There is a change in the structure of chromosomes. Types of mutations, causes, examples

Changes in the structural organization of chromosomes. Chromosomal mutations

Despite the evolutionary proven mechanism that allows maintaining the constant physicochemical and morphological organization of chromosomes in a number of cell generations, this organization can change under the influence of various influences. Changes in the structure of the chromosome, as a rule, are based on the initial violation of its integrity - breaks, which are accompanied by various rearrangements called chromosomal mutations or aberrations.

Chromosome breaks occur regularly in the course of crossing over, when they are accompanied by the exchange of corresponding regions between homologues (see Section 3.6.2.3). Violation of crossing over, in which chromosomes exchange unequal genetic material, leads to the emergence of new linkage groups, where individual sections fall out - division - or doubling - duplications(Fig. 3.57). With such rearrangements, the number of genes in the linkage group changes.

Chromosome breaks can also occur under the influence of various mutagenic factors, mainly physical (ionizing and other types of radiation), some chemical compounds, and viruses.

Rice. 3.57. Types of chromosomal rearrangements

Violation of the integrity of the chromosome may be accompanied by a rotation of its section, located between two breaks, by 180 ° - inversion. Depending on whether this area includes the centromere region or not, there are pericentric and paracentric inversions(Fig. 3.57).

A fragment of a chromosome separated from it during a break can be lost by a cell during the next mitosis if it does not have a centromere. More often, such a fragment is attached to one of the chromosomes - translocation. Often, two damaged non-homologous chromosomes mutually exchange detached sections - reciprocal translocation(Fig. 3.57). It is possible to attach a fragment to its own chromosome, but in a new place - transposition(Fig. 3.57). Thus, various types of inversions and translocations are characterized by a change in the localization of genes.

Chromosomal rearrangements, as a rule, are manifested in a change in the morphology of chromosomes, which can be observed under a light microscope. Metacentric chromosomes turn into submetacentric and acrocentric and vice versa (Fig. 3.58), ring and polycentric chromosomes appear (Fig. 3.59). A special category of chromosomal mutations are aberrations associated with centric fusion or separation of chromosomes, when two non-homologous structures are combined into one - robertsonian translocation, or one chromosome forms two independent chromosomes (Fig. 3.60). With such mutations, not only chromosomes with a new morphology appear, but their number in the karyotype also changes.

Rice. 3.58. Changing the shape of chromosomes

as a result of pericentric inversions

Rice. 3.59. Ring formation ( I) and polycentric ( II) chromosomes

Rice. 3.60. Chromosomal rearrangements associated with centric fusion

or separation of chromosomes cause changes in the number of chromosomes

in the karyotype

Rice. 3.61. A loop formed during the conjugation of homologous chromosomes that carry unequal hereditary material in the corresponding regions as a result of chromosomal rearrangement

The described structural changes in chromosomes, as a rule, are accompanied by a change in the genetic program received by the cells of a new generation after the division of the mother cell, since the quantitative ratio of genes changes (during divisions and duplications), the nature of their functioning changes due to a change in the relative position in the chromosome (during inversion and transposition) or with a transition to another linkage group (during translocation). Most often, such structural changes in chromosomes adversely affect the viability of individual somatic cells of the body, but chromosomal rearrangements occurring in the precursors of gametes have especially serious consequences.

Changes in the structure of chromosomes in the precursors of gametes are accompanied by a violation of the process of conjugation of homologues in meiosis and their subsequent divergence. So, division or duplication of a section of one of the chromosomes is accompanied by the formation of a loop by a homologue with excess material during conjugation (Fig. 3.61). Reciprocal translocation between two non-homologous chromosomes leads to the formation during conjugation not of a bivalent, but of a quadrivalent, in which the chromosomes form a cross figure due to the attraction of homologous regions located on different chromosomes (Fig. 3.62). Participation in reciprocal translocations of a larger number of chromosomes with the formation of a polyvalent is accompanied by the formation of even more complex structures during conjugation (Fig. 3.63).

