What causes genomic mutations. Gene mutations: causes, examples, classification

The hereditary information of a cell is recorded in the form of a DNA nucleotide sequence. There are mechanisms to protect DNA from external influences in order to avoid damage to genetic information, however, such violations occur regularly, they are called mutations.

Mutations- changes that have arisen in the genetic information of the cell, these changes can have a different scale and are divided into types.

Mutation types

Genomic mutations- changes concerning the number of whole chromosomes in the genome.

Chromosomal mutations- changes relating to regions within the same chromosome.

Gene mutations- changes occurring within a single gene.

As a result of genomic mutations, there is a change in the number of chromosomes within the genome. This is due to a malfunction of the division spindle, thus, homologous chromosomes do not diverge to different poles of the cell.

As a result, one cell acquires twice as many chromosomes as it should (Fig. 1):

Rice. 1. Genomic mutation

The haploid set of chromosomes remains the same, only the number of sets of homologous chromosomes (2n) changes.

In nature, such mutations are often fixed in the offspring; they occur most often in plants, as well as in fungi and algae (Fig. 2).

Rice. 2. Higher plants, mushrooms, algae

Such organisms are called polyploid, polyploid plants can contain from three to one hundred haploid sets. Unlike most mutations, polyploidy most often benefits the body, polyploid individuals are larger than normal ones. Many cultivars of plants are polyploid (Fig. 3).

Rice. 3. Polyploid crop plants

A person can artificially induce polyploidy by influencing plants with colchicine (Fig. 4).

Rice. 4. Colchicine

Colchicine destroys the spindle fibers and leads to the formation of polyploid genomes.

Sometimes during division, non-disjunction in meiosis may occur not for all, but only for some chromosomes, such mutations are called aneuploid. For example, the mutation trisomy 21 is typical for a person: in this case, the twenty-first pair of chromosomes does not diverge, as a result, the child receives not two twenty-first chromosomes, but three. This leads to the development of Down syndrome (Fig. 5), as a result of which the child is mentally and physically handicapped and sterile.

Rice. 5. Down syndrome

A variety of genomic mutations is also the division of one chromosome into two and the fusion of two chromosomes into one.

Chromosomal mutations are divided into types:

- deletion- loss of a chromosome segment (Fig. 6).

Rice. 6. Deletion

- duplication- duplication of some part of the chromosomes (Fig. 7).

Rice. 7. Duplication

- inversion- rotation of a chromosome region by 180 0, as a result of which the genes in this region are located in a reverse sequence compared to the norm (Fig. 8).

Rice. 8. Inversion

- translocation- moving any part of the chromosome to another place (Fig. 9).

Rice. 9. Translocation

With deletions and duplications, the total amount of genetic material changes, the degree of phenotypic manifestation of these mutations depends on the size of the altered areas, as well as on how important genes got into these areas.

During inversions and translocations, the amount of genetic material does not change, only its location changes. Such mutations are evolutionarily necessary, since mutants often can no longer interbreed with the original individuals.

Bibliography

1. Mamontov S.G., Zakharov V.B., Agafonova I.B., Sonin N.I. Biology, 11th grade. General biology. Profile level. - 5th edition, stereotypical. - Bustard, 2010.
2. Belyaev D.K. General biology. A basic level of. - 11th edition, stereotypical. - M.: Education, 2012.
3. Pasechnik V.V., Kamensky A.A., Kriksunov E.A. General biology, grades 10-11. - M.: Bustard, 2005.
4. Agafonova I.B., Zakharova E.T., Sivoglazov V.I. Biology 10-11 class. General biology. A basic level of. - 6th ed., add. - Bustard, 2010.

1. Internet portal "genetics.prep74.ru" ()
2. Internet portal "shporiforall.ru" ()
3. Internet portal "licey.net" ()

Homework

1. Where are genome mutations most common?
2. What are polyploid organisms?
3. What are the types of chromosomal mutations?

There are various methods to detect genetic mutations. Southern blotting described above is used to detect large genomic mutations. Other methods use PCR-amplified or cloned DNA. Mutations can be detected directly by sequencing (determining the primary structure of DNA macromolecules) or using radioisotope and fluorescent systems.

They can also be identified by comparing the sequence tumor DNA with DNA isolated from normal tissues, or by comparison with the normal DNA sequence described in the literature (for example, in databases posted on the Internet).

Analysis of conformational polymorphism single stranded- a radioisotope method for determining mutations, based on a change in the shape (conformation) of mutant DNA, which can be detected by electrophoresis. To do this, normal and tumor DNA is cloned by PCR, denatured and examined using gel electrophoresis. The mutant DNA changes its conformation to a non-normal shape and acquires non-normal mobility on electrophoresis.

These changes are easily identified when radioautographs. The figure below illustrates the technique for analyzing the conformational polymorphism of single-stranded (single-stranded) DNA.

Denatured high performance liquid chromatography- a new method for detecting mutations that does not require the use of radioactive substances. In this study, normal and tumor DNA are amplified (cloned) by PCR, mixed and denatured to form a mixture of single-stranded DNA molecules. Then slow annealing is carried out, as a result of which double-stranded DNA is formed again.

When pairing the thread normal DNA with the tumor thread at the site of the mutation, mating is disturbed - the so-called heteroduplex. This heteroduplex has a melting point that differs from that of normal and tumor DNA, i.e., homoduplex molecules, and due to this it can be easily determined using chromatography.

Other detection methods mutations- denaturing gradient gel electrophoresis, allele-specific oligonucleotide analysis and allele-specific amplification - based on the detection of differences in the sequences of normal and tumor DNA.

Each of these methods(with the exception of direct sequencing) is a means of screening for the presence of a mutation, but does not determine its type or the nature of the sequence disorder. Currently, instruments and methods have been developed that allow us to study large fragments of the genome and exponentially increase our ability to detect mutations.

These include molecular genetic analysis of DNA(microarray analysis) using gene chips, or biochips, and the transgenomic WAVE DNA Fragmentation Analysis System, developed in California by Transgenomic.

Analysis of the conformation of single-stranded DNA.
Left - normal alleles have the same sequence and, accordingly, the same conformation, form two identical stripes.
The mutant allele is shown on the right. The dark and light segments have a slightly different sequence and, therefore, migrate in the gel at different rates.
As a result, four stripes are formed. This technique is sensitive for detecting differences of several base pairs.

