Genetics with the Fundamentals of Breeding - M.P. Myhun - 2008

CHAPTER IV. Variability and Its Genetic Foundations

4.2. Types of Mutations and Their Classification

The types of Mutations and their Classification depend on the criteria chosen as The basis of the new classification system. The most solid genetic foundation is provided by the classification based on The Nature of changes within the genotype. It has been established that the totality of all genes localized in the haploid set of Chromosomes constitutes a genetic system termed The Genome, as proposed by H. Winkler in 1920. Furthermore, genes have also been discovered in the Cell Cytoplasm, known as the plasmone and plastome. In addition to these genetic systems, the chromosomal structures of Cells and their karyotypes are considered material bearers of heredity. The existence of various genetic systems gives rise to different Types of Mutations.

I. Depending on Changes in the genotype, the following types of mutations are distinguished:

1) Gene or point mutations — changes in the Introduction/20.html">DNA Structure within the limits of a gene;

2) chromosomal mutations — disruptions in Chromosome structure;

3) genomic mutations — changes in the number of chromosomes or chromosome sets.

1) Gene (point) mutations. If the material basis of a gene is a DNA fragment, then mutations of this gene are nothing other than damage to METABOLISM/28.html">The Genetic Code, which may manifest as changes in nucleotide sequences (rearrangements); loss, duplication, or insertion of NUCLEOTIDES. Based on the nature of changes in The nucleotide sequence, Gene Mutations are subdivided into:

a) transitions — the substitution of one purine base for another purine, or a pyrimidine for another pyrimidine;

b) transversions — the substitution of a purine base for a pyrimidine or vice versa;

c) insertions — The addition of one or more nucleotides;

d) deletions — the loss of one or more nucleotides;

e) rearrangement of adjacent nucleotides.

The vast majority of gene mutations are recessive and persist within heterozygotes. Accumulating in the population, they find phenotypic expression in the homozygous state and drive evolutionary changes.

2) Chromosomal mutations (aberrations). These are characterized by alterations in the morphological structures of chromosomes caused by breaks and rearrangements of chromosomal material within the cells of an Organism. Structural rearrangements occurring within a single chromosome represented in The Cell by one, two, or more homologs are called intrachromosomal, whereas those involving nonhomologous chromosomes within individual karyotypes are termed interchromosomal. Intrachromosomal rearrangements include: a) deficiencies; b) duplications; c) inversions; d) insertions.

a) Deficiencies — the loss of a specific chromosomal segment; if two breaks occur within the interior of a chromosome and the intervening segment is lost, such a deficiency is termed a deletion;

b) duplication — a situation where one of the chromosome segments is represented by two or a significantly greater number of copies;

c) inversion (rearrangement) — A change in the order of gene arrangement within a chromosome. This involves a chromosome break at two points and the Rotation of the chromosomal fragment by 180 degrees, As a result of which the genes in the corresponding fragment are arranged in reverse order;

d) insertion — fragments of a chromosomal strand are transferred from one site to another within the chromosome while retaining the original gene order within each fragment. Interchromosomal rearrangements include translocations, which are reciprocal exchanges between nonhomologous chromosomes.

3) Genomic mutations — caused by a change in the number of chromosomes in the genomes of organisms, leading to The formation of aneuploids, as well as an increase in the number of genomes in cells, resulting in polyploids, or a reduction of the chromosome number by half, yielding haploids. Changes in the chromosomal composition of cells occur due to disruptions in The Mechanism of karyokinesis, and include:

a) uneven disjunction of chromosomes during Meiosis or mitosis;

b) nuclear division without subsequent Cell Division;

c) an increase in chromosome number without their subsequent Separation;

d) fusion of somatic cells and their nuclei.

