BASICS OF MEDICAL BIOLOGY - 2012

Human Variability as a Property of Life and a Genetic Phenomenon: Phenotypic and Genotypic Variability

Individuals of any species differ from one another in many traits, and humans are no exception. People vary in Skin, eye, and Hair color, body and organ mass and linear dimensions, Blood Cell counts, arterial and venous blood pressure levels, erythrocyte Antigens (Blood Groups, Rh system), enzyme activity, and the histocompatibility complex. These and other differences define human individuality, which must be taken into account in transplant surgery, transfusiology, forensic medicine, and MEDICAL Genetic Counseling.

These differences are driven by both genetic and environmental factors. Both factors are equally important, yet their relative roles in trait development vary. While all traits are genetically determined, some are exclusively genetic in nature—meaning that under all environmental conditions compatible with life, a given genotype produces the same phenotype (such as blood groups). Other differences are also genetically determined, but the degree of their phenotypic expression is influenced by the environment. For instance, THE RED BLOOD cell count in peripheral blood often correlates with altitude above sea level, although the Organism's inherent ability to adjust this count within certain limits depending on atmospheric oxygen partial pressure is determined by its genotype. There are also numerous differences shaped by both HEREDITY AND ENVIRONMENT. Human height is controlled by several dominant non-allelic genes (polygenes) and is likewise influenced by Nutrition. The onset of Hereditary diseases stems from Mutations occurring spontaneously or induced by mutagenic factors at the CELLULAR AND MOLECULAR levels. Understanding such mutations, their mechanisms, and their frequencies in populations helps predict the manifestation of pathological traits in offspring, determine genetic risk levels, and assess the necessity of prenatal Diagnosis.

Consequently, the exact same hereditary information can manifest differently under altered conditions. For example, in Himalayan rabbits and Siamese cats, the pigmentation pattern of the fur across different body parts is determined by ambient Temperature (the fur is dark on cooled areas because these organisms possess a mutant enzyme, tyrosinase) (Fig.). Thus, what is inherited is not a pre-formed trait, but a specific type of reaction to environmental stimuli.

The range of Variability within which a single genotype can produce different phenotypes depending on environmental conditions is called the norm of reaction. In some cases, depending on the overall genotype and external conditions, a single Gene may exhibit Various Forms of phenotypic expression: from the near-total absence of The Genome-controlled trait in the phenotype to its full manifestation. In the primrose, the genotype is such that red flowers appear at 15–20 °C and white flowers at higher temperatures, yet blue, indigo, violet, or yellow flowers are never observed under any temperature. This defines the norm of reaction for this plant regarding flower pigmentation.

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Fig. 23. Changes in the fur coloration of a Himalayan rabbit under METABOLISM/18.html">The Influence of temperature:

1 — a rabbit raised at standard ambient temperature (approx. 20 °C); 2 — a rabbit raised at a high ambient temperature (approx. 32 °C); 3 — a rabbit with a shaved patch on its back, with the skin cooled under a sterile bandage; 4 — a rabbit with pigmented fur in the cooled body area.

The degree to which a trait is expressed when a genotype is realized under various environmental conditions is termed expressivity. Expressivity refers to the degree of phenotypic manifestation of a gene and is closely linked to trait variability within the norm of reaction. It can manifest as shifts in morphological, biochemical, immunological, pathological, and other parameters. For instance, the chlorine content in human sweat typically does not exceed 40 mmol/L, whereas in the hereditary disease cystic fibrosis (with the exact same genotype), it ranges from 40 to 150 mmol/L. The hereditary disorder phenylketonuria (a disturbance of Amino acid metabolism) can exhibit varying degrees of expression (i.e., variable expressivity): primarily determined by differing phenylalanine levels in the blood, and secondarily ranging from mild intellectual disability to profound imbecility (where individuals are capable of only basic self-care skills). Expressivity may be constant (such as the presence of erythrocyte antigens) or variable (such as blood chlorine or sugar levels, or fluctuations in Metabolic waste products in phenylketonuria, galactosemia, and other hereditary disorders).