Genomic mutations are mutations that result in the addition or loss of one, several, or complete haploid set of chromosomes. Different types of genomic mutations are called heteroploidy and polyploidy.
Genomic mutations are associated with a change in the number of chromosomes. For example, in plants, the phenomenon of polyploidy is often found - a multiple change in the number of chromosomes. In polyploid organisms, the haploid set of chromosomes n in cells is repeated not 2, as in diploids, but a much larger number of times 3n, 4n, 5n and up to 12n. Polyploidy is a consequence of a violation of hodamitosis or meiosis: when the division spindle is destroyed, the duplicated chromosomes do not diverge, but remain inside the undivided cell. The result is gametes with 2n chromosomes. When such a gamete fuses with a normal n, the offspring will have a triple set of chromosomes. If a genomic mutation occurs not in germ cells, but in somatic cells, then clones of a polyploid cell line appear in the body. Often, the rate of division of these cells outstrips the rate of division of normal 2n diploid cells. In this case, a rapidly dividing line of polyploid cells forms a malignant tumor. If it is not removed or destroyed, then due to rapid division, polyploid cells will crowd out normal ones. This is how many forms of cancer develop. The destruction of the mitotic spindle can be caused by radiation, the action of a series chemical substances- mutagens.
Genomic mutations in the animal and plant world are diverse, but only 3 types of genomic mutations have been found in humans: tetraploidy, triploidy and aneuploidy. Wherein

of all variants of aneuploidy, only trisomy for autosomes, polysomy for sex chromosomes of three-, tetra- and pentasomy are found, and only monosomy-X is found from monosomy

42. Chromosomal aberration mutations and their classification. Causes and mechanisms of occurrence

The role of chromosomal mutations in the development of pathology in humans and the role in the evolutionary process.
Chromosomal mutations
aberrations, rearrangements - changes in the position of chromosome sections; lead to changes in the size and shape of chromosomes. Both parts of one chromosome and parts of different, non-homologous chromosomes can be involved in these changes; therefore, chromosomal rearrangement mutations are divided into intra- and interchromosomal.

A. Intrachromosomal mutations
1. Chromosomal duplications - doubling of a section of a chromosome.
2. Chromosomal deletions - the loss of a chromosome of any site.
Chromosomal inversions - a break in the chromosome, turning the torn off section by 180 ° and embedding it in its original place.

B. Interchromosomal mutations
1. Translocation - exchange of sites between non-homologous chromosomes in meiosis.
2. Transposition - the inclusion of a section of a chromosome into another, non-homologous chromosome without mutual exchange.

48. Gene mutations and their classification. Causes and mechanisms of gene mutations. Mouton. Consequences of gene mutations.

Gene, or point, mutations are associated with a change in the composition or sequence of nucleotides within a DNA segment - a gene. A nucleotide within a gene may be replaced by another or lost, an extra nucleotide may be inserted, and so on. Gene mutations can lead to the fact that the mutant gene either stops working and then the corresponding mRNA and protein are not formed, or a protein with altered properties is synthesized, which leads to a change in the phenotypic characteristics of the individual. As a result of gene mutations, new alleles are formed, which is of great evolutionary importance.
As a result of gene mutations, substitutions, deletions and insertions of one or more nucleotides, translocations, duplications and inversions occur. various parts gene. If a single nucleotide is changed by a mutation, it is called a point mutation. Point mutations with base substitutions are divided into two classes: purine-to-purine or pyrimidine-to-pyrimidine transitions and purine-to-pyrimidine transversions or vice versa. Due to the degeneracy of the genetic code, there can be three genetic consequences of point mutations: preservation of the meaning of a codon, a synonymous replacement of a nucleotide, a change in the meaning of a codon, leading to a replacement of an amino acid in the corresponding place in the polypeptide chain, a missense mutation, or the formation of a meaningless codon with premature termination of a nonsense mutation. There are three meaningless codons in the genetic code: amber - UAG, ocher - UAA and opal - UGA. In accordance with this, mutations are also named, leading to the formation of meaningless triplets, for example, an amber mutation.
MUTON, the elementary unit of mutation, i.e. the smallest section of the genetic. material, change to-rogo represents the mutation caught phenotypically and leads to dysfunction to. - l. gene. The term M., proposed by S. Benzer in 1957, has fallen into disuse, since it has been established that the unit of mutation is a pair of nucleotides in a double-stranded DNA molecule or one nucleotide, if genetic. the material of the body is represented by a single-stranded DNA molecule, some bacteriophages or RNA URNA-containing viruses.
51. Population genetics. Population-statistical method for studying human heredity Hardy-Weiberg law.

Population genetics is a branch of genetics that studies the distribution of allele frequencies and their change under the influence of the driving forces of evolution: mutagenesis, natural selection, genetic drift and the migration process. It also takes into account subpopulation structures and the spatial structure of the population. Population genetics attempts to explain adaptation and specialization and is one of the main components of the synthetic theory of evolution. The formation of population genetics was most influenced by: Sewell Wright, John Haldane, Ronald Fisher, Sergei Sergeevich Chetverikov; the key patterns that determine allele frequencies in populations are formulated by Godfrey Hardy and Wilhelm Weinberg.
Population-statistical method for studying human genes.
This method is used to study the genetic structure of human populations or individual families. It allows you to determine the frequency of individual genes in populations. The population method makes it possible to study the genetic structure of human populations, to reveal the relationship between individual populations, and also sheds light on the history of human distribution around the planet. The method is widely used in clinical genetics, because intrafamilial analysis of incidence is inseparable from the study of hereditary pathology both in countries with a large population and in relatively isolated population groups.
In this regard, all genes are divided into the following 2 groups:
having a universal distribution.
found locally, mainly in strictly defined areas.
The essence of the method is to study the frequencies of genes and genotypes in various population groups using the methods of variation statistics, which provides the necessary information about the frequency of heterozygosity and the degree of polymorphism in humans. In particular, in the heterozygous state in populations there is a significant number of recessive alleles, which causes the development of various

hereditary diseases, the frequency of which depends on the concentration of the recessive gene in the population and increases significantly with closely related marriages. Mutations can be passed on to offspring over many generations, resulting in genetic heterogeneity that underlies population polymorphism.
According to the Hardy-Weinberg law of 1980, a population maintains a constant ratio of genotype frequencies from generation to generation, if no factors disturb this balance.

2pq + q? = 1
Where is p? - proportion of homozygotes for one of the alleles; p is the frequency of this allele; q? - proportion of homozygotes for the alternative allele; q is the frequency of the corresponding allele; 2pq - proportion of heterozygotes.
The vast majority of recessive alleles are present in the population in a latent heterozygous state. So, albinos are born with a frequency of 1:20,000, but one out of every 70 inhabitants of European countries is heterozygous for this allele.
If the gene is located on the sex chromosome, then a different picture is observed: in men, the frequency of homozygous recessives is quite high. So, in the population of Muscovites in the 1930s. 7% of colorblind males and 0.5% homozygous recessive colorblind females were present.
In human populations, very interesting research blood groups. There is an assumption that their distribution in various parts of the globe was influenced by plague and smallpox epidemics. The least resistant to the plague were people of the I blood group 00; on the contrary, the smallpox virus most often infects carriers of group II AA, A0. The plague was especially rampant in countries such as India, Mongolia, China, Egypt, and therefore there was a "culling" of the 0 allele as a result of increased mortality from the plague of people with blood type I. Smallpox epidemics covered mainly India, Arabia, tropical Africa, and after the arrival of Europeans - and America.
In countries with malaria, as you already know, the Mediterranean, Africa, there is a high frequency of the gene that causes sickle cell anemia.
There is evidence that Rh negative is less common in populations living in conditions of high prevalence of various infectious diseases, including malaria. And in populations living in highlands and other areas where infections are a rare thing, there is an increased percentage of Rh-negative people.

