Sections: Biology

Lesson objectives

1. To acquaint students with the forms of hereditary variability, their causes and effects on the body. To develop in schoolchildren the ability to classify the forms of variability, to compare them with each other; give examples illustrating the manifestation of each of them;
2. To form knowledge about the types of mutations;
3. Formulate the law of homologous series and explain its meaning;
4. Convince high school students that the mutation process is very important for the evolution of the organic world and human selection work.

Demonstrations

• Scheme of different types of chromosomal mutations.
• Scheme of polyploidization.
• Homological series in hereditary variability.

Terms Genotypic variability, mutation, gene mutations, genomic mutations, chromosomal mutations:

• inversion;
• deletion;
• duplication;
• translocation.

Tasks for students:

1. Formulate the law of homologous series and give examples.
2. Get acquainted with the biography of N.I. Vavilov and know his main scientific discoveries.
3. Make a table "Forms of variability"

1. Organizing time.
2. Testing knowledge and skills.

Front work

1. What does genetics study?
2. What does the term heredity mean? - variability?
3. What forms of variability do you know?
4. What does reaction rate mean?
5. What are the patterns of modification variability?
6. How does changing conditions affect quantitative and qualitative traits? Give examples
7. What is a reaction rate? Why does the diversity of qualitative traits depend to a small extent on the influence of environmental conditions?
8. What is the practical significance in agriculture of the value of the reaction rate of animals and plants?

Individual work on the computer - test work

Fill in the chart:

Work of students on computers with the application 1 . (During the lesson, tasks 1-5 are completed).

1. Learning new material

The concept of hereditary variability includes genotypic and cytoplasmic variability. The first is divided into mutational, combinative, correlative. Combinative variability occurs with crossing over, independent divergence of chromosomes in meiosis, and random fusion of gametes during sexual reproduction. The composition of mutational variability includes genomic, chromosomal and gene mutations. The term mutation was introduced into science by G. de Vries. His biography and main scientific achievements are located in the section. Genomic mutations are associated with the occurrence of polyploids and aneuploids. Chromosomal mutations are determined by interchromosomal changes - translocation or intrachromosomal rearrangements: deletion, duplication, inversion. Gene mutations are explained by changes in the sequence of nucleotides: an increase or decrease in their number (deletion, duplication), insertion of a new nucleotide, or rotation of a section within a gene (inversion). Cytoplasmic variability is associated with DNA, which is found in the plastids and mitochondria of the cell. The hereditary variability of related species and genera obeys the law of Vavilov's homologous series.

Modification variability reflects changes in the phenotype without affecting the genotype. Opposite to it is another form of variability - genotypic, or mutational (according to Darwin - hereditary, indefinite, individual), changing the genotype. Mutation is a persistent hereditary change in genetic material.

Individual changes in the genotype are called mutations.

The concept of mutations was introduced into science by the Dutchman de Vries. Mutations are hereditary changes that lead to an increase or decrease in the amount of genetic material, to a change in nucleotides or their sequence.

Mutation classification

• Mutations by the nature of manifestation: Dominant, recessive.
• Mutations at the place of their occurrence: somatic, generative.
• Mutations by the nature of appearance: spontaneous, induced.
• Mutations by adaptive value: beneficial, harmful, neutral. (Lethal, semi-lethal.)

Most of the resulting mutations are recessive and unfavorable for the organism, they can even cause its death. In combination with an allelic dominant gene, recessive mutations do not appear phenotypically. Mutations occur in sex and somatic cells. If mutations occur in germ cells, they are called generative and manifest themselves in the generation that develops from germ cells. Changes in vegetative cells are called somatic mutations. Such mutations lead to a change in the trait of only a part of the organism that develops from the changed cells. In animals, somatic mutations are not transmitted to subsequent generations, since a new organism does not arise from somatic cells. It is different in plants: in hybrid cells of plant organisms, replication and mitosis can be carried out in different nuclei somewhat differently. Over the course of a number of cell generations, individual chromosomes are lost and certain karyotypes are selected that can be preserved for many generations.