The exact same trait may manifest in some organisms while remaining unexpressed in others carrying the same gene. The quantitative measure of a gene's phenotypic expression is called penetrance. Penetrance is the frequency with which a gene phenotypically manifests among individuals carrying that gene in their genotype. It is a quantitative metric characterized by The ratio of individuals in whom the given gene is phenotypically expressed to the total number of individuals in whom the gene could potentially appear (accounting for recessives in homozygotes, and dominants in both homozygotes and heterozygotes). If, for example, a mutant gene appears in all individuals, it is referred to as 100% penetrance; in other cases, it is termed incomplete penetrance, specifying the percentage of individuals exhibiting the gene. For example, blood group determination in humans via the ABO, MN, and Rh systems shows 100% penetrance, whereas hereditary disorders show variable penetrance: Epilepsy — 67%, Diabetes Mellitus — 65%, congenital hip dislocation — 20%, and Gout — 20% in men and 0% in women, etc.

The terms "expressivity" and "penetrance" were introduced in 1927 by N.V. Timofeeff-Ressovsky.

The expressivity and penetrance of traits are determined by:

a) the presence of specific genes and their alleles in the organism's genotype;

b) the interacting gene network within the genotype, particularly the presence of polygenic systems;

c) the IMPACT OF ENVIRONMENTAL factors exerting a modifying effect on trait expression.

The fact that a single genotype can serve as the foundation for developing different phenotypes carries profound medical significance. It means that a burdened heredity does not necessarily have to manifest; everything depends on the conditions the individual experiences. In many cases, a disease as the phenotypic expression of hereditary information can be prevented or mitigated through prophylactic measures, such as dietary regulation, therapeutic Physical Exercise, or medication. The realization of hereditary information directly depends on the environment, and their interdependence can be summarized by the following principles:

1) Because organisms are open systems that exist as a unified whole within their environment, the realization of hereditary information unfolds under environmental influence.

2) A single genotype is capable of producing different phenotypes, governed by the conditions under which the individual's genotype is realized during ontogeny.

3) Organisms can only develop traits that are predetermined by their genotype.

4) Phenotypic variability within the norm of reaction occurs for each specific trait.

5) Environmental factors can influence either the degree of expression of a hereditary trait in organisms (expressivity) or the quantitative manifestation of traits (penetrance).

Variability is the ability of organisms within a species to exist in various trait variants and to acquire new properties and features during development. There are two forms of variability: hereditary and non-hereditary. The former is linked to changes in the genotype, while the latter involves the phenotype. Darwin referred to non-hereditary variability as definite, and hereditary variability as indefinite. Non-hereditary variability includes phenotypic (modification) variability, whereas hereditary variability encompasses genotypic variability. Genotypic variability is further divided into mutational and combinative. While all forms of variability alter the phenotype, in genotypic variability these alterations are driven by changes in the genotype, whereas in phenotypic variability they are caused by environmental factors.

Phenotypic variability (modification). Modifications are phenotypic changes that arise under the Influence of Environmental conditions. The Scope of Modification Variability is limited by the norm of reaction. Modification changes in a trait are not inherited, yet their range—the norm of reaction—is genetically determined and inherited. Modification changes are unrelated to alterations in the genotype. As a rule, modification variability is adaptive in nature; it corresponds to living conditions and serves a protective, functional purpose.

Under external influences, the growth, mass, and coloration of animals and plants undergo phenotypic changes. The Emergence of modifications is tied to environmental factors impacting the enzymatic reactions occurring within the organism, thereby shifting their course. This explains, among other things, the varying flower colors in primroses and pigment deposition in the fur of Himalayan rabbits. Examples of modification variability in humans include enhanced skin pigmentation under ultraviolet radiation, and The Development of the muscular and skeletal systems resulting from physical exertion.

Modificational variability should also include phenocopies (R. Goldschmidt, 1935) — non-heritable phenotypic changes caused by environmental factors that resemble genetically determined phenotypic variations (mutations). In other words, during development under the Influence of External factors, a trait dependent on a specific genotype can change in such a way that it mimics features characteristic of other changes, namely mutations. The development of phenocopies can be influenced by various environmental factors — climatic, physical, chemical, and biological. Certain infectious diseases (such as rubella or Toxoplasmosis) contracted by a mother during Pregnancy can also trigger phenocopies of A number of Hereditary diseases and developmental defects in newborns. Phenocopies occur with high frequency at specific (phenocritical) stages of ontogenesis, where different factors at the same stage of ontogenesis cause identical phenocopies, while at different stages the same agent produces different phenocopies.

