The genotype includes a large number of various genes, which in turn act as a single whole. The founder of genetics, Mendel, described in his writings that he discovered only one possibility of interaction of allelic genes - when there is absolute dominance (predominance) of one of the alleles, while the second remains completely recessive (passive, i.e. does not participate in interaction).
Allelic genes and main types of their interaction
Each gene has two states - A and a, so they form one pair, and each member of the pair is called an allele. Thus, genes located in the same loci (regions) of homologous chromosomes and determining alternative development of the same trait are called allelic.
In the simplest case, a gene is represented by two alleles. For example, the purple and white color of a pea flower are dominant and recessive traits, respectively, for two alleles of the same gene.
Allelic genes
Most people on globe know that genes are passed on hereditary traits parents to their offspring, and this applies not only to humans, but to all living beings on the planet. These microscopic structural units represent a certain segment of DNA that determines the sequence of polypeptides (chains of more than 20 amino acids that make up DNA). The nature and methods of interaction of genes are quite complex, and the slightest deviations from the norm can lead to genetic diseases.
Biology and medicine
Allele (allelomorph, allele): one of two (or more) sequence-differing forms of a particular gene. Alternative forms of sequences located in the same loci (identical regions) on homologous chromosomes. Humans have two sets of chromosomes. one from each parent. The equivalent sequences in the two sets may be different, for example due to single nucleotide polymorphisms.
Therefore, the alleles are alternative options gene at each locus.
What is an allele
Alleles (from Greek word allelon - mutually), or allelomorphs, - different shapes the same gene (in the singular - allele).
In the simplest case, a gene is represented by two alleles (for example, the alleles determining green and yellow colors peas in the experiments of G. Mendel). An example of a three-allelic gene is gene. determining a person’s blood group system AB0 (read “A-B-zero”). At different combinations These alleles form the first blood group (00), the second (A0, AA), the third (B0, BB) and the fourth (AB).
What are allelic genes
An allele is one of the forms of a gene that determines one of many variants of the development of a particular trait. Alleles are usually divided into dominant and recessive - the first completely corresponds to a healthy gene, while the recessive includes various mutations of its gene, leading to a “malfunction” in its operation. Multiple allelism also occurs, in which geneticists identify more than two alleles.
An organism with the same allelic genes is considered homozygous, and an organism with different alleles is considered heterozygous.
And determining alternative development options for the same trait. In a diploid organism, there may be two identical alleles of the same gene, in which case the organism is called homozygous, or two different alleles, resulting in a heterozygous organism. The term “allele” was proposed by V. Johansen (1909)
Normal diploid somatic cells contain two alleles of one gene (according to the number of homologous chromosomes), and haploid gametes contain only one allele of each gene. For characters that obey Mendel's laws, one can consider dominant And recessive alleles. If the genotype of an individual contains two different alleles (the individual is a heterozygote), the manifestation of the trait depends on only one of them - the dominant one. A recessive allele affects the phenotype only if it is on both chromosomes (the individual is homozygous). In more complex cases, other types of allelic interactions are observed (see below).
Types of allelic interactions
- Complete Domination- interaction of two alleles of one gene, when the dominant allele completely excludes the manifestation of the effect of the second allele. The phenotype contains only the trait determined by the dominant allele.
- Incomplete dominance- the dominant allele in the heterozygous state does not completely suppress the effect of the recessive allele. Heterozygotes have an intermediate character of the trait.
- Overdominance- a stronger manifestation of the trait in a heterozygous individual than in any homozygous one.
- Codominance- the manifestation in hybrids of a new trait caused by the interaction of two different alleles of one gene. The phenotype of heterozygotes is not something intermediate between the phenotypes of different homozygotes.
Multiple alleles
Multiple allelism is the existence of more than two alleles of a given gene in a population. There are not two allelic genes in a population, but several. They arise as a result of different mutations of one locus. Genes from multiple alleles interact with each other in different ways.
In populations of both haploid and diploid organisms, there are usually many alleles for each gene. This follows from the complex structure of the gene - replacement of any of the nucleotides or other mutations lead to the appearance of new alleles. Apparently, only in very rare cases does any mutation have such a strong effect on the functioning of a gene, and the gene turns out to be so important that all its mutations lead to the death of its carriers. Thus, for the well-studied human globin genes, several hundred alleles are known, only about a dozen of them lead to serious pathologies.
Lethal alleles
Lethal alleles are those whose carriers die due to developmental disorders or diseases associated with the operation of this gene. There are all transitions between lethal alleles and alleles that cause hereditary diseases. For example, patients with Huntington's chorea (an autosomal dominant trait) usually die within 15-20 years after the onset of the disease from complications, and some sources suggest that this gene is lethal.
Allele designation
Typically, an allele is designated by abbreviating the name of the corresponding gene to one or more letters; to distinguish a dominant allele from a recessive one, the first letter in the designation of the dominant one is capitalized.
see also
Notes
Literature
- Biological encyclopedic Dictionary. - M.: “ Soviet encyclopedia", 1986.
