Gene classification

The accumulated knowledge about the structure, functions, nature of interaction, expression, mutability, and other properties of genes gave rise to several variants of the classification of genes.

According to the localization of genes in the structures of the cell, nuclear genes located in the chromosomes of the nucleus and cytoplasmic genes are distinguished, the localization of which is associated with chloroplasts and mitochondria.

By their functional meaning, structural genes are distinguished, characterized by unique sequences of nucleotides encoding their protein products, which can be identified using mutations that disrupt the function of the protein, and regulatory genes - nucleotide sequences that do not encode specific proteins, but regulate the action of the gene (inhibition, increase activity, etc.).

According to the effect on physiological processes in the cell, lethal, conditionally lethal, supervital genes, mutator genes, antimutator genes, etc. are distinguished.

It should be noted that any biochemical and biological processes in the body are under gene control. Thus, cell division (mitosis, meiosis) is controlled by several tens of genes; groups of genes control the restoration of genetic damage to DNA (repair). Oncogenes and tumor suppressor genes are involved in the processes of normal cell division. The individual development of an organism (ontogenesis) is controlled by many hundreds of genes. Mutations in genes lead to altered synthesis of protein products and disruption of biochemical or physiological processes.

Homeotic mutations in Drosophila made it possible to discover the existence of genes whose normal function is to select or maintain a certain path of embryonic development along which cells follow. Each developmental pathway is characterized by the expression of a certain set of genes, the action of which leads to the appearance of the final result: eyes, head, chest, abdomen, wing, legs, etc. Studies of the genes of the Drosophila bithorax complex by the American geneticist Lewis showed that this is a giant cluster of closely linked genes, which function is necessary for normal segmentation of the chest (thorax) and abdomen (abdomen). Such genes are called homeobox genes. Homeobox genes are located in DNA groups and manifest their action in a strictly sequential manner. Such genes are also found in mammals, and they have high homology (similarity).

Functions of genes

In the process of realizing the hereditary information contained in a gene, a number of its properties are manifested. Determining the possibility of developing a separate quality inherent in a given cell or organism, a gene is characterized by discreteness of action (from the Latin discretus - divided, discontinuous), discontinuity (introns and exons). The discreteness of the hereditary material, the assumption of which was expressed by G. Mendel, implies its divisibility into parts that are elementary units - genes. Currently, a gene is considered as a unit of genetic function. It represents the minimum amount of hereditary material that is necessary for the synthesis of tRNA, rRNA, or a polypeptide with certain properties. The gene is responsible for the formation and transmission by inheritance of a separate trait or properties of cells, organisms of a given species. In addition, a change in the structure of a gene that occurs in different parts of it ultimately leads to a change in the corresponding elementary trait.

Due to the fact that a gene contains information about the amino acid sequence of a particular polypeptide, its action is specific. However, in some cases, the same nucleotide sequence can determine the synthesis of not one, but several polypeptides. This is observed in the case of alternative splicing in eukaryotes and in overlapping genes in phages and prokaryotes. Obviously, this ability should be assessed as multiple, or pleiotropic, action of a gene (although traditionally the pleiotropic action of a gene is usually understood as the participation of its product, a polypeptide, in various biochemical processes related to the formation of various complex traits). For example, the participation of an enzyme in accelerating a certain reaction (see Fig.), Which is a link in several biochemical processes, makes the results of these processes dependent on the normal functioning of the gene encoding this protein. Violation of the A> B reaction, catalyzed by protein b, as a result of gene mutation leads to the shutdown of the subsequent stages of the formation of traits D and E.

Determining the possibility of mRNA transcription for the synthesis of a specific polypeptide chain, a gene is characterized by a dosage of action, i.e. the quantitative dependence of the result of its expression on the dose of the corresponding allele of this gene. An example is the dependence of the degree of impairment of the transport properties of hemoglobin in humans with sickle cell anemia on the dose of the HbS allele. The presence in the human genotype of a double dose of this allele, leading to a change in the structure of β-globin chains of hemoglobin, is accompanied by a gross violation of the shape of erythrocytes and the development of a clinically pronounced picture of anemia up to death. In carriers of only one HbS allele, with a normal second allele, the shape of erythrocytes changes only slightly and anemia does not develop, and the body is characterized by almost normal viability.

V. Johansen in $ 1909.

Definition 1

Gene Is a specific region of a nucleic acid molecule (DNA or RNA) that determines the hereditary characteristics of a living organism.

