Examples of hereditary. Hereditary variability: features and significance

DEAD SOULS

A small chaise with a middle-aged gentleman of good appearance, not fat, but not thin, drove into the provincial town of NN. The arrival made no impression on the inhabitants of the city. The visitor stopped at a local tavern. During dinner, a new visitor asked the servant in the most detailed way, who used to run this institution, and who now, how much income and what kind of owner. Then the visitor found out who was the governor in the city, who was the chairman of the chamber, who was the prosecutor, that is, "he did not miss a single significant official."

In addition to the city authorities, the visitor was interested in all the large landowners, as well as the general state of the region: whether there were any epidemics in the province or general famine. After dinner and a long rest, the gentleman wrote down his rank, first and last name on a piece of paper to report to the police. Going down the stairs, the floorman read: "Collegiate adviser Pavel Ivanovich Chichikov, landowner, according to his own needs."

The next day Chichikov devoted visits to all city officials. He testified his respect even to the inspector of the medical board and the city architect.

Pavel Ivanovich showed himself to be a good psychologist, since in almost every house he left the most favorable impressions about himself - "he was very skillfully able to flatter everyone." At the same time, Chichikov avoided talking about himself, but if the conversation turned to his person, he got off with general phrases and somewhat bookish turns. The visitor began to receive invitations to the houses of officials. The first was an invitation to the governor. Getting ready, Chichikov very carefully put himself in order.

During the reception, the guest of the city managed to show himself to be a skillful interlocutor, he successfully made a compliment to the governor's wife.

The male society was divided into two parts. The thin men followed the ladies and danced, while the thick men mostly concentrated at the gaming tables. Chichikov joined the latter. Here he met most of his old acquaintances. Pavel Ivanovich also met the wealthy landowners Manilov and Sobakevich, about whom he immediately made inquiries from the chairman and postmaster. Chichikov quickly charmed both and received two invitations to visit.

The next day the newcomer went to the chief of police, where from three o'clock in the afternoon they played whist until two in the morning. There Chichikov met Nozdryov, "a broken fellow, whom you began to say to him after three or four words." In turn, Chichikov visited all the officials, and a good opinion developed about him in the city. He could show a secular person in any situation. Whatever the conversation turned to, Chichikov was able to support it. Moreover, "he knew how to clothe all this with some degree, he knew how to behave well."

Everyone was pleased with the arrival of a decent person. Even Sobakevich, who in general was rarely satisfied with his surroundings, recognized Pavel Ivanovich as "a most pleasant person." This opinion in the city persisted until one strange circumstance led the inhabitants of the city of NN into bewilderment.

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• Summary of dead souls chapter 1
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• Summary of chapter 1 dead souls

variability called the common property of all living organisms to acquire differences between individuals of the same species.

Ch. Darwin singled out the following main types of variability: definite (group, non-hereditary, modification), indefinite (individual, hereditary, mutational) and combined. Hereditary variability includes such changes in the characteristics of living beings that are associated with changes in (i.e., mutations) and are transmitted from generation to generation. The transfer of material from parents to offspring must be very accurate, otherwise the species cannot be preserved. However, sometimes there are quantitative or qualitative changes in the DNA, and the daughter cells get distorted compared to the parental genes. Such errors in the hereditary material are passed on to the next generation and are called mutations. An organism that has received new properties as a result of mutations is called a mutant. Sometimes these changes are clearly visible phenotypically, for example, the absence of pigments in the skin and hair - albinism. But most often, mutations are recessive and appear in the phenotype only when they are present in the homozygous state. The existence of hereditary changes was known. All of it follows from the doctrine of hereditary changes. Hereditary variability is a necessary prerequisite for natural and. However, at the time of Darwin there were still no experimental data on heredity and the laws of inheritance were not known. This made it impossible to strictly distinguish between different forms of variability.

mutation theory was developed in the early twentieth century by the Dutch cytologist Hugo de Vries. have a number of properties:

Mutations occur suddenly, and any part of the genotype can mutate.
Mutations are more often recessive and less often dominant.
Mutations can be harmful, neutral or beneficial to the organism.
Mutations are passed down from generation to generation.
Mutations can take place under the influence of both external and internal influences.

