the teloblastic method, but only externally, is the method of formation of mesoderm not by dividing teloblasts, but by the appearance at the edges of the blastopore of an unpaired dense primordium (group of cells), which is subsequently divided into two symmetrical stripes of cells. With the enterocoel method, the mesoderm rudiment can be paired or unpaired; in some cases, two symmetrical coelomic sacs are formed, and in others, one common coelomic sac is first formed, which is subsequently divided into two symmetrical halves.

  Derivatives of germ layers. The further fate of the three germ layers is different.

  From the ectoderm develop: all nervous tissue; outer layers of skin and its derivatives (hair, nails, tooth enamel) and partially mucous membrane oral cavity, nasal and anal cavities.

  The endoderm gives rise to the lining of the entire digestive tract - from the oral cavity to the anus - and all its derivatives, i.e. thymus, thyroid gland, parathyroid glands, trachea, lungs, liver and pancreas.

  From the mesoderm are formed: all types of connective tissue, bone and cartilage tissue, blood and the vascular system; all types muscle tissue; excretory and reproductive systems, dermal layer of skin.

  In an adult animal there are very few organs of endodermal origin that do not contain nerve cells originating from the ectoderm. Each important organ also contains derivatives of the mesoderm - blood vessels, blood, and often muscles, so that the structural isolation of the germ layers is preserved only at the stage of their formation. Already at the very beginning of their development, all organs acquire a complex structure, and they include derivatives of all germ layers

Postembryonic period of ontogenesis. Basic processes: growth, formation of definitive structures, puberty, reproduction, aging.

• Postnatal ontogeny - the period of development of an organism from birth to death. It combines two stages: a) the stage of early postnatal ontogenesis; b) stage of late postnatal ontogenesis. Early postnatal ontogenesis begins with the birth of the organism and ends with the onset of structural and functional maturity of all organ systems, including the reproductive system. Its duration in humans is 13-16 years. Early postnatal ontogenesis may include the basic processes of organogenesis, differentiation and growth (for example, in kangaroos) or only growth, as well as differentiation of later maturing organs (gonads, secondary sexual characteristics). In many animals, metamorphosis occurs during postembryonic development. Late postnatal ontogenesis includes adulthood, aging and death. Postembryonic development is characterized by: 1) intensive growth; 2) establishing definitive (final) body proportions; 3) a gradual transition of organ systems to functioning in a mode characteristic of a mature organism.

  Height - this is an increase in the mass and linear dimensions of an individual (organism) due to an increase in mass, but mainly in the number of cells, as well as non-cellular formations. To describe growth, growth curves (changes in body mass or length during ontogenesis), indicators of absolute and relative growth over a certain period of time, and specific growth rate are used.

  The growth of an individual is characterized eitherisometric - uniform growth of body parts and organs, orallometry - uneven growth of body parts.Allometry It can be negative (for example, slow growth of the head in relation to the body in a child) and positive (for example, accelerated growth of horns in ruminants). Growth rate usually decreases with age. Animals with indefinite growth grow throughout their lives (molluscs, crustaceans, fish, amphibians). In animals with a certain height, growth stops at a certain age (insects, birds, mammals). However, there is no sharp line between definite and uncertain growth. Humans, mammals, and birds can still increase somewhat in size after growth ceases. Growth processes are controlled by the genotype, while at the same time depending on environmental conditions. Human growth, determined by a combination of hereditary and environmental factors, reveals variability (age, gender, group, intragroup or individual and epochal). The growth and development of an organism can also be indirectly influenced by its genotype through the synthesis of biologically active substances - hormones. These are neurosecrets produced by nerve cells, hormones of the endocrine glands. Hormones can influence both metabolic processes (biosynthesis) and the expression of other genes, which in turn affect growth. There is a relationship between all endocrine glands, regulated by the principle of feedback. Thus, pituitary hormones affect the endocrine function of the gonads, thyroid gland and adrenal glands. The pituitary gland produces somatotropic hormone, the deficiency of which leads to dwarfism - nanism, and the excess - to gigantism.