- Inge-Vechtomov S. G. Genetics with the basics of selection. - M.: “ graduate School", 1989.
| Genetics | |
|---|---|
| Introduction Story Related Topics List of organizations List of genetic terms | |
| Key Components | Chromosome Nucleotide RNA Genome |
| Fields of genetics | Classical genetics Conservation genetics Environmental genetics Immunogenetics Molecular genetics Population genetics Quantitative genetics |
Most people on the globe know that genes transmit the hereditary characteristics of parents to their offspring, and this applies not only to humans, but to all living beings on the planet. These microscopic structural units represent a certain segment of DNA that determines the sequence of polypeptides (chains of more than 20 amino acids that make up DNA). The nature and methods of interaction of genes are quite complex, and the slightest deviations from the norm can lead to genetic diseases. Let's try to understand the essence of genes and the principles of their behavior.
The concept of “allelicity,” according to Greek terminology, implies reciprocity. It was introduced by the Danish scientist Wilhelm Johansen at the beginning of the twentieth century. The term “gene,” as well as “genotype” and “phenotype,” was coined by the same Johansen. In addition, he discovered the important law of “pure line” heredity.
Based on numerous experiments with plant material, it was found that the same genes within a locus (the same section of a chromosome) can take on various shapes, which have a direct impact on the diversity of variations of any parental trait. Such genes were called alleles, or alleles. In creatures whose organism is diploid, that is, it has paired sets of chromosomes, allelic genes can be present either two identical or two different. In the first case, they talk about the homozygous type, in which the inherited characteristics are identical. In the second case, the type is heterozygous. Its hereditary traits vary because the copies of genes on the chromosomes are different from one another.
Dominant principle of heredity
The human body is diploid. The cells of our body (somatic) include two allelic genes.
Only gametes (sex cells) contain a single allele that determines the sex characteristic. When male and female gametes fuse, a zygote is obtained in which there is a double set of chromosomes, that is, 46, including 23 maternal and 23 paternal. Of these, 22 pairs are homological (identical) and 1 is sexual. If she received the XX chromosome set, a female individual develops, and if XY, then a male. Each chromosome, as noted above, contains 2 alleles. For convenience, they were divided into two types - dominant and recessive. The former are much stronger than the latter. The hereditary information contained in them turns out to be prevalent. What characteristics a nascent individual will inherit from its parents depends on whose allelic genes (father or mother) were dominant. This is the simplest way for alleles to interact.
Other types of inheritance
Each parent can be a carrier of homozygous or heterozygous genes for dominant or recessive traits. A child who has received dominant and recessive allelic genes from homozygous parents will inherit only dominant traits.
Simply put, if the dominant person in a couple is dark color hair, and recessive - light, all children will be born only dark-haired. In the case when one of the parents has a dominant gene of the heterozygous type, and the other - homozygous, their children will be born with a dominant and recessive trait of approximately 50 X 50. In our example, the couple may have both dark-haired and blond children. If both parents have heterozygous dominant and recessive genes, every fourth child will inherit recessive traits, that is, will be fair-haired. This rule of inheritance is very important, since there are many diseases transmitted through genes, and one of the parents may be the carrier. Such pathologies include dwarfism, hemochromatosis, hemophilia and others.
How are alleles designated?
In genetics, alleles are usually denoted by the first letters of the name of the gene of which they are forms. The dominant allele is written with a capital letter. Nearby indicate serial number modified gene form. The word “allele” in Russian can be used in both the feminine and masculine gender.
Types of allelic interactions
The interaction of allelic genes can be divided into several types:
What is allelic exclusion
It happens that in homogametic individuals containing germ cells with the same set of chromosomes, one of them becomes little or completely inactive. Regarding people, this condition is observed in women, while, say, in butterflies, on the contrary, in males. With allelic exclusion, only one of the two chromosomes is expressed, and the second becomes a so-called Barr body, that is, an inactive unit twisted into a spiral. This structure is called mosaic. In medicine, this can be seen in B lymphocytes, which can synthesize antibodies only to certain antigens. Each such lymphocyte chooses between the activity of either the paternal allele or the maternal one.
Multiple allelism
In nature, a widespread phenomenon is when the same gene has not two, but more forms. In plants this is manifested by a variety of stripes on leaves and petals, in animals - various combinations colors In humans, a striking example of multiple allelism is the inheritance of a child's blood type. Its system is designated ABO and is controlled by a single gene. Its locus is designated I, and allelic genes are designated IA, IB, IO. Combinations of IO IO give the first blood group, IA IO and IA IA - the second, IB IO and IB IB - the third, and IA IB - the fourth. In addition, Rh is determined in humans. Positive is given by combinations of 2 allelic genes with the sign “+” or 1+ and 1-. Rh negative is produced by two allelic genes with the “-” trait. The Rh system is controlled by CDE genes, and the D gene often causes Rh conflict between the fetus and the mother if her blood is Rh negative and the fetus is Rh positive. In such cases, in order for the second and subsequent pregnancies to be successfully completed, the woman is given special therapy.