There are several types of mutations according to the level of occurrence:

1. Genomic mutations - change in ploidy, i.e. chromosome numbers (numerical chromosome aberrations), which are especially common in plants;
2. Chromosomal mutations - changes in the structure of chromosomes (structural chromosomal aberrations);
3. Gene mutations - changes in individual genes;

Genomic mutations

Polyploidy is a multiple increase in the number of chromosomes.
Aneuploidy is the loss or appearance of extra chromosomes as a result of a violation of meiosis.

Occur due to a change in the number or structure of chromosomes. Changes in ploidy are observed in disorders of chromosome divergence.

Chromosomal diseases

• generative mutations
• XXY; HUU - Klinefelter's syndrome.
• XO - Shershevsky-Turner syndrome.

Autosomal mutations

• Patau syndrome (on chromosome 13).
• Edwards syndrome (on chromosome 18).
• Down syndrome (on chromosome 21).

Klinefelter syndrome.

XXY and XXXY - Klinefelter's syndrome. The frequency of occurrence is 1:400 - 1:500. The karyotype is 47, XXY, 48, XXXY, etc. The phenotype is male. Female body type, gynecomastia. Tall, relatively long arms and legs. Weakly developed hairline. Intelligence is reduced.

Shershevsky-Turner syndrome

X0 - Shereshevsky-Turner syndrome (monosomy X). The frequency of occurrence is 1:2000 - 1:3000. Karyotype 45,X. The phenotype is female. Somatic signs: height 135 - 145 cm, pterygoid skin fold on the neck (from the back of the head to the shoulder), low position of the ears, underdevelopment of primary and secondary sexual characteristics. In 25% of cases, there are heart defects and anomalies in the functioning of the kidneys. The intellect rarely suffers.

Patau syndrome - Trisomy on the 13th chromosome (Patau syndrome) is found in newborns with a frequency of about 1:5000 - 1:7000 and is associated with a wide range of malformations. SP is characterized by multiple congenital malformations of the brain and face. This is a group of early disorders in the formation of the brain, eyeballs, bones of the brain and facial parts of the skull. The circumference of the skull is usually reduced. Forehead sloping, low; the palpebral fissures are narrow, the nose bridge is sunken, the auricles are low and deformed. A typical sign of SP is cleft lip and palate.

Down's syndrome - A disease caused by an anomaly of the chromosome set (a change in the number or structure of autosomes), the main manifestations of which are mental retardation, a peculiar appearance of the patient and congenital malformations. One of the most common chromosomal diseases, occurs on average with a frequency of 1 in 700 newborns. A transverse fold is often found on the palm

Chromosomal mutations

There are several types of chromosomal mutations associated with changes in the structure of chromosomes:

• deletion - loss of a portion of a chromosome;
• duplication - doubling of a section of a chromosome;
• inversion - rotation of a chromosome segment by 180 degrees;
• translocation - the transfer of a section of a chromosome to another chromosome.
• transposition - movement in one chromosome.

Deletions and duplications change the amount of genetic material. Phenotypically, they appear depending on how large the corresponding sections of chromosomes are and whether they contain important genes. Duplications can lead to the creation of new genes. During inversions and translocations, the amount of genetic material does not change, but its location changes. Such mutations also play an important role, since the crossing of mutants with the original forms is difficult, and their F1 hybrids are most often sterile.

Deletions. In humans, as a result of a deletion:

• Wolf's syndrome - a lost section of the large chromosome 4 -
• syndrome "cat's cry" - with a deletion in chromosome 5. Cause: chromosomal mutation; loss of a chromosome fragment in the 5th pair.
  Manifestation: abnormal development of the larynx, feline-like screams, I in early childhood, lag in physical and mental development

Inversions

• This is a change in the structure of the chromosome, caused by a 180 ° turn of one of its internal sections.
• Similar chromosomal rearrangement is a consequence of two simultaneous breaks in one chromosome.

Translocations

• During translocation, regions of nonhomologous chromosomes are exchanged, but the total number of genes does not change.

Base replacement

1. phenylketonuria. Manifestation: impaired cleavage of phenylalanine; this is due to dementia caused by hyperphenylalaninemia. With a timely prescribed and observed diet (nutrition, low phenylalanine) and the use of certain medications, the clinical manifestations of this disease are practically absent.
2. sickle cell anemia.
3. Morfan syndrome.

Genetic(point) mutations are associated with changes in the nucleotide sequence. The normal gene (peculiar to the wild type) and the mutant genes that arise from it are called alleles.

