A pair of allelic genes are determinant genes. Allelic and non-allelic genes (definitions)

The genotype includes a large number of different genes, which in turn act as a single whole. Mendel, in his writings, described that he discovered only one possibility of interaction of allelic genes - when absolute dominance (predominance) of one of the alleles occurs, while the second remains completely recessive (passive, i.e. does not participate in the interaction). But let’s say right away that the phenotypic manifestation of genes (external, noticeable to the eye) cannot depend on only one or a pair of genes, because it is a consequence of the interaction of the whole system.

In reality, proteins and enzymes interact, not genes.

There are only 2 types - the first consists in the interaction of allelic genes, the second, respectively, non-allelic. It is only necessary to understand the material side of this issue, because it is not some concepts from a textbook that interact, but proteins that are synthesized according to a certain program in the cytoplasm of cells, and the number of these proteins is in the millions. The program itself, according to which proteins will be synthesized, and, as a result, their further interaction will develop, is embedded in genes that issue external commands, located in the chromosomes of cells (ultrascopic organelles of cells).

What genes are called allelic?

Allelic genes are genes that occupy the same “locations” (or loci) on chromosomes. Every living organism has allelic genes in pairs. The interaction of allelic genes can occur in several ways, which are called: codominance, overdominance, complete and incomplete dominance.

Allelic genes interact according to the principle if the action of the dominant gene completely overlaps the action of the recessive one. Incomplete dominance can be called a relationship in which it is not completely suppressed and takes, albeit minimally, part in the formation of phenotypic characteristics.

Codominance occurs when allelic genes exhibit their properties independently of each other. Probably the most illustrative example of codominance is the AB0 blood system, in which both A and B genes function independently of each other.

Overdominance is an increase in the quality of the phenotypic manifestations of a dominant gene if it is “in conjunction” with a recessive one. That is, if there are 2 alleles in one allele, then they manifest themselves worse than a dominant gene that is “in conjunction” with a recessive one.

Multiple allelism

As mentioned earlier, each living creature can have only 2 allelic genes, but there can be much more than two alleles - this phenomenon is called multiple allelism. Let us say right away that only one pair of alleles can exhibit phenotypic characteristics, that is, while some are working, others are resting.

Almost always, homologous (identical) alleles are responsible for the development and manifestation of the same trait, but they differ in the quality of its manifestation. Also, multiple allelism is characterized by various forms of gene interaction. That is, although they are responsible for the same trait, they, firstly, manifest it in different ways, and secondly, using different methods (complete, incomplete dominance, and so on).

It would seem, why such confusion? It's simple - only one pair of homologous alleles can enter the reproductive cell of a living creature, but which of all the available ones decides the case. It is thanks to this that variability of species is achieved, which plays a major role in the evolution of living beings.

A pair of genes that determines alternative (opposite) traits is called an allelic pair, and this pairing phenomenon is called allelism.

Each gene can be in two states - A and a, so they are a pair, and each of them is called an allele. These allelic genes are located in the same regions of chromosomes and regulate the alternative development of the same trait.

In the most primitive case, a gene consists of two alleles. For example, the white and purple colors of pea seeds are dominant and recessive traits, respectively, of two alleles of one gene. There are three-allelic genes, for example, a gene that regulates whether a person has a certain blood group according to the ABO system. There may be more alleles. So, for the gene that determines the production of hemoglobin, 20-30 types are known. But no matter how many alleles each gene has, the reproductive cell contains only one allele, according to the rule of gamete purity. Accordingly, in a diploid cell of an organism there are only two alleles obtained from each of the parent organisms.

Interaction

The interaction of allelic genes is a phenomenon in which several allelic genes are responsible for the manifestation of one trait. If these alleles are of the same gene, then the interactions are called allelic; if they are different, they are called non-allelic. There are the following main forms of interaction of allelic genes: complete dominance, incomplete dominance, codominance.

Genetics- a science that studies genes, mechanisms of inheritance of traits and variability of organisms. During the process of reproduction, a number of traits are passed on to the offspring. It was observed back in the nineteenth century that living organisms inherit the characteristics of their parents. The first to describe these patterns was G. Mendel.

Heredity– the property of individual individuals to transmit their characteristics to their offspring through reproduction (through reproductive and somatic cells). This is how the characteristics of organisms are preserved over a number of generations. When transmitting hereditary information, its exact copying does not occur, but variability is always present.

Variability– the acquisition by individuals of new properties or the loss of old ones. This is an important link in the process of evolution and adaptation of living beings. The fact that there are no identical individuals in the world is due to variability.

Inheritance of characteristics is carried out using elementary units of inheritance - genes. The set of genes determines the genotype of an organism. Each gene carries encoded information and is located in a specific place in the DNA.