In the case of inversion, the bivalent that occurs in prophase I of meiosis forms a loop that includes a mutually inverted section (Fig. 3.64).

Conjugation and subsequent divergence of structures formed by altered chromosomes leads to the appearance of new chromosomal rearrangements. As a result, gametes, receiving defective hereditary material, are not able to ensure the formation of a normal organism of a new generation. The reason for this is a violation of the ratio of genes that make up individual chromosomes, and their relative position.

However, despite the usually unfavorable consequences of chromosomal mutations, sometimes they turn out to be compatible with the life of the cell and the organism and provide the possibility of the evolution of the chromosome structure that underlies biological evolution. So, divisions small in size can be preserved in a heterozygous state for a number of generations. Duplications are less harmful than division, although a large amount of material in an increased dose (more than 10% of the genome) leads to the death of the organism.

Rice. 3.64. Chromosome conjugation during inversions:

I- paracentric inversion in one of the homologues, II- peridentric inversion in one of the homologues

Often, Robertsonian translocations are viable, often not associated with a change in the amount of hereditary material. This can explain the variation in the number of chromosomes in the cells of organisms of closely related species. For example, in different species of Drosophila, the number of chromosomes in the haploid set ranges from 3 to 6, which is explained by the processes of chromosome fusion and separation. Perhaps the essential moment in the appearance of the species Homo sapiens there were structural changes in chromosomes in his ape-like ancestor. It has been established that two arms of the large second human chromosome correspond to two different chromosomes of modern great apes (chimpanzees 12 and 13, gorillas and orangutans 13 and 14). Probably, this human chromosome was formed as a result of a centric fusion, similar to the Robertsonian translocation, of two simian chromosomes.

Translocations, transpositions and inversions lead to a significant variation in the morphology of chromosomes, which underlies their evolution. Analysis of human chromosomes has shown that its 4th, 5th, 12th, and 17th chromosomes differ from the corresponding chimpanzee chromosomes by pericentric inversions.

Thus, changes in the chromosomal organization, which most often have an adverse effect on the viability of the cell and organism, with a certain probability can be promising, be inherited in a number of generations of cells and organisms and create prerequisites for the evolution of the chromosomal organization of the hereditary material.

Mutational variability occurs in the event of the appearance of mutations - persistent changes in the genotype (i.e. DNA molecules), which can affect entire chromosomes, their parts or individual genes.

Mutations can be beneficial, harmful, or neutral. According to the modern classification, mutations are usually divided into the following groups.

1. Genomic mutations associated with a change in the number of chromosomes. Of particular interest is POLYPLOIDY - a multiple increase in the number of chromosomes, i.e. instead of a 2n chromosome set, a set of 3n,4n,5n or more appears. The occurrence of polyploidy is associated with a violation of the mechanism of cell division. In particular, nondisjunction of homologous chromosomes during the first division of meiosis leads to the appearance of gametes with a 2n set of chromosomes.

Polyploidy is widespread in plants and much less frequently in animals (roundworm, silkworm, some amphibians). Polyploid organisms, as a rule, are characterized by larger sizes, increased synthesis of organic substances, which makes them especially valuable for breeding work.

A change in the number of chromosomes associated with the addition or loss of individual chromosomes is called aneuploidy. An aneuploidy mutation can be written as 2n-1, 2n+1, 2n-2, etc. Aneuploidy is characteristic of all animals and plants. In humans, a number of diseases are associated with aneuploidy. For example, Down's disease is associated with the presence of an extra chromosome in the 21st pair.

2. Chromosomal mutations - this is a rearrangement of chromosomes, a change in their structure. Separate sections of chromosomes can be lost, doubled, change their position.

Schematically, this can be shown as follows:

ABCDE normal gene order

ABBCDE duplication of a segment of a chromosome

ABDE loss of one section

ABEDC 180 degree turn

ABCFG region exchange with non-homologous chromosome

Like genomic mutations, chromosomal mutations play a huge role in evolutionary processes.

