The law of homologous series of hereditary variability. Abstracts for state exams for biology students
Homological series in hereditary variability law, open Russian geneticist N.I. Vavilov in 1920 established a pattern establishing parallelism (similarity) in hereditary (genotypic) variability in related organisms. In Vavilov's formulation, the law reads: "Species and genera that are genetically close to each other are characterized by identical series of hereditary variability with such regularity that, knowing the series of forms for one species, one can foresee the finding of identical forms in other species and genera." At the same time, the closer the relationship between species, the more complete the similarity (homology) in the series of their variability. The law summarizes a huge amount of material on the variability of plants (cereals and other families), but it turned out to be true for the variability of animals and microorganisms.
The phenomenon of parallel variability in closely related genera and species is explained by their common origin and, consequently, by the presence of a large part of the same genes, obtained from a common ancestor and not changed in the process. When mutated, these genes give similar traits. Parallelism in genotypic variability in related species is manifested by parallelism in phenotypic variability, i.e., similar characters (phenotypes).
Vavilov's law is the theoretical basis for choosing directions and methods for obtaining economically valuable features and properties of cultivated plants and pets.
Mutations that occur naturally without affecting the body of various factors are called spontaneous. The main feature of the manifestation of spontaneous mutations is that genetically close species and genera are characterized by the presence of similar forms of variability. The pattern of the presence of homologous series in hereditary variability was established by the outstanding geneticist and breeder, Academician N.I. Vavilov (1920). He found that homologous series exist not only at the species and genus levels in plants, but can also be found in mammals and humans.
The essence of the law is that genetically close genera and species are characterized by homologous (similar) series in hereditary variability. Similar genotypic variability is based on a similar genotype in closely related forms (i.e., a set of genes, their position in homologous loci). Therefore, knowing the forms of variability, for example, a number of mutations in species within the same genus, one can assume the presence of the same mutations in other species of a given genus or family. Similar mutations in genetically related species N.I. Vavilov called homologous series in hereditary variability. Examples:
1) representatives of the cereal family have a similar genotype. Similar mutations are observed within the genera of this family (wheat, rye, oats, etc.). These include the following: naked-grained, awnless, lodging, different consistency and color of grain, etc. Awnless forms of wheat, rye, oats, and rice are especially common;
2) similar mutations occur in humans and mammals: short-toed (sheep, humans), albinism (rats, dogs, humans), diabetes(rats, humans), cataracts (dogs, horses, humans), deafness (dogs, cats, humans), etc.
The law of homological series of hereditary variability is universal. Medical genetics uses this law to study diseases in animals and develop treatments for them in humans. It has been established that oncogenic viruses are transmitted through germ cells, integrating into their genome. At the same time, the offspring develop comorbidities similar to those of the parents. The DNA nucleotide sequence has been studied in many closely related species, and the degree of similarity is more than 90%. This means that the same type of mutations can be expected in related species.
The law has wide application in plant breeding. Knowing the nature of hereditary changes in some varieties, it is possible to predict similar changes in related varieties by acting on them with mutagens or using gene therapy. In this way, beneficial changes can be brought about in them.
Modification variability(according to Ch. Darwin - a certain variability) - is a change in the phenotype under the influence of environmental factors that are not inherited, and the genotype remains unchanged.
Changes in the phenotype under the influence of environmental factors in genetically identical individuals are called modifications. Modifications are otherwise called changes in the degree of expression of a trait. The appearance of modifications is due to the fact that environmental factors (temperature, light, moisture, etc.) affect the activity of enzymes and, within certain limits, change the course of biochemical reactions. Modification variability is adaptive in nature, in contrast to mutational variability.
Modification examples:
1) the arrowhead has 3 types of leaves, which differ in shape, depending on the action of the environmental factor: arrow-shaped, located above the water, oval - on the surface of the water, linear - immersed in water;
2) in a Himalayan rabbit, in place of shaved white wool, when it is placed in new conditions (temperature 2 C), black hair grows;
3) when using certain types of feed, body weight and milk yield of cows increase significantly;
4) lily of the valley leaves on clay soils wide, dark green, and on poor sandy ones - narrow and pale in color;
5) Dandelion plants relocated high up in the mountains, or in areas with a cold climate, do not reach normal sizes, and grow dwarfed.