Genes have a number of specific properties:

1. Different traits are encoded by different genes;
2. Constancy - in the absence of a mutating effect, the hereditary material is transmitted unchanged;
3. Lability – the ability to succumb to mutations;
4. Specificity - a gene carries special information;
5. Pleiotropy – one gene encodes several traits;

Under the influence of environmental conditions, the genotype gives different phenotypes. The phenotype determines the degree to which the organism is influenced by environmental conditions.

Allelic genes

The cells of our body have a diploid set of chromosomes; they, in turn, consist of a pair of chromatids, divided into sections (genes). Different forms of the same genes (for example, brown/blue eyes), located in the same loci of homologous chromosomes, are called allelic genes. In diploid cells, genes are represented by two alleles, one from the father and one from the mother.

Alleles are divided into dominant and recessive. The dominant allele determines which trait will be expressed in the phenotype, and the recessive allele is inherited, but does not manifest itself in a heterozygous organism.

There are alleles with partial dominance, such a condition is called codominance, in which case both traits will appear in the phenotype. For example, flowers with red and white inflorescences were crossed, resulting in red, pink and white flowers in the next generation (pink inflorescences are a manifestation of codominance). All alleles are designated by letters of the Latin alphabet: large - dominant (AA, BB), small - recessive (aa, bb).

Homozygotes and heterozygotes

Homozygote is an organism in which alleles are represented only by dominant or recessive genes.

Homozygosity means having the same alleles on both chromosomes (AA, bb). In homozygous organisms, they code for the same traits (for example, the white color of rose petals), in which case all offspring will receive the same genotype and phenotypic manifestations.

Heterozygote is an organism in which alleles have both dominant and recessive genes.

Heterozygosity is the presence of different allelic genes in homologous regions of chromosomes (Aa, Bb). The phenotype of heterozygous organisms will always be the same and is determined by the dominant gene.

For example, A – brown eyes, and – blue eyes, an individual with genotype Aa will have brown eyes.

Heterozygous forms are characterized by splitting, when when crossing two heterozygous organisms in the first generation we get the following result: by phenotype 3:1, by genotype 1:2:1.

An example would be the inheritance of dark and light hair if both parents have dark hair. A is a dominant allele for dark hair, and is recessive (blond hair).

R: Aa x Aa

G: A, a, a, a

F: AA:2Aa:aa

*Where P – parents, G – gametes, F – offspring.

According to this diagram, you can see that the probability of inheriting a dominant trait (dark hair) from parents is three times higher than a recessive one.

Diheterozygote- a heterozygous individual that carries two pairs of alternative characteristics. For example, Mendel's study of the inheritance of traits using pea seeds. The dominant characteristics were yellow color and smooth seed surface, while the recessive characteristics were green color and rough surface. As a result of the crossing, nine different genotypes and four phenotypes were obtained.

Hemizygote- this is an organism with one allelic gene, even if it is recessive, it will always manifest itself phenotypically. Normally they are present on sex chromosomes.

Difference between homozygote and heterozygote (table)

Reproduction and crossing of homozygotes and heterozygotes leads to the formation of new characteristics that are necessary for living organisms to adapt to changing environmental conditions. Their properties are necessary when breeding crops and breeds with high quality indicators.

Alleles(allelic genes) are different forms of the same gene. An allele is one form of a particular gene. Different genes can have different numbers of alleles. If there are more than two alleles of a gene, then they say multiple allelism.

In diploid (containing a double set of chromosomes) cells of the body, there are two alleles of each gene. Alleles of the same gene are located at the same loci (locations) on homologous chromosomes.

If two alleles of one gene in the cells of an organism are the same, then such an organism (or cell) is called homozygous. If the alleles are different, then the organism is called heterozygous.

Alleles of one gene, being in one organism, interact with each other, and this interaction determines how the trait determined by the corresponding gene will manifest itself. The most common type of interaction is complete dominance, in which one allelic gene manifests itself and completely suppresses the expression of another allelic gene. In this case, the first one is called dominant, and the second - recessive.

In genetics, it is customary to denote a dominant gene with a capital letter (for example, A), and a recessive gene with a small letter (a). If an individual is heterozygous, then its genotype will be Aa. If homozygous, then AA or aa. In the case of complete dominance, genotypes AA and Aa will have the same phenotype.

In addition to complete dominance, there are other types of interaction of alleles: incomplete dominance, codominance, overdominance, complementation of alleles and some others. In case incomplete dominance a heterozygous genotype will have an intermediate value of the trait. For example, the parent forms of plants have white (aa) and red (AA) flowers, and their hybrid in the first generation (Aa) has pink flowers. In this case, none of the alleles fully manifested themselves, but they were not completely suppressed either.

At co-dominance two allelic genes, once in one organism, fully express themselves. As a result, the body synthesizes two different proteins that determine the same trait. Something similar happens with overdominance and interallelic complementation.

