What is the biological significance of non-hereditary variability. Lecture: Variability, its types and biological significance. What will we do with the received material?

Variability is a universal property of living systems associated with changes in phenotype and genotype that arise under the influence of the external environment or as a result of changes in hereditary material. There are non-hereditary and hereditary variability.

Non-hereditary variability. Non-hereditary, or group (certain), or modification variability– these are changes in phenotype under the influence of environmental conditions. Modification variability does not affect the genotype of individuals. The genotype, while remaining unchanged, determines the limits within which the phenotype can change. These limits, i.e. opportunities for the phenotypic manifestation of a trait are called reaction norm And are inherited. The reaction norm sets the boundaries within which a specific characteristic can change. Different signs have different reaction norms - broad or narrow. For example, signs such as blood type and eye color do not change. The shape of the mammalian eye varies slightly and has a narrow reaction rate. The milk yield of cows can vary over a fairly wide range depending on the conditions under which the breed is kept. Other quantitative characteristics may also have a wide reaction rate - growth, leaf size, number of grains in the cob, etc. The wider the reaction norm, the more opportunities an individual has to adapt to environmental conditions. That is why there are more individuals with an average expression of the trait than individuals with its extreme expressions. This is well illustrated by the number of dwarfs and giants in humans. There are few of them, while there are thousands of times more people with a height in the range of 160-180 cm.

The phenotypic manifestations of a trait are influenced by the combined interaction of genes and environmental conditions. Modification changes are not inherited, but are not necessarily of a group nature and do not always appear in all individuals of a species under the same environmental conditions. Modifications ensure the adaptation of the individual to these conditions.

Hereditary variability(combinative, mutational, indeterminate).

Combinative variability occurs during the sexual process as a result of new combinations of genes that arise during fertilization, crossing over, conjugation, i.e. during processes accompanied by recombinations (redistribution and new combinations) of genes. As a result of combinative variability, organisms arise that differ from their parents in genotypes and phenotypes. Some combinational changes may be harmful to an individual. For the species, combinative changes are, in general, useful, because lead to genotypic and phenotypic diversity. This promotes the survival of species and their evolutionary progress.

Mutational variability associated with changes in the sequence of nucleotides in DNA molecules, loss and insertion of large sections in DNA molecules, changes in the number of DNA molecules (chromosomes). Such changes themselves are called mutations. Mutations are inherited.

Among the mutations are:

– genetic– causing changes in the sequence of DNA nucleotides in a specific gene, and consequently in the mRNA and protein encoded by this gene. Gene mutations can be either dominant or recessive. They can lead to the appearance of signs that support or inhibit the vital functions of the body;

– generative mutations affect germ cells and are transmitted during sexual reproduction;

– somatic mutations do not affect germ cells and are not inherited in animals, but in plants they are inherited during vegetative propagation;

– genomic mutations (polyploidy and heteroploidy) are associated with changes in the number of chromosomes in the karyotype of cells;

– chromosomal mutations are associated with rearrangements in the structure of chromosomes, changes in the position of their sections resulting from breaks, loss of individual sections, etc.

The most common gene mutations result in changes, loss or insertion of DNA nucleotides in a gene. Mutant genes transmit different information to the site of protein synthesis, and this, in turn, leads to the synthesis of other proteins and the emergence of new characteristics. Mutations can occur under the influence of radiation, ultraviolet radiation, and various chemical agents. Not all mutations are effective. Some of them are corrected during DNA repair. Phenotypically, mutations appear if they do not lead to the death of the organism. Most gene mutations are recessive. Phenotypically manifested mutations are of evolutionary significance, providing individuals with either advantages in the struggle for existence, or, conversely, leading to their death under the pressure of natural selection.

The mutation process increases the genetic diversity of populations, which creates the preconditions for the evolutionary process.

The frequency of mutations can be increased artificially, which is used for scientific and practical purposes.

MINISTRY OF HEALTH OF THE REPUBLIC OF BELARUS

BELARUSIAN STATE MEDICAL UNIVERSITY

DEPARTMENT OF PATHOLOGICAL PHYSIOLOGY

S. A. Zhadan, T. N. Afanasyeva, F. I. Vismont

ROLE OF HEREDITARY IN PATHOLOGY

Educational and methodological manual

Minsk BSMU 2012

GENERAL CHARACTERISTICS OF HEREDITARY PATHOLOGY

Medical genetics and its tasks

Heredity- this is the property of living beings and cells of the body to transmit their characteristics (anatomical and physiological characteristics) to descendants. It provides relative stability of the species. The material carriers of hereditary information are genes - sections of the DNA molecule.