  4th stage of embryogenesis - stage of definitive (final) organogenesis , on which the formation of permanent organs occurs. The very complex processes occurring at this final stage of embryogenesis are the object of study of special embryology. In this section we will limit ourselves to considering the “fate” of the primary organs of the embryo.

  From the ectoderm develop: the epidermis of the skin and its derivatives - feathers, hair, nails, skin and mammary glands, and the nervous system. The anterior (expanded) section of the neural tube is transformed into the brain, the rest of it (anterior and middle sections) into the spinal cord. Endoderm gives rise to the inner lining of the digestive and respiratory systems, secreting cells of the digestive glands. Somites undergo the following transformations: the dermatome forms the dermis (deep layer of skin); the sclerotome is involved in the formation of the skeleton (cartilaginous, then bone); the myotome gives rise to skeletal muscles. The urinary organs develop from the nephrotome.

  Unsegmented mesoderm (splanchnotome) gives rise to the pleura, peritoneum, pericardium, and participates in the development of the cardiovascular and lymphatic systems.

  Puberty - the process of formation of the reproductive function of the human body, manifested by the gradual development of secondary sexual characteristics and ending with the onset of puberty. In humans, the period of puberty is called transitional, or puberty, and its duration averages about 5 years. The age range of puberty is subject to individual fluctuations (for girls from 8 - 10 to 16 - 17 years, for boys from 10 - 12 to 19 - 20 years). The appearance of secondary sexual characteristics in girls from 8 to 10 years old, in boys from 10 to 12 years old is called early puberty (it is usually associated with constitutional factors).

  An important sign of pubertal development – establishment of regular gonadal activity, which manifests itself in girls as menstruation, and in boys as ejaculation. The intrasecretory activity of the gonads in both sexes is also manifested by phase changes in the growth rates of individual skeletal segments, as a result of whichDefinitive (structure) proportions of the body are established and secondary sexual characteristics are formed. Secondary sexual characteristics include mainly changes in the skin (in particular, the scrotum) and its derivatives (it is during the period of maturation that the mane grows in a lion, the development of the so-called sexual skin in monkeys, and antlers in a deer). The first signs of pubertal development in boys, along with an increase in the size of the testicles and acceleration of total growth, are the intensification of hair growth and changes in the scrotum. The average age period for the appearance of individual signs in 50% of those examined was: voice mutation - 12 years 3.5 months, pubic hair growth - 12 years 9.5 months, enlargement of the thyroid cartilage of the larynx - 13 years 3.5 months, axillary hair growth - 13 years 9.5 months and facial hair – 14 years 2 months. Studying the duration and rate of formation of secondary sexual characteristics, V. G. Sidamon Eristavi found that the rate of development of individual signs of puberty has its “peaks”.

  Human reproductive function - reproduction of one's own kind. The ability of humans as a species to transmit one half of the genetic information of the future generation from father to mother is ensured by the physiological characteristics of the reproductive function of the male body. The reproductive function of the female body ensures the process of fertilization, intrauterine development of the fetus, the birth of a child and its feeding with breast milk. A distinctive feature of the human reproductive function from other physiological functions of the body is that its normal functioning leads to the fusion of germ cells of male and female organisms in the process of sexual reproduction. Oocytes and sperm are called female and male reproductive cells, or gametes. Male and female gametes in their mature form contain a haploid number of chromosomes, that is, half the normal number. The haploid number of chromosomes in gametes is formed during the process of spermatogenesis and oogenesis (Fig. 16.1). In the male body, meiotic division of spermatogenic cells occurs continuously throughout life after the onset of puberty (puberty). In contrast, in an oocyte, the haploid number of chromosomes is formed immediately before the ovulation of the egg from the follicle. As a result of the ability of the oocyte and sperm to unite with each other during fertilization, a zygote is formed in the female reproductive tract. This process is called fertilization. The zygote contains a diploid number of chromosomes, as in any somatic cell of the human and animal body. Two chromosomes from the diploid number in the zygote, namely the sex X and Y chromosomes, determine the male or female sex of the future individual in the new generation. The female reproductive cell contains only X chromosomes, while the male cell contains X and Y chromosomes. Chromosomes contain genes that pass on the genetic characteristics of one generation to the next.