52 Human medical genetics. The concept of hereditary and non-hereditary human diseases. Medical genetic counseling. Diagnostic methods.

Human medical genetics is a field of medicine, a science that studies the phenomena of heredity and variability in various human populations, features of the manifestation and development of normal and pathological signs, the dependence of diseases on genetic predisposition and conditions environment. The task of honey genetics is the identification, study, prevention and treatment of hereditary diseases, the development of ways to prevent the impact of negative environmental factors on human heredity.
Hereditary Variability is due to the occurrence of different types of mutations and their combinations in subsequent crosses. In each sufficiently long in a number of generations of the existing population of individuals, various mutations spontaneously and not directed arise, which are later combined more or less randomly with different hereditary properties already existing in the population.
The whole variety of individual differences is based on hereditary variability, which include: both sharp qualitative differences, not connected with each other by transitional forms, and purely quantitative differences, forming continuous series, in which close members of the series can differ from each other as little as desired; b both changes in individual features and properties, and interrelated changes in a number of features; as changes that have an adaptive value, as well as changes that are "indifferent" or even reduce the viability of their carriers. All these types of hereditary changes constitute the material of the evolutionary process. V individual development organism, the manifestation of hereditary traits and properties is always determined not only by the main genes responsible for these traits and properties, but also by their interaction with many other genes that make up the genotype of the individual, as well as by the environmental conditions in which the organism develops.
The concept of non-hereditary variability includes those changes in traits and properties that in individuals or certain groups of individuals are caused by the influence of external factors - nutrition, temperature, light, humidity, etc. Such non-hereditary traits of modification in their specific manifestation in each individual are not inherited , they develop in individuals of subsequent generations only under the conditions in which they arose. Such variability is also called modification. For example, the color of many insects darkens at low temperatures, and brightens at high temperatures; however, their offspring will be colored regardless of the color of the parents, according to the temperature at which they themselves developed. There is another form of non-hereditary variability - the so-called long-term modifications, which are often found in unicellular organisms, but are occasionally observed in multicellular organisms. They arise under the influence of external influences, for example, temperature or chemical, and are expressed in qualitative or quantitative deviations from the original form, usually gradually fading during subsequent reproduction. They

apparently based on changes in relatively stable cytoplasmic structures. The limits of non-hereditary changes are determined by the norm of the reaction of the genotype to environmental conditions.
Medical genetic counseling is one of the types of specialized assistance to the population, aimed primarily at preventing the appearance of children with hereditary pathology in the family. For this purpose, a forecast is made for the birth of a child with a hereditary disease in a given family, parents are explained the likelihood of this event and are assisted in making a decision. In the case of a high probability of the birth of a sick child, parents are advised either to refrain from childbearing, or to conduct prenatal diagnosis, if it is possible with this type of pathology.

53. Monogenic, chromosomal and multifactorial human diseases, mechanisms of their occurrence and manifestation.

Monogenic diseases with a hereditary predisposition are also determined by one mutant gene, but their manifestation requires the obligatory action of a specific environmental factor, which can be considered as specific in relation to this disease. These diseases are relatively few, they are inherited according to the laws of Mendel, their prevention and treatment are sufficiently developed and effective. Given the important role of environmental factors in the manifestation of these diseases, they should be considered as hereditary pathological reactions to the action of external factors. This may be a perverted response to pharmacological drugs sulfonamides, primaquine, etc., to atmospheric pollution, polycyclic hydrocarbons, to nutrients and additives lactose, chocolate, alcohol, physical cold, ultraviolet rays and biological vaccines, allergen factors.
Causes of gene pathologies
Most gene pathologies are caused by mutations in structural genes that perform their function through the synthesis of polypeptides - proteins. Any mutation of a gene leads to a change in the structure or amount of the protein.
The onset of any gene disease is associated with the primary effect of the mutant allele.
The main scheme of gene diseases includes a number of links:
mutant allele > altered primary product > chain of biochemical processes in the cell > organs > organism
As a result of gene mutation at the molecular level, the following options are possible:
abnormal protein synthesis

overproduction of a gene product

lack of production of the primary product

production of a reduced amount of a normal primary product.
Not ending at the molecular level in the primary links, the pathogenesis of gene diseases continues at the cellular level. In various diseases, the point of application of the action of the mutant gene can be both individual cell structures - lysosomes, membranes, mitochondria, peroxisomes, and human organs.
Clinical manifestations of gene diseases, the severity and speed of their development depend on the characteristics of the genotype of the organism, the age of the patient, environmental conditions, nutrition, cooling, stress, overwork and other factors.
A feature of genes, as well as all hereditary diseases in general, is their heterogeneity. This means that the same phenotypic manifestation of a disease can be due to mutations in different genes or different mutations within the same gene. For the first time, the heterogeneity of hereditary diseases was identified by S. N. Davidenkov in 1934.
The general frequency of gene diseases in the population is 1-2%. Conventionally, the frequency of gene diseases is considered high if it occurs with a frequency of 1 case per 10,000 newborns, medium - 1 per 10,000 - 40,000, and then - low.
Monogenic forms of gene diseases are inherited in accordance with the laws of G. Mendel. According to the type of inheritance, they are divided into autosomal dominant, autosomal recessive and linked to the X or Y chromosomes.
Classification
Genetic diseases in humans include numerous metabolic diseases. They may be associated with impaired metabolism of carbohydrates, lipids, steroids, purines and pyrimidines, bilirubin, metals, etc. There is still no unified classification of hereditary metabolic diseases.
Diseases of amino acid metabolism
The largest group of hereditary metabolic diseases. Almost all of them are inherited in an autosomal recessive manner. The cause of diseases is the insufficiency of one or another enzyme responsible for the synthesis of amino acids. These include:
phenylketonuria - a violation of the conversion of phenylalanine to tyrosine due to a sharp decrease in the activity of phenylalanine hydroxylase

alkaptonuria - a violation of the metabolism of tyrosine due to reduced activity of the homogentisinase enzyme and the accumulation of homotentisic acid in the tissues of the body

oculocutaneous albinism - due to the lack of tyrosinase enzyme synthesis.
Carbohydrate metabolism disorders
galactosemia - the absence of the enzyme galactose-1-phosphate-uridyltransferase and the accumulation of galactose in the blood

glycogen disease - a violation of the synthesis and breakdown of glycogen.
Diseases associated with impaired lipid metabolism
Niemann-Pick disease - decreased activity of the enzyme sphingomyelinase, degeneration of nerve cells and impaired activity nervous system