6) with an excess content of potassium in the soil, plant growth increases, and if there is a lot of iron in the soil, then a brownish tint appears on the white petals.
Mod properties:
1) modifications can occur in a whole group of individuals, because these are group changes in the severity of signs;
2) the changes are adequate, i.e. correspond to the type and duration of exposure to a certain environmental factor (temperature, light, soil moisture, etc.);
3) modifications form a variation series, therefore they are referred to as quantitative changes in features;
4) modifications are reversible within one generation, i.e. with a change in external conditions in individuals, the degree of expression of signs changes. For example, in cows with a change in feeding, milk yield may change, in humans, under the influence of ultraviolet rays, a tan, freckles, etc. appear;
5) modifications are not inherited;
6) modifications are adaptive (adaptive) in nature, i.e., in response to changes in environmental conditions, individuals exhibit phenotypic changes that contribute to their survival. For example, domestic rats adapt to poisons; hares change seasonal color;
7) are grouped around the average value.
Under the influence of the external environment, to a greater extent, the length and shape of the leaves, height, weight, etc.
However, under the influence of the environment, signs can change within certain limits. reaction rate are the upper and lower bounds within which the attribute can change. These limits, in which the phenotype can change, are determined by the genotype. Example 1: milk yield from one cow is 4000–5000 l / year. This indicates that the variability of this trait is observed within such limits, and the reaction rate is 4000–5000 L/year. Example 2: if the height of the stem of a tall oat variety varies from 110 to 130 cm, then the reaction rate of this trait is 110–130 cm.
Different signs have different norms of reaction - wide and narrow. Wide reaction rate- leaf length, body weight, milk yield of cows, etc. Narrow reaction rate- the fat content of milk, the color of seeds, flowers, fruits, etc. Quantitative signs have a wide reaction rate, and qualitative ones have a narrow reaction rate.
Statistical analysis of modification variability on the example of the number of spikelets in an ear of wheat
Since modification is a quantitative change in a trait, it is possible to perform a statistical analysis of modification variability and derive the average value of modification variability, or a variation series. Variation series variability of the trait (i.e., the number of spikelets in the ears) - the arrangement in a row of ears according to the increase in the number of spikelets. The variational series consists of separate variants (variations). If we count the number of individual variants in the variation series, we can see that the frequency of their occurrence is not the same. Options ( variations) is the number of spikelets in ears of wheat (single expression of the trait). Most often, the average indicators of the variation series are found (the number of spikelets varies from 14 to 20). For example, in 100 ears, you need to determine the frequency of occurrence of different options. According to the results of calculations, it can be seen that most often there are spikes with an average number of spikelets (16–18):
The top row shows the options, from smallest to largest. The bottom row is the frequency of occurrence of each option.
The distribution of a variant in a variation series can be shown visually using a graph. The graphical expression of the variability of a trait is called variation curve, which reflects the limits of variation and the frequency of occurrence of specific variations of the trait (Fig. 36) .
V
Rice. 36 . Variation curve of the number of spikelets in an ear of wheat
In order to determine the average value of the modification variability of wheat ears, it is necessary to take into account the following parameters:
P is the number of spikelets with a certain number of spikelets (the frequency of occurrence of the trait);
n is the total number of series options;
V is the number of spikelets in an ear (options forming a variational series);
M - the average value of modification variability, or the arithmetic mean of the variation series of ears of wheat is determined by the formula:
M=–––––––––– (average value of modification variability)
2x14+7x15+22x16+32x17+24x18+8x19+5x20
M=––––––––––––––––––––––––––––––––––––––– = 17, 1 .
The average value of modification variability has a practical application in solving the problem of increasing the productivity of agricultural plants and animals.
Vavilov's law of homological series
An important theoretical generalization of N. I. Vavilov’s research is his theory of homological series. According to the law of homological series of hereditary variability formulated by him, not only genetically close species, but also genera of plants form homological series of forms, i.e., there is a certain parallelism in the genetic variability of species and genera. Close species due to the great similarity of their genotypes (almost the same set of genes) have similar hereditary variability. If all the known variations of characters in a well-studied species are arranged in a certain order, then in other related species one can find almost all the same variations in the variability of characters. For example, the variability of the ear awn is approximately the same in soft, durum wheat and barley.