Variability is a property of an organism and its cells, manifested in the emergence of new characteristics.

IN Currently, about 2000 types of hereditary pathologies are known

And genetically determined syndromes. Their number is constantly growing, and dozens of new forms of hereditary diseases are described every year.. The main reasons contributing to the increase in the growth of hereditary pathology are:

– significant advances in medicine in the treatment and prevention of many infectious diseases;

– increasing environmental pollution with mutagenic agents;

– increase in average human life expectancy.

Along with this, the improvement of diagnostic methods and advances in molecular biology make it possible to identify the genetic nature of a number of serious diseases that were not previously associated with genomic abnormalities (for example, chromosomal diseases).

Genetics is the science of heredity and variability of an organism. The branch of genetics that deals with the study of human heredity and variability from the point of view of pathology is called medical genetics.

The main objectives of medical genetics are:

1. Study of hereditary forms of pathology, their etiology, pathogenesis, improvement of diagnostics, development of methods of prevention and treatment.

2. Study of the causes and mechanisms of hereditarily determined predisposition and resistance to various (including infectious diseases) diseases.

3. Study of the role and significance of the genetic apparatus in the development of adaptation reactions, compensation and decompensation phenomena.

4. Detailed comprehensive study of the processes of mutagenesis and antimutagenesis, their role in the development of diseases.

5. Study of a number of general biological problems: molecular genetic mechanisms of carcinogenesis, the role of the genetic apparatus in the phenomena of tissue incompatibility, autoimmune reactions of the body, etc.

The concept of hereditary and congenital pathology. Phenocopies

The concepts of “hereditary diseases” and “congenital diseases” are far from clear-cut.

Congenital diseases are any diseases that appear immediately after the birth of a child. They can be hereditary and non-hereditary.

To the number hereditary diseases include only those based on structural changes in the genetic material. Some of them clinically manifest themselves already in the first days after birth, others in adolescence, adulthood, and sometimes in old age.

Non-hereditary diseases are caused by the effect of unfavorable environmental factors on the developing fetus during pregnancy and do not affect its genetic apparatus.

Phenocopies, reasons for their development

In medical genetics, another concept is distinguished - phenocopies. Phenocopy is a clinical syndrome that occurs under the influence of environmental factors during embryonic development, similar in its manifestations to a hereditary disease, but having a non-genetic origin. For example, such anomalies as “cleft palate”, “cleft lip” can be both hereditary (Patau syndrome) and non-hereditary, resulting from a disorder of embryonic development. Hypothyroidism is inherited as an autosomal recessive trait, but can also occur as a phenocopy in people living in areas where drinking water is low in iodine. Early deafness can be inherited as a recessive or dominant trait, or can occur as a phenocopy in children born to women who had rubella during pregnancy.

Thus, phenocopies are diseases that are superficially similar to hereditary diseases, but are not associated with changes in the genotype.

The causes of phenocopy may be:

– oxygen starvation of the fetus (intrauterine hypoxia), causing the development of serious defects in the structure of the brain and skull, microcephaly;

– endocrine disorders in the body of a pregnant woman (the likelihood of giving birth to a sick child in such a woman is approximately 2.5 times higher);

– infectious diseases of a pregnant woman (toxoplasmosis, rubella, syphilis, etc.), especially in the early period of pregnancy, causing in a significant percentage of cases (up to 60–70%) severe deformities (microcephaly, deaf-muteness, cleft of the soft palate, etc.);

– severe mental trauma and prolonged emotional stress of a woman during pregnancy;

– drugs with cytotoxic or antimetabolic effects;

– chronic alcoholism of parents (developmental defects in children of non-drinking parents are about 2%, in moderate drinkers - up to 9%, in heavy drinkers - 74%), etc.

Classification of diseases taking into account the relationship of hereditary and environmental factors in their development. Concept

O penetrance and expressivity

IN In the development of the disease, as in the life of a healthy organism, two main factors take part: environmental influences

(external factor ) and heredity ( internal factor).