Aneuploidy is a change in the chromosome number that is not a multiple of the haploid set. This phenomenon occurs as a result of disruptions in the regular distribution and disjunction of chromosomes during meiosis. Aneuploids that lack one chromosome from a homologous pair are called monosomics (2n—1). Fertilization of an egg lacking one chromosome by a sperm with a normal chromosome number yields monosomics. If two (a pair of) homologous chromosomes are missing, they are called nullisomics (2n-2). An increase in the chromosome set by a single chromosome results in trisomics (2n+1), and by two chromosomes — tetrasomics (2n+2).

Aneuploidy disrupts the gene dosage balance within organism genotypes, as well as the regular reduction of the chromosome number during meiosis, leading to degeneration. However, in species capable of vegetative, apomictic, or parthenogenetic reproduction, individual aneuploids can be inherited across vegetative generations and play a positive role in evolution. According to some data, up to 6% of all human fertilizations result in aneuploid zygotes, the majority of which perish during early embryonic development. Nevertheless, well-known hereditary human diseases are associated with the trisomy of specific chromosomes, such as chromosome 21 (Down syndrome), chromosome 18 (Edwards syndrome), and chromosome 13 (Patau syndrome), as well as Klinefelter and Turner syndromes, which are linked to sex chromosome nondisjunction. Aneuploids are widely used in plant genetic research to determine the chromosomal localization of specific genes.

Polyploidy is a change in the number of chromosome sets. If complete nondisjunction of chromosomes occurs during meiosis, Gametes with an unreduced chromosome number are formed. Such gametes contain a doubled number of chromosomes, and upon fertilization, they give rise to organisms with three or four chromosome sets. For example, if two diploid gametes fuse, the zygote receives 4 genomes (2n+2n) and becomes tetraploid; if (2n+1n), it is a triploid; (3n+2n) yields a pentaploid, (3n+3n) a hexaploid, and so forth. Polyploids are rarely

found in animals (such as earthworms and bugs, which can reproduce both sexually and parthenogenetically), but they are widespread in the plant kingdom. About 50% of plant species are polyploids. Humans predominantly utilize polyploid plants for consumption: wheat, potatoes, oats, sugarcane, strawberries, plums, cherries, apples, pears, lemons, and many others. The proportion of polyploid species increases from south to north, which is explained by the significantly greater genetic Variability and adaptability of polyploids compared to diploids. In the Arctic, 72% of studied plants are polyploids; in the high-altitude Pamirs, 86%; and in the Altai, 65%. The general pattern of hereditary variability caused by polyploidy involves an increase in cell size, certain plant Organs, and vegetative mass. The negative effect of polyploidization manifests as a reduction in seed productivity and an extended vegetative period. The reproductive patterns of polyploids indicate that there are several types of them:

a) autopolyploids;

b) allopolyploids.

Autopolyploids are organisms produced by multiplying the haploid chromosome set of the same species.

Under natural conditions, autopolyploids arise among plants with any mode of reproduction, but they are best preserved in self-pollinating plants and under asexual reproduction. In autopolyploids, sexual reproduction is generally significantly hindered. This is due to the fact that in the karyotypes of polyploid cells, each chromosome is represented not by two, but by several homologs. Gametes comprising multiple homologous chromosomes have low viability, and their combinations lead to nonviable zygotes. Polyploid plant forms can be obtained artificially using the alkaloid colchicine, although they exhibit reduced fertility. Different plant species have varying optimal ploidy levels, and humans exploit these phenomena for both scientific and practical purposes. Japanese geneticist H. Kihara obtained triploid seedless watermelons by crossing tetraploid and diploid forms!

Allopolyploids are organisms that combine chromosome sets from different species and genera within their cells. Hybrids resulting from the crossing of different species and genera are called distant hybrids, or amphidiploids. The chromosome sets of allopolyploids differ not only in chromosome number but also in their genetic composition, and their offspring are sterile (such as the mule).