With multiple allelism, the relationships between alleles may be ambiguous. Firstly, if there is exclusively complete dominance, then one gene may be dominant in relation to another, but recessive in relation to a third. In this case, rows are built (A > a" > a"" > a"" ...), which reflect the relationships of dominance. For example, coat color in many animals and eye color are inherited.

Secondly, in one pair of alleles there may be a relationship of complete dominance, and in the other - codominance. Thus, human blood groups are determined by a gene that exists in three forms (alleles): I 0, I A, I B. Genes I A and I B are dominant in relation to I 0, but interact with each other according to the principle of codominance. As a result, if a person has the genotype I 0 I 0, then he will have the 1st blood group. If I A I A or I A I 0, then 2nd. I B I B and I B I 0 define the 3rd group. People with genotype I A I B have blood group 4.

The frequency of occurrence of allelic genes in a population may vary. Often recessive genes are rare and are essentially mutations of the main allele. Many mutations are harmful. However, it is mutant genes that create the material for the action of natural selection and, as a consequence, the process of evolution.

In a hypothetical ideal population (in which natural selection does not operate, which has an unlimitedly large size, is isolated from other populations, etc.), the frequency of genotypes (for a particular gene) does not change and obeys Hardy-Weinberg law. According to this law, the distribution of genotypes in the population will fit into the equation: p 2 + 2pq + q 2 = 1. Here p and q are the frequencies (expressed in fractions of one) of alleles in the population, p 2 and q 2 are the frequencies of the corresponding homozygotes, and 2pq - frequency of heterozygotes.

ALLELES(Greek allēlōn - mutually; synonym allelomorphs) - various forms of gene state, occupying identical sections in homologous, paired chromosomes and determining the commonality of the biochemical processes of development of a particular trait. Each gene can exist in at least two allelic states, determined by its structure. The presence of allelic genes determines phenotypic differences among individuals.

The terms “allelomorphs”, “allelomorphic pair”, “allelomorphism” were proposed by Bateson and Saunders (W. Bateson, J. Saunders, 1902). Subsequently, Johannsen (W. L. Johannsen, 1909) proposed replacing them with shorter ones - “alleles”, “allelic pair”, “allelism”.

In its original meaning, the term “alleles” denoted only genes that determine a pair of alternative Mendelian traits (see Mendelian laws). Despite the fact that in essence the terms “gene” and “allele” should be synonymous, the term “allele” is used to designate a specific type of gene. The concept “gene” refers to the locus (see) of the chromosome as such, regardless of the number of existing alleles of this gene.

Each homologous chromosome can contain only one allele of a given gene. Since diploid organisms have two chromosomes of each type (homologous chromosomes), the cells of these organisms contain two alleles of each gene. An allelic pair is formed at fertilization and can consist of identical or non-identical alleles. In the first case, we talk about the allele in a homozygous state, in the second - in a heterozygous state. In addition, allelism in the hemizygous state can be detected in males of diploid organisms. This is due to the fact that in humans the pair of sex chromosomes (XY chromosomes) is not homologous. As a result, in cases where an allelic pair cannot be made, the expression of genes does not depend on whether they are dominant or recessive (see Dominance). An individual who has one or more of these unpaired genes, but is diploid in the remaining genes, is called hemizygous.

The name (nomenclature) of genes usually corresponds to their final effects (phenotypes), and English terminology is used. Thus, the recessive gene causing achondroplasia can be called achondroplasia. For ease of writing genetic formulas, alleles are designated by symbols. A recessive allele is usually designated by the first lowercase letter of the name of a given gene; in particular, for the achondroplasia gene, the symbol can be a. If the symbol a has already been used previously to designate other genes of a given species, then the symbol ac or some other symbol can be taken.

The dominant gene is designated in one of the following ways: the same, but with a capital letter (L), the same letter with a superscript + (a+); sign + with the superscript symbol of the recessive allele (+a) or most often just sign +. Thus, the genetic formula for an individual heterozygous for the mutant recessive albinism gene will be c/+, for an albino c/c, and for a person with normal pigmentation +/+.

A gene that is commonly found in nature and ensures the normal development and viability of an organism is called a normal or wild-type allele.

A normal allele can mutate (see Mutagenesis). As a result of a series of successive mutations (see), a series of alleles of one locus may arise. This phenomenon is called multiple allelism. Therefore, in order to determine diverse changes in a gene, it is necessary to study many individuals - carriers of different members of a series of multiple alleles. People with blood type A are divided into three subgroups. This is due to the presence in human populations of three different alleles of the IA gene - IA1, IA2 and IA3. For the other allele of this system, IB, three different allelic forms are also known, which leads to the identification of three groups of people with blood type B.