Taking into account the proportion of internal and external factors in the development of the disease, the following groups of diseases are distinguished (N. P. Bochkov, 2002):

1. Actually hereditary diseases. The cause of these diseases are abnormalities in the genetic apparatus of the cell, i.e. mutations (genetic, chromosomal and genomic). The environment determines only penetrance (frequency of manifestation of an abnormal gene in a population of individuals possessing this gene)

And expressiveness(the degree of expression of the action of the gene in a particular individual). This group includes such monogenically caused diseases as alkaptonuria, phenylketonuria, hepatocerebral dystrophy, hemophilia, etc., as well as all chromosomal diseases.

2. Ecogenetic diseases. This group of hereditary diseases is caused by a mutation, the effect of which is manifested only when the body is exposed to a certain environmental factor specific to a given mutant gene. For these diseases, both genetic and environmental

the component is presented one-factorially: an individual gene - an environmental factor specific to a given gene. Such diseases include, for example, sickle cell anemia (a semi-dominantly inherited hemoglobinopathy). In heterozygous HbS carriers, hemolytic crises leading to anemia occur only under conditions of hypoxia or acidosis. In hereditary fermentopathy associated with deficiency of glucose-6-phosphate dehydrogenase, a similar role can be played by the use of oxidizing drugs, consumption of faba beans, and sometimes a viral infection.

3. Diseases with hereditary predisposition. They are the result of the interaction of genetic and environmental factors, both of which are numerous. Sometimes these diseases are called multifactorial, or multifactorial. These include the overwhelming number of diseases of mature and old age: hypertension, atherosclerosis, coronary heart disease, gastric ulcer and 12 ring intestines, malignant neoplasms, etc.

There is no clear difference between the second and third groups of diseases. They are often combined into one group of diseases with hereditary predisposition, distinguishing between monogenic and polygenic predisposition.

4. Diseases caused by environmental factors from which the body has no means of defense (extreme). These are various injuries

(mechanical, electrical), diseases arising under the influence of ionizing radiation, burns, frostbite, especially dangerous infections, etc. The genetic factor in these cases determines only the severity of the disease, its outcome, and in some cases the likelihood of occurrence.

Classification of hereditary forms of pathology

IN Due to the complex nature of hereditary pathology, there are two main principles of its classification: clinical and genetic.

Clinical principle of classification implies the division of hereditary forms of pathology depending on the organ or system most involved in the pathological process. In accordance with this criterion, hereditary diseases of the nervous system, diseasesmusculoskeletalapparatus, skin, blood, etc.

The basis genetic classification Hereditary diseases are based on an etiological principle, namely the type of mutations and the nature of their interaction with the environment. In accordance with this criterion, all hereditary pathologies can be divided into groups:

1) gene diseases, caused by gene mutations;

2) chromosomal diseases, resulting from chromosomal or genomic mutations;

3) diseases with hereditary predisposition (multifactorial)

- develop in individuals with an appropriate combination of “predisposing” hereditary and “manifesting” external factors;

4) genetic diseases of somatic cells;

5) diseases of genetic incompatibility of mother and fetus.

Each of these groups is in turn subdivided according to more detailed genetic characteristics and mode of inheritance.

Etiology of hereditary forms of pathology. Mutations, their types. The concept of mutagens

Individual genes, chromosomes, and the genome as a whole are constantly undergoing various changes. Despite the fact that DNA repair (restoration) mechanisms exist, some damage and errors remain. Changes in the sequence and number of nucleotides in DNA are called mutations.

Mutations are a persistent, abrupt change in the hereditary apparatus of a cell, not associated with the usual recombination of genetic material.

All mutations are classified according to several criteria. 1. Due to the occurrence distinguish between spontaneous and induced

Spontaneous mutations - These are mutations that arise spontaneously under the influence of natural mutagens of exogenous or endogenous origin. The cause of such mutations may be cosmic radiation, radioactive isotopes, endogenous chemical mutagens (peroxides and free radicals - automutagens) formed in the body during metabolism. Age plays a significant role in the occurrence of spontaneous mutations. In men, gene mutations accumulate in their germ cells as they age. In women, the dependence of gene mutations on age was not noted, but a clear connection was revealed between the age of the mother and the frequency of chromosomal diseases in the offspring.

Induced mutations - These are mutations caused by targeted effects on the body of factors of various origins - physical, chemical or biological mutagens. The prevalence of some mutagens in the human environment is presented in the appendix. 1.