  Aging is an irreversible process of gradual inhibition of the basic functions of the body (regeneration, reproductive, etc.), as a result of which the body loses the ability to maintain homeostasis, resist stress, illness and injury, which makes death inevitable.

Basic concepts in developmental biology (hypotheses of preformationism and epigenesis). Modern ideas about the mechanisms of embryonic development.

1. What is combinative variability? Give examples. Name the sources of combinative variability.

Combinative variability is variability in the offspring caused by the emergence of new combinations of genes of the parents.

The sources of combinative variability are: crossing over, independent divergence of chromosomes in anaphase I of meiosis, random combination of gametes during fertilization. The first two processes ensure the formation of gametes with different combinations of genes. Random fusion of gametes leads to the formation of zygotes with different combinations of genes from both parents. As a result, hybrids develop new combinations of parental traits, as well as new traits that the parents did not have. The structure of the genes does not change.

An example of combinative variability is the birth of children with blood group I or IV to heterozygous parents with blood groups II and III (the offspring have new characteristics that differ from the parents). Another example would be the appearance of flies with a gray body, rudimentary wings and a black body, normal wings when crossing a diheterozygous Drosophila (gray body, normal wings) with a black male with rudimentary wings. IN in this case As a result of crossing over, the offspring developed new combinations of parental traits.

2. Define the concepts “mutation”, “mutagenesis”, “mutagen”. What groups are mutagens usually divided into? Give examples.

A mutation is an inherited change in the genetic material of an organism.

Mutagenesis is the process of occurrence of mutations.

Mutagen is a factor that leads to the occurrence of mutations in living organisms.

3. Describe the main types of gene, chromosomal and genomic mutations.

● Gene mutations are changes in the nucleotide sequence of DNA within one gene. This is the most common type of mutation and the most important source of hereditary variability in organisms. Gene mutations include insertions, deletions, and nucleotide substitutions.

● Chromosomal mutations are changes in the structure of chromosomes. There are intrachromosomal and interchromosomal mutations. Intrachromosomal mutations include: loss of a chromosome section (deletion), double or multiple repetition of a chromosome fragment (duplication), rotation of a chromosome section by 180° (inversion). Interchromosomal mutations include the exchange of sections between two non-homologous chromosomes (translocation).

● Genomic mutations are changes in the number of chromosomes in cells. Among genomic mutations, polyploidy and heteroploidy are distinguished.

Polyploidy is an increase in the number of chromosomes in cells, a multiple of the haploid set. For example, 3n (triploidy), 4n (tetraploidy), 6n (hexaploidy), 8n (octaploidy). Polyploidy is common mainly in plants. Polyploid forms have large leaves, flowers, fruits and seeds, are characterized by increased resistance to adverse environmental factors.

Heteroploidy (aneuplody) is a change in the number of chromosomes that is not a multiple of the haploid set. For example, 2n – 2 (nullisomy, if a pair of homologous chromosomes is missing), 2n – 1 (monosomy), 2n + 1 (trisomy), 2n + 2 (tetrasomy), 2n + 3 (pentasomy).

4. What types of mutations are distinguished by origin? By type of mutated cells? By influence on the viability and fertility of organisms?

● Based on their origin, spontaneous and induced mutations are distinguished. Spontaneous mutations occur spontaneously throughout the life of an organism under normal environmental conditions. Induced mutations are those that are artificially caused using mutagenic factors under experimental conditions. Induced mutations occur many times more often than spontaneous ones.