With gene mutations, the following structural changes occur:

gene mutation

For example, sickle cell anemia is the result of a single base substitution in the b-chain of blood globin (adenine is replaced by thymine). During deletion and duplication, the sequence of triplets is shifted and mutants with a “frameshift” appear, i.e. shifts of boundaries between codons - all subsequent amino acids change from the place of mutation.

The primary structure of hemoglobin in healthy (1) and patients with sickle cell anemia (2).

1. - val-gis-ley-tre - pro-glut. to-ta- glu-liz
2. - val-gis-ley-tre - valine- glu-liz

Mutation in the beta hemoglobin gene

Morfan's syndrome

The high release of adrenaline, characteristic of the disease, contributes not only to the development of cardiovascular complications, but also to the appearance in some individuals of special fortitude and mental endowment. Methods of treatment are unknown. It is believed that Paganini, Andersen, Chukovsky were sick with it

Hemophilia

Mutagens are factors that cause mutations: biological, chemical, physical.

Experimentally, the mutation rate can be increased. Under natural conditions, mutations occur with sudden changes in temperature, under the influence of ultraviolet radiation, and for other reasons. However, in most cases, the true causes of mutations remain unknown. Currently, methods have been developed to increase the number of mutations by artificial means. For the first time, a sharp increase in the number of hereditary changes that occur was obtained under the influence of X-rays.

• Physical factors (various types of ionizing radiation, ultraviolet radiation, X-rays)
• Chemical factors (insecticides, herbicides, lead, drugs, alcohol, certain drugs and other substances)
• Biological factors (viruses of smallpox, chickenpox, mumps, influenza, measles, hepatitis, etc.)

Eugenics.

Eugenics is the science of improving the breed of mankind.

Eugenics in Greek means the birth of the best. This scandalous science is looking for ways to improve the hereditary qualities of a person using genetic principles. It has always been difficult for it to remain a pure science: its development was closely followed by politics, which disposed of its fruits in its own way.

In ancient Sparta, the selection of people was carried out more radically, destroying babies who did not have the physical qualities necessary for a future warrior. The father of eugenics, which put it on a scientific basis, was Francis Galton in 1869. After analyzing the pedigrees of hundreds of talented people, he came to the conclusion that genius abilities are inherited.

Today, eugenics aims to eradicate hereditary diseases in the human race. Any biological species will be on the verge of destruction if its existence conflicts with nature. Almost half of newborns out of a thousand are born with some kind of hereditary pathology. In the world, 2 million such children are born every year. Among them - 150 thousand with Down syndrome. It has long been known to everyone that it is easier to prevent the birth of a child than to deal with ailments. But such opportunities have appeared only in our time. Prenatal diagnosis and genetic counseling help to solve the problem of the advisability of childbirth.

Modern possibilities of medical genetic counseling make it possible to determine the risk of hereditary diseases during pregnancy planning.

Nikolay Ivanovich Vavilov

Nikolai Ivanovich Vavilov (1887-1943) - Russian botanist, geneticist, plant grower, geographer. Formulated the law of homologous series of hereditary variability. He created the doctrine of the centers of origin of cultivated plants.

The Russian scientist N. I. Vavilov established an important regularity known as the law of homological series in hereditary variability: species and genera that are genetically close (related to each other by the unity of origin) are characterized by similar series in hereditary variability. On the basis of this law, one can foresee the discovery of similar changes in related species and genera. He compiled a table of homologous series in the family

cereals. In animals, this pattern also manifests itself: for example, in rodents there are homologous series in terms of coat color.

Law of homologous series

Studying the hereditary variability of cultivated plants and their ancestors, N.I. Vavilov formulated the law of homological series: “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, one can foresee the presence of parallel forms in other species and genera.”

Using the family of cereals as an example, Vavilov showed that similar mutations are found in a number of species of this family. So, the black color of seeds is found in rye, wheat, barley, corn and others, with the exception of oats, wheatgrass, and millet. The elongated shape of the grain is found in all studied species. Animals also have similar mutations: albinism and lack of hair in mammals, short-fingeredness in cattle, sheep, dogs, birds. The reason for the appearance of similar mutations is the common origin of genotypes.

Thus, the detection of mutations in one species provides a basis for searching for similar mutations in related plant and animal species.