TO physical mutagens include ionizing radiation (α-, β- and γ-rays, X-rays, neutrons) and UV radiation. The peculiarity of ionizing radiation is that it can induce mutations

V low doses that do not cause radiation damage.

TO chemical mutagens include alcohols, acids, heavy metals, salts and other compounds. Chemical mutagens are found in the air (arsenic, fluorine, hydrogen sulfide, lead, etc.), soil (pesticides and other

chemicals), food, water. It has been established that many drugs have pronounced mutagenic activity (Appendix 2). A very strong mutagen is cigarette smoke condensate, which contains benzopyrene. Smoke condensation and surface crusts formed when frying fish and beef contain tryptophan pyrolysates, which are chemical mutagens. The peculiarity of chemical mutagens is that their effect depends on the dose and stage of the cell cycle. The higher the dose of mutagen, the stronger the mutagenic effect.

TO biological mutagens include bacterial toxins, measles, rubella, influenza, herpes viruses, and antigens of certain microorganisms.

The main medical consequences of mutagenesis in various cell types are presented in Fig. 1.

2. Based on the type of cells in which the mutation occurred, gametic, somatic and mosaic mutations are distinguished.

Gametic mutations arise in germ cells. They are inherited by descendants and are usually found in all cells of the body. Their consequences affect the fate of the offspring and cause hereditary diseases.

Rice. 1 . Medical consequences of mutagenesis in various cell types

Somatic mutations occur in somatic cells, are random in nature, and can occur at any stage of development, starting from the zygote. They are not inherited.

Mosaic mutations- These are mutations that occur in the cells of the embryo or fetus. As a result, cell lines with different genotypes arise. Some cells in the body have a normal karyotype, while others have an abnormal karyotype. The earlier in ontogenesis a somatic mutation occurs, the more cells contain this mutation and the more pronounced its manifestations.

3. According to their significance, pathogenic, neutral and beneficial mutations are distinguished.

Pathogenic mutations lead to the death of the embryo (or fetus) or to the development of hereditary and congenital diseases. They are divided into lethal, semi-lethal, non-lethal. Lethality can occur at the level of gametes, zygotes, embryos, fetuses, and also after birth.

Neutral Mutations usually do not affect the vital functions of the body (for example, mutations that cause the appearance of freckles on the skin, changes in hair color, or change in the iris).

Favorable Mutations increase the viability of an organism or species (for example, the dark color of the skin of the inhabitants of the African continent).

4 . Depending on the volume of damaged material mutations are divided into genetic (changes in individual genes), chromosomal (structural chromosomal aberrations), genomic (numerical chromosomal aberrations).

Antimutagenesis. Mechanisms of action of antimutagens

Antimutagenesis is the process of suppressing spontaneous and induced mutations. Substances with such properties are called antimutagens. Some of them are given in the appendix. 3.

There are different principles for classifying antimutagens:

1) by origin: exogenous and endogenous, intracellular and extracellular;

2) mechanism of action;

3) chemical structure and anticarcinogenic properties.

Exogenous antimutagens include:

– essential amino acids (methionine, histidine, arginine, glutamic acid, etc.);

– vitamins and provitamins (mainly A, E, C, K);

– polyunsaturated fatty acids;

– trace elements (Se), cobalt chloride;

– alimentary fiber;

2) penetrating the body through the respiratory route (phytoncides);

3) entering the human body orally during pharmacotherapy or prophylactic use:

– medications (streptomycin, chloramphenicol, etc., used in small

– specially synthesized drugs (bemitil);

– biologically active additives(indole-3-carbinol, etc.);

– synthetic antimutagens (ionol, dibunol, etc.).

Endogenous antimutagens include:

1) damaged DNA repair system;

2) antioxidant system;

3) enzyme systems;

4) cellular metabolites;

5) thyroid hormones, melatonin;

6) embryonic substances (Co);

7) S-containing compounds (glutathione).

Mechanisms of action of antimutagens

The main mechanisms of action of antimutagens include:

1. Inactivation of mutagens of external origin and protection of DNA from their damaging effects(dismutagens). In most cases, dismutagens stably bind to the mutagen and remove it from the body (extracts of parsley, beets, radishes, celery, plums, blueberries, apples).

2. Suppression of the process of formation of true mutagens from previous non-mutagenic substances(vitamins C, E, tannins,

some phenols).

3. Inhibits the activity of free radicals that can damage DNA(antioxidants: superoxide dismutase, glutathione peroxidase, catalase, vitamin C, A,α-tocopherol, β-carotene, E, melatonin, etc.).