Gaucher disease is the accumulation of cerebrosides in the cells of the nervous and reticuloendothelial system, due to a deficiency of the enzyme glucocerebrosidase.
Hereditary diseases of purine and pyrimidine metabolism
gout

Lesch-Nyhan syndrome.
Diseases of connective tissue metabolism disorders
marfan's syndrome

fingers", arachnodactyly - damage to the connective tissue due to a mutation in the gene responsible for the synthesis of fibrillin

Mucopolysaccharidoses are a group of connective tissue diseases associated with impaired metabolism of acid glycosaminoglycans.
Fibrodysplasia is a connective tissue disease associated with its progressive ossification as a result of a mutation in the ACVR1 gene.
Inherited disorders of circulating proteins
hemoglobinopathy - hereditary disorders of hemoglobin synthesis. Their quantitative structural and qualitative forms are distinguished. The former are characterized by a change in the primary structure of hemoglobin proteins, which can lead to impaired stability and function of sickle cell anemia. With qualitative forms, the structure of hemoglobin remains normal, only the rate of synthesis of thalassemia globin chains is reduced.
Hereditary diseases of metal metabolism
Konovalov-Wilson disease, etc.
Syndromes of malabsorption in the digestive tract
cystic fibrosis

lactose intolerance, etc.
Chromosomal diseases include diseases caused by genomic mutations or structural changes in individual chromosomes. Chromosomal diseases result from mutations in the germ cells of one of the parents. No more than 3-5% of them are passed from generation to generation. Chromosomal abnormalities are responsible for approximately 50% of spontaneous abortions and 7% of all stillbirths.
All chromosomal diseases are usually divided into two groups: anomalies in the number of chromosomes and violations of the structure of chromosomes.
Chromosome number anomalies
Diseases caused by a violation of the number of autosomes of non-sex chromosomes
Down syndrome - trisomy on chromosome 21, signs include: dementia, growth retardation, characteristic appearance, changes in dermatoglyphics

Patau syndrome - trisomy on chromosome 13, characterized by multiple malformations, idiocy, often - polydactyly, violations of the structure of the genital organs, deafness; almost all patients do not live up to one year

Edwards syndrome - trisomy on chromosome 18, the lower jaw and mouth opening are small, the palpebral fissures are narrow and short, the auricles are deformed; 60% of children die before the age of 3 months, only 10% live up to a year, the main cause is respiratory arrest and disruption of the heart.
Diseases associated with a violation of the number of sex chromosomes
Shereshevsky-Turner syndrome - the absence of one X chromosome in women 45 XO due to a violation of the divergence of the sex chromosomes; the signs include short stature, sexual infantilism and infertility, various somatic disorders of micrognathia, a short neck, etc.

polysomy on the X chromosome - includes trisomy karyoty 47, XXX, tetrasomy 48, XXXX, pentasomy 49, XXXXX, there is a slight decrease in intelligence, an increased likelihood of developing psychoses and schizophrenia with an unfavorable type of course

polysomy on the Y chromosome - like polysomy on the X chromosome, includes trisomy karyoty 47, XYY, tetrasomy 48, XYYY, pentasomy 49, XYYYY, clinical manifestations are also similar to polysomy X chromosome

Klinefelter's syndrome - polysomy on X- and Y-chromosomes in boys 47, XXY; 48, XXYY and others, signs: eunuchoid body type, gynecomastia, weak hair growth on the face, in the armpits and on the pubis, sexual infantilism, infertility; mental development lags behind, but sometimes intelligence is normal.
Diseases caused by polyploidy
triploidy, tetraploidy, etc.; the reason is a violation of the meiosis process due to a mutation, as a result of which the daughter sex cell receives a diploid 46 set of chromosomes instead of haploid 23, that is, 69 chromosomes in men karyotype 69, XYY, in women - 69, XXX; almost always fatal before birth.
Chromosome structure disorders

Translocations are exchange rearrangements between non-homologous chromosomes.
Deletions are the loss of a segment of a chromosome. For example, "cat's cry" syndrome is associated with a deletion of the short arm of the 5th chromosome. A sign of it is the unusual cry of children, reminiscent of meowing or the cry of a cat. This is due to the pathology of the larynx or vocal cords. The most typical, in addition to the "cat's cry", is mental and physical underdevelopment, microcephaly is an abnormally reduced head.
Inversions are 180 degree rotations of a segment of a chromosome.
Duplications are doublings of a portion of a chromosome.
Isochromosomy - chromosomes with repeated genetic material in both arms.
The occurrence of ring chromosomes is the connection of two terminal deletions in both arms of the chromosome.

Currently, more than 700 diseases are known in humans caused by changes in the number or structure of chromosomes. About 25% are due to autosomal trisomies, 46% - to the pathology of the sex chromosomes. Structural adjustments account for 10.4%. The most common chromosomal rearrangements are translocations and deletions.

Polygenic diseases earlier - diseases with a hereditary predisposition are caused by both hereditary factors and, to a large extent, environmental factors. In addition, they are associated with the action of many genes, so they are also called multifactorial. The most common multifactorial diseases include: rheumatoid arthritis, coronary heart disease, hypertension and peptic ulcer, liver cirrhosis, diabetes mellitus, bronchial asthma, psoriasis, schizophrenia, etc.

Polygenic diseases are closely associated with inborn errors of metabolism, some of which may manifest as metabolic diseases.

The spread of polygenic hereditary diseases
This

a group of diseases currently accounts for 92% of the total number of human hereditary pathologies. With age, the frequency of diseases increases. In childhood, the percentage of patients is at least 10%, and in the elderly - 25-30%.
The spread of multifactorial diseases in different human populations can vary significantly due to differences in genetic and environmental factors. As a result of genetic processes occurring in human populations, selection, mutation, migration, genetic drift, the frequency of genes that determine hereditary predisposition can increase or decrease until their complete elimination.
Features of polygenic diseases
The clinical picture and the severity of the course of multifactorial human diseases depending on gender and age are very different. However, despite their diversity, there are the following common features:
High incidence of disease in the population. So, about 1% of the population suffers from schizophrenia, 5% from diabetes, more than 10% from allergic diseases, about 30% from hypertension.
Clinical polymorphism of diseases varies from latent subclinical forms to pronounced manifestations.
Features of the inheritance of diseases do not correspond to Mendelian patterns.
The degree of manifestation of the disease depends on the gender and age of the patient, the intensity of the work of his endocrine system, adverse factors of the external and internal environment, for example, poor nutrition, etc.
Genetic prediction of polygenic diseases
Genetic prognosis in multifactorial diseases depends on the following factors:
the lower the frequency of the disease in the population, the higher the risk for relatives of the proband

the stronger the severity of the disease in the proband, the greater the risk of developing the disease in his relatives

the risk to relatives of the proband depends on the degree of relationship with the affected family member

the risk for relatives will be higher if the proband belongs to the less affected sex.
The polygenic nature of diseases with a hereditary predisposition is confirmed using genealogical, twin and population-statistical methods. Quite objective and sensitive twin method. Using the twin method, a hereditary predisposition to certain infectious diseases tuberculosis, poliomyelitis and many common diseases - coronary heart disease, rheumatoid arthritis, diabetes mellitus, peptic ulcer, schizophrenia, etc.