Interpretation by N.I. Vavilov. Species and genera 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 finding of parallel forms in other species and genera. The closer the relationship, the more complete the similarity in the series of variability.
Modern interpretation of the law
Related species, genera, families have homologous genes and gene orders in chromosomes, the similarity of which is the more complete, the evolutionarily closer compared taxa. The homology of genes in related species is manifested in the similarity of the series of their hereditary variability (1987).
The Significance of the Law
1. The law of homological series of hereditary variability makes it possible to find the necessary features and variants in an almost infinite variety of forms. various kinds both cultivated plants and domestic animals, and their wild relatives.
2. It makes it possible to successfully search for new varieties of cultivated plants and breeds of domestic animals with certain required traits. This is the enormous practical significance of the law for crop production, animal husbandry and selection.
3. Its role in the geography of cultivated plants is comparable to that of Periodic system elements of D. I. Mendeleev in chemistry. By applying the law of homologous series, it is possible to establish the center of origin of plants by related species with similar characters and forms, which probably develop in the same geographical and ecological setting.
Ticket 4
Inheritance of traits during the divergence of sex chromosomes (primary and secondary non-disjunction of the X chromosomes in Drosophila)
As noted earlier, when a white-eyed female Drosophila is crossed with a red-eyed male in F1 all daughters have red eyes, and all sons who receive their only X- a chromosome from the mother, eyes are white. However, sometimes in such a crossing, single red-eyed males and white-eyed females appear, the so-called exceptional flies with a frequency of 0.1-0.001%. Bridges suggested that the appearance of such "exceptional individuals" is due to the fact that during meiosis, both X chromosomes in their mother fell into one egg, i.e. there was a nondisjunction X-chromosome. Each of these eggs can be fertilized either by sperm X- a chromosome, or Y-chromosome. As a result, 4 types of zygotes can be formed: 1) with three X- chromosomes - XXX; 2) with two mother X- chromosomes and Y-chromosome XXI; 3) from one paternal X-chromosome; 4) without X- chromosomes, but Y-chromosome.
XXI are normal fertile females. XO- male, but sterile. This shows that Drosophila Y The chromosome does not contain sex-determining genes. When crossing XXI females with normal red-eyed males ( XY) Bridges found among the offspring 4% of white-eyed females and 4% of red-eyed males. The rest of the offspring consisted of red-eyed females and white-eyed males. The author explained the appearance of such exceptional individuals by the secondary nondisjunction X-chromosomes in meiosis, because females taken in crossing ( XXY), arose due to the primary nondisjunction of chromosomes. Secondary nondisjunction of chromosomes in such females during meiosis is observed 100 times more often than primary.
In a number of other organisms, including humans, nondisjunction of sex chromosomes is also known. Of the 4 types of descendants resulting from non-divergence X-chromosomes in women, individuals that do not have any X chromosomes are lost during embryonic development. Zygotes XXX develop in women who are more likely to have mental defects and infertility. From zygotes XXI inferior men develop - Klinefelter's syndrome - infertility, mental retardation, eunuchoid physique. Descendants from one X-chromosome often die in embryonic development, rare survivors are women with Shereshevsky-Turner syndrome. They are short, infantile, barren. In man Y-chromosomes contain genes that determine the development of the male organism. With absence Y-Chromosome development proceeds according to the female type. Sex chromosome nondisjunction occurs more frequently in humans than in Drosophila; on average, for every 600 boys born, there is one with Klinefelter's syndrome.
N.I. Vavilov, studying hereditary variability in cultivated plants and their ancestors, discovered a number of patterns that made it possible to formulate the law of homologous series of hereditary variability: “Species and genera that are genetically close are characterized by similar series of hereditary variability with such regularity that, knowing a number of forms within one species, it is possible to foresee the finding of parallel forms in other species and genera. The closer genera and species are genetically located in the general system, the more complete is the similarity in the series of their variability. Entire families of plants are generally characterized by a certain cycle of variability passing through all the genera and species that make up the 30 family.