4. Increasing the activity of enzyme systems that neutralize mutagens, carcinogens and other genotoxic compounds. The universal mechanism for inactivating xenobiotics is provided by microsomal liver enzymes, which metabolize up to 75% of all drugs.

5. Antimutagens that reduce errors in DNA repair and replication,

activation and correction of repair (reparagens) . Towards reparations

Antimutagens found in some foods (for example, corn, cottonseed, sunflower, soybean and other vegetable oils) include:

– vanillin, cyanamaldehyde and other aldehydes formed during the oxidation of saturated fatty acids. These substances stimulate genetic recombination, temporarily inhibit cell division, increasing the time of DNA repair;

– cobalt salts, which increase the efficiency of error-free DNA repair (found in sufficient quantities in onions, cabbage, tomatoes, lettuce, potatoes, black currants and pears).

6. Antimutagens with an unknown mechanism of action.In recent years, the polyfunctionality of some antimutagens has been established (the phenolic component of green tea - epigallocatechin galate, isocyanates from cruciferous vegetables - sulforane and phenolic isocyanate, etc.). Antimutagens act as free radical scavengers, suppress the synthesis of metabolic activation of xenobiotics and stimulate their detoxification, modulate DNA repair, influence transcription factors and signaling pathways involved in apoptosis and cell cycle regulation, suppress inflammation and angiogenesis.

Thus, the main antimutagens include:

1) compounds that neutralize the mutagen before it reacts with the molecule

2) substances that remove damage to the DNA molecule caused by a mutagen or increase its resistance to mutagen;

3) compounds that prevent the body from converting indirect mutagens into true ones.

GENE DISEASES

Gene diseases are a group of diseases, heterogeneous in clinical manifestations, caused by mutations at the gene level. The basis for combining them into one group is the etiological genetic characteristics and, accordingly, the patterns of inheritance in families and populations.

Etiology of gene diseases

The causes of gene diseases are gene mutations that can affect structural, transport And embryonic proteins as well as enzymes.

Gene mutations are molecular changes in the structure of DNA. They are caused by changes in the chemical structure of the gene, namely specific

The term “genotype” was proposed in 1909 by the Danish geneticist Wilhelm Johansen. He also introduced the terms: “gene”, “allele”, “phenotype”, “line”, “pure line”, “population”.

Genotype is the totality of genes of a given organism. A person has about 100 thousand genes.

The genotype as a single functional system of the body has developed in the process of evolution. A sign of a systematic genotype is the interaction of genes.

Allelic genes (more precisely, their products - proteins) can interact with each other:

as part of chromosomes - an example is complete and incomplete linkage of genes;
in a pair of homologous chromosomes - examples are complete and incomplete dominance, codominance (independent manifestation of allelic genes).

Nonallelic genes interact in the following forms:

1.19. Variability, its types and biological significance

Variability is a universal property of living systems associated with variations in phenotype and genotype that arise under the influence of the external environment or as a result of changes in hereditary material. There are hereditary and non-hereditary variability.

Hereditary variability can be combinative, mutational, or uncertain.

Combinative variability arises as a result of new combinations of genes during sexual reproduction, crossing over and other processes accompanied by gene recombinations. As a result of combinative variability, organisms arise that differ from their parents in genotypes and phenotypes.

Mutational variability is associated with changes in the sequence of nucleotides in DNA molecules, loss and insertion of large sections in DNA molecules, changes in the number of DNA molecules (chromosomes). Such changes themselves are called mutations. Mutations are inherited.

Mutations are distinguished:

Genetic, causing changes in a specific gene. Gene mutations can be either dominant or recessive. They can support or, conversely, inhibit the vital functions of the body;
generative, affecting germ cells and transmitted through sexual reproduction;
somatic, not affecting germ cells. In animals they are not inherited, but in plants they are inherited during vegetative propagation;
genomic (polyploidy and heteroploidy), associated with changes in the number of chromosomes in the karyotype of cells;
chromosomal, associated with rearrangements in the structure of chromosomes, changes in the position of their sections resulting from breaks, loss of individual sections, etc.