Phenocopies are disruptions in the normal course of ontogenetic processes caused by various factors without a specific change in the genotype. The presence of phenocopies often complicates diagnosis; therefore, physicians must be aware that phenocopies are not inherited and require a different Treatment approach than hereditary diseases.

A special group of modificational variability is formed by durable modifications. For instance, exposing pupae of the Colorado potato beetle to high or low temperatures alters the coloration of the adult insects. This trait persists across several generations before reverting to the original color. Interestingly, this trait is transmitted to offspring only when the temperature acts on female specimens and is not passed down if the factor affects males exclusively. Thus, durable modifications bear resemblance to CYTOPLASMIC INHERITANCE.

Genotypic, or hereditary, variability is conventionally divided into combinatorial and mutational.

Combinatorial variability is associated with The formation of new gene combinations within the genotype. It naturally arises during sexual reproduction, achieved primarily through the following mechanisms:

a) independent assortment of Chromosomes during Meiosis;

b) their random combination during Fertilization;

c) gene recombination via Crossing-over; the genes themselves remain unchanged, but new random combinations of them lead to the emergence of organisms with altered genotypes and phenotypes.

Combinatorial variability is widespread in nature. Asexually reproducing microorganisms have developed unique mechanisms (such as transformation and Transduction) that also lead to combinatorial variability. All this highlights the immense significance of combinatorial variability for evolution and speciation. In human populations, phenotypic polymorphism is achieved As a result of combinatorial variability.

The phenomenon of heterosis is a prime example of combinatorial variability. Heterosis (alteration, transformation), or "hybrid vigor," can be observed in the first generation when hybridizing representatives of different populations, breeds, or varieties. It manifests as increased vitality, enhanced growth, and other superior traits.

Mutational Variability

A mutation is a sudden, abrupt change in a hereditary trait caused by an alteration in the genetic material. Mutagenesis is the process by which mutations arise. A mutant is an organism that has altered its phenotype as a result of a mutation. Mutations were first described in the evening primrose (Oenothera lamarckiana) and the term was introduced into science by the Dutch botanist H. de Vries (1901), one of the three scientists who rediscovered the laws of inheritance established by G. Mendel. Evolution — The process of forming new species, breeds, and varieties — is intrinsically linked to mutational variability. Mutations are known in all classes of animals, plants, and Viruses. Mutations are precisely what drive the polymorphism of human populations: varying skin pigmentation, hair and eye colors, Nose and lip shapes, and so on. Clinically, mutations manifest as hereditary diseases. Unlike modifications:

1. Mutations arise suddenly and abruptly, without transitional states compared to the original form of the trait.

2. New traits are stable and inherited across a series of generations.

3. These are qualitative, discrete changes; they do not form continuous series nor do they cluster around the mean value of the trait.

4. Mutations occur in various directions and can arise repeatedly.

5. They have an individual character, meaning they appear in any single individual living under the exact same conditions as other members of the same species.

6. Mutations are not adaptive responses to environmental factors and generally lack adaptive value.

There are several classifications of mutations:

I. According to their phenotypic manifestation, they are distinguished as:

- morphological (changes in Structure);

- physiological (changes in life processes);

- biochemical (changes in chemical composition).

II. According to the site of origin:

- somatic (in somatic Cells of the body);

- generative (in Germ Cells).

III. By their significance for the organism:

- conditionally beneficial;

- neutral;

- harmful (lethal and sublethal).

IV. By their manifestation in the genotype:

- dominant;

- recessive (the majority).

V. By their localization within The Cell:

- nuclear;

- cytoplasmic.

VI. By the mode of origin:

- spontaneous (involuntary);

- induced.

VII. By changes in the genotype:

- Gene Mutations — changes in the molecular structure of a gene (codon deletion, extra codon, nucleotide deletion, nucleotide substitution, codon substitution);

- chromosomal mutations — changes in Chromosome structure (deletions, duplications, inversions, translocations);

- genomic mutations — changes in chromosome number (polyploidy, haploidy, aneuploidy).

Genomic mutations

The haploid set of chromosomes, as well as the total set of genes contained within it, is called the genome. Mutations that cause changes in the chromosome number are called genomic. These include haploidy, polyploidy, and heteroploidy (aneuploidy).

Polyploidy is an increase in the haploid chromosome number through The addition of entire sets of chromosomes as a result of meiotic disruption.