This law can be illustrated by the example of the Bluegrass family, which includes wheat, rye, barley, oats, millet, etc. Thus, the black color of the caryopsis was found in rye, wheat, barley, corn and other plants, the elongated shape of the caryopsis was found in all the studied species of the family. The law of homological series in hereditary variability allowed N.I. Vavilov himself to find a number of previously unknown forms of rye, based on the presence of these characters in wheat. These include: awned and awnless ears, grains of red, white, black and purple color, mealy and vitreous grain, etc.
The law discovered by N.I. Vavilov is valid not only for plants, but also for animals. So, albinism is found not only in different groups of mammals, but also in birds and other animals. Short-fingeredness is observed in humans, cattle, sheep, dogs, birds, lack of feathers in birds, scales in fish, wool in mammals, etc.
The law of homological series of hereditary variability is of great importance for breeding practice. It allows predicting the presence of forms not found in a given species, but characteristic of closely related species, that is, the law indicates the direction of the search. Moreover, the desired form can be found in the wild or obtained by artificial mutagenesis. For example, in 1927, the German geneticist E. Baur, based on the law of homologous series, suggested the possible existence of an alkaloid-free form of lupine that could be used for animal feed. However, such forms were not known. It has been suggested that non-alkaloid mutants are less resistant to pests than bitter lupine plants, and most of them die before flowering.
Based on these assumptions, R. Zengbush began the search for alkaloid-free mutants. He examined 2.5 million lupine plants and identified among them 5 plants with a low content of alkaloids, which were the ancestors of fodder lupine.
Later studies showed the effect of the law of homological series on the level of variability of morphological, physiological and biochemical characteristics of a wide variety of organisms - from bacteria to humans.
Artificial obtaining of mutations
In nature, spontaneous mutagenesis is constantly going on. However, spontaneous mutations are rare. For example, in Drosophila, the white eye mutation occurs at a rate of 1:100,000 gametes; in humans, many genes mutate at a rate of 1:200,000 gametes.In 1925, G.A. Nadson and G.S. Filippov discovered the mutagenic effect of radium rays on hereditary variability in yeast cells. Of particular importance for the development of artificial mutagenesis were the works of G. Meller (1927), who not only confirmed the mutagenic effect of radium rays in experiments on Drosophila, but also showed that irradiation increases the frequency of mutations hundreds of times. In 1928, L. Stadler used X-rays to obtain mutations. Later, the mutagenic effect of chemicals was also proven. These and other experiments have shown the existence of a large number of factors called mutagenic capable of causing mutations in various organisms.
All mutagens used to obtain mutations are divided into two groups:
physical - radiation, high and low temperature, mechanical impact, ultrasound;
chemical- various organic and inorganic compounds: caffeine, mustard gas, heavy metal salts, nitrous acid, etc.
induced mutagenesis has great importance. It makes it possible to create a valuable source material for breeding, hundreds of highly productive varieties of plants and animal breeds, increase the productivity of a number of producers of biologically active substances by 10-20 times, and also reveals ways to create means of protecting humans from the action of mutagenic factors.
MUTATIONAL VARIABILITY
Plan
The difference between mutations and modifications.
Mutation classification.
Law of N.I. Vavilov
Mutations. The concept of mutation. mutagenic factors.
Mutations - These are sudden, persistent, natural or artificial changes in genetic material that occur under the influence of mutagenic factors .
Types of mutagenic factors:
A) physical– radiation, temperature, electromagnetic radiation.
B) chemical factors - substances that cause poisoning of the body: alcohol, nicotine, formalin.
IN) biological- viruses, bacteria.
The difference between mutations and modifications
Mutation classification
There are several classifications of mutations.
I Classification of mutations by value: beneficial, harmful, neutral.
Useful mutations lead to increased resistance of the organism and are the material for natural and artificial selection.
Harmful mutations reduce viability and lead to the development of hereditary diseases: hemophilia, sickle cell anemia.
II Classification of mutations by localization or place of occurrence: somatic and generative.