This is explained by the fact that each chromosome in the hybrid cells lacks its specific homolog. Chromosomes remain as univalents, leading to irregular chromosome distribution during meiotic division, which prevents the formation of genetically balanced gametes. However, if the chromosome number in the cells of an interspecific hybrid is doubled, each of these doubled chromosomes forms a sister pair, rendering the hybrid fertile.

A classic example of an allopolyploid is the cabbage-radish hybrid produced by G. D. Karpechenko (1924). He crossed the radish (Raphanus sativus), 2n = 18, with cabbage (Brassica oleracea), 2n = 18. Their gametes each contain 9 chromosomes, but the hybrids are completely sterile because the cabbage chromosome set lacks homologs for the radish chromosomes, and vice versa. In this case, each chromosome behaves entirely independently during meiosis—as a univalent—and normal gamete development does not occur, leaving the hybrids sterile. Among A large number of sterile hybrids, G. D. Karpechenko discovered a few normally fertile ones. Cytological studies showed that in such hybrids, the zygote consists of two complete sets from both radish and cabbage (9R + 9C + 9R + 9C = 36 chromosomes). Each radish chromosome 9R conjugates with its counterpart 9R from the radish, forming viable homologous pairs 2R (or 18R), and the cabbage chromosomes behave similarly, forming 18C. The resulting 36-chromosome hybrid is not only fertile but also constant, meaning it does not segregate during reproduction because the radish and cabbage chromosomes do not recombine with one another. Thus, an opportunity arose to utilize distant Hybridization to synthesize new forms that do not exist in nature: triticale (rye-wheat), radish-cabbage, and wheat-wheatgrass hybrids. In the animal kingdom, interspecific hybridization followed by the formation of stable and fertile allopolyploids is uncommon, but there are instances of creating new animal breeds and hybrids through interspecific crosses: domestic cattle and zebras, cattle and yaks, carp and goldfish hybrids, as well as fine-wooled sheep and wild bighorn sheep. Nevertheless, fertile allopolyploids in animals remain quite rare. At the same time, this approach allows not only for the synthesis of novel plant species but also for the artificial restoration of already existing ones.

The experimental restoration of existing species based on genome recombination of known forms is termed species resynthesization. The Water/144.html">Origin of the cultivated plum remained a mystery for a long time, as no wild relatives existed in nature, but Crane and Lawrence demonstrated that it arose through the natural hybridization of the blackthorn and cherry plum, followed by chromosome doubling. Both the resynthesis and Synthesis of the plum have been artificially recreated.

The Current state of distant hybridization is grounded in the achievements and capabilities of Genetic Engineering, with the creation of transgenic organisms becoming an increasingly widespread phenomenon.

Haploidy. A haploid is an organism whose somatic cells contain a haploid set of non-homologous chromosomes.

Haploids develop from a single cell, bypassing the fertilization stage, originating from an egg cell, synergids, antipodals, or a pollen grain. Haploidy can be natural or artificially induced. Natural haploidy occurs in The life cycle of most eukaryotes, as well as in male insects, bees, and ants. One of the Characteristic Features of haploids is the reduced size of all cells and organs; both dominant and recessive traits immediately manifest in the phenotype (being homozygous for all genes). There are also polyhaploids, which can be derived from allopolyploids. Haploidy is of significant interest to geneticists and breeders working with higher plants. This is because harmful and beneficial recessive traits are easily detected in haploids. A haploid plant free of deleterious mutations can be converted into a diploid using colchicine. It is precisely through this pathway—The conversion of haploids into diploids—that new forms of tomatoes, tobacco, and cotton have been obtained.

II. According to their localization in Eukaryotic cells, mutations are classified into:

a) nuclear — mutations of the nuclear genetic apparatus (discussed above);

b) cytoplasmic — plastome and plasmon mutations.

Plasmon Mutations arise as a result of mutations in genes localized in the cell cytoplasm, excluding Plastids. An example is Mitochondrial DNA, which contains a complex of regulatory and structural genes and undergoes autonomous synthesis of certain types of Proteins independently of nuclear genes; it is here that mutations occur.