Law of homologous series

1. What mutant forms should arise in closely related species?
2. Who is the founder of the law of homologous series?
3. How does the law say?

Homework.

1. Section 24
2. Find examples of mutations in nature.

On June 4, he made a presentation "The law of homological series in hereditary variability." This is one of those works that are considered fundamental and are the theoretical basis for biological research. The essence of the law is that species and genera that are genetically close (connected to each other by the unity of origin) are characterized by similar series in hereditary variability. Student enthusiasm for the study of cereals, and then cruciferous, legumes, and pumpkin allowed Vavilov and his students to find mutations that are similar in related species, and then genera. In the table developed as a result of the experiments, Vavilov noted the mutations, the manifestation of which was found in these species, with a “+” sign, and empty spaces indicate that such mutations should be, but have not yet been detected. A table with empty cells that will be filled in with the further development of science. Where have we met with something like this? Of course, in chemistry, the famous periodic table! The regularity of the two laws is confirmed by science. "Empty" cells are filled, and this is the basis for practical selection. Durum wheat is known only in the spring form, but on the basis of the law, durum wheat in the winter form should also exist in nature. Indeed, it was soon discovered on the border of Iran and Turkey. Pumpkins and melons are characterized by simple and segmented fruits, but watermelon of this form was not described at the time of Vavilov. But segmented watermelons have been found in the southeast of the European part of Russia. The culture is dominated by the cultivation of three-sprout beets, the crops of which require weeding and removal of two extra shoots. But among the relatives of beets in nature there were also single-sprouted forms, so scientists were able to create a new variety of single-sprouted beets. Awnlessness of cereal crops is a mutation that has proven beneficial in the introduction of machine harvesting when the machinery is less clogged. Breeders, using the Vavilov law, found awnless forms and created new varieties of awnless cereals. The facts of parallel variability in close and distant species were already known to C. Darwin. For example, the same coat color of rodents, albinism in representatives of different groups of the animal world and humans (a case of albinism in blacks is described), lack of plumage in birds, lack of scales in fish, similar coloring of fruits of fruit and berry crops, variability of root crops, etc. Reason parallelism in variability lies in the fact that the basis of homologous traits is the presence of similar genes: the genetically closer the species and genera, the more complete the similarity in the series of variability. Hence - the cause of homologous mutations - the common origin of genotypes. Living nature in the process of evolution was programmed, as it were, according to one formula, regardless of the time of origin of species. The law of homological series in hereditary variability by NI Vavilov was not only a confirmation of Darwin's theory of the origin of species, but also expanded the idea of ​​hereditary variability. Nikolai Ivanovich can again be proclaimed: “Thanks to Darwin!”, but also “Continuing Darwin!” Let's go back to 1920. The recollections of eyewitnesses are interesting. Alexandra Ivanovna Mordvinkina, who was present at the congress of the Saratov Agricultural Institute (later a candidate of biological sciences), recalled: “The congress opened in the largest auditorium of the university. Not a single report subsequently made such a strong impression on me as the speech of Nikolai Ivanovich. He spoke with inspiration, everyone listened with bated breath, it was felt that something very big and new in science was opening before us. When stormy, long-lasting applause broke out, Professor Vyacheslav Rafailovich Zelensky said: “It is biologists who greet their Mendeleev.” The words of Nikolai Maksimovich Tulaikov especially imprinted in my memory: “What can be added to this report? I can say one thing: Russia will not perish if it has such sons as Nikolai Ivanovich. Nikolai Vladimirovich Timofeev-Resovsky, an excellent geneticist who knew Vavilov not only by work, but also personally, spoke confidentially to close acquaintances: “Nikolai Ivanovich was a wonderful person and great martyr, an excellent plant breeder and gatherer, a traveler, a brave and universal favorite, but his series - the law is not at all homological, but analogous series, yes, sir! What is homology? This similarity is based on a common origin. What is an analogy? The similarity of external signs, which is determined by a similar habitat, but not by kinship. So who is right? Vavilov! One can only admire the depth of his biological mind! Changing just one term in the title also changes the essence of the law. According to the law of homological series, all people are equal, because they are of the same biological origin, and