Germ cells possess a haploid set of chromosomes (n), whereas zygotes and all somatic cells are characterized by a diploid set (2n). Polyploid forms exhibit an increase in chromosome number that is a multiple of the haploid set: 3n — triploid (triploidy), 4n — tetraploid (tetraploidy), 5n — pentaploid (pentaploidy), 6n — hexaploid (hexaploidy), and so on. Apparently, the evolution of a number of flowering plants proceeded via polyploidization. Most cultivated plants are polyploids.

In breeding practice, to obtain polyploids, plants are treated with critical temperatures, ionizing radiation, and chemical agents (the alkaloid colchicine being the most widespread).

Forms that arise from an increase in the chromosome number of a single genome are called autopolyploids. Another form of polyploidy is also known — allopolyploidy, which is the increase in chromosome number originating from two genomes of two different species of organisms.

Polyploid forms are also known in animals. The evolution of certain groups of Protozoa, notably Ciliates and radiolarians, occurred through polyploidization. In some Multicellular animals, polyploid forms have been created artificially (the silkworm), and certain human somatic cells can be polyploid. Triploids and tetraploids (3n, 4n) are found among aborted fetuses, stillbirths, and infants with Congenital Malformations. Such mutations are lethal or sublethal.

Heteroploidy (aneuploidy) is A change in the chromosome number that is not a multiple of the haploid set as a result of impaired meiosis and mitosis.

An increase by a single homologous chromosome (for a given pair) is called trisomy. If trisomy occurs in a single chromosome pair, such an organism is called a trisomic, and its chromosome Complement will be 2n + 1. Trisomy can affect any chromosome, or even multiple chromosomes. A double trisomic has a chromosome complement of 2n + 2, a triple trisomic 2n + 3, and so on. The phenomenon of trisomy was first described in the Jimson weed (Datura). Trisomy is also known in other PLANT AND ANIMAL species, as well as in humans. Examples of trisomics in humans include individuals with Down, Patau, and Edwards syndromes. Trisomics are most often non-viable because they exhibit a series of pathological alterations (developmental defects) incompatible with life (lethal or sublethal).

The loss of a single chromosome from a pair in a diploid set is called monosomy, and the organism is a monosomic, with a karyotype of 2n - 1. In the absence of two different chromosomes, the organism will be a double monosomic (2n - 2). If both homologous chromosomes are lost from the diploid set, the organism is called a nullisomic. Such organisms are non-viable.

Consequently, heteroploidy leads to structural changes and a decrease in organismal viability. In humans, such disruptions in the balanced chromosome set cause Chromosomal Disorders.

Chromosomal aberrations arise as a result of chromosomal rearrangements. These are the consequence of chromosome breakage leading to fragment formation, which subsequently rejoin, though without restoring the normal chromosomal structure. The following types of chromosomal aberrations are distinguished: intrachromosomal: deficiency (deletion), duplication, inversions; interchromosomal: translocations.

Deletions occur as a result of the loss of a particular segment by a chromosome. Deletions in the middle of a chromosome are lethal, while the loss of minor segments causes disorders known as Chromosomal diseases.

Duplications are associated with the incorporation of an extra, duplicated chromosomal segment. This also leads to the emergence of new Linkage groups and an increased dosage of individual genes.

Inversions are observed when chromosomes break and the detached segment rotates by 180°. If the break occurs at a single site, the fragment attaches to the chromosome with its opposite end; if it occurs at two sites, the middle fragment reverses and attaches to the breakpoints with its opposite ends, which also results in the formation of a new group of linked genes. N. P. Dubinin established that inversions are widespread, notably in natural populations of Drosophila, and likely play a role in species evolution.

A translocation is the relocation of a chromosome segment to another site on the same chromosome (intrachromosomal translocation) or, more commonly, to another chromosome (interchromosomal translocation). A symmetric translocation involves the centric segment (containing the centromere) of one chromosome joining with the acentric fragment (lacking a centromere) of another. An asymmetric translocation involves the fusion of centric segments with centric ones, and acentric with acentric. Centric fusion is one of the most common types of chromosomal rearrangements in humans, resulting in the formation of chromosomes with two or three centromeres (dicentrics, tricentrics). A Robertsonian translocation is the centric fusion between the long arms of acrocentric chromosomes. This is precisely how the translocation form of Down syndrome arises (translocation of an extra chromosome 21 onto one of the chromosomes of the D or G group). The fusion of the arms of a chromosome that has lost its telomeres produces ring chromosomes, which are closed in a ring shape. Horizontal division of the centromere produces monocentric chromosomes with two genetically identical arms, known as isochromosomes. For prognostic purposes regarding offspring, it is essential to determine the specific type of mutation, which is achieved by analyzing the karyotype of the patient and their parents.