54. Asexual and sexual reproduction. Forms of asexual reproduction, definition, essence, biological significance.

Reproduction is the property of organisms to produce offspring. Two forms of reproduction: sexual and asexual. Sexual reproduction - the change of generations and the development of organisms based on the merger of specialized - germ cells and zygote formation. With asexual reproduction, a new individual appears from non-specialized cells: somatic, non-sexual; body.
Asexual reproduction, or agamogenesis, is a form of reproduction in which an organism reproduces itself on its own, without any participation of another individual.
Reproduction by division
Division is characteristic primarily of unicellular organisms. As a rule, it is carried out by a simple cell division in two. Some protozoa, such as foraminifera, divide into more cells. In all cases, the resulting cells are completely identical to the original. The extreme simplicity of this method of reproduction, associated with the relative simplicity of the organization of unicellular organisms, makes it possible to multiply very quickly. So, in favorable conditions, the number of bacteria can double every 30-60 minutes. An asexually reproducing organism is capable of endlessly reproducing itself until a spontaneous change in the genetic material occurs - a mutation. If this mutation is favorable, it will be preserved in the offspring of the mutated cell, which will be a new cell clone. In same-sex reproduction, one parent organism is involved, which is able to form many organisms identical to it.
Reproduction by spores
Often asexual reproduction of bacteria is preceded by the formation of spores. Bacterial spores are dormant cells with reduced metabolism, surrounded by a multilayered membrane, resistant to desiccation and other adverse conditions that cause the death of ordinary cells. Sporulation serves both to survive such conditions and to settle bacteria: once in a suitable environment, the spore germinates, turning into a vegetative dividing cell.
Asexual reproduction with the help of unicellular spores is also characteristic of various fungi and algae. Spores in many cases are formed by mitosis of mitospores, and sometimes especially in fungi in huge quantities; when germinating, they reproduce the mother organism. Some fungi, such as the noxious plant pest Phytophthora, form motile, flagellated spores called zoospores or vagrants. After swimming in droplets of moisture for some time, such a tramp “calms down”, loses flagella, becomes covered with a dense shell and then, under favorable conditions, germinates.
Vegetative reproduction
Another variant of asexual reproduction is carried out by separating from the body of its part, consisting of a larger or smaller number of cells. They develop into adults. An example is budding in sponges and coelenterates or propagation of plants by shoots,

cuttings, bulbs or tubers. This form of asexual reproduction is commonly referred to as vegetative reproduction. Basically, it is similar to the process of regeneration. Vegetative propagation plays an important role in the practice of crop production. So, it may happen that a sown plant, for example, an apple tree, has some successful combination of traits. In the seeds of this plant, this fortunate combination will almost certainly be broken, since the seeds are formed as a result of sexual reproduction, and this is associated with the recombination of genes. Therefore, when breeding apple trees, vegetative propagation is usually used - layering, cuttings or grafting buds on other trees.
budding
Some species of unicellular organisms are characterized by such a form of asexual reproduction as budding. In this case, mitotic division of the nucleus occurs. One of the formed nuclei moves into the emerging local protrusion of the mother cell, and then this fragment buds. The daughter cell is significantly smaller than the mother cell, and it takes some time for it to grow and complete the missing structures, after which it takes on the form characteristic of a mature organism. Budding is a type of vegetative reproduction. Many lower fungi reproduce by budding, such as yeasts and even multicellular animals, such as freshwater hydra. When yeast buds, a thickening forms on the cell, gradually turning into a full-fledged daughter cell yeast. On the body of the hydra, several cells begin to divide, and gradually a small hydra grows on the mother individual, in which a mouth with tentacles and an intestinal cavity are formed, connected with the intestinal cavity of the “mother”.
Fragmentation division of the body
Some organisms can reproduce by dividing the body into several parts, and from each part a full-fledged organism grows, in all respects similar to the parent individual - flat and annelids, echinoderms.

Sexual reproduction is a process in most eukaryotes associated with the development of new organisms from germ cells.
The formation of germ cells, as a rule, is associated with the passage of meiosis at some stage of the life cycle of the organism. In most cases, sexual reproduction is accompanied by the fusion of germ cells, or gametes, while a double set of chromosomes is restored relative to gametes. Depending on the systematic position of eukaryotic organisms, sexual reproduction has its own characteristics, but as a rule, it allows you to combine genetic material from two parental organisms and allows you to get offspring with a combination of properties that are absent in parental forms.
The effectiveness of combining genetic material in offspring obtained as a result of sexual reproduction is facilitated by:
chance meeting of two gametes

random arrangement and divergence to the poles of division of homologous chromosomes during meiosis

crossing over between chromatids.

Such a form of sexual reproduction as parthenogenesis does not involve the fusion of gametes. But since the organism develops from the germ cell of the oocyte, parthenogenesis is still considered sexual reproduction.
In many groups of eukaryotes, the secondary extinction of sexual reproduction has occurred, or it occurs very rarely. In particular, the Deuteromycetes division of fungi combines an extensive group of phylogenetic ascomycetes and basidiomycetes that have lost their sexual process. Until 1888, it was assumed that among the terrestrial higher plants, sexual reproduction was completely lost in sugar cane. The loss of sexual reproduction in any group of metazoans has not been described. However, many species of lower crustaceans are known - daphnia, some types of worms that, under favorable conditions, can reproduce parthenogenetically for tens and hundreds of generations. For example, some species of rotifers for millions of years reproduce only parthenogenetically, even forming new species!
In a number of polyploid organisms with an odd number of sets of chromosomes, sexual reproduction plays a small role in maintaining genetic variability in the population due to the formation of unbalanced sets of chromosomes in gametes and in offspring.
The ability to combine genetic material during sexual reproduction has great importance for selection of model and economically important organisms.

55. Sexual reproduction, its evolutionary significance. Forms of sexual reproduction in unicellular and multicellular organisms are conjugation, copulation. The biological significance of sexual reproduction.

The main significance of sexual reproduction in evolution is not just to increase the number of individuals, but to expand the gene pool, further contributing to natural selection.