The most common gene mutations result in changes, loss or insertion of DNA nucleotides in a gene. Mutant genes transmit different information to the site of protein synthesis, and this, in turn, leads to the synthesis of other proteins and the emergence of new characteristics. Mutations can occur under the influence of radiation, ultraviolet radiation, and various chemical agents. Not all mutations are effective. Some of them are corrected during DNA repair. Phenotypically, mutations appear if they do not lead to the death of the organism. Most gene mutations are recessive. Phenotypically manifested mutations are of evolutionary significance, either providing individuals with advantages in the struggle for existence, or, conversely, leading to their death under the pressure of natural selection.

The mutation process increases the genetic diversity of populations, which creates the preconditions for the evolutionary process.

The frequency of mutations can be increased artificially, which is used for scientific and practical purposes.

Non-hereditary variability

Non-hereditary, or group (defined), or modification variability is changes in the phenotype under the influence of environmental conditions. Modification variability does not affect the genotype of individuals. The extent to which the phenotype can change is determined by the genotype. These limits are called reaction norms. The reaction norm sets the boundaries within which a specific characteristic can change. Different signs have different reaction norms - broad or narrow. For example, the variability of the mammalian eye is small and has a narrow reaction norm. The milk yield of cows can vary within fairly wide limits depending on the conditions of the breed.

The phenotypic manifestations of a trait are influenced by the combined interaction of genes and environmental conditions. The degree to which a trait is expressed is called expressiveness. The frequency of manifestation of a trait (%) in a population where all its individuals carry a given gene is called penetrance. Genes can be expressed with varying degrees of expressivity and penetrance. For example, the baldness gene can manifest itself with a penetrance of 100% or 50%, which depends on specific environmental conditions, the number and interaction of genes responsible for the development of the trait.

Modification changes are not inherited in most cases, but are not necessarily of a group nature and do not always appear in all individuals of a species under the same environmental conditions. Modifications ensure the adaptation of the individual to these conditions.

Remember how genotype influences the manifestation of traits. What are mutations? How do mutations occur at the molecular genetic level?

In the process of individual development, some signs do not appear immediately and change throughout life. With the same genotype, different phenotypes can be formed. For example, if two organisms of the same genotype are kept under different conditions, they will differ in phenotype. Plants grown from seeds of the same variety and even from one individual can differ in height, flowering time, and fruit size.

Variability is the ability of an organism to change in the process of individual development under the influence of various environmental conditions.

Types of variability. The phenotype is the result of the interaction of the genotype with various environmental conditions. Depending on the nature of the influencing conditions, changes may or may not be inherited. If changes affect only the phenotype of the organism, then they are not inherited. In this case, the genotype is preserved, and changes that arise during the process of individual development are not transmitted to the offspring. If changes affect the genotype of an organism, that is, its genes change, then such changes are inherited. Hence, two types of variability are distinguished - non-hereditary and hereditary.

Non-hereditary variability occurs in organisms under the direct influence of environmental conditions. For example, a white hare grows white fur in winter at low temperatures, i.e. pigment is not formed in the hair (Fig. 111). In the spring, when the temperature rises, pigment begins to be produced, and the wool turns brown. Such variability of organisms is always adequate to environmental conditions and is adaptive. It promotes the survival of individuals. Thus, the white fur of the white hare allows it to be invisible to its enemies against the background of white snow.

Rice. 111. Non-hereditary variability: 1 - change in coat color in a white hare; 2 - dandelions grown on fertile (right) and poor (left) soil

Non-hereditary variability appears gradually. These changes appear in many individuals in one group, i.e. they are massive. Thus, all dandelions grown on fertile soil in the garden have large growth and large inflorescences, and, conversely, on poor soil - low plants with small baskets (Fig. 111).

Hereditary variability. Unlike non-hereditary variability, hereditary variability affects the genotype and is inherited. It can be combinative and mutational.

Combinative variability is associated with the emergence of new combinations of traits in organisms due to the combination of their genes. As a result, offspring develop traits that their parents may not have had. For example, dachshunds come in both long-haired and short-haired varieties of different colors (Fig. 112). In humans, green, blue and brown eye colors can be combined with light and dark hair in different combinations.

Rice. 112. Hereditary genetic variability in color and coat length in dachshunds

Combinative variability determines the diversity of individuals of one species. It contributes to the appearance of such characteristics that are used by humans in breeding new varieties of plants and animal breeds.

Mutations are also considered hereditary variability. You have already become familiar with the peculiarity of this variability at the molecular genetic level of the organization of life. The genotype of any organism is exposed to external factors that can cause “errors” in the structure of chromosomes or genes. As a result, a change in the genotype occurs and a new trait arises - a mutation. Different types of mutations occur in plants, animals, and humans (Fig. 113).