belong to the homo sapiens species, that is, everyone is equally smart, capable and talented, etc., but they have external differences: in height, proportions between body parts etc. According to the law of analogous series, people are outwardly similar, because they have a similar habitat, but a different origin. And this is already room for chauvinism, racism, nationalism, up to genocide. And the Vavilov law says that the pygmy of Africa and the basketball player of America are of the same genetic root, and one cannot be placed over the other - this is unscientific! The validity of the universal biological regularity discovered by Vavilov has been confirmed by modern research not only in plants, but also in animals. Modern geneticists believe that the law reveals boundless prospects for scientific knowledge, generalization and foresight” (Professor M. E. Lobanov). Another fundamental work by N. I. Vavilov, “Plant Immunity to Infectious Diseases” (1919), belongs to the Saratov period. On the title page of the book, Nikolai Ivanovich wrote: "Dedicated to the memory of the great researcher of immunity, Ilya Ilyich Mechnikov." No great scientist sees himself as a stand-alone in science. So Vavilov, thanks to Mechnikov, asked himself the question, can plants have protective forces if animals have them? In search of an answer to the question, he conducted research on cereals according to an original method and, summarizing practice and theory, laid the foundations of a new science - phytoimmunology. The work was of purely practical importance - to use the natural immunity of plants as the most rational and cost-effective way to control pests. The young scientist created an original theory of the physiological immunity of plants to infectious diseases, and the study of genotypic immunity formed the basis of the doctrine. N. I. Vavilov studied the reaction of the "host" to the introduction of the parasite, the specificity of this reaction, and found out whether the entire series is immune, or only certain types of this series. Nikolai Ivanovich attached particular importance to group immunity, believing that in breeding it is important to breed varieties that are resistant not to one race, but to a whole population of physiological races, and such resistant species should be sought in the plant's homeland. Science later confirmed that wild species - relatives of cultivated plants - have natural immunity and are less susceptible to infectious diseases. It is the introduction of resistance genes into plants that modern breeders are engaged in, using the theory of N. I. Vavilov and genetic engineering methods. The scientist was interested in the development of immunity issues throughout his scientific activity: “The doctrine of plant immunity to infectious diseases” (1935), “The laws of natural plant immunity to infectious diseases (keys to finding immune forms)” (published only in 1961. ). Academician Petr Mikhailovich Zhukovsky rightly noted: “In the Saratov period, although it was short (1917-1921), the star N. I. Vavilov - a scientist. Later Vavilov would write: "I migrated from Saratov in March 1921 with the entire laboratory of 27 people." He was elected head of the Bureau of Applied Botany of the Agricultural Scientific Committee in Petrograd. From 1921 to 1929 - Professor of the Department of Genetics and Breeding of the Leningrad Agricultural Institute. In 1921, V. I. Lenin sent two scientists to a conference in America, one of them - N. I. Vavilov. The report on genetic research made him popular among the scientists of the conference. In America, his performances were accompanied by a standing ovation, similar to the one that was later for Chkalov. “If all Russians are like that, then we need to be friends with them,” American newspapers shouted. In the 20-30s. N. I. Vavilov also manifests himself as a major organizer of science. He was actually the founder and permanent leader of the All-Union Institute of Plant Industry (VIR). In 1929, the All-Union Academy of Agricultural Sciences (VASKhNIL) was created on the basis of the All-Union Institute of Experimental Agronomy, which had previously been organized by Vavilov. He was elected the first president (from 1929 to 1935). With the direct participation of the scientist, the Institute of Genetics of the USSR Academy of Sciences was organized. In a short time, Vavilov's talent created a scientific school of geneticists, which became the world's leading one. All the initial work in our country in the field of genetics was carried out by him or under his direction. In VIR, the method of experimental polyploidy was first used, and G. D. Karpechenko began work on its use in distant hybridization. Vavilov insisted on starting work on the use of the phenomenon of heterosis and interline hybridization. Today it is the ABC of selection, but then it was the beginning. Over 30 years of scientific activity, about 400 works and articles have been published! Phenomenal memory, encyclopedic knowledge, knowledge of almost twenty languages, aware of all the innovations in science. He worked 18-20 hours a day. Mom scolded him: “You don’t even have time to sleep ...,” recalls Vavilov’s son.