Sexual reproduction creates a higher genetic variability in a population. As a result of a number of processes, the genes originally carried by the parents end up in a new combination in the offspring. It is due to recombination within the litter that numerous genetic differences are found, which increases the adaptive potential of the population and the species as a whole.

In unicellular organisms, two forms of sexual reproduction are distinguished - copulation and conjugation.
During copulation, the sexual process is carried out with the help of specialized germ cells - gametes. In unicellular organisms, they arise by repeated division of the cell-organism.
Gametes differ significantly from individual cells in size, shape, they may not have a number of organelles. For example, in

For genetic research a person is an inconvenient object, since in a person: experimental crossing is impossible; a large number of chromosomes; puberty comes late; a small number of descendants in each family; equalization of living conditions for offspring is impossible.

A number of research methods are used in human genetics.

genealogical method

The use of this method is possible in the case when direct relatives are known - the ancestors of the owner of the hereditary trait ( proband) on the maternal and paternal lines in a number of generations or the descendants of the proband also in several generations. When compiling pedigrees in genetics, a certain system of notation is used. After compiling the pedigree, its analysis is carried out in order to establish the nature of the inheritance of the trait under study.

Conventions adopted in the preparation of pedigrees:
1 - man; 2 - woman; 3 - gender not clear; 4 - the owner of the studied trait; 5 - heterozygous carrier of the studied recessive gene; 6 - marriage; 7 - marriage of a man with two women; 8 - related marriage; 9 - parents, children and the order of their birth; 10 - dizygotic twins; 11 - monozygotic twins.

Thanks to the genealogical method, the types of inheritance of many traits in humans have been determined. Thus, polydactyly (an increased number of fingers), the ability to roll the tongue into a tube, brachydactyly (short fingers due to the absence of two phalanges on the fingers), freckles, early baldness, fused fingers, cleft lip, cleft palate, cataracts of the eyes, are inherited according to the autosomal dominant type. fragility of bones and many others. Albinism, red hair, susceptibility to polio, diabetes mellitus, congenital deafness, and other traits are inherited as autosomal recessive.

The dominant trait is the ability to roll the tongue into a tube (1) and its recessive allele is the absence of this ability (2).
3 - pedigree for polydactyly (autosomal dominant inheritance).

A number of traits are inherited sex-linked: X-linked inheritance - hemophilia, color blindness; Y-linked - edge hypertrichosis auricle, webbed toes. There are a number of genes located in homologous regions of the X and Y chromosomes, such as general color blindness.

The use of the genealogical method showed that in a related marriage, compared with an unrelated one, the likelihood of deformities, stillbirths, and early mortality in the offspring increases significantly. In related marriages, recessive genes often go into a homozygous state, as a result, certain anomalies develop. An example of this is the inheritance of hemophilia in the royal houses of Europe.

- hemophilic; - carrier woman

twin method

1 - monozygotic twins; 2 - dizygotic twins.

Children born at the same time are called twins. They are monozygotic(identical) and dizygotic(variegated).

Monozygotic twins develop from one zygote (1), which is divided into two (or more) parts during the crushing stage. Therefore, such twins are genetically identical and always of the same sex. Monozygotic twins are characterized by a high degree of similarity ( concordance) in many ways.

Dizygotic twins develop from two or more eggs that are simultaneously ovulated and fertilized by different spermatozoa (2). Therefore, they have different genotypes and can be either the same or different sex. Unlike monozygotic twins, dizygotic twins are characterized by discordance - dissimilarity in many ways. Data on the concordance of twins for some signs are given in the table.

signs	Concordance, %
	Monozygotic twins	dizygotic twins
Normal
Blood group (AB0)	100	46
eye color	99,5	28
Hair color	97	23
Pathological
Clubfoot	32	3
"Hare Lip"	33	5
Bronchial asthma	19	4,8
Measles	98	94
Tuberculosis	37	15
Epilepsy	67	3
Schizophrenia	70	13

As can be seen from the table, the degree of concordance of monozygotic twins for all the above characteristics is significantly higher than that of dizygotic twins, but it is not absolute. As a rule, the discordance of monozygotic twins occurs as a result of intrauterine development disorders of one of them or under the influence of the external environment, if it was different.

Thanks to the twin method, a person's hereditary predisposition to a number of diseases was clarified: schizophrenia, epilepsy, diabetes mellitus and others.

Observations on monozygotic twins provide material for elucidating the role of heredity and environment in the development of traits. Moreover, the external environment is understood not only as physical factors of the environment, but also as social conditions.

Cytogenetic method

Based on the study of human chromosomes in normal and pathological conditions. Normally, a human karyotype includes 46 chromosomes - 22 pairs of autosomes and two sex chromosomes. The use of this method made it possible to identify a group of diseases associated either with a change in the number of chromosomes or with changes in their structure. Such diseases are called chromosomal.

Blood lymphocytes are the most common material for karyotypic analysis. Blood is taken in adults from a vein, in newborns - from a finger, earlobe or heel. Lymphocytes are cultivated in a special nutrient medium, which, in particular, contains substances that “force” lymphocytes to intensively divide by mitosis. After some time, colchicine is added to the cell culture. Colchicine stops mitosis at the metaphase level. It is during metaphase that the chromosomes are most condensed. Next, the cells are transferred to glass slides, dried and stained with various dyes. Coloring can be a) routine (chromosomes stain evenly), b) differential (chromosomes acquire transverse striation, with each chromosome having an individual pattern). Routine staining allows you to identify genomic mutations, determine the group belonging of the chromosome, and find out in which group the number of chromosomes has changed. Differential staining allows you to identify chromosomal mutations, determine the chromosome to the number, find out the type of chromosomal mutation.

In cases where it is necessary to conduct a karyotypic analysis of the fetus, cells of the amniotic (amniotic) fluid are taken for cultivation - a mixture of fibroblast-like and epithelial cells.

Chromosomal diseases include: Klinefelter syndrome, Turner-Shereshevsky syndrome, Down syndrome, Patau syndrome, Edwards syndrome and others.

Patients with Klinefelter's syndrome (47, XXY) are always male. They are characterized by underdevelopment of the sex glands, degeneration of the seminiferous tubules, often mental retardation, high growth (due to disproportionately long legs).

Turner-Shereshevsky syndrome (45, X0) is observed in women. It manifests itself in slowing down puberty, underdevelopment of the gonads, amenorrhea (absence of menstruation), infertility. Women with Turner-Shereshevsky syndrome are small in stature, the body is disproportionate - the upper body is more developed, the shoulders are wide, the pelvis is narrow - the lower limbs are shortened, the neck is short with folds, the "Mongoloid" incision of the eyes and a number of other signs.