Mutations are divided into several types:

Point (gene) mutations are changes in individual genes. This can happen when one or more nucleotide pairs in a DNA molecule are replaced, dropped or inserted.
Chromosomal mutations are changes in parts of a chromosome or whole chromosomes. Such mutations can occur as a result of deletion - loss of part of the chromosome, duplication - doubling of any part of the chromosome, inversion - rotation of the chromosome section by 1800, translocation - tearing off part of the chromosome and moving it to a new position, for example, attachment to another chromosome.
mutations consist in changing the number of chromosomes in the haploid set. This can occur due to the loss of a chromosome from the genotype, or, conversely, an increase in the number of copies of any chromosome in the haploid set from one to two or more. A special case of genomic mutations - polyploidy - an increase in the number of chromosomes by a factor. The concept of mutations was introduced into science by the Dutch botanist de Vries. In an aspen (primrose) plant, he observed the appearance of sharp, spasmodic deviations from the typical form, and these deviations turned out to be hereditary. Further studies on various objects - plants, animals, microorganisms showed that the phenomenon of mutational variability is characteristic of all organisms.
Chromosomes are the material basis of the genotype. Mutations are changes that occur in chromosomes under the influence of external factors or. Mutational variability is newly occurring changes in the genotype, while combinations are new combinations of parental genes in the zygote. Mutations affect various aspects of the structure and functions of the body. For example, in Drosophila, mutational changes in the shape of the wings (up to their complete disappearance), body color, development of bristles on the body, shape of the eyes, their color (red, yellow, white, cherry), as well as many physiological signs (lifespan, fertility) are known. ).

They take place in different directions and in themselves are not adaptive, beneficial changes for the body.

Many emerging mutations are unfavorable for the organism and can even cause its death. Most of these mutations are recessive.

Most mutants have reduced viability and are weeded out by natural selection. Evolution or new breeds and varieties require those rare individuals that have favorable or neutral mutations. the significance of mutations lies in the fact that they create hereditary changes that are the material for natural selection in nature. Mutations are also necessary for individuals with new properties valuable to humans. Artificial mutagenic factors are widely used to obtain new breeds of animals, plant varieties and strains of microorganisms.

Combination variability also refers to hereditary forms of variability. It is due to the rearrangement of genes during the fusion of gametes and the formation of a zygote, i.e. during the sexual process.

1. What is heredity?

Answer. Heredity is the property of organisms to repeat in a number of generations similar types of metabolism and individual development as a whole. It is provided by self-reproduction of the material units of heredity - genes localized in specific structures of the cell nucleus (chromosomes) and cytoplasm. Together with variability, heredity ensures the constancy and diversity of life forms and underlies the evolution of living nature.

2. What is variability?

Answer. Variability - a variety of signs and properties in individuals and groups of individuals of any degree of kinship. Variation is inherent in all living organisms. Distinguish variability: hereditary. and non-hereditary. individual and group. Hereditary variability is due to the occurrence of mutations, non-hereditary - by the influence of environmental factors. The phenomena of heredity and variability underlie evolution.

Questions after § 46

1. What types of variability do you know?

Answer. There are two types of variability: modification (phenotypic) and hereditary (genotypic).

Changes in the traits of an organism that do not affect its genes and cannot be passed on to the next generations are called modification, and this type of variability is called modification.

The following main characteristics of modification variability can be listed:

– modification changes are not passed on to descendants;

- modification changes occur in many individuals of the species and depend on the impact of the environment;

- modification changes are possible only within the limits of the reaction norm, i.e., ultimately they are determined by the genotype

Hereditary variability is due to changes in the genetic material and is the basis of the diversity of living organisms, as well as the main cause of the evolutionary process, since it supplies material for natural selection.

The occurrence of changes in the hereditary material, i.e., in DNA molecules, is called mutational variability. Moreover, changes can occur both in individual molecules (chromosomes), and in the number of these molecules. Mutations occur under the influence of various factors of the external and internal environment.

2. What are the main signs of modification variability?

Answer. Most often, quantitative traits are subject to modifications - height, weight, fertility, etc. A classic example of modification variability is the variability in the shape of the leaves of an arrowhead plant that takes root under water. One arrowhead individual has three types of leaves, depending on where the leaf develops: under water, on the surface or in the air. These differences in the shape of the leaves are determined by the degree of their illumination, and the set of genes in the cells of each leaf is the same.

For various signs and properties of the organism, a greater or lesser dependence on environmental conditions is characteristic. For example, in humans, the color of the iris and the blood type are determined only by the corresponding genes, and living conditions cannot influence these signs. But height, weight, physical endurance strongly depend on external conditions, for example, on the quality of nutrition, physical activity, etc.

3. What is the reaction rate?

Answer. The limits of modification variability of any trait are called the reaction norm. The reaction rate is genetically determined and inherited.

The variability of a trait is sometimes very large, but it cannot go beyond the limits of the reaction norm. In some traits, the reaction rate is very wide (for example, shearing wool from sheep, milkiness of cows), while other features are characterized by a narrow reaction rate (hair color in rabbits).

A very important conclusion follows from the above. It is not the trait itself that is inherited, but the ability to manifest this trait under certain conditions, in other words, the norm of the body's reaction to external conditions is inherited.

4. What forms of hereditary variability do you know?

Answer. Hereditary variability manifests itself in two forms - combinative and mutational.