3. Gene mutations associated with a change in the composition or sequence of DNA nucleotides within a gene. Gene mutations are the most important of all mutation categories.

Protein synthesis is based on the correspondence between the arrangement of nucleotides in a gene and the order of amino acids in a protein molecule. The occurrence of gene mutations (changes in the composition and sequence of nucleotides) changes the composition of the corresponding enzyme proteins and, as a result, leads to phenotypic changes. Mutations can affect all features of the morphology, physiology and biochemistry of organisms. Many human hereditary diseases are also caused by gene mutations.

Mutations in natural conditions are rare - one mutation of a particular gene per 1000-100000 cells. But the mutation process goes on constantly, there is a constant accumulation of mutations in genotypes. And if we take into account that the number of genes in the body is large, then we can say that in the genotypes of all living organisms there is a significant number of gene mutations.

Mutations are the largest biological factor that determines the enormous hereditary variability of organisms, which provides material for evolution.

The causes of mutations can be natural disturbances in cell metabolism (spontaneous mutations) and the action of various environmental factors (induced mutations). Factors that cause mutations are called mutagens. Mutagens can be physical factors - radiation, temperature .... Biological mutagens include viruses capable of transferring genes between organisms of not only close, but distant systematic groups.

Human economic activity has brought a huge amount of mutagens into the biosphere.

Most mutations are unfavorable for the life of an individual, but sometimes mutations occur that may be of interest to breeding scientists. Currently, methods of site-directed mutagenesis have been developed.

1. According to the nature of the change in the phenotype, mutations can be biochemical, physiological, anatomical and morphological.

2. According to the degree of adaptability, mutations are divided into beneficial and harmful. Harmful - can be lethal and cause the death of the organism even in embryonic development.

More often, mutations are harmful, since traits are normally the result of selection and adapt the organism to its environment. Mutation always changes adaptation. The degree of its usefulness or uselessness is determined by time. If a mutation enables the organism to better adapt, gives a new chance to survive, then it is "picked up" by selection and fixed in the population.

3. Mutations are direct and reverse. The latter are much less common. Usually, a direct mutation is associated with a defect in the function of the gene. The probability of a secondary mutation in the opposite direction at the same point is very small, other genes mutate more often.

Mutations are more often recessive, since dominant ones appear immediately and are easily "rejected" by selection.

4. According to the nature of the change in the genotype, mutations are divided into gene, chromosomal and genomic.

Gene, or point, mutations - a change in a nucleotide in one gene in a DNA molecule, leading to the formation of an abnormal gene, and, consequently, an abnormal protein structure and the development of an abnormal trait. A gene mutation is the result of a "mistake" in DNA replication.

The result of a gene mutation in humans are diseases such as sickle cell anemia, phenylketonuria, color blindness, hemophilia. As a result of a gene mutation, new alleles of genes arise, which is important for the evolutionary process.

Chromosomal mutations - changes in the structure of chromosomes, chromosomal rearrangements. The main types of chromosomal mutations can be distinguished:

a) deletion - loss of a chromosome segment;

b) translocation - the transfer of part of the chromosomes to another non-homologous chromosome, as a result - a change in the linkage group of genes;

c) inversion - rotation of a chromosome segment by 180 °;

d) duplication - doubling of genes in a certain region of the chromosome.

Chromosomal mutations lead to a change in the functioning of genes and are important in the evolution of a species.

Genomic mutations - changes in the number of chromosomes in a cell, the appearance of an extra or loss of a chromosome as a result of a violation in meiosis. A multiple increase in the number of chromosomes is called polyploidy (3n, 4/r, etc.). This type of mutation is common in plants. Many cultivated plants are polyploid in relation to their wild ancestors. An increase in chromosomes by one or two in animals leads to anomalies in the development or death of the organism. Example: Down syndrome in humans - trisomy for the 21st pair, in total there are 47 chromosomes in a cell. Mutations can be obtained artificially with the help of radiation, X-rays, ultraviolet, chemical agents, and thermal exposure.