Plastome mutations are mutations of genes localized in plastids. Mutations in plastome genes can alter photosynthetic processes, producing progeny with variegated leaf traits.

III. According to phenotypic manifestation and effect on the organism:

a) morphological; b) physiological; c) biochemical.

These can alter the expression of any external trait, affect the Functions of individual organs (growth, development), cause various changes in the Chemical composition of Cells and Tissues, and so forth.

IV. According to their effect on viability:

a) beneficial — promote the preservation and spread of the species;

b) neutral — do not affect viability;

c) sublethal — reduce viability;

d) lethal — result in death.

V. According to their relation to the norm (the so-called wild type):

a) forward mutations, in which wild-type genes transform into allelic forms;

b) reverse mutations, which restore the wild phenotype.

Example. The dominant red eye color gene in Drosophila mutates to white, and then back to redSelection/selection.files/image028.jpg" width="58"/>

VI. According to their manifestation in the heterozygote:

a) dominant mutations, b) recessive mutations.

Recessive mutations occur more frequently than dominant ones; the Mutational Process generally proceeds from dominance to recessiveness. Dominant mutations manifest immediately in F, already in the heterozygous state. Recessive mutations appear only when the mutated gene is in the homozygous state.

VII. Depending on the type of cells in which mutations arise:

a) germline (generative) — arise in sex cells and their precursors, and are transmitted to subsequent generations;

b) somatic — arise in somatic cells and spread through their mitotic division.

VIII. Depending on the mode of origin:

a) spontaneous — mutations that arise autogenetically.

Their cause may be constant Background ionizing and ultraviolet radiation, radioactive terrestrial isotopes, or chemical compounds (introduced artificially against parasites or synthesized within the organism as its own metabolites). Natural mutations are the result of potential errors in the functioning of the cell's physiological systems. The efficiency of molecular-genetic processes (Replication, repair, recombination, Transcription, Translation) substantially depends on the individual CHARACTERISTICS OF THE genotype and its environmental conditions, which explains the varying frequency of spontaneous mutations under specific conditions. Spontaneous mutations arise with a frequency of (1–10^-5 — 10^-9) per gene. The mutation rate depends on the genotype, the physiological state of the cell or organism, age, and developmental stage. In humans, many genes mutate with a frequency of 1:200,000 gametes, and the average mutation frequency per locus per generation is 4·10^-6.

When considered relative to each individual gene, these figures are insignificant; however, given that the haploid chromosome set contains several thousand genes and each of them mutates with a frequency of at least 1:1,000,000, the cumulative total becomes quite substantial.

b) induced mutations — arise in response to the action of various environmental factors (Physical and Chemical), and their frequency is significantly higher than that of spontaneous mutations. Induced mutagenesis was first applied in 1925 by G. A. Nadson and G. S. Filippov. In 1927, H. Muller demonstrated the mutagenic effect of ionizing radiation, and subsequently the mutagenic effect of ultraviolet irradiation was also elucidated and studied using PLANT AND ANIMAL subjects.

The Effect of chemical compounds on mutation induction was first demonstrated by V. Sakharov (1932) and M. Lobashev (1939). S. Gershenzon discovered the potent mutagenic effect of exogenous DNA. Intensive study of chemical mutagenesis began in 1946 when I. Rapoport discovered supermutagens — chemical compounds (such as ethyleneimine) that induce mutations at a frequency of up to 100%.

Later, substances known as antimutagens were discovered, which attenuate the effects of chemical and physical Mutagenic Factors.

Induced mutagenesis is one of the PATHWAYS FOR OBTAINING new initial forms for breeding, and it is also used in genetic analysis for labeling genetic material.

Today, mutagenesis and ecology are of great importance to human life. An increased concentration of mutagens in the environment contributes to an escalating genetic load across all populations, including humans.