Rice. 113. Hereditary mutational variability in different organisms: 1 - Drosophila with the “wingless” mutation (on the left - a normal winged individual); 2 - grape variety with an increased set of chromosomes in cells (on the left - grapes with a normal set of chromosomes); 3 - image of the mutation of polydactyly (polydactyly) in Pope Sixtus II in Raphael’s painting “The Sistine Madonna”

Mutations are associated not only with errors in DNA reduplication and protein synthesis, but also with disturbances in chromosomes during cell division. Sometimes, when exposed to chemicals, the nucleus of a plant cell begins to divide faster than the cell itself. As a result, cells with a double set of chromosomes appear. From them, plants develop that are distinguished by a significantly larger size of flowers, fruits and leaves than specimens with a normal set of chromosomes (Fig. 113, 2). This has a positive meaning both for the plants themselves and for humans when growing them in fields and gardens.

Mutational variability is spasmodic; there is no gradual change in the characteristics of organisms. Mutations are individual and occur in single individuals. Exposure to the same external conditions causes different mutations in each organism. For example, irradiation of wheat grains before sowing with X-rays leads in some cases to the formation of defective ears, in another case to the absence of an ear, in a third case to the formation of a larger ear. Thus, mutational variability is not predictable. In terms of their significance, mutations can be indifferent to organisms, that is, unnecessary, or useful, but most often they are harmful, because they reduce the viability of mutant organisms.

So, the development of a trait in any organism is the result of the interaction of its genotype with the external environment. The genotype and environment, interacting, determine the development of the organism's phenotype.

Biological significance of heredity and variability. Heredity and variability are two opposing properties of an organism that form a single whole in nature. Heredity is realized in the process of reproduction, and variability - in the process of individual development of the organism. Heredity ensures the stability of the organism, its hereditary program and the transmission of certain characteristics over generations. Its implementation is based on DNA reduplication and the behavior of chromosomes in meiosis. The accuracy of these processes is a guarantee of the stability of the properties and functions of the body. Thus, heredity as a property of living things is realized at all levels of its organization. Heredity is conservative and is aimed at preserving the characteristics of an organism unchanged.

Variability is the phenomenon of instability of the hereditary properties of a living thing. It arises in the process of individual development of the organism. Non-hereditary variability is continuous. Changes arise as a result of the direct influence of the environment on the body. It is characterized by a series of gradual transitions from one form to another. The biological significance of non-hereditary variability is an increase in the adaptive capabilities of the organism and the diversity of characteristics in individuals belonging to the same species.

The genotype is a fairly stable and conservative system, and the process of DNA reduplication is close to perfection. The persistence of the gene is of great biological significance. It ensures the constancy of the species and its immutability in relatively stable conditions. At the same time, the gene also has the ability to undergo mutations and new combinations during sexual reproduction, which leads to a change in the genotype. Hereditary changes are unpredictable. Hereditary variability is discontinuous and individual. The differences between individuals are sharply expressed, and there are no intermediate forms.

Mutational variability is of particular importance. It occurs randomly when various factors influence the genotype. Mutations are not adequate to the influence of factors, they are rare and varied. Under natural conditions, each individual gene mutates very rarely. At first glance, it may seem that changes in genes are unimportant for an individual. But in reality, the body has several thousand genes. If we consider that mutations can occur in any of them, the total number of mutations increases sharply. Mutations are often harmful because they change the adaptive characteristics of organisms. However, it is mutations that create a reserve of hereditary variability and play an important role in the process of historical development of the organic world on Earth.

Exercises based on the material covered

Define variability. What features does non-hereditary variability have?
When does combinative variability occur?
How does mutational variability differ from combinative variability?
Compare two properties of an organism: heredity and variability. Which one is primary and which is secondary?
In any organism you can find signs typical of its parents. However, even among the offspring of the same parental pair, it is difficult to find two absolutely identical individuals unless they are twins. What is this connected with?
Which of the two types of variability is more important for the historical development of the organic world on Earth? Justify your answer.