Lethal allelic genes
Alleles whose carriers die due to genetic diseases caused by these genes are called lethal. In humans they cause Huntington's disease. In addition to lethal ones, there are also so-called semi-lethal ones. They can cause death, but only under certain conditions, e.g. high temperatures environment. If these factors can be avoided, semi-lethal genes do not cause the death of the individual.
And determining alternative development options for the same trait. In a diploid organism, there may be two identical alleles of the same gene, in which case the organism is called homozygous, or two different alleles, resulting in a heterozygous organism. The term “allele” was proposed by V. Johansen (1909)
Normal diploid somatic cells contain two alleles of one gene (according to the number of homologous chromosomes), and haploid gametes contain only one allele of each gene. For characters that obey Mendel's laws, one can consider dominant And recessive alleles. If the genotype of an individual contains two different alleles (the individual is a heterozygote), the manifestation of the trait depends on only one of them - the dominant one. A recessive allele affects the phenotype only if it is on both chromosomes (the individual is homozygous). In more complex cases, other types of allelic interactions are observed (see below).
Types of allelic interactions
- Complete Domination- interaction of two alleles of one gene, when the dominant allele completely excludes the manifestation of the effect of the second allele. The phenotype contains only the trait determined by the dominant allele.
- Incomplete dominance- the dominant allele in the heterozygous state does not completely suppress the effect of the recessive allele. Heterozygotes have an intermediate character of the trait.
- Overdominance- a stronger manifestation of the trait in a heterozygous individual than in any homozygous one.
- Codominance- the manifestation in hybrids of a new trait caused by the interaction of two different alleles of one gene. The phenotype of heterozygotes is not something intermediate between the phenotypes of different homozygotes.
Multiple alleles
Multiple allelism is the existence of more than two alleles of a given gene in a population. There are not two allelic genes in a population, but several. They arise as a result of different mutations of one locus. Genes from multiple alleles interact with each other in different ways.
In populations of both haploid and diploid organisms, there are usually many alleles for each gene. This follows from the complex structure of the gene - replacement of any of the nucleotides or other mutations lead to the appearance of new alleles. Apparently, only in very rare cases does any mutation have such a strong effect on the functioning of a gene, and the gene turns out to be so important that all its mutations lead to the death of its carriers. Thus, for the well-studied human globin genes, several hundred alleles are known, only about a dozen of them lead to serious pathologies.
Lethal alleles
Lethal alleles are those whose carriers die due to developmental disorders or diseases associated with the operation of this gene. There are all transitions between lethal alleles and alleles that cause hereditary diseases. For example, patients with Huntington's chorea (an autosomal dominant trait) usually die within 15-20 years after the onset of the disease from complications, and some sources suggest that this gene is lethal.
Allele designation
Typically, an allele is designated by abbreviating the name of the corresponding gene to one or more letters; to distinguish a dominant allele from a recessive one, the first letter in the designation of the dominant one is capitalized.
see also
Notes
Literature
- Biological encyclopedic dictionary. - M.: “Soviet Encyclopedia”, 1986.
- Inge-Vechtomov S. G. Genetics with the basics of selection. - M.: “Higher School”, 1989.
| Genetics | |
|---|---|
| Introduction Story Related Topics List of organizations List of genetic terms | |
| Key Components | Chromosome Nucleotide RNA Genome |
| Fields of genetics | Classical genetics Conservation genetics Environmental genetics Immunogenetics Molecular genetics Population genetics Quantitative genetics |
An allele is one of the forms of a gene that determines one of many variants of the development of a particular trait. Usually alleles are divided into dominant and - the first completely corresponds to a healthy gene, then it includes various mutations of its gene, leading to a “malfunction” in its operation. Multiple allelism also occurs, in which geneticists identify more than two alleles.
With multiple allelism, diploid organisms have two alleles inherited from their parents in different combinations.
An organism with the same allelic genes is considered homozygous, and an organism with different alleles is considered heterozygous. A heterozygote is distinguished by the manifestation of a dominant trait in phenotype and concealment. With complete dominance, the heterozygous organism has a dominant phenotype, while with incomplete dominance it is intermediate between the recessive and dominant allele. Thanks to a pair of homologous alleles that enter the germ cell of an organism, the species of living beings are variable and capable of evolution.
Interaction of allelic genes
There is only one possibility for the interaction of these genes - with the absolute dominance of one allele over the second, which remains in a recessive state. The basics of genetics include no more than two types of interaction of allelic genes - allelic and non-allelic. Since the allelic genes of every living organism are always present in a pair, their interaction can occur in the manner of codominance, overdominance, as well as complete and incomplete dominance.
Only one pair of allelic genes is capable of manifesting phenotypic characteristics - while some are resting, others are working.
The interaction of alleles with complete dominance occurs only when the dominant gene completely overlaps the recessive one. Interaction with incomplete dominance occurs when a recessive gene, partially involved in the formation of traits, is incompletely suppressed.
Codominance occurs with a separate manifestation of the properties of allelic genes, while overdominance represents an increase in the quality of the phenotypic characteristics of a dominant gene that is in conjunction with a recessive gene. Thus, two dominant genes in the same allele will perform worse than a dominant gene supplemented by a recessive one.