Down syndrome is one of the most common chromosomal diseases. It develops as a result of trisomy on chromosome 21 (47; 21, 21, 21). The disease is easily diagnosed, as it has a number of characteristic features: shortened limbs, a small skull, a flat, wide nose, narrow palpebral fissures with an oblique incision, the presence of a fold of the upper eyelid, and mental retardation. Violations of the structure of internal organs are often observed.

Chromosomal diseases also occur as a result of changes in the chromosomes themselves. Yes, deletion R-arm of autosome number 5 leads to the development of the "cat's cry" syndrome. In children with this syndrome, the structure of the larynx is disturbed, and in early childhood they have a kind of “meowing” voice timbre. In addition, there is a retardation of psychomotor development and dementia.

Most often, chromosomal diseases are the result of mutations that have occurred in the germ cells of one of the parents.

Biochemical method

Allows you to detect metabolic disorders caused by changes in genes and, as a result, changes in the activity of various enzymes. Hereditary metabolic diseases are divided into diseases of carbohydrate metabolism (diabetes mellitus), metabolism of amino acids, lipids, minerals, etc.

Phenylketonuria refers to diseases of amino acid metabolism. The conversion of the essential amino acid phenylalanine to tyrosine is blocked, while phenylalanine is converted to phenylpyruvic acid, which is excreted in the urine. The disease leads to rapid development dementia in children. Early diagnosis and diet can stop the development of the disease.

Population-statistical method

It is a method of studying the distribution of hereditary traits (inherited diseases) in populations. An essential point when using this method is the statistical processing of the obtained data. Under population understand the totality of individuals of the same species, living in a certain territory for a long time, freely interbreeding with each other, having a common origin, a certain genetic structure and, to one degree or another, isolated from other such populations of individuals of a given species. A population is not only a form of existence of a species, but also a unit of evolution, since the basis of microevolutionary processes culminating in the formation of a species are genetic transformations in populations.

The study of the genetic structure of populations deals with a special section of genetics - population genetics. In humans, three types of populations are distinguished: 1) panmictic, 2) demes, 3) isolates, which differ from each other in number, frequency of intra-group marriages, the proportion of immigrants, and population growth. Population big city corresponds to the panmictic population. The genetic characteristics of any population includes the following indicators: 1) gene pool(the totality of genotypes of all individuals of a population), 2) gene frequencies, 3) genotype frequencies, 4) phenotype frequencies, marriage system, 5) factors that change gene frequencies.

To determine the frequencies of occurrence of certain genes and genotypes, hardy-weinberg law.

Hardy-Weinberg law

In an ideal population, from generation to generation, a strictly defined ratio of frequencies of dominant and recessive genes (1), as well as the ratio of frequencies of genotypic classes of individuals (2) is preserved.

p + q = 1, (1)
R 2 + 2pq + q 2 = 1, (2)

where p— frequency of occurrence of the dominant gene A; q- the frequency of occurrence of the recessive gene a; R 2 - the frequency of occurrence of homozygotes for the dominant AA; 2 pq- frequency of occurrence of Aa heterozygotes; q 2 - the frequency of occurrence of homozygotes for the recessive aa.

The ideal population is a sufficiently large, panmictic (panmixia - free crossing) population in which there are no mutation process, natural selection and other factors that disturb the balance of genes. It is clear that ideal populations do not exist in nature; in real populations, the Hardy-Weinberg law is used with amendments.

The Hardy-Weinberg law, in particular, is used to roughly count the carriers of recessive genes for hereditary diseases. For example, phenylketonuria is known to occur at a rate of 1:10,000 in a given population. Phenylketonuria is inherited in an autosomal recessive manner, therefore, patients with phenylketonuria have the aa genotype, that is q 2 = 0.0001. From here: q = 0,01; p= 1 - 0.01 = 0.99. Carriers of the recessive gene have the Aa genotype, that is, they are heterozygotes. The frequency of occurrence of heterozygotes (2 pq) is 2 0.99 0.01 ≈ 0.02. Conclusion: in this population, about 2% of the population are carriers of the phenylketonuria gene. At the same time, you can calculate the frequency of occurrence of homozygotes for the dominant (AA): p 2 = 0.992, just under 98%.

A change in the balance of genotypes and alleles in a panmictic population occurs under the influence of constantly acting factors, which include: the mutation process, population waves, isolation, natural selection, gene drift, emigration, immigration, inbreeding. It is thanks to these phenomena that an elementary evolutionary phenomenon arises - a change in the genetic composition of a population, which is the initial stage in the process of speciation.

Human genetics is one of the most intensively developing branches of science. It is the theoretical basis of medicine, reveals the biological basis of hereditary diseases. Knowing the genetic nature of diseases allows you to make an accurate diagnosis in time and carry out the necessary treatment.

Go to lectures №21"Variability"

The most significant changes in the genetic apparatus occur during genomic mutations, i.e. when the number of chromosomes in the set changes. They can concern either individual chromosomes ( aneuploidy), or whole genomes ( euploidy).

In animals, the main diploid the level of ploidy, which is associated with the predominance of their sexual mode of reproduction. polyploidy in animals it is extremely rare, for example, in roundworms and rotifers. haploidy at the organismal level, it is also rare in animals (for example, drones in bees). Haploid are the germ cells of animals, which has a deep biological meaning: due to the change in nuclear phases, the optimal level of ploidy is stabilized - diploid. The haploid number of chromosomes is called the base number of chromosomes.

In plants, haploids spontaneously arise in populations at a low frequency (maize has 1 haploid per 1000 diploids). The phenotypic features of haploids are determined by two factors: external similarity with the corresponding diploids, from which they differ in smaller sizes, and the manifestation of recessive genes that are in their homozygous state. Haploids are usually sterile, because they lack homologous chromosomes and meiosis cannot proceed normally. Fertile gametes in haploids can be formed in the following cases: a) when chromosomes diverge in meiosis according to type 0- n(i.e. the entire haploid set of chromosomes goes to one pole); b) with spontaneous diploidization of germ cells. Their fusion leads to the formation of diploid offspring.

Many plants have a wide range of ploidy levels. For example, within the genus Poa (bluegrass), the number of chromosomes ranges from 14 to 256, i.e. basic number of chromosomes ( n= 7) increases by several tens of times. However, not all chromosome numbers are optimal and ensure the normal viability of individuals. There are biologically optimal and evolutionarily optimal levels of ploidy. In sexual species, they usually coincide (diploidy). In facultatively apomictic species, the evolutionary optimal is often the tetraploid level, which allows for the possibility of a combination of sexual reproduction and apomixis (i.e., parthenogenesis). It is the presence of an apomictic form of reproduction that explains the wide distribution of polyploidy in plants, since. in sexual species, polyploidy usually leads to sterility due to disturbances in meiosis, while in apomicts, meiosis does not occur during gamete formation, and they are often polyploid.