The law of homological series N.I. Vavilov. Russian biologist N.I. Vavilov established the nature of the occurrence of mutations in closely related species: "Genera and species that are genetically close are characterized by similar series of hereditary variability with such regularity that, knowing the number of forms within one species, one can foresee the presence of parallel forms in other species and genera."

The discovery of the law facilitated the search for hereditary deviations. Knowing the variability and mutations in one species, one can foresee the possibility of their appearance in related species, which is important in breeding.

Changes in the structure of chromosomes include deletions, translocations, inversions, duplications, insertions.

Deletions these are changes in the structure of chromosomes in the form of the absence of its site. In this case, the development of a simple deletion or a deletion with a duplication of a section of another chromosome is possible.

In the latter case, the reason for the change in the structure of the chromosome, as a rule, is crossing over in meiosis in the translocation carrier, which leads to the appearance of an unbalanced reciprocal chromosomal translocation. Deletions can be localized at the end or in the interior of the chromosome and are usually associated with mental retardation and malformations. Small deletions in the telomere region are relatively often found in nonspecific mental retardation in combination with developmental microanomalies. Deletions can be detected by routine chromosome acquisition, but microdeletions can only be identified by microscopic examination in prophase. In cases of submicroscopic deletions, the missing site can only be detected using molecular probes or DNA analysis.

microdeletions are defined as small chromosomal deletions, distinguishable only in high quality preparations in metaphase. These deletions are more common in multiple genes, and the patient's diagnosis is suggested based on unusual phenotypic manifestations that appear to be associated with a single mutation. Williams, Langer-Gidion, Prader-Willi, Rubinstein-Taybi, Smith-Magenis, Miller-Dicker, Alagille, DiGeorge syndromes are caused by microdeletions. Submicroscopic deletions are invisible on microscopic examination and are only detected using specific DNA testing methods. Deletions are recognized by the absence of staining or fluorescence.

Translocations represent a change in the structure of chromosomes in the form of a transfer of chromosomal material from one to another. There are Robertsonian and reciprocal translocations. Frequency 1:500 newborns. Translocations can be inherited from parents or occur de novo in the absence of pathology in other family members.

Robertsonian translocations involve two acrocentric chromosomes that coalesce close to the centromere region with subsequent loss of non-functional and highly truncated short arms. After translocation, the chromosome consists of long arms, which are made up of two spliced chromosomes. Thus, the karyotype has only 45 chromosomes. The negative consequences of losing short arms are unknown. Although carriers of the Robertsonian translocation generally have a normal phenotype, they are at increased risk of miscarriage and abnormal offspring.

Reciprocal translocations result from the breakdown of non-homologous chromosomes in combination with the reciprocal exchange of lost segments. Carriers of a reciprocal translocation usually have a normal phenotype, but they also have an increased risk of having offspring with chromosomal abnormalities and miscarriages due to abnormal chromosome segregation in germ cells.

Inversions- changes in the structure of chromosomes that occur when it breaks at two points. The broken section is turned over and joined to the rupture site. Inversions occur in 1:100 newborns and may be peri- or paracentric. With pericentric inversions, breaks occur on two opposite arms, and the part of the chromosome containing the centromere rotates. Such inversions are usually detected in connection with a change in the position of the centromere. In contrast, with paracentric inversions, only the area located on one shoulder is involved. Carriers of inversions usually have a normal phenotype, but they may have an increased risk of spontaneous miscarriage and the birth of offspring with chromosomal abnormalities.