For quite a long time, before the structure of nucleic acids was clarified and the genetic code was discovered, the gene was considered an indivisible unit of heredity, recombinations and mutations. But later it was found that mutation processes can affect not the entire gene, but only a certain part of it. Also, it was possible to establish that during crossing over, homologous chromosomes can exchange both whole genes and their individual parts. But functionally, a gene is a complete unit. Any, even the most insignificant, violations of the structure of the gene change the information encoded in it or even lead to its loss.

Types of genes

Genes are divided into structural which encode the structure of proteins and ribonucleic acids, and regulatory , which are the site of attachment of enzymes and other biologically active compounds. In addition, they affect the activity of structural genes and are involved in the processes of DNA replication and transcription. Regulatory genes are usually small in size (several tens of base pairs). The sizes of structural genes reach hundreds and thousands of nucleotides.

Genes properties

In the course of many years of research, scientists have come to the conclusion that genes can exhibit different qualities (properties) in different situations. As a result of scientific research, concepts of the basic properties of genes were formulated. We bring them to your attention.

1. The gene is discrete in its action. This means that it is isolated in its activity from other genes.
2. The gene exerts its influence in a specific way. He is responsible only for a strictly defined, encoded by him, characteristic of the organism.
3. The gene is capable of acting in a dosed manner. It can enhance the degree of manifestation of a particular trait if there is an increase in the number of dominant alleles.
4. A single gene can influence the development of several different traits. This phenomenon is called multiple gene action.
5. Different genes can influence the development of the same trait in the same way. That is, one trait can be encoded by several genes. These genes are called multiple genes, or polygenes.
6. A gene is capable of interacting with other genes. This leads to the appearance of other, new signs. This interaction is carried out indirectly. It occurs with the help of the products of their reactions synthesized under their control.
7. The action of a gene can be modified by changing its location (position effect) or by various environmental factors.

Genome of prokaryotes

The genome of prokaryotes is more complex in structure and includes both structural and regulatory genes. For example, the E. coli DNA strand contains $ 3,800,000 base pairs, while the number of structural genes is about a thousand. Almost half the length of its molecule does not carry any hereditary information, these are the sections that lie between genes, called spacers .

Eukaryotic genome

The genome of eukaryotes has a much more complex structure: there is more DNA in their nucleus, which means there are more structural and regulatory genes. For example, the genome of Drosophila consists of almost $ 180,000,000 of base pairs and includes about $ 10,000 of structural genes. And in the human genome, there are more than one hundred thousand structural genes.

The concept of "dominant and recessive gene"

If a trait encoded by a genome manifests itself in a phenotype immediately, then this gene is called dominant (dominant). If the sign does not manifest itself (it is oppressed by the dominant one), then such a sign will be recessive ... It can manifest itself in the phenotype only if it is homozygous for a recessive trait.

Remark 1

Sometimes a variant happens when none of the genes has advantages and a special variant of the trait is manifested in the phenotype. In this case, we are dealing with incomplete dominance (instead of white or red flowers, pink flowers are obtained).

The human genotype includes a huge number of genes that carry information about the properties and qualities of our body. Despite such a large number, they interact as a single integral system.

From the school course in biology, we know the laws of Mendel, who studied the patterns of inheritance of traits. In the course of his research, the scientist discovered dominant and recessive genes. Some are able to suppress the manifestation of others.

In fact, the interaction of genes goes far beyond Mendelian laws, although all the rules of inheritance are respected. You can see a difference in the nature of the cleavage by phenotype, because the type of interaction may differ.

Gene characteristics

A gene is a unit of heredity, it has certain characteristics:

1. The gene is discrete. It determines the degree of development of a particular trait, including the features of biochemical reactions.
2. Has a gradual effect. Accumulating in the cells of the body, it can lead to an increase or decrease in the manifestation of the symptom.
3. All genes are strictly specific, that is, they are responsible for the synthesis of a certain protein.
4. One gene can have multiple effects, affecting the development of several traits at once.
5. Different genes can take part in the formation of one trait.
6. All genes can interact with each other.
7. The manifestation of the action of the gene is influenced by the external environment.

Genes are capable of acting at two different levels. The first is the genetic system itself, which determines the state of genes and their work, stability and variability. The second level can be considered already when working in the cells of the body.

Types of interaction of allelic genes

All cells in our body have a diploid set of chromosomes (also called double). The 23 chromosomes of the egg are fused with the same number of sperm chromosomes. That is, each trait is represented by two alleles, so they are called allelic genes.

Such allelic pairs are formed during fertilization. They can be either homozygous, that is, consisting of the same alleles, or heterozygous, if different alleles are included.

The forms of interaction of allelic genes are clearly presented in the table.