In some plant genera, the species form polyploid series with chromosome numbers that are multiples of the base number. For example, such a series exists in wheat: Triticum monococcum 2 n= 14 (einkorn wheat); Tr. durum 2 n= 28 (durum wheat); Tr. aestivum 2 n= 42 (soft wheat).

Distinguish between autopolyploidy and allopolyploidy.

Autopolyploidy

Autopolyploidy is an increase in the number of haploid sets of chromosomes of one species. The first mutant, an autotetraploid, was described at the beginning of the 20th century. G. de Vries at evening primrose. It had 14 pairs of chromosomes instead of 7. Further study of the number of chromosomes in representatives of different families revealed the wide distribution of autopolyploidy in the plant world. With autopolyploidy, either an even (tetraploids, hexaploids) or odd (triploids, pentaploids) increase in chromosome sets occurs. Autopolyploids differ from diploids in the larger size of all organs, including reproductive ones. This is based on an increase in cell size with increasing ploidy (nuclear plasma index).

Plants react differently to an increase in the number of chromosomes. If, as a result of polyploidy, the number of chromosomes becomes higher than optimal, then autopolyploids, showing individual signs of gigantism, are generally less developed, as, for example, 84-chromosomal wheat. Autopolyploids often exhibit some degree of sterility due to disruptions in meiosis during the maturation of germ cells. Sometimes highly polyploid forms generally turn out to be unviable and sterile.

Autopolyploidy is the result of a disruption in the process of cell division (mitosis or meiosis). Mitotic polyploidy results from nondisjunction of daughter chromosomes in prophase. If it occurs during the first division of the zygote, then all cells of the embryo will be polyploid; if at later stages, then somatic mosaics are formed - organisms whose body parts consist of polyploid cells. Mitotic polyploidization of somatic cells can occur at different stages of ontogeny. Meiotic polyploidy is observed when meiosis is lost or replaced by mitosis or some other type of non-reductive division during the formation of germ cells. Its result is the formation of unreduced gametes, the fusion of which leads to the appearance of polyploid offspring. Such gametes are most often formed in apomictic species, and as an exception in sexual species.

Very often, autotetraploids do not interbreed with the diploids from which they are descended. If the crossing between them still succeeds, then as a result, autotriploids arise. Odd polyploids, as a rule, are highly sterile and are not capable of seed reproduction. But for some plants, triploidy appears to be the optimum level of ploidy. Such plants show signs of gigantism compared to diploids. Examples are triploid aspen, triploid sugar beet, some varieties of apple trees. Reproduction of triploid forms is carried out either through apomixis or through vegetative reproduction.

For the artificial production of polyploid cells, a strong poison is used - colchicine, obtained from the autumn colchicum plant (Colchicum automnale). Its action is truly universal: you can get polyploids from any plant.

Allopolyploidy

Allopolyploidy- this is a doubling of the set of chromosomes in distant hybrids. For example, if a hybrid has two different AB genomes, then the polyploid genome will be AABB. Interspecific hybrids often turn out to be sterile, even if the species taken for crossing have the same chromosome numbers. This is explained by the fact that the chromosomes of different species are not homologous, and therefore the processes of conjugation and divergence of chromosomes are disturbed. Violations are even more pronounced when the numbers of chromosomes do not match. If the hybrid spontaneously duplicates the chromosomes in the egg, then an allopolyploid containing two diploid sets of parental species will be obtained. In this case, meiosis proceeds normally, and the plant will be fertile. Similar allopolyploids S.G. Navashin proposed calling them amphidiploids.

It is now known that many naturally occurring polyploid forms are allopolyploidy, for example, 42-chromosome common wheat is an amphidiploid that arose from crossing a tetraploid wheat and a diploid related species of Aegilops (Aegilops L.) followed by doubling the set of chromosomes of a triploid hybrid .

The allopolyploid nature has been established in a number of cultivated plant species, such as tobacco, rape, onion, willow, etc. Thus, allopolyploidy in plants is, along with hybridization, one of the mechanisms of speciation.

Aneuploidy

Aneuploidy denote a change in the number of individual chromosomes in the karyotype. The occurrence of aneuploids is a consequence of improper divergence of chromosomes in the process of cell division. Aneuploids often arise in the offspring of autopolyploids, which, due to incorrect divergence of multivalents, give rise to gametes with abnormal numbers of chromosomes. As a result of their merger, aneuploids arise. If one gamete has a set of chromosomes n+ 1, and the other - n, then from their merger, trisomic- diploid with one extra chromosome in the set. If a gamete with a set of chromosomes n- 1 merges with normal ( n), then it is formed monosomic A diploid with a lack of one chromosome. If two homologous chromosomes are missing in the set, then such an organism is called nullisomic. In plants, both monosomic and trisomic are often viable, although the loss or addition of one chromosome causes certain changes in the phenotype. The effect of aneuploidy depends on the number of chromosomes and the genetic makeup of the extra or missing chromosome. The more chromosomes in a set, the less sensitive plants are to aneuploidy. Trisomics in plants are somewhat less viable than normal individuals, and their fertility is reduced.

Monosomes in cultivated plants, such as wheat, are widely used in genetic analysis to determine the localization of various genes. In wheat, as well as in tobacco and other plants, monosomic series have been created, consisting of lines, in each of which some chromosome of the normal set has been lost. Nullisomics with 40 chromosomes (instead of 42) are also known in wheat. Their viability and fertility are reduced depending on which of the 21st pair of chromosomes is missing.

Aneuploidy in plants is closely related to polyploidy. This is clearly seen in the example of bluegrass. Within the genus Roa, species are known that make up polyploid series with chromosome numbers that are multiples of one basic number ( n= 7): 14, 28, 42, 56. In bluegrass meadow, euploidy is almost lost and replaced by aneuploidy. The number of chromosomes in different biotypes of this species varies from 50 to 100 and is not a multiple of the main number, which is associated with aneuploidy. Aneuploid forms are preserved due to the fact that they reproduce parthenogenetically. According to geneticists, aneuploidy is one of the mechanisms of genome evolution in plants.

In animals and humans, a change in the number of chromosomes has much more serious consequences. An example of monosomy is Drosophila with a deficiency of the 4th chromosome. It is the smallest chromosome in the set, but it contains the nucleolar organizer and therefore forms the nucleolus. Its absence causes a decrease in the size of flies, a decrease in fertility and a change in the series morphological features. However, the flies are viable. The loss of one homologue from other pairs of chromosomes has a lethal effect.

In humans, genomic mutations usually lead to severe hereditary diseases. So, monosomy on the X chromosome leads to Shereshevsky-Turner syndrome, which is characterized by physical, mental and sexual underdevelopment of carriers of this mutation. A trisomy on the X chromosome has a similar effect. The presence of an extra 21st chromosome in the karyotype leads to the development of the well-known Down syndrome. (More details are given in the lecture “