Rules for Recording Normal and abnormal human karyotypes:

1. At the very beginning of the genetic karyotype formula, the total chromosome number is indicated, followed by the sex chromosome composition. For example: 46, XX denotes a normal female karyotype; 46, XY denotes a normal male karyotype; 47, XXY denotes the karyotype of a male with Klinefelter syndrome; 45, XO denotes the karyotype of a female with Turner syndrome.

2. An additional autosome is denoted by its respective number and a "+" sign. The notation 47, XY, +21 signifies a male karyotype with an extra chromosome 21 (Down syndrome). The loss of an entire chromosome is denoted by a "-" sign. Thus, the notation 45, XY, -21 signifies a male karyotype with monosomy for chromosome 21.

3. Chromosomal arms are designated by "p" (short arm) and "q" (long arm).

4. A translocation is designated by "t" with details specified in parentheses. For example: 45, XX, t(14;21) designates a female carrier of a balanced 14/21 translocation.

5. Mosaicism is designated by a fraction. The formula 45, XO/46, XX signifies a female mosaic for Turner syndrome.

Human chromosomal disorders associated with structural chromosome abnormalities include:

1) cri-du-chat syndrome (5p-) - deletion of the short arm of chromosome 5: multiple developmental defects combined with abnormal laryngeal development, causing the infant's cry to resemble a cat's meow;

2) Orbeli syndrome (13q-) - deletion of the long arm of chromosome 13: microcephaly (small Brain size), broad nasal bridge, prominent upper jaw, ptosis (drooping eyelid).

Gene mutations, or transgenations, alter The structure of the gene itself. Mutations can affect DNA molecules of varying lengths. The smallest segment whose alteration leads to a mutation is called a muton (a single nucleotide pair). A change in The nucleotide sequence causes alterations in the triplet sequence and ultimately changes The Genetic Code. Disruptions in Introduction/20.html">DNA Structure lead to mutations only when repair does not take place.

The MAIN TYPES OF gene mutations include substitutions, insertions, deletions, and duplications of nucleotide pairs. In all cases, they alter the DNA nucleotide sequence. These changes are transcribed into an mRNA molecule and often lead to The production of an altered polypeptide. The most typical gene mutations are base substitutions and frameshift mutations. The latter are associated with the loss or insertion of one or more NUCLEOTIDES. Mutations involving the replacement of a purine base with another purine (A↔G) or a pyrimidine base with another pyrimidine (T↔C) are called transitions. When a purine base is replaced by a pyrimidine and vice versa, the mutations are called transversions (A↔T, A↔C, G↔C, and G↔T). Base substitution mutations lead to missense and nonsense mutations. A missense mutation is a nucleotide substitution in the coding region of a gene that may result in the replacement of an amino acid in the protein molecule. A nonsense mutation is a nucleotide substitution in the coding region of a gene that leads to the formation of a nonsense (stop) codon. There are mutations where a nucleotide substitution in the coding region of a gene does not alter the meaning of the genetic code due to its redundancy. Gene mutations cause Sickle-Cell Anemia, as well as hemophilia, color blindness, albinism, phenylketonuria, alkaptonuria, and galactosemia. These disorders are called gene or molecular diseases because they are caused by alterations in gene structure (the DNA molecule).

The majority of mutations associated with the Evolution of the organic world are transgenations (gene mutations). Here are a few examples of mutations widely used in studying the Patterns of inheritance of traits. In Drosophila, which normally has red eyes, mutants with white eyes, ivory eyes, etc., have appeared. This gave rise to a large series of alleles comprising more than 10 mutant eye-color variants. Albinism in humans and animals is a classic gene mutation.