Mutational variability is a change in the DNA of a cell (a change in the structure and number of chromosomes). Arise under the influence of ultraviolet, radiation (X-rays), etc. They are inherited, serve as material for natural selection (the mutation process is one of the driving forces of evolution).

Combination variability occurs when the genes of the father and mother are recombined (mixing). Sources:

1) Crossing over during meiosis (homologous chromosomes closely approach and change areas).

2) Independent divergence of chromosomes during meiosis.

3) Random fusion of gametes during fertilization.

5. What are the reasons for combinative variability?

Answer. The basis of combinative variability is the sexual process, which results in a huge set of diverse genotypes.

Let's take a human as an example. Each human cell contains 23 maternal and 23 paternal chromosomes. During the formation of gametes, only 23 chromosomes will fall into each of them, and how many of them will be from the father and how many from the mother is a matter of chance. This is the first source of combinative variability.

The second reason is crossing over. Not only does each of our cells carry the chromosomes of grandparents, a certain part of these chromosomes received, as a result of crossing over, part of their genes from homologous chromosomes that previously belonged to another line of ancestors. Such chromosomes are called recombinant. Participating in the formation of the organism of a new generation, they lead to unexpected combinations of features that neither the paternal nor the maternal organism had.

Finally, the third reason for combinative variability is the random nature of the meetings of certain gametes in the process of fertilization.

All three processes underlying combinative variability operate independently of each other, creating a huge variety of all possible genotypes.

Named this type of variability uncertain, since it is initially impossible to determine what changes will appear, in addition, they are always individual.

In each sufficiently long-existing set of individuals, various mutations spontaneously and undirectedly arise, which are subsequently combined more or less randomly with different hereditary properties already existing in the set.

Variability due to the occurrence of mutations is called mutational, and due to further recombination of genes as a result of crossing - combinative.

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  Combinative variability - variability that occurs due to the recombination of genes during the fusion of gametes. Main reasons:

  • independent segregation of chromosomes during meiosis;
  • a random meeting of gametes, and as a result of this, a combination of chromosomes during fertilization;
  • recombination of genes due to crossing over.

  Mutational variability

  Mutational variability - variability caused by the action of mutagens on the body, resulting in mutations (reorganization of the reproductive structures of the cell). Mutagens are physical, chemical and biological.

  mutation theory

  The main provisions of the mutation theory in 1901-1903 were developed by Hugo de Vries, and wrote about it in his work The Mutation Theory. This work rejected the then current understanding of inheritance as the main mechanism of variability in Darwin's theory. Instead, he introduced the term "mutation", denoting the unexpected appearance of new traits in the phenotype, not caused by heredity. The main provisions of the theory:

  1. Mutations occur suddenly, abruptly, as discrete changes in traits.
  2. Unlike non-hereditary changes, mutations are qualitative changes that are passed down from generation to generation.
  3. Mutations manifest themselves in different ways and can be both beneficial and harmful, both dominant and recessive.
  4. The probability of detecting mutations depends on the number of individuals studied.
  5. Similar mutations can occur repeatedly.
  6. Mutations are undirected (spontaneous), that is, any part of the chromosome can mutate, causing changes in both minor and vital signs.

  Almost any change in the structure or number of chromosomes, in which the cell retains the ability to reproduce itself, causes a hereditary change in the characteristics of the organism. According to the nature of the change in the genome, that is, the totality of genes contained in the haploid set of chromosomes, gene, chromosomal and genomic mutations are distinguished.

  Role in evolution

  The whole variety of individual differences is based on hereditary variability, which include:

  • Both sharp qualitative differences, not connected with each other by transitional forms, and purely quantitative differences, forming continuous series, in which close members of the series can differ from each other as little as desired;
  • Both changes in individual traits and properties (independent variability) and interrelated changes in a number of traits (correlative variability);
  • Both changes that have an adaptive value (adaptive variability) and changes that are “indifferent” or even reduce the viability of their carriers (non-adaptive variability).

  All these types of hereditary changes constitute the material of the evolutionary process (see Microevolution). In the individual development of an organism, the manifestation of hereditary traits and properties is always determined not only by the main genes responsible for these traits and properties, but also by their interaction with many other genes that make up the genotype of the individual, as well as by the environmental conditions in which the organism develops.

  Accuracy is undeniably important in the transmission of genetic information over a number of generations, however, excessive conservation of genetic information contained in individual genetic loci can be harmful to the organism and the species as a whole.

  The evolutionarily established relationships between the accuracy of the functioning of genetic systems and the frequency of errors that occur when reproducing the genetic information of individual genetic loci are clearly balanced among themselves, and it has already been established that in a number of cases they are adjustable. Programmed and random inherited changes in the genome, called mutations, can be accompanied by enormous quantitative and qualitative changes in gene expression.