Ring chromosomes are rare, but their formation is possible from any human chromosome. Ring formation is preceded by deletions at each end. The ends are then “glued together” to form a ring. Phenotypic manifestations with ring chromosomes vary from mental retardation and multiple developmental anomalies to normal or minimally pronounced changes, depending on the amount of "lost" chromosomal material. If the ring replaces the normal chromosome, this leads to the development of partial monosomy. The phenotypic manifestations in these cases are often similar to those seen with deletions. If a ring is added to normal chromosomes, the phenotypic manifestations of partial trisomy occur.

duplication called the excess amount of genetic material belonging to one chromosome. Duplications may result from abnormal segregation in carriers of translocations or inversions.

Insertions(inserts) are changes in the structure of chromosomes that occur when there is a break at two points, while the broken section is built into the break zone on the other part of the chromosome. Three discontinuity points are required to form an insertion. One or two chromosomes may be involved in this process.

Telomeric, subtelomeric deletions. Since chromosomes are closely intertwined during meiosis, small deletions and duplications near the ends are relatively common. Subtelomeric chromosomal rearrangements are more often (5-10%) found in children with moderate or severe mental retardation of unclear etiology without pronounced dysmorphic signs.

Submicroscopic subtelomeric deletions (less than 2-3 Mb) are the second most common cause of mental retardation after trisomy 21. Clinical manifestations of this chromosome structure change in some of these children include prenatal growth retardation (about 40% of cases) and a family history of mental retardation ( 50% of cases). Other symptoms occur in about 30% of patients and include microcephaly, hypertelorism, nose, ear or hand defects, cryptorchidism, and short stature. After ruling out other causes of developmental delay, the FISH method using multiple telomeric probes in metaphase is recommended.

The article was prepared and edited by: surgeon

Chromosome breaks can also occur under the influence of various mutagenic factors, mainly physical (ionizing and other types of radiation), some chemical compounds, and viruses.

Rice. 3.57. Types of chromosomal rearrangements

Rice. 3.58. Changing the shape of chromosomes

as a result of pericentric inversions

Rice. 3.59. Ring formation ( I) and polycentric ( II) chromosomes

Rice. 3.60. Chromosomal rearrangements associated with centric fusion

or separation of chromosomes cause changes in the number of chromosomes

in the karyotype

Rice. 3.61. A loop formed during the conjugation of homologous chromosomes that carry unequal hereditary material in the corresponding regions as a result of chromosomal rearrangement

In the case of inversion, the bivalent that occurs in prophase I of meiosis forms a loop that includes a mutually inverted section (Fig. 3.64).

Rice. 3.64. Chromosome conjugation during inversions:

I- paracentric inversion in one of the homologues, II- peridentric inversion in one of the homologues

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A change in the number of chromosomes in a cell means a change in the genome. (Therefore, such changes are often called genomic mutations.) Various cytogenetic phenomena associated with changes in the number of chromosomes are known.

Autopolyploidy

Autopolyploidy is the repeated repetition of the same genome, or basic number of chromosomes ( X).

This type of polyploidy is characteristic of lower eukaryotes and angiosperms. In multicellular animals, autopolyploidy is extremely rare: in earthworms, some insects, some fish and amphibians. Autopolyploids in humans and other higher vertebrates die in the early stages of intrauterine development.

In most eukaryotic organisms, the main number of chromosomes ( x) matches the haploid set of chromosomes ( n); while the haploid number of chromosomes is the number of chromosomes in the cells formed in the chord of meiosis. Then in diploid (2 n) contains two genomes x, and 2 n=2x. However, in many lower eukaryotes, many spore and angiosperms, diploid cells contain not 2 genomes, but some other number. The number of genomes in diploid cells is called the genomic number (Ω). The sequence of genomic numbers is called polyploid near.