Different alleles exhibit varying mutation rates. For instance, in humans, the mutation causing dwarfism occurs at a rate of 5–13 per million Gametes, muscular dystrophy at 8–11, microcephaly at 27, and retinoblastoma (retinal tumor) at 3–12 per million gametes, among others. For each allele, the mutation rate is more or less constant, fluctuating within the range of 10^-5–10^-7. However, because the body contains a vast number of genes, mutations occur quite frequently. Thus, in Higher Plants and animals, up to 10% of gametes carry some new spontaneous changes.

Somatic and Germline Mutations, Mosaicism

Mutations arise in the cells of any tissue in a multicellular organism and at various stages of its development. Depending on the cell type, a distinction is made between somatic and germline mutations. Somatic mutations are mutations that occur in somatic (non-germ) cells. Their mechanism of origin is associated with mitotic abnormalities. Gene, chromosomal, and genomic somatic mutations are distinguished. The Effect of a somatic mutation depends on the organism's age: the earlier it arises, the greater the potential harm. In some cases, cells with an accelerated rate of growth and division are formed. These cells can give rise to tumors—benign ones, which do not significantly affect the organism as a whole, or malignant ones, which lead to cancerous diseases.

Somatic mutations are not inherited through sexual reproduction, but they are inherited through vegetative propagation, being transmitted exclusively to those cells that originate via mitosis from the mutant cell. Somatic mutations utilized in vegetative propagation are applied in plant breeding. For instance, I.V. Michurin developed the 'Antonovka 600-gram' apple cultivar.

Somatic mutations frequently lead to mosaicism. Mosaicism refers to the emergence of genetically distinct cell lines, and the resulting organism is called a mosaic. In mosaics, only that part of the organism derived from the division of the mutant cell is altered. Individuals with different colored eyes are examples of mosaics. Mosaicism also occurs in Down syndrome, where a certain proportion of cells have a normal karyotype (46) while others are abnormal (47). The earlier a mutation arises during ontogeny, the larger the group of cells it affects.

Generative (gametic) mutations occur in gametes or in the cells from which they originate, primarily as a result of meiotic abnormalities. Generative mutations are inherited through sexual reproduction. Dominant mutations manifest themselves as early as the first generation. Recessive mutations do not appear in the first generation because they remain in a heterozygous state (Aa); however, they may emerge In the second and subsequent generations once they transition to the homozygous state (aa).

Spontaneous and Induced Mutations

Spontaneous (natural) mutations arise arbitrarily under natural conditions. Among spontaneous mutations, gene mutations hold the greatest evolutionary significance. They increase the number of alleles at a specific locus and, consequently, expand the population's gene pool, heterozygosity, and intrapopulation variability. Mutational capability (mutability) is an inherent property of a gene. Any gene can mutate at any time, in various directions, and for undetermined reasons. In this regard, spontaneous mutations are termed random. The average frequency of spontaneous mutations is low—ranging from 10^-4 to 10^-6 per locus per generation in higher organisms. However, because the genome contains a vast number of genes, the total absolute number of mutations is substantial. Assuming an average human mutation rate of 10^-5, each individual carries between 1 and 10 novel mutations absent in either parent. Different genes within the same organism mutate at varying frequencies, yet for any given gene, the average mutation rate remains relatively constant. Given the large number of nucleotides within a single gene, the theoretical number of mutations per gene can be significant. According to the international Phenylalanine Hydroxylase Mutation Consortium, over 400 mutations of the phenylalanine hydroxylase gene had been identified by 2000, including 255 missense mutations, 22 nonsense mutations, 47 splice site mutations, and others.

Spontaneous mutations occur independently of their adaptive value and can be beneficial, harmful, or neutral. The majority of mutations are deleterious. This is because mutations arise within an integrated and coadapted genetic system, where each gene influences the entire system and vice versa. This genetic system has been shaped by evolution under the control of natural Selection to ensure optimal population adaptation to the environment. Consequently, any alteration to this genetic system is far more likely to be harmful than beneficial. When harmful mutations manifest phenotypically, they come under the scrutiny of natural selection and are eliminated through the death or reduced fertility of their carriers. The eliminative action of natural selection in humans is especially pronounced during early embryonic development. While 41.5% of all pregnancies end in Spontaneous Abortion, chromosomal abnormalities account for 5–6% of these—which is tenfold higher than The rate of live births with chromosomal syndromes (0.5–0.6% among live births). The spontaneous mutation rate depends on parental age, particularly paternal age. In women over 35, the likelihood of having a child with Down syndrome increases sharply.