Interaction type	The nature of the interaction	Example
Complete domination	The dominant gene completely suppresses the manifestation of the recessive one.	Inheritance of pea color, human eye color.
Incomplete dominance	The dominant gene does not completely suppress the expression of the recessive gene.	Coloring of flowers in a night beauty (flower).
Codominance	In a heterozygous state, each of the allelic genes causes the development of a trait it controls.	Blood group inheritance in humans.
Overdominance	In the heterozygous state, the signs appear brighter than in the homozygous state.	A striking example is the phenomenon of heterosis in the animal and plant world, sickle cell anemia in humans.

Complete and incomplete dominance

One can speak of complete dominance in the case when one of the genes can provide the manifestation of a trait, and the second is not able to do so. A strong gene is called dominant, and its opponent is called recessive.

In this case, inheritance occurs entirely according to Mendel's laws. For example, the color of pea seeds: in the first generation we see all the peas are green, that is, this color is the dominant feature.

If, during fertilization, the gene for brown eyes and blue eyes gets together, then the child's eyes will be brown, because this allele completely suppresses the gene that is responsible for blue eyes.

With incomplete dominance, the manifestation of an intermediate trait can be seen in heterozygotes. For example, when crossing a dominant homozygous night beauty with red flowers with the same individual, only with a white corolla, you can see pink hybrids in the first generation. The dominant red trait does not completely suppress the manifestation of recessive white, so in the end, something in between is obtained.

Co-dominance and over-dominance

This interaction of genes, in which each provides its own trait, is called codominance. All genes in one allelic pair are absolutely equivalent. Neither can suppress the action of the other. It is this interaction of genes that we observe in the inheritance of blood groups in humans.

Gene O ensures the manifestation of the 1st blood group, gene A - the second, gene B - the third, and if genes A and B fall together, then neither can suppress the manifestation of the other, therefore a new trait is formed - 4th blood group.

Overdominance is another example of allelic gene interactions. In this case, heterozygous individuals for this trait have a more vivid manifestation of it in comparison with homozygous ones. This interaction of genes underlies such a phenomenon as heterosis (the phenomenon of hybrid strength).

When two varieties of tomatoes are crossed, for example, a hybrid is obtained that inherits the characteristics of both original organisms, since the characteristics become heterozygous. In the next generation, splitting according to characteristics will already take place, so it will not be possible to get the same offspring.

In the animal kingdom, the sterility of such hybrid forms can be observed altogether. Such examples of gene interaction can be found often. For example, when a donkey and a mare are crossed, a mule is born. He inherited all the best qualities of his parents, but he himself cannot have offspring.

In humans, sickle cell anemia is inherited by this type.

Non-allelic genes and their interactions

Genes that are located on different pairs of chromosomes are called non-allelic. If they happen to be together, they may well influence each other.

The interaction of non-allelic genes can be carried out in different ways:

1. Complementarity.
2. Epistasis.
3. Polymeric action.
4. Pleiotropy.

All these types of gene interactions have their own distinctive features.

Complementarity

In this interaction, one dominant gene complements another, which is also dominant, but not allelic. When they get together, they contribute to the manifestation of a completely new trait.

One can give an example of the manifestation of color in sweet pea flowers. The presence of pigment, which means that the color of the flower is provided by a combination of two genes - A and B. If at least one of them is absent, then the corolla will be white.

In humans, such an interaction of non-allelic genes is observed during the formation of the organ of hearing. Normal hearing can only be present if both genes - D and E - are present in a dominant state. In the presence of only one dominant or both in a recessive state, hearing is absent.

Epistasis

This interaction of non-allelic genes is completely opposite to the previous interaction. In this case, one non-allelic gene is able to suppress the expression of the other.

The forms of gene interaction in this variant can be different:

• Dominant epistasis.
• Recessive.

In the first type of interaction, one dominant gene suppresses the manifestation of another dominant. Recessive genes are involved in recessive epistasis.

According to this type of interaction, the color of the fruit in pumpkin, the color of the coat in horses, is inherited.

Polymeric action of genes

This phenomenon can be observed when several dominant genes are responsible for the manifestation of the same trait. If at least one dominant allele is present, then the trait will certainly appear.

The types of gene interaction in this case can be different. One of them is cumulative polymerization, when the degree of manifestation of a trait depends on the number of dominant alleles. This is how the color of wheat grains or the color of the skin in humans is inherited.

Everyone knows that all people have a different skin color. For some, it is completely light, some have dark skin, and representatives of the Negroid race are completely black. Scientists are of the opinion that skin color is determined by the presence of three different genes. For example, if all three are present in the genotype in a dominant state, then the skin is the darkest, like that of blacks.