For example, in cereals x = 7 the following polyploid series are known (the + sign indicates the presence of a polyploid of a certain level)

Distinguish between balanced and unbalanced autopolyploids. Balanced polyploids are called polyploids with an even number of chromosome sets, and unbalanced - polyploids with an odd number of chromosome sets, for example:

unbalanced polyploids		balanced polyploids
haploids	1 x	diploids	2 x
triploids	3 x	tetraploids	4 x
pentaploids	5 x	hexaploids	6 x
hectaploids	7 x	octoploids	8 x
enneaploids	9 x	decaploids	10 x

Autopolyploidy is often accompanied by an increase in the size of cells, pollen grains and the overall size of organisms, an increased content of sugars and vitamins. For example, the triploid aspen ( 3X = 57) reaches gigantic dimensions, is durable, its wood is resistant to decay. Among cultivated plants, both triploids (a number of varieties of strawberries, apple trees, watermelons, bananas, tea, sugar beets) and tetraploids (a number of varieties of rye, clover, and grapes) are widespread. Under natural conditions, autopolyploid plants are usually found in extreme conditions (in high latitudes, in high mountains); moreover, here they can displace normal diploid forms.

The positive effects of polyploidy are associated with an increase in the number of copies of the same gene in cells, and, accordingly, in an increase in the dose (concentration) of enzymes. However, in some cases, polyploidy leads to inhibition of physiological processes, especially at very high levels of ploidy. For example, 84 chromosome wheat is less productive than 42 chromosome wheat.

However, autopolyploids (especially unbalanced ones) are characterized by reduced fertility or complete infertility, which is associated with impaired meiosis. Therefore, many of them are only capable of vegetative reproduction.

Allopolyploidy

Allopolyploidy is the repeated repetition of two or more different haploid chromosome sets, which are denoted by different symbols. Polyploids obtained as a result of distant hybridization, that is, from crossing organisms belonging to different species, and containing two or more sets of different chromosomes, are called allopolyploids.

Allopolyploids are widely distributed among cultivated plants. However, if somatic cells contain one genome from different species (for example, one genome BUT and one - AT ), then such an allopolyploid is sterile. The infertility of simple interspecific hybrids is due to the fact that each chromosome is represented by one homologue, and the formation of bivalents in meiosis is impossible. Thus, with distant hybridization, a meiotic filter arises that prevents the transmission of hereditary inclinations to subsequent generations sexually.

Therefore, in fertile polyploids, each genome must be doubled. For example, in different wheat species, the haploid number of chromosomes ( n) is equal to 7. Wild wheat (einkorn) contains 14 chromosomes in somatic cells of only one doubled genome BUT and has the genomic formula 2 n = 14 (14BUT ). Many allotetraploid durum wheats contain 28 chromosomes of duplicated genomes in somatic cells. BUT and AT ; their genomic formula 2 n = 28 (14BUT + 14AT ). Soft allohexaploid wheats contain 42 chromosomes of doubled genomes in somatic cells BUT , AT , and D ; their genomic formula 2 n = 42 (14 A+ 14B + 14D ).

Fertile allopolyploids can be obtained artificially. For example, a radish-cabbage hybrid, synthesized by Georgy Dmitrievich Karpechenko, was obtained by crossing radish and cabbage. The radish genome is symbolized R (2n = 18 R , n = 9 R ), and the cabbage genome as a symbol B (2n = 18 B , n = 9 B ). Initially, the resulting hybrid had the genomic formula 9 R + 9 B . This organism (amphiploid) was sterile, since 18 single chromosomes (univalents) and not a single bivalent were formed during meiosis. However, in this hybrid, some gametes turned out to be unreduced. When such gametes were fused, a fertile amphidiploid was obtained: ( 9 R + 9 B ) + (9 R + 9 B ) → 18 R + 18 B . In this organism, each chromosome was represented by a pair of homologues, which ensured the normal formation of bivalents and the normal divergence of chromosomes in meiosis: 18 R + 18 B → (9 R + 9 B ) and ( 9 R + 9 B ).

Currently, work is underway to create artificial amphidiploids in plants (eg, wheat-rye hybrids (triticale), wheat-couch hybrids) and animals (eg, hybrid silkworms).