Mutations can be dominant or recessive. Under conditions of complete dominance and full penetrance, dominant mutations manifest in the first generation, immediately becoming subject to positive or negative selection. Most mutations are recessive, persist within the population in a heterozygous state, remain phenotypically silent, and stay hidden from natural selection until they transition to the homozygous state (aa). In their heterozygous state, recessive mutations form a reserve of hereditary variability that is utilized in each generation during sexual reproduction to generate novel gene combinations (new genotypes). THE CONCEPT OF a mutation being "harmful" is somewhat relative: a recessive allele may be lethal in the homozygous state while enhancing organismal survival in the heterozygous state.

Endogenous factors of spontaneous mutagenesis include: 1) DNA Replication errors; 2) DNA Repair errors; 3) DNA recombination errors; 4) the action of mutator and antimutator genes;

5) transposition of Mobile Genetic Elements (MGEs).

Induced (artificial) mutations are caused by humans when organisms are experimentally exposed to mutagens at levels deliberately exceeding permissible doses. The frequency of induced mutations is several orders of magnitude higher than that of spontaneous ones. Artificial mutagenesis serves as an important source of initial material in plant and microorganism breeding.

Mutagens

Mutagens (mutagenic agents) are factors that induce mutations. Many of them are also carcinogens, meaning they are capable of causing malignant tumors. Mutagens are categorized into physical, chemical, and biological types.

Physical Mutagens include all Types of ionizing radiation (gamma and X-rays, electrons, positrons, protons, neutrons), ultraviolet radiation, as well as high and low temperatures. Ionizing radiation is the most hazardous. All forms of ionizing radiation possess high penetrance and distinct biological activity. As they pass through human Tissues, they transfer their energy to tissue atoms, causing excitation and ionization. Proliferating tissues—such as lymphoid and hematopoietic tissues—are particularly sensitive. Ionizing radiation affects all cellular components, but the nuclear chromosomes are especially vulnerable. It induces breaks in DNA molecules, resulting in chromosomal aberrations and point mutations. Mutations are induced at any dose level, with the number of mutations increasing in proportion to the dose. The genetic effect of low doses is cumulative. Any application of ionizing radiation requires strict adherence to radiation safety protocols and radioprotective measures for both patients and medical staff.

Ultraviolet rays do not cause ionization; instead, they excite the electron shells of atoms, increasing their reactivity and potentially leading to mutations. UV rays with a wavelength of approximately 260 nm possess the highest mutagenic activity because DNA absorbs this specific region of the spectrum.

Chemical Mutagens comprise A wide variety of chemical substances used in agriculture as herbicides (maleic hydrazide) and pesticides (DDT), in medicine as Pharmaceuticals (cytostatic and antimitotic agents), in industry (benzene, heavy metals such as cadmium, mercury, lead, and nickel), in the food industry (food additives), and in everyday life (varnishes, paints). To prevent mutagenesis, every chemical compound intended for use is tested for mutagenicity.

The most potent mutagens (supermutagens) include ethyleneimine, diethyl sulfate, nitrosoethylurea, nitrosomethylurea, hydrogen peroxide, and mustard gas. A second group consists of substances structurally similar to the nitrogenous bases of Nucleic Acids that act directly upon them: 5-bromouracil, 5-fluorodeoxyuridine, and 5-bromodeoxyuridine. A third group includes acridines and their derivatives, such as acridine yellow and ethidium bromide. A fourth group comprises nitrous acid, formaldehyde, and hydroxylamine.

Biological Mutagens include viruses, live Vaccines, toxins from various organisms (particularly Molds), Bacteria, protozoa, and helminths. Live vaccines are biological preparations derived from bacteria or viruses with attenuated virulence.

Genetic monitoring

The condition of atmospheric air, Water, and soil in many regions of Ukraine is unsatisfactory. Numerous pollutants have been shown to induce gene mutations in tests using Salmonella strains. In several instances, drinking water was found to be genotoxic. The unfavorable ecological situation in Ukraine has been profoundly exacerbated by the Chernobyl nuclear power plant catastrophe (1986). The country's demographic situation has evolved into an acute crisis; since 1991, mortality rates have exceeded birth rates. The number of individuals with Genetic Disorders resulting from the combined effects of chemical and physical mutagens has grown, increasing the genetic load within populations. Impaired heredity has become a significant factor negatively impacting public health. Damage to the hereditary material of somatic cells can lead to a sharp rise in malignancies, premature Aging, and a weakened immune system, all of which depress the birth rate and drive biological regression. Damage to the hereditary material of germ cells can trigger the gradual degeneration and even extinction of individual populations.