● According to the type of mutated cells, somatic and generative mutations are distinguished. Somatic mutations occur in somatic cells. They can manifest themselves in the individual itself and be transmitted to offspring during vegetative propagation. Generative mutations occur in germ cells and are transmitted to offspring during sexual reproduction.

● Based on the effect on the viability and fertility of individuals, lethal, semi-lethal, neutral and beneficial mutations are distinguished. Lethal mutations lead to the death of the organism (for example, in humans, the absence of X chromosomes in the set causes the death of the fetus in the third month of embryonic development). Semi-lethal mutations reduce the viability of mutants (hemophilia, congenital diabetes mellitus, etc.). Neutral mutations do not have a significant effect on the viability and fertility of individuals (the appearance of freckles). Beneficial mutations increase the adaptability of organisms to environmental conditions (mutations that cause immunity to certain pathogens - HIV, malarial plasmodium, etc.)

5. What is the fundamental difference between combinative and mutational variability? How does mutational variability differ from modification variability?

The occurrence of mutational variability is based on changes in genetic material: the structure of genes, the structure or number of chromosomes changes. Combinative variability is caused by the emergence of new combinations of parental genes in the offspring, while the gene structure, structure and number of chromosomes remain unchanged.

Mutations, as opposed to modifications:

● inherited;

● do not develop gradually, but arise suddenly;

● do not form continuous series of variability and do not have reaction norms;

● are undirected (undefined);

● manifest themselves individually, and are not of a mass nature.

6. What is the essence and practical significance law of homological series of hereditary variability?

The essence of the law of homological series of hereditary variability is that species and genera that are close genetically and related by common origin are characterized by similar series of hereditary variability. Knowing what forms of variability occur in one species, one can predict the presence of similar forms in other species.

N.I. Vavilov’s law is of great practical importance for breeding and agriculture, since it predicts the presence of certain forms of variability in plants and animals. Knowing the nature of variability in one or several closely related species, one can purposefully search for forms that are not yet known in a given species, but have already been discovered in related forms. Thanks to the law of homological series, medicine and veterinary medicine have the opportunity to transfer knowledge about the mechanisms of development, course and methods of treating diseases of some species (in particular, humans) to others, closely related ones.

7. In einkorn wheat, gametes contain 7 chromosomes. How many chromosomes are contained in the somatic cells of einkorn wheat mutants if nullisomy led to the emergence of the mutant form? Monosomy? Triploidy? Trisomy? Tetraploidy? Tetrasomy?

With nullisomy, somatic cells contain a set of 2n – 2 (14 – 2 = 12 chromosomes), with monosomy – 2n – 1 (14 – 1 = 13 chromosomes), with triploidy – 3n (21 chromosomes), with trisomy – 2n + 1 (14 + 1 = 15 chromosomes), with tetraploidy – 4n (28 chromosomes), with tetrasomy – 2n + 2 (14 + 2 = 16 chromosomes).

8*. The black coat color in cats dominates over the red one; heterozygous cats have a tortoiseshell coloration - black spots alternate with red ones. The genes that control coat color are located on the X chromosome. Theoretically, cats, i.e. there should not be males with tortoiseshell coloration (why?), but sometimes they are born. How to explain this phenomenon? What other features (besides unusual coloring) do you think are characteristic of tortoiseshell cats?

Tortoiseshell coloring is caused by the simultaneous presence of two different alleles in the genotype - dominant (for example, A - black wool) and recessive (a - red wool). Normally, cats (i.e. females) have two X chromosomes, while males have only one (set of sex chromosomes - XY). Therefore, cats can be black (X A X A), red (X a X a) or tortoiseshell (X A X a), and cats can only be black (X A Y) or red (X a Y).

However, sometimes, due to non-disjunction of sex chromosomes in meiosis (during the formation of gametes in one of the parents), male kittens are born with a set of sex chromosomes XXY. Such cats may have a tortoiseshell coloration (X A X a Y). However, in most cats with a set of sex chromosomes XXY, due to the presence of an extra X chromosome, the process of spermatogenesis is disrupted, and they are infertile (sterile).

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