Biology [Complete reference book for preparing for the Unified State Exam] Lerner Georgy Isaakovich

3.6.1. Variability, its types and biological significance

Non-hereditary variability . Non-hereditary, or group (certain), or modification variability– these are changes in phenotype under the influence of environmental conditions. Modification variability does not affect the genotype of individuals. The genotype, while remaining unchanged, determines the limits within which the phenotype can change. These limits, i.e. opportunities for the phenotypic manifestation of a trait are called reaction norm And are inherited. The reaction norm sets the boundaries within which a specific characteristic can change. Different signs have different reaction norms - broad or narrow. For example, signs such as blood type and eye color do not change. The shape of the mammalian eye varies slightly and has a narrow reaction rate. The milk yield of cows can vary over a fairly wide range depending on the conditions under which the breed is kept. Other quantitative characteristics may also have a wide reaction rate - growth, leaf size, number of grains in the cob, etc. The wider the reaction norm, the more opportunities an individual has to adapt to environmental conditions. That is why there are more individuals with an average expression of the trait than individuals with its extreme expressions. This is well illustrated by the number of dwarfs and giants in humans. There are few of them, while there are thousands of times more people with a height in the range of 160-180 cm.

Hereditary variability (combinative, mutational, indeterminate).

Among the mutations are:

– generative mutations affect germ cells and are transmitted during sexual reproduction;

– somatic mutations do not affect germ cells and are not inherited in animals, but in plants they are inherited during vegetative propagation;

– genomic mutations (polyploidy and heteroploidy) are associated with changes in the number of chromosomes in the karyotype of cells;

– chromosomal mutations are associated with rearrangements in the structure of chromosomes, changes in the position of their sections resulting from breaks, loss of individual sections, etc.

The mutation process increases the genetic diversity of populations, which creates the preconditions for the evolutionary process.

The frequency of mutations can be increased artificially, which is used for scientific and practical purposes.

EXAMPLES OF TASKS

Part A

A1. Modification variability is understood as

1) phenotypic variability

2) genotypic variability

3) reaction norm

4) any changes in the characteristic

A2. Indicate the characteristic with the widest reaction norm

1) the shape of a swallow's wings

2) eagle beak shape

3) time for the hare to molt

4) the amount of wool a sheep has

A3. Please indicate the correct statement

1) environmental factors do not affect the genotype of an individual

2) it is not the phenotype that is inherited, but the ability to manifest it

3) modification changes are always inherited

4) modification changes are harmful

A4. Give an example of a genomic mutation

1) the occurrence of sickle cell anemia

2) the appearance of triploid forms of potatoes

3) creation of a tailless dog breed

4) birth of an albino tiger

A5. Changes in the sequence of DNA nucleotides in a gene are associated

1) gene mutations

2) chromosomal mutations

3) genomic mutations

4) combinative rearrangements

A6. A sharp increase in the percentage of heterozygotes in a cockroach population can result from:

1) increase in the number of gene mutations

2) the formation of diploid gametes in a number of individuals

3) chromosomal rearrangements in some members of the population

4) change in ambient temperature

A7. Accelerated skin aging in rural residents compared to urban residents is an example

1) mutational variability

2) combinational variability

3) gene mutations under the influence of ultraviolet radiation

4) modification variability

A8. The main cause of a chromosomal mutation may be

1) nucleotide replacement in a gene

2) change in ambient temperature

3) disruption of meiosis processes

4) insertion of a nucleotide into a gene

Part B

IN 1. What examples illustrate modification variability?

1) human tan

2) birthmark on the skin

3) the thickness of the fur of a rabbit of the same breed

4) increase in milk yield in cows

5) six-fingered humans

6) hemophilia

AT 2. Indicate events related to mutations

1) multiple increase in the number of chromosomes

2) change of undercoat of a hare in winter

3) replacement of an amino acid in a protein molecule

4) the appearance of an albino in the family

5) growth of the root system of a cactus

6) formation of cysts in protozoa

VZ. Correlate the feature characterizing variability with its type

Part WITH

C1. In what ways can one achieve an artificial increase in the frequency of mutations and why should this be done?

C2. Find errors in the given text. Correct them. Indicate the numbers of sentences in which errors were made. Explain them.

1. Modification variability is accompanied by genotypic changes. 2. Examples of modification are lightening hair after long exposure to the sun, increasing the milk yield of cows with improved feeding. 3. Information about modification changes is contained in genes. 4. All modification changes are inherited. 5. The manifestation of modification changes is influenced by environmental factors. 6. All signs of one organism are characterized by the same reaction norm, i.e. limits of their variability.

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