In the Caucasian race, judging by the color of our skin, there are no dominant alleles.

It has long been found out that the interaction of non-allelic genes by the type of polymerization affects most of the quantitative traits in humans. These include: height, body weight, intellectual ability, body resistance to infectious diseases, and some others.

It can only be noted that the development of such signs depends on environmental conditions. A person may have a predisposition to being overweight, but with adherence to the diet, it is possible to avoid this problem.

Pleiotropic action of genes

For a long time, scientists were convinced that the types of gene interaction are rather ambiguous and very versatile. Sometimes it is impossible to predict the manifestation of certain phenotypic traits, because it is not known how genes interact with each other.

This statement is only emphasized by the fact that one gene can influence the formation of several traits, that is, have a pleiotropic effect.

It has long been noticed that the presence of red pigment in beets is necessarily accompanied by the presence of the same, but only in the leaves.

In humans, a disease such as Marfan's syndrome is known. It is associated with a defect in a gene that is responsible for the development of connective tissue. As a result, it turns out that wherever this tissue is in the body, problems can be observed.

Such patients have long "spider" fingers, dislocation of the lens of the eye, heart defect are diagnosed.

The influence of environmental factors on the action of genes

The influence of external environmental factors on the development of organisms cannot be denied. These include:

• Nutrition.
• Temperature.
• Light.
• Chemical composition of the soil.
• Humidity, etc.

Environmental factors are fundamental in the processes of selection, heredity and variability.

When we consider the forms of interaction between allelic genes or non-allelic genes, we must always take into account the influence of the environment. An example can be given: if primrose plants are crossed at a temperature of 15-20 degrees, then all hybrids of the first generation will have a pink color. At a temperature of 35 degrees, all plants will turn out white. So much for the influence of the environmental factor on the manifestation of signs, here it is no longer important which gene is dominant. In rabbits, it turns out, the color of the coat also depends on the temperature factor.

Scientists have long been working on the question of how the manifestations of signs can be controlled by exerting various external influences. This can provide the ability to control the development of congenital signs, which is especially important for humans. Why not use your knowledge to prevent some hereditary ailments from manifesting?

All types of interactions of allelic genes, and not only them, can be so different and multifaceted that it is impossible to attribute them to any specific type. One can only say that all these interactions are equally complex both in humans and in representatives of all types of plants and animals.

1) By the nature of the interaction in the allelic pair:

Dominant (a gene capable of suppressing the manifestation of a recessive gene allelic to it);

Recessive (gene, the expression of which is suppressed by the dominant gene allelic to it).

2) Functional classification:

modulator genes- genes that promote the spread of the tumor in the body, but are not directly responsible for the malignant transformation of the cell.

Inhibitor genes (suppressor genes, anti-oncogenes) - genes encoding key regulatory proteins, the loss of which entails a violation of proliferation control; suppress gene activity.

Gene-intensifier - enhances the activity of some genes.

Gene modifier- a gene that does not have its own expression in the phenotype, but has an intensifying or weakening effect on the expression of other genes.

Regulator gene- a gene that modifies or regulates the activity of other genes.

Protein synthesis is controlled operator genes ... The collection of working genes - operators and structural genes - is called an operon.

Allele of a gene. Multiple alleles as a result of a change in the nucleotide sequence of a gene. Gene polymorphism as a variant of norm and pathology. Examples.

Alleles (paired genes) - genes located in the same locus of homologous chromosomes and causing the formation of alternative traits (for example, genes that determine the yellow and green color of pea seeds in the experiments of G. Mendel). During meiosis, allelic genes fall into different gametes. When crossing individuals, the traits determined by allelic genes obey the laws of Mendale's splitting.

Multiple alleles- one of the types of interaction of allelic genes, in which a gene can be represented not by two alleles (as in cases of complete or incomplete dominance), but by a much larger number of them;

Examples: 1 . multiple alleles of rabbit coloration. Allele C provides black body coloration; allele ch - the so-called Himalayan coloration, when the ears, the tip of the muzzle, the tips of the paws and the tail are black; allele c causes albinism. The C allele dominates the other two, and the ch allele dominates the allele. 2. Inheritance of blood groups.

It is customary to call genes polymorphic, which are represented in a population by several varieties - alleles, which determines a variety of traits within a species.