The demographic situation and public health status in Ukraine demand urgent state-level measures to protect the population's gene pool. By presidential decree, a targeted comprehensive program of genetic monitoring (genetic surveillance and control) in Ukraine was approved. One of the most critical components of this program

is the establishment of a state genetic monitoring service. The objective of genetic monitoring is to analyze the mutagenic state of the environment and monitor mutagenic shifts within human populations.

To reduce mutation risks: 1) all new chemicals and pharmaceuticals are screened for mutagenicity; 2) antimutagens are employed. Antimutagens are factors that decrease mutation rates either by neutralizing the mutagen before it reacts with DNA or by repairing mutagen-induced DNA damage. Vitamins, glutamine, serotonin, reserpine, and certain physical factors (such as daylight) exhibit antimutagenic activity. Comutagens are substances that enhance the EFFECTS OF ENVIRONMENTAL mutagens, even though they lack intrinsic mutagenic activity of their own. Such effects are produced by compounds of both natural and artificial origin, as well as organic and inorganic nature. Comutagenesis refers to the Amplification of the damaging effects of mutagens under the influence of non-mutagenic compounds. For instance, ascorbic acid (Vitamin C) enhances the cytogenic effects of mutagens (such as cyclophosphamide) in human lymphocyte cultures, while caffeine increases methotrexate-induced sister chromatid exchanges and micronuclei formation. The presence of comutagens in the environment can exacerbate the Adverse effects of chemical, physical, biological, and other mutagens to which humans are exposed.

It is known that mutations occur in various directions. This diversity is explained by a regularity discovered by N.I. Vavilov in 1920.

Law of Homologous Series in Hereditary Variation (N.I. Vavilov, 1920)

The Law of Homologous Series in Hereditary Variation reflects the general directionality of the Mutational Process across All living organisms, which is determined by:

a) the universality of the genetic code, the general organizational scheme of genes, and the processes governing the Realization of Genetic information;

b) the chromosomal Organization of hereditary material;

c) identical Cell Division processes in eukaryotes;

d) homologous Mechanisms of Recombination and mutation;

e) homologous processes of gametogenesis and fertilization...

Comparing the traits of various cultivated plant varieties and their wild relatives, N.I. Vavilov noticed that the compared plants shared many common hereditary variations, which led him to formulate the law of homologous series in hereditary variability:

1. Genetically close species and genera are characterized by similar series of hereditary variability with such regularity that, knowing a series of forms within one species, one can predict the existence of parallel forms in other species and genera. The closer organisms are genetically within the general system of genera and species, the more complete the similarity in their series of variability.

N.I. Vavilov expressed his law with the formula:

where G represents different species (genera) of organisms, and a, b, c represent various variable traits.

2. Entire families of organisms are generally characterized by a specific cycle of homologous forms of variability observed across all genera and species belonging to the given taxonomic group.

The law of homologous series in hereditary variability is directly relevant to The Study of human hereditary diseases. The issues of treatment and Prevention of Hereditary Disorders cannot be resolved without research on animals with hereditary anomalies similar to those observed in humans.

According to N.I. Vavilov's law, phenotypes analogous to human hereditary diseases should also be found in animals. Indeed, many pathological conditions identified in animals can serve as models for human hereditary diseases. For instance, sex-linked hemophilia is observed in dogs. Albinism has been recorded in many species of rodents, cats, dogs, and a number of birds. Mice, cattle, and horses are used to study muscular dystrophy; rabbits, rats, and mice for epilepsy; and eye structural anomalies are studied in many rodent species, dogs, pigs, and other animals. Hereditary deafness exists in guinea pigs, mice, and dogs. Craniofacial malformations in humans homologous to cleft lip and cleft palate are observed in the facial Skeleton of mice, dogs, and pigs. Metabolic hereditary diseases, such as obesity and diabetes mellitus, affect mice. In addition to already known mutations, exposing laboratory animals to mutagenic factors makes it possible to induce many new anomalies similar to those encountered in humans and to study them experimentally.