The processing of extensive material of observations and experiments, a detailed study of the variability of numerous Linnaean species (Linneons), a huge amount of new facts obtained mainly from the study of cultivated plants and their wild relatives, allowed N.I. Vavilov to bring together all known examples of parallel variability and formulate a general law, which he called the "Law of homological series in hereditary variability" (1920), reported by him at the Third All-Russian Congress of Breeders, held in Saratov. In 1921 N.I. Vavilov was sent to America to attend the International Congress on Agriculture, where he delivered a report on the law of homologous series. The law of parallel variability of closely related genera and species, established by N.I. Vavilov and associated with a common origin, developing the evolutionary teachings of Charles Darwin, was duly appreciated by world science. It was perceived by the audience as the largest event in the world biological science, which opens up the widest horizons for practice.

The law of homological series, first of all, establishes the foundations of the taxonomy of the huge variety of plant forms that the organic world is so rich in, allows the breeder to get a clear idea of ​​​​the place of each, even the smallest, systematic unit in the plant world and judge the possible diversity of the source material for selection.

The main provisions of the law of homological series are as follows.

"one. Species and genera 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 occurrence of parallel forms in other species and genera. The closer genera and linneons are genetically located in the general system, the more complete is the similarity in the series of their variability.

2. Whole families of plants are generally characterized by a certain cycle of variability passing through all the genera and species that make up the family.

Even at the III All-Russian Congress on Selection (Saratov, June 1920), where N.I. Vavilov reported his discovery for the first time, all participants of the congress recognized that “like the periodic table (periodic system)” the law of homological series will allow predicting the existence, properties and structure of still unknown forms and species of plants and animals, and highly appreciated the scientific and practical significance of this law . Modern advances in molecular cell biology make it possible to understand the mechanism of the existence of homological variability in similar organisms - what exactly is the basis for the similarity of future forms and species with existing ones - and meaningfully synthesize new forms of plants that are not found in nature. Now a new content is being introduced into Vavilov's law, just as the advent of quantum theory has given a new, deeper content to Mendeleev's periodic system.

homologous series). Formulated in 1920 by N. I. Vavilov, who discovered that the hereditary variability of plants is similar in closely related species and genera of the grass family. It manifests itself in a change in similar characters with such regularity that, knowing the forms of plants in representatives of one species, one can foresee the appearance of these forms in other related species and genera. The closer the species are to each other by origin, the more clearly this similarity is manifested. So, in different types of wheat (for example, soft and durum), rows of similar hereditary changes are revealed in the awn of the ear (awned, semi-awned, awnless), its color (white, red, black, gray ears), the shape and texture of the grain, early maturity, cold resistance , responsiveness to fertilizers and so on.

Similar variability in the awning of the ear in soft wheat (1-4), durum wheat (5-8) and six-row barley (9-12) (according to N. I. Vavilov).

The parallelism of variability is more weakly expressed in different genera within the family (for example, wheat, barley, rye, oats, wheatgrass and other genera from the family of cereals) and even weaker in different families within the order (higher taxonomic rank). In other words, in accordance with the law of homology series, related species due to the great similarity of their genomes (almost identical sets of genes) have a similar potential variability of traits, which is based on similar mutations of homologous (orthologous) genes.

N. I. Vavilov pointed out the applicability of homological series of laws to animals as well. Obviously, this is a universal law of variability, covering all the kingdoms of living organisms. The validity of this law is vividly illustrated by genomics, which reveals the similarity of the primary structure of the DNA of closely related species. The law of homology series finds further development in the modular (block) principle of the theory of molecular evolution, according to which the genetic material diverges through duplications and subsequent combinatorics of DNA sections (modules).

The law of homology series helps to purposefully search for hereditary changes necessary for selection. It indicates to breeders the direction of artificial selection, facilitates the production of forms that are promising for the selection of plants, animals and microorganisms. For example, guided by the law of homology series, scientists have created alkaloid-free (non-bitter) varieties of fodder lupins for pasture animals, while simultaneously enriching the soil with nitrogen. The law of homology series also helps to navigate in the choice of model objects and specific genetic systems (genes and traits) for modeling and searching for therapy for human hereditary diseases, such as metabolic diseases, neurodegenerative diseases, etc.

Lit .: Vavilov N. I. The law of homological series in hereditary variability. M., 1987.

S. G. Inge-Vechtomov.

Homologous 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 generalizes 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 in them, 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 traits and properties in cultivated plants and domestic animals.