Currently, population genetic studies have identified more than 50 different alleles that control the synthesis of α- or β-polypeptide chains of the hemoglobin molecule or the enzyme glucose-6-phosphate dehydrogenase in humans.

The main form of interaction between alleles is dominance (see Dominance). The normal (wild) allele is usually dominant over the mutant allele. Depending on the nature of the interaction of normal alleles with mutant ones, amorphs, hypomorphs, hypermorphs, antimorphs and neomorphs are distinguished. Amorphs are completely recessive alleles; hypomorphs have the same properties as normal alleles, only to a weakened degree; hypermorphs produce more primary products in the cell compared to the normal allele; antimorphs suppress the manifestation of the effects of normal alleles, aneomorphs - alleles with new functions, their effects differ not quantitatively, but qualitatively from the effects of the normal allele.

Although no fundamental differences in the effects of dominant and recessive alleles have been identified, the end products of their activity (effects) are different. This is especially clear in enzymes. The transformation of a normal dominant allele into a mutant recessive one often results in the synthesis of an inactive enzyme. If heterozygotes exhibit the effects of both alleles, then this pattern of gene action is called codominant (see Codominance).

The only known exception to the rule of codominant action of autosomal genes is, apparently, genetic control of the synthesis of immunoglobulin polypeptide chains. The immunoglobulin molecule consists of 2 heavy and 2 light full-peptide chains, the synthesis of which is controlled by two pairs of autosomal unlinked genes, and in each cell only one of the allelic genes of these loci is active. This allelic exclusion of autosomal genes is obviously associated with the specificity of immunoglobulin biosynthesis.

In the history of the development of the doctrine of alleles, a major role was played by the discovery of the phenomenon of stepwise allelism (N.P. Dubinin, A.S. Serebrovsky and others, 1929-1934). In this case, the development of the method of interallelic complementation (see Mutation analysis) made it possible to show that during mutations the gene can not change as a whole, but through changes in its individual parts. This marked the beginning of the doctrine of the complex structure of the gene and significantly changed the old concepts of the essence of the allele. With different changes in the same gene region, homoalleles arise. In this case, there is no recombination between alleles (see). When different places within a gene change, heteroalleles appear.

Pseudoalleles are closely linked loci that have similar phenotypic effects. Their similarity to alleles is that they are usually transmitted together as one unit, although in rare cases they can recombine as a result of crossing over. In cis- and trans-positions (see Molecular genetics), pseudo-alleles cause different phenotypes. In cis-heterozygotes (ab/++), mutant pseudoalleles exhibit a wild or normal phenotype, and in trans-heterozygotes (a+/+b) - a mutant phenotype. A group of closely linked loci is called a series of pseudoalleles, or a complex gene locus.

Genes with the same function and localization in individuals of different species are called homologous. The presence of homologous genes in individuals of different species is explained by their origin from common parental forms. For example, mutations in genes that control the synthesis of the enzyme tyrosinase, which is involved in the formation of the melanin pigment, lead to the inactivity of this enzyme and, as a result, to the appearance of albinism in various species. Homologous genes also control the synthesis of factors VIII and IX of the blood coagulation system in humans and other mammals. Mutations in these genes cause the development of hemophilia A and B.

For most genes, a multiplicity of manifestation effects has been established, as a result of which mutant genes cause the occurrence of various syndromes (see Pleiotropy). The visible effects of some genes do not in all cases manifest themselves phenotypically in carriers of these genes (see Gene penetrance). The degree of manifestation of the effects of allelic genes is often influenced by other non-allelic genes - modifier genes. The latter themselves do not have any visible manifestation effects, but are capable of enhancing or weakening the effects of the so-called. the main genes that control the formation of alternative Mendelian traits. The formation of a certain trait may also depend on the interaction of two or more dominant non-allelic genes, each of which does not have an independent manifestation, but controls the occurrence of one of the links in a sequential chain of biochemical reactions. Such genes are called complementary. The trait they control manifests itself phenotypically only if all the dominant alleles of these loci are present in the body.

Thus, the presence in a population of diverse forms of genes that make up allelic pairs, the complex nature of the relationships within this pair, the influence on the manifestation of this pair of non-allelic genes is the main reason for the existence of phenotypic differences among individuals of this population for a certain trait.

Bibliography

Gershkovich I. Genetics, trans. from English, M., 1968, bibliogr.; Dubinin N.P. General genetics, M., 1970; Lobashev M. E. Genetics. L., 1967; Medvedev N.N. Practical genetics, M., 1966, bibliogr.; Harris H. Polymorphism and protein evolution, J. med. Genet., v. 8, p. 444, 1971, bibliogr.; Wagner R. P. a. Mitchell N.K. Genetics and metabolism, N.Y., 1964.

B.V. Konyukhov.