Somatic arise in the cells of the body and affect only part of the body, while individuals of the mosaic develop: different eyes, hair color. These mutations are inherited only during vegetative propagation (in currants).
Generative – occur in germ cells or in cells from which gametes are formed. They are divided into nuclear and extranuclear (mitochondrial, plastid).
III Mutations according to the nature of the change in the genotype: chromosomal, genomic, gene.
Genetic (or point) not visible under a microscope, are associated with a change in the structure of the gene. These mutations result from the loss of a nucleotide, the insertion or substitution of one nucleotide for another. These mutations lead to gene diseases: color blindness, phenylketonuria.
Chromosomal (perestroika) associated with changes in the structure of chromosomes. May happen:
Deletion: - loss of a chromosome segment;
Duplication - duplication of a chromosome segment;
Inversion - rotation of a part of the chromosome by 180 0 ;
Translocation - exchange of segments of nonhomologous chromosomes and merger two non-homologous chromosomes into one.
Causes of chromosomal mutations: the occurrence of two or more chromosome breaks and their subsequent connection, but in the wrong order.
Genomic mutations lead to a change in the number of chromosomes. Distinguish heteroploidy And polyploidy.
heteroploidy associated with a change in the number of chromosomes, on several chromosomes - 1.2.3. Causes: no segregation of chromosomes in meiosis:
- Monosomy - decrease in the number of chromosomes by 1 chromosome. The general formula of the chromosome set is 2n-1.
- Trisonomy - an increase in the number of chromosomes by 1. The general formula is 2n + 1 (47 chromosomes Clanfaiter syndrome; trisonomy on 21 pairs of chromosomes - Down syndrome (multiple signs birth defects that reduce the viability of the body and impaired mental development).
Polyploidy - multiple change in the number of chromosomes. In polyploid organisms, the haploid (n) set of chromosomes in cells is repeated not 2 times, as in diploid ones, but 4-6 times, sometimes much more - up to 10-12 times.
The emergence of polyploids is associated with a violation of mitosis or meiosis. In particular, non-separation of homologous chromosomes during meiosis leads to the formation of gametes with an increased number of chromosomes. In diploid organisms, this process can produce diploid (2n) gametes.
It is widely found in cultivated plants: buckwheat, sunflower, etc., as well as in wild plants.
The law of N.I. Vavilov (the law of homologous series of hereditary variability).
/Since ancient times, researchers have observed the existence of similar features in different types and genera of the same family, such as melons that look like cucumbers, or watermelons that look like melons. These facts formed the basis of the law of homologous series in hereditary variability.
Multiple allelism. Parallel variability. A gene can be in more than two states. The variety of alleles for a single gene is called multiple allelism. Different alleles determine different degrees of the same trait. The more alleles the individuals of populations carry, the more plastic the species is, the better adapted it is to changing environmental conditions.
Multiple allelism underlies parallel variability - a phenomenon in which similar characters appear in different species and genera of the same family. N.I. Vavilov systematized the facts of parallel variability./
N.I. Vavilov compared species of the Zlaki family. He found out that if soft wheat has winter and spring forms, awned and awnless, then the same forms are necessarily found in durum wheat. Moreover, the composition of features. By which forms differ within species and genus, it often turns out to be the same in other genera. For example, the forms of rye and barley repeat the forms of different types of wheat, while forming the same parallel or homologous series of hereditary variability.
The systematization of facts allowed N.I. Vavilov to formulate law of homologous series in hereditary variability (1920): species and genera that are genetically close are characterized by similar series of hereditary variability with such regularity. That, knowing a number of forms within one species, it is possible to foresee the finding of parallel forms in other species and genera.
The homology of hereditary traits of closely related species and genera is explained by the homology of their genes, since they originated from the same parent species. In addition, the mutation process in genetically close species proceeds similarly. Therefore, they have similar series of recessive alleles and, as a result, parallel traits.
Derivation from Vavilov's law: each species has certain boundaries of mutational variability. No mutation process can lead to changes that go beyond the spectrum of hereditary variability of the species. So, in mammals, mutations can change the color of the coat from black to brown, red, white, striping, spotting may occur, but the appearance of a green color is excluded.