The silkworm is an object of intensive selection work. It should be noted that in this species (as in most butterflies), females have a heterogametic sex ( XY), while males are homogametic ( XX). For the rapid reproduction of new silkworm breeds, induced parthenogenesis is used - unfertilized eggs are removed from females even before meiosis and heated to 46 ° C. Only females develop from such diploid eggs. In addition, androgenesis is known in the silkworm - if the egg is heated to 46 ° C, the nucleus is killed by X-rays, and then inseminated, then two male nuclei can penetrate the egg. These nuclei fuse together to form a diploid zygote ( XX), from which the male develops.

The silkworm is known to be autopolyploidy. In addition, Boris Lvovich Astaurov crossed the silkworm with the wild handicap of the tangerine silkworm, and as a result, fertile allopolyploids (more precisely, allotetraploids) were obtained.

In the silkworm, the yield of silk from male cocoons is 20-30% higher than from female cocoons. V.A. Strunnikov, using induced mutagenesis, brought out a breed in which males in X- chromosomes carry different lethal mutations (system of balanced lethals) - their genotype l1+/+l2. When such males are crossed with normal females ( ++/ Y) only future males hatch from eggs (their genotype l1+/++ or l2/++), and females die at the embryonic stage of development, because their genotype or l1+/Y, or + l2/Y. To breed males with lethal mutations, special females are used (their genotype + l2/++ Y). Then, when such females and males with two lethal alleles in their offspring are crossed, half of the males die, and half carry two lethal alleles.

There are breeds of silkworms in which Y-chromosome has an allele for dark egg color. Then dark eggs ( XY, from which females should hatch), are discarded, and only light ones are left ( XX), which later give male cocoons.

Aneuploidy

Aneuploidy (heteropolyploidy) is a change in the number of chromosomes in cells that is not a multiple of the main chromosome number. There are several types of aneuploidy. At monosomy one of the chromosomes of the diploid set is lost ( 2 n - 1 ). At polysomy one or more chromosomes are added to the karyotype. A special case of polysomy is trisomy (2 n + 1 ), when instead of two homologues there are three of them. At nullisomy Both homologues of any pair of chromosomes are missing ( 2 n - 2 ).

In humans, aneuploidy leads to the development of severe hereditary diseases. Some of them are associated with a change in the number of sex chromosomes (see Chapter 17). However, there are other diseases:

Trisomy on the 21st chromosome (karyotype 47, + 21 ); Down syndrome; the frequency among newborns is 1:700. Slowed physical and mental development, wide distance between the nostrils, wide bridge of the nose, development of the fold of the eyelid (epicant), half-open mouth. In half of the cases, there are violations in the structure of the heart and blood vessels. Immunity is usually lowered. The average life expectancy is 9-15 years.

Trisomy on the 13th chromosome (karyotype 47, + 13 ); Patau syndrome. The frequency among newborns is 1:5.000.

Trisomy on the 18th chromosome (karyotype 47, + 18 ); Edwards syndrome. The frequency among newborns is 1:10,000.

haploidy

Reducing the number of chromosomes in somatic cells to the main number is called haploidy. There are organisms haplobionts, for which haploidy is a normal state (many lower eukaryotes, gametophytes of higher plants, male Hymenoptera insects). Haploidy as an anomalous phenomenon occurs among sporophytes of higher plants: in tomato, tobacco, flax, Datura, and some cereals. Haploid plants are characterized by reduced viability; they are practically sterile.

Pseudopolyploidy(false polyploidy)

In some cases, a change in the number of chromosomes can occur without a change in the amount of genetic material. Figuratively speaking, the number of volumes changes, but the number of phrases does not change. Such a phenomenon is called pseudopolyploidy. There are two main forms of pseudopolyploidy:

1. Agmatopolyploidy. It is observed if large chromosomes break up into many small ones. Found in some plants and insects. In some organisms (for example, in roundworms), fragmentation of chromosomes occurs in somatic cells, but the original large chromosomes are preserved in germ cells.

2. Fusion of chromosomes. It is observed if small chromosomes are combined into large ones. Found in rodents.

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Changes in the structural organization of chromosomes. Chromosomal mutations

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