Usually, the cause of differences (polymorphism) of genes is changes in individual nucleotides in the DNA molecule, which leads to a change in the properties of the gene (sometimes for the better, and more often for the worse). Some changes inevitably cause genetic diseases and are manifested already from birth (for example, cystic fibrosis, muscular dystrophy, etc.), these are the so-called monogenic diseases, others do not lead to diseases, but are a factor of predisposition to certain diseases (malignant tumors, cardiovascular vascular, allergic and other diseases). In this case, for the development of the disease, certain external conditions are necessary - the nature of the diet, the intake of toxins and oncogenes (tobacco smoke, alcohol) in the body, lack of vitamins, etc. These diseases are called multifactorial. Under certain conditions (a sufficiently long period of time is required - hundreds or thousands of years), mutant genes can spread in populations and become fairly common allelic variants, providing the basis for gene polymorphism.

A genotype is not just a mechanical set of genes, it is a historically formed system of genes interacting with each other. More precisely, it is not the genes themselves (sections of DNA molecules) that interact, but the products formed on their basis (RNA and proteins).

Both allelic genes and non-allelic genes can interact.

Types of gene interactions

Type of gene interaction		The nature of the interaction	Cleavage by phenotype at F 2	Genotypic composition of phenotypic classes	Example
Allelic gene interactions
Complete domination		Dominant allele A suppresses recessive allele a	3:1	3A-: 1aa	Pea seed color inheritance
Incomplete dominance		The trait in the heterozygous form is less pronounced than in the homozygous one	1:2:1	1AA: 2Aa: 1aa	Inheritance of the color of the flowers of the night beauty
Codominance		In a heterozygous state, each of the allelic genes causes the development of a trait it controls	1:2:1	1I A I A: 2I A I B: 1I B I B	Inheritance of blood groups in humans
Interaction of non-allelic genes
Cooperation		Dominant genes from different pairs (A, B), being present in the genotype together, cause the formation of a new trait. Each present separately, genes A and B cause the development of their traits	9:3:3:1	9A-B-: 3A-bb: 3aaB-: 1aabb	Inheriting the shape of the comb of chickens
Complementarity		Dominant genes from different pairs (A, B), being present in the genotype together, cause the formation of a new trait. Each being present separately, genes A and B do not cause the development of the trait.	9:7	(9A-B-): (3A-BB + 3aaB- + 1aabb)	Inheritance of the color of sweet pea flowers
Epistasis	dominant	Genes from one allelic pair suppress the action of genes from another	13:3	(9I-C- + 3I-cc + 1iicc): (3cci-)	Inheritance of the color of plumage of chickens
	recessive		9:3:4	9A-C-: 3aaC-: (3A-cc + 1aacc)	Inheritance of coat color in house mice
Polymerism		Simultaneous action of several non-allelic genes	15:1	(9A 1 -A 2 + 3A 1 -a 2 a 2 + 3a 1 a 1 A 2 -): 1a 1 a 1 a 2 a 2	Human skin color inheritance

Allelic gene interactions

There are three types of interactions between allelic genes: complete dominance, incomplete dominance, and codominance.

1. Complete domination- a phenomenon when a dominant gene completely suppresses the work of a recessive gene, as a result of which a dominant trait develops.
2. Incomplete dominance- the phenomenon when the dominant gene does not completely suppress the work of the recessive gene, as a result of which an intermediate trait develops.
3. Codominance (independent manifestation)- a phenomenon when both alleles are involved in the formation of a trait in a heterozygous organism. A person with a series of multiple alleles has a gene that determines a blood group. In this case, the genes that determine the blood groups A and B are codominant in relation to each other and both are dominant in relation to the gene that determines the blood group 0.

Interaction of non-allelic genes

There are four types of interactions between non-allelic genes: cooperation, complementarity, epistasis, and polymerization.

Cooperation- a phenomenon when, with the mutual action of two dominant non-allelic genes, each of which has its own phenotypic manifestation, a new trait is formed.

Complementarity- a phenomenon when a trait develops only with the mutual action of two dominant non-allelic genes, each of which separately does not cause the development of a trait.

Epistasis- the phenomenon when one gene (both dominant and recessive) suppresses the action of another (non-allelic) gene (both dominant and recessive). Suppressor gene(suppressor) can be dominant (dominant epistasis) or recessive (recessive epistasis).

Polymerism- a phenomenon when several non-allelic dominant genes are responsible for a similar effect on the development of the same trait. The more such genes are present in the genotype, the brighter the trait is. The phenomenon of polymerization is observed when quantitative traits are inherited (skin color, body weight, milk yield of cows).

In contrast to polymerization, there is such a phenomenon as pleiotropy- multiple gene action, when one gene is